Bookmark and Share

Air conditioning is used throughout the world to counter the negative effects caused by heat and humidity. Without air conditioning people tire easily and feel lethargic, resulting in low morale and productivity. As a HVAC-R pro, one of your duties will be to install, operate, maintain, and repair air-conditioning systems to provide the required comfort in working spaces. In order to provide this comfortable environment, you need to have an understanding of the principles and theory of air conditioning, be able to recognize system components and controls, and understand how they work within the system. This course will provide you with the information required to meet those requirements. Also, covered in this course are the basic types of ductwork systems that deliver the conditioned air necessary to cool a specified area.

When you have completed this course, you will be able to:


1.0.0 Principles of Air Conditioning

2.0.0 Air-Conditioning Systems

3.0.0 Major System Components and Controls

4.0.0 Automotive Air Conditioning

5.0.0 Ductwork

Review Questions


Air conditioning is the process of conditioning the air in a space to maintain a predetermined temperature-humidity relationship to meet comfort or technical requirements. This warming and cooling of the air is usually referred to as winter and summer air conditioning.

1.1.0 Temperature

Temperature, humidity, and air motion are interrelated in their effects on health and comfort. The term given to the net effects of these factors is effective temperature. This effective temperature cannot be measured with a single instrument; therefore, a psychrometric chart aids in calculating the effective temperature when given sufficient known conditions relating to air temperatures and velocity.

Research has shown that most persons are comfortable in air where the effective temperature lies within a narrow range. The range of effective temperatures that most people feel comfortable in is called the COMFORT ZONE. Since winter and summer weather conditions are markedly different, the summer zone varies from the winter zone. The specific effective temperature within the zone at which most people feel comfortable is called the COMFORT LINE (Figure 1).

Figure 1 — Comfort zones and lines.

1.2.0 Humidity

When air is at a high temperature and saturated with moisture, it makes people feel uncomfortable. However, people usually feel quite comfortable at the same temperature with fairly dry air. As dry air passes over the surface of the skin, it evaporates the moisture sooner than damp air, producing a greater cooling effect. However, if the air is too dry it causes discomfort. When air is too dry, it causes the surface of the skin to become dry and irritated.

Humidity is the amount of water vapor in a given volume of air. Relative humidity is the amount of water vapor in a given amount of air in comparison to the amount of water vapor the air would hold at a temperature if it were saturated.

Relative humidity may be remembered as a fraction or percentage of water vapor in the air; that is, DOES HOLD divided by CAN HOLD. Relative humidity is determined by using a sling psychrometer. It consists of a wet-bulb thermometer and a dry-bulb thermometer, as shown in Figure  2. The wet-bulb thermometer is an ordinary thermometer similar to the dry-bulb thermometer, except that the bulb is enclosed in a wick that is wet with distilled water.

The wet bulb is cooled as the moisture evaporates from it while it is being spun through the air. This action causes the wet-bulb thermometer to register a lower temperature than the dry-bulb thermometer. For certain conditions, tables and charts have been designed that use these two temperatures to arrive at a relative humidity.

 A comfort zone chart is shown in Figure 3. The comfort zone is the range of effective temperatures that feel comfortable to the majority of adults. In looking over the chart, note that the comfort zone represents a considerable area. The charts show the wet- and dry-bulb temperature combinations that are also comfortable to the majority of adults. The summer comfort zone extends from 66°F effective temperature to 75°F effective temperature for 98 percent of all persons. The winter comfort zone extends from 63°F effective temperature to 71°F effective temperature for 97 percent of all persons.

Figure 3 — Comfort zone chart.

1.2.1 Dew-Point Temperature

The dew point depends on the amount of water vapor in the air. If the air at a certain temperature is not saturated (maximum water vapor at that temperature) and the air temperature falls, a point is finally reached saturating the air for the new and lower temperature, and moisture condensation begins. This is the dew-point temperature of the air for the quantity of water vapor present.

1.2.2 Relationship of Wet-Bulb,

Dry-Bulb, and Dew-Point Temperatures A definite relationship exists between the wet-bulb, dry-bulb, and dew-point temperatures. These relationships are as follows:

 To humidify air is to increase its water vapor content. To dehumidify air is to decrease its water vapor content. The device used to add moisture to the air is a humidifier, and the device used to remove the moisture from the air is a dehumidifier. The control device, sensitive to various degrees of humidity, is called a humidistat.

Methods for humidifying air in air-conditioning units usually consist of an arrangement that causes air to pick up moisture. One arrangement consists of a heated water surface over which conditioned air passes and picks up a certain amount of water vapor by evaporation, depending upon the degree of humidifying required. A second arrangement to humidify air is to spray or wash the air as it passes through the air-conditioning unit.

During the heat of the day, the air usually absorbs moisture. As the air cools at night, it may reach the dew point and give up moisture, which is deposited on objects. This principle is used in dehumidifying air by mechanical means.

Dehumidifying equipment for air conditioning usually consists of cooling coils within the air conditioner. As warm, humid air passes over the cooling coils (Figure 4), its temperature drops below the dew point and some of its moisture condenses into water on the surface of the coils. The condensing moisture gives up latent heat that creates a part of the cooling load that must be overcome by the air-conditioning unit. For this reason, the relative humidity of the air entering the air conditioner has a definite bearing on the total cooling load. The amount of water vapor that can be removed from the air depends upon the air over the coils and the temperature of the coils.

Figure 4 — Air conditioning cooling coils.

1.3.0 Purity of Air

The air should be free from all foreign materials, such as ordinary dust, rust, animal and vegetable matter, and pollen. It should also be free of carbon (soot) from poor combustion, fumes, smoke, and gases. These types of pollution alone are harmful to the human body but they also carry bacteria and harmful germs, which can cause additional dangers. During air conditioning, the outside air brought into a space or the re-circulating air within a space should be filtered.

Air in an air conditioner may be purified or cleaned by filters, air washing, or electricity.

The types of filters that can be used consist of permanent or throwaway. They are usually made of fibrous material that collects the particles of dust and other foreign matter from the air as it passes through the filter. In some cases, the fibers are dry, while in others they have a viscous (sticky) coating. Filters usually have a large dust holding capacity. When permanent filters become dust-laden they can be cleaned. Throwaway filters are only one-time filters and are discarded when they become dust laden.

Water sprays can be used to recondition the air by washing and cleaning it. These sprays may also serve to humidify or dehumidify the air to some extent.

In some large air-conditioning systems, air is cleaned by electricity. In this type of system, electrical precipitators remove the dust particles from the air. The air is first passed between plates where the dust particles are charged with electricity. Then the air is passed through a second set of oppositely charged plates that attract and remove the dust particles (Figure 5). This method is by far the best method of air cleaning, but the most expensive.

Figure 5 — Electrostatic filter diagram.

1.4.0 Circulation of Air

The velocity of the air is the primary factor that determines what temperature and humidity are required to produce comfort. (The chart in Figure 3 is based on an air movement of 15 to 25 feet per minute.) We know from experience that a high velocity of air produces a cooling effect on human beings. However, air velocity does not produce a cooling effect on a surface that does not have exposed moisture. A fan does not cool the air, but merely increases its velocity. The increased velocity of air passing over the skin surfaces evaporates moisture at a greater rate thereby cooling the individual. For this reason, circulation of air has a decided influence on comfort conditions. Air can be circulated by gravity or mechanical means.

When air is circulated by gravity, the heavier cold tends to settle to the floor, forcing the warm and lighter air to the ceiling. When the air at the ceiling is cooled by some sort of refrigeration, it will settle to the floor and cause the warm air to rise. The circulation of the air by this method will eventually stop when the temperature of the air at the ceiling is the same as the temperature on the floor. Axial or radial fans can also be used to circulate the air. When axial or radial fans are mounted in an enclosure, they are often called blowers.

Test Your Knowledge

1. What is the term used for the range of effective temperatures that most people feel comfortable in?

A. Best temperature
B. Comfort zone
C. Area of comfort
D. Health range

2. Using electricity to clean the air of large air-conditioning systems is the best method and the cheapest.

A. True
B. False


- To Table of Contents -


As an HVAC-R tech, it is necessary that you be able to recognize the basic types of air-conditioning systems and understand the operation, maintenance, and repair methods and procedures.

A complete air-conditioning system includes a means of refrigeration, one or more heat transfer units, air filters, a means of air distribution, an arrangement for piping the refrigerant and heating medium, and controls to regulate the proper capacity of these components. In addition, the application and design requirements that an air-conditioning system must meet make it necessary to arrange some of these components to condition the air in a certain sequence.

For example, an installation that requires reheating of the conditioned air must be arranged with the reheating coil on the downstream side of the dehumidifying coil; otherwise, it is impossible to reheat the cooled and dehumidified air.

There has been a tendency by many designers to classify an air-conditioning system by referring to one of its components. For example, the air-conditioning system that includes a dual duct arrangement to distribute the conditioned air; is referred to as a dual duct system. This classification makes no reference to the type of refrigeration, the piping arrangement, or the type of controls.

For the purpose of classification, the following definitions are used:

2.1.0 Self-Contained Air-Conditioning Units

Air-conditioning units that are self-contained may be divided into two types: window mounted and floor-mounted units. Window-mounted air-conditioning units usually range from 4,000 to 36,000 Btu per hour in capacity (Figure 6). The use of windows to install these units is not a necessity. They may be installed in transoms or directly in the outside walls (commonly called a "through-the-wall" installation). A package type of room air conditioner, showing airflow patterns for cooling, ventilating, and exhausting services, is shown in Figure 7.

Figure 6 — Window air conditioner.

Figure 7 — Airflow patterns of a package type air conditioner.

In construction and operating principles, the window unit is a small and simplified version of much larger systems. As shown in Figures 8 and 9, the basic refrigeration components are present in the window unit. The outside air cools the condenser coils. The room air is circulated by a fan that blows across the evaporator coils. Moisture, condensed from the humid air by these coils, is collected in a pan at the bottom of the unit; it is usually drained to the back of the unit and discharged.

Figure 8 — Refrigerant cycle of a package air conditioner.

Figure 9 — Air-handling components of a package room air conditioner.

Most window units are equipped with thermostats that maintain a fixed dry-bulb temperature and moisture content in an area within reasonable limits. These units are installed so there is a slight tilt of the unit towards the outside, toward the condenser, to assist in drainage of the condensate. It is a good idea to mount the unit on the eastside of the building to take advantage of the afternoon shade. These units require very little mechanical attention before they are put into operation. Window units are normally operated by the user, who should be properly instructed on their use.

Floor-mounted air-conditioning units range in size from 24,000 to 360,000 Btu per hour and are also referred to as PACKAGE units, as the entire system is located in the conditioned space. Like window units, these larger units contain a complete system of refrigeration components.

A self-contained unit with panels removed is shown in Figure 10. These units normally use either a water-cooled or air-cooled condenser. Self-contained units should be checked regularly to ensure they operate properly. Filters should be renewed or cleaned weekly or more often if necessary. Always stop the blower when changing filters to keep loose dust from circulating through the system. When the filters are permanent, they should be returned to the shop for cleaning. At least once a year, the unit should be serviced.

If the unit has been designed with a spray humidifier, spray nozzle, water strainers, and cooling coils, each device should be cleaned each month to remove water solids and scale. Cooling coil casings, drain pans, fan scrolls, and fan wheels should be wire brushed and repainted when necessary. Oiling and greasing of the blower and motor bearings should be performed as required.

Figure 10 — Floor-mounted air-conditioning unit.

2.2.0 Heat Pumps

A heat pump removes heat from one place and puts it into another. A domestic refrigerator is considered to be a heat pump because it removes heat from inside a box and releases it on the outside. The only difference between a refrigerator and a residential or commercial heat pump is that the latter can reverse its system. The heat pump is one of the most modern means of heating and cooling. Using no fuel, the electric heat pump automatically heats or cools, as determined by outside temperature.

The air type of unit works on the principle of removing heat from the atmosphere. No matter how cold the weather, some heat can always be extracted and pumped indoors to provide warmth. To cool during the hot months, this cycle is merely reversed with the unit removing heat from the area to be cooled and exhausting it to the outside air. The heat pump is designed to control the moisture in the air and to remove dust and pollen. Cool air, provided during hot weather, enters the area with uncomfortable moisture removed. In winter, when a natural atmosphere is desirable, air is not dried out when pumped indoors.

The heat pump is simple in operation (Figure 11). In summer, the evaporator is cooling and the condenser outside is giving off heat the evaporator picked up. During the winter the condenser outside is picking up heat from the outside air because its temperature is lower than that of the outside air (until it reaches the balance point). This heat is then sent to the evaporator by the compressor and is given off into the conditioned space.

Figure 11 — Basic heat pump operation.

A reversing valve is the key to this operation. The compressor always pumps in one direction, so the reversing valve changes the hot-gas direction from the condenser to the evaporator, as indicated by the setting on the thermostat. The setting of the thermostat assures the operator of a constant temperature through an automatic change from heating to cooling anytime outside conditions warrant.

2.2.1 Heating Cycle

The initial heating demand of the thermostat starts the compressor. The reversing valve is de-energized during the heating mode. The compressor pumps the hot refrigerant gas through the indoor coil, where heat is released into the indoor air stream. This supply of warmed air is distributed through the conditioned space. As the refrigerant releases its heat, it changes into a liquid, which is then transported to the outdoor coil. The outdoor coil absorbs heat from the air blown across the coil by the outdoor fan. The refrigerant changes from a liquid into a vapor, as it passes through the outdoor coil. The vapor returns to the compressor where it increases temperature and

 pressure. The hot refrigerant is then pumped back to the indoor coil to start another cycle. A graphic presentation of the nine steps of the cycle is shown in Figure 12.

Figure 12 — Heating cycle.

2.2.2 Cooling Cycle

Once the thermostat is put in the cooling mode, the reversing valve is energized. A cooling demand starts the compressor. The compressor pumps hot high-pressure gas to the outdoor coil where heat is released by the outdoor fan. The refrigerant changes into a liquid, which is transported to the indoor blower. The refrigerant absorbs heat from the indoor air of the supply air, which is distributed throughout the controlled space. This temperature change removes moisture from the air and forms condensate, which must be piped away. The compressor suction pressure draws the cool vapor back into the compressor where the temperature and pressure are greatly increased. This completes the cooling refrigerant cycle. Figure 13 shows a graphic presentation of the nine steps of the cycle.

Figure 13 — Cooling cycle.

2.2.3 Defrost Cycle

Heat pumps operating at temperatures below 45°F accumulate frost or ice on the outdoor coil. The relative humidity and ambient temperature affect the degree of accumulation. This ice buildup restricts the airflow through the outdoor coil, which consequently affects the system operating pressures. The defrost control detects this restriction and switches the unit into a defrost mode to melt the ice.

The reversing valve is energized and the machine temporarily goes into the cooling cycle, where hot refrigerant flows to the outdoor coil. The outdoor fan stops at the same time, thus allowing the discharge temperature to increase rapidly to shorten the length of the defrost cycle. If there is supplemental heat, a defrost relay activates it to offset the cooling released by the indoor coil.

2.2.4 Supplemental

Heat As the outside temperature drops, the heat pump runs for longer periods until it eventually operates continually to satisfy the thermostat. The system "balance point" is when the heat pump capacity exactly matches the heating loss. The balance point varies between homes, depending on actual heat loss and the heat pump capacity. However, the balance point usually ranges between 15°F and 40°F. Either electric heat or fossil fuels provide the auxiliary heat. Conventional heat pump applications use electric heaters downstream from the indoor coil. This design prevents damaging head pressures when the heat pump and auxiliary heat run simultaneously. The indoor coil can only be installed downstream from the auxiliary heat if a Fuelmaster® control system is used. This control package uses a two stage heat thermostat with the first stage controlling heat pump operation and the second stage controlling furnace operation.

2.3.0 Chilled-Water Systems

Water chillers (Figures 14 and 15) are used for air conditioning large tonnage capacities and for central refrigeration plants serving a number of zones, each with its individual air-cooling and air-circulating units. An example is a large hospital with wings off a corridor. Air conditioning may be necessary in operating rooms, treatment suites, and possibly some recovery wards. Chilled water-producing and water-circulating equipment is in a mechanical equipment room. Long mains with many joints between condensing equipment and conditioning units increase the chance of leaks. Expensive refrigerant has to be replaced. It may be better to provide water-cooling equipment close to the condensing units and to circulate chilled water to remote air-cooling coils. Chilled water is circulated to various room-located coils by a pump, and the temperature of the air leaving each coil may be controlled by a thermostat that controls a water valve or stops and starts each cooling coil fan motor.

Figure 14 — Rotary screw compressor unit.

Figure 15 — Two-stage semi-hermetic centrifugal unit.

2.3.1 Types of Coolers

For chilled-water air conditioning, the two most commonly used water coolers (evaporators) are the flooded shell-and-tube (Figure 16) and dry-expansion (Figure 17). The disadvantage to using the flooded shell-and-tube cooler is that it needs more refrigeration than other systems of equal size. Furthermore, when the load falls off, water in tubes may freeze, causing the tubes to split.

2.3.2 Controls

Flooded coolers should be controlled with a low-pressure float control-afloat valve placed so the float is about the same level as the predetermined refrigerant level. The float, as a pilot, moves a valve in the liquid line to control the flow of refrigerant to the evaporator. Automatic or thermostatic expansion valves control the dry-expansion coolers. The refrigerant is inside the tubes; therefore, freezing of water on the tubes is less likely to cause damage.

2.3.3 Condensers

The primary purpose of the condenser is to liquefy the refrigerant vapor. The heat added to the refrigerant in the evaporator and compressor must be transferred to some other medium from the condenser. This medium is the air or water used to cool the condenser.

2.3.4 Water-Cooled Condensers

Condensing water must be noncorrosive, clean, inexpensive, below a certain maximum temperature, and available in sufficient quantity. The use of corrosive or dirty water results in high maintenance costs for condensers and piping. Dirty water can generally be economically filtered if it is noncorrosive. Corrosive water, if it is clean, can sometimes be economically treated to neutralize its corrosive properties. An inexpensive source of water that must be filtered and chemically treated will probably not be economical to use without some means of conservation, such as an evaporative condenser or a cooling tower.

Water circulated in evaporative condensers and cooling towers must always be treated to reduce the formation of scale, algae, and chalky deposits. However, overtreatment of water can waste costly chemicals and result in just as much maintenance as under treatment. Shell-and-Coil Condensers

A shell-and-coil water-cooled condenser (Figure 16) is simply a continuous copper coil mounted inside a steel shell. Water flows through the coil, and the refrigerant vapor from the compressor is discharged inside the shell to condense on the outside of the cold tubes. In many designs, the shell also serves as a liquid receiver.

Figure 16 — Shell-and-coil condenser.

The shell-and-coil condenser has a low manufacturing cost, but is difficult to service in the field. If a leak develops in the coil, the head from the shell must be removed and the entire coil pulled from the shell to find and repair the leak. A continuous coil is a nuisance to clean, whereas straight tubes are easy to clean with mechanical tube cleaners. In summary, it may be difficult to maintain a high rate of heat transfer with a shell-and-coil condenser, with some types of cooling water. Shell-and-Tube Condensers

The shell-and-tube water-cooled condenser shown in Figure 17 permits a large amount of condensing surface to be installed in a comparatively small space. The condenser consists of a large number of 3/4- or 5/8-inch tubes installed inside a steel shell. The water flows inside the tubes while the vapor flows outside around the nest of tubes. The vapor condenses on the outside surface of the tubes and drips to the bottom of the condenser, which may be used as a receiver for the storage of liquid refrigerant. Shell-and-tube condensers are used for practically all water-cooled refrigeration systems.

Figure 17 — Shell-and-tube condenser.

To obtain a high rate of heat transfer through the surface of a condenser, it is necessary for the water to pass through the tubes at a fairly high velocity. For this reason, the tubes in shell-and-tube condensers are separated into several groups with the same water traveling in series through each of these various groups. A condenser having four groups of tubes is known as a four-pass condenser because the water flows back and forth along its length four times. Four-pass condensers are common although any reasonable number of passes may be used. The fewer the number of water passes in a condenser, the greater the number of tubes in each pass.

The friction of water flowing through a condenser with a few passes is lower than in one having a large number of passes. This means a lower power cost in pumping the water through a condenser with a smaller number of passes. Tube-Within-A-Tube Condensers

The use of tube-within-a-tube for condensing purposes is popular because it is easy to make. Water passing through the inner tube along with the exterior air condenses the refrigerant in the outer tube (Figure 18). This "double cooling" improves efficiency of the condenser. Water enters the condenser at the point where the refrigerant leaves the condenser. It leaves the condenser at the point where the hot vapor from the compressor enters the condenser. This arrangement is called counter-flow design.

Figure 18 — Tube-within-a-tube condenser.

The rectangular type of tube-within-a-tube condenser uses a straight hard copper pipe with manifolds on the ends. The water pipes can be clean mechanically after the manifolds have been removed

2.4.0 Cleaning Water-Cooled Condensers

As an HVAC-R tech, you may be assigned to some activities where water-cooled condensers are used in the air-conditioning system. One of your jobs will probably include cleaning the condensers.

Water contains many impurities that can vary, depending on the location. Especially harmful to components are lime and iron. These two impurities form a hard scale on the walls of water tubes that reduces the efficiency of the condenser. Condensers can be cleaned mechanically or chemically.

Scale on tube walls of condensers with removable heads is removed by attaching a round steel brush to a rod and by working it in and out of the tubes. After the tubes have been cleaned with a brush, flush them by running water through them. Some scale deposits are harder to remove than others, and a steel brush may not do the job. Several types of tube cleaners for removing hard scale can usually be purchased from local sources. Be sure that the type selected does not injure water tubes.

When the condenser tubes are not accessible for mechanical cleaning, the simplest method for removing scale and dirt is to use inhibited acid to clean the coils or tubes. This is called chemical action.



 Prevent chemical solution from splashing in eyes and on skin or clothing.

Figure 19 — Gravity method.

Equipment and connections for circulating inhibited acid through the condenser using gravity flow, as shown in Figure 19, are as follows:

  1. A rubber or plastic bucket for mixing solution. Do not use galvanized materials because prolonged contact with acid deteriorates such surfaces.
  2. A crock or wooden bucket for catching the drainage residue.
  3. One-inch steel pipe that is long enough to make the connections shown.
  4. Fittings for 1-inch steel pipe. The vent pipe shown should be installed at the highest connection of the condenser.

Figure 20 — Forced Circulation Method.

Equipment and connections for circulating inhibited acid through the condenser using forced circulation, as shown in Figure 20, are as follows:

  1. A pump suitable for this application. A centrifugal pump and a 1/2-horsepower motor are recommended (30 gallons per minute at 35-foot head capacity).
  2. A non-galvanized metal tank, stone or porcelain crock, or wooden barrel with a capacity of about 50 gallons with ordinary bronze or copper screening to keep large pieces of scale or dirt from getting into the pump intakes.
  3. A one-inch pipe that is long enough to make the piping connections shown.
  4. Fittings for 1-inch steel globe valves. The vent pipe, as shown, should be installed at the higher connection of the condenser.

Remember, when you are cleaning with acid make sure to follow the usual precautions observed when handling acids. It stains hands and clothing and attacks concrete and if an inhibitor is not present, it reacts with steel. Therefore, use every precaution to prevent spilling or splashing. When splashing might occur, cover the surfaces with burlap or boards. Gas produced during cleaning that escapes through the vent pipe is not harmful but prevents any liquid or spray from being carried through with the gas. The basic formula should be maintained as closely as possible, but a variation of 5 percent is permissible. The inhibited acid solution is made up of the following:

Place the required amount of water in a non-galvanized metal tank or wooden barrel, and add the necessary amount of inhibitor powder while stirring the water. Continue stirring the water until the powder is completely dissolved; then add the required quantity of acid.



NEVER add water to acid; this mistake may cause an explosion.

When gravity flow is used to charge the system with an acid solution, introduce the inhibited acid as shown in Figure 19. Do not add the solution faster than the vent can exhaust the gases generated during cleaning. When the condenser has been filled, allow the solution to remain overnight.

When forced circulation is used (Figure 20), the valve in the vent pipe should be fully opened while the solution is introduced into the condenser but must be closed when the condenser is completely charged and the solution is circulated by the pump. When a centrifugal pump is used, the valve in the supply line may be fully closed while the pump is running.

The solution should be allowed to stand or be circulated in the system overnight for cleaning out average scale deposits. The cleaning time also depends on the size of the condenser to be cleaned. For extremely heavy deposits, forced circulation is recommended, and the time should be increased to 24 hours. The solution acts more rapidly if it is warm, but the cleaning action is just as thorough with a cold solution if adequate time is allowed.

After the solution has been allowed to stand or has been circulated for the required time through the condenser, it should be drained and the condenser thoroughly flushed with water. To clean condensers with removable heads by using inhibited acid, use the above procedure without removing the heads.

However, exercise extra precaution when flushing out the condenser with clear water after the acid has been circulated through the condenser to ensure removal of the acid from all water passages.

2.5.0 Maintenance

A well-planned maintenance program avoids unnecessary downtime, prolongs the life of the unit, and reduces the possibility of costly equipment failure. It is recommended that a maintenance log be maintained for recording the maintenance activities. This action provides a valuable guide and aids in obtaining extended length of service from the unit.

This section describes specific maintenance procedures, which must be performed as a part of the maintenance program of the unit. Use and follow the manufacturer’s manual for the unit undergoing maintenance. When specific directions or requirements are furnished, follow them. Before performing any of these operations, ensure that the power to the unit is disconnected, unless otherwise instructed.



When maintenance checks and procedures must be completed with the electrical power on, care must be taken to avoid contact with energized components or moving parts. Failure to exercise caution when working with electrically powered equipment may result in serious injury or death.

2.5.1 Coil Cleaning

Refrigerant coils must be cleaned at least once a year or more frequently if the unit is located in a dirty environment. This action helps maintain unit operating efficiency and reliability.

The relationship between regular coil maintenance and efficient/reliable unit operation is as follows:

The following equipment is required to clean condenser coils: a soft brush and either a garden pump-up sprayer or a high-pressure sprayer. In addition, a high-quality detergent must be used. Follow the manufacturer's recommendations for mixing to make sure the detergent is alkaline with a pH value less than 8.5.

Specific steps required for cleaning the condenser coils are as follows:

  1. Disconnect the power to the unit.



Open the unit disconnect switch. Failure to disconnect the unit from the electrical power source may result in severe electrical shock and possible injury or death.

  1. Remove enough panels from the unit to gain access to the coil.
  2. Protect all electrical devices, such as motors and controllers, from dust and spray.
  3. Straighten coil fins with a fin rake, if necessary.
  4. Use a soft brush to remove loose dirt and debris from both sides of the coil.
  5. Mix the detergent with water according to the manufacturer's instructions. The detergent and water solution may be heated to a maximum of 150°F to improve its cleaning ability.



Do not heat the detergent and water solution to temperatures in excess of 150°F. High-temperature liquids sprayed on the coil exterior raise the pressure within the coil and may cause it to burst. Should this occur, the result could be both injury to personnel and equipment damage.

  1. Place the detergent and water solution in the sprayer. If a high-pressure sprayer is used, be sure to follow these guidelines:



Do NOT spray motors or other electrical components. Moisture from the spray can cause component failure.

  1. Spray the side of the coil where the air leaves first then spray the other side (where the air enters). Allow the detergent and water solution to stand on the coil for 5 minutes.
  2. Rinse both sides of the coil with cool water.
  3. Inspect the coil and if it still appears dirty, repeat Steps 8 and 9.
  4. Remove the protective covers installed in Step 3.
  5. Replace all unit panels and parts, and restore electrical power to the unit.

2.5.2 Fan Motors

The inspection of fan motors should be conducted periodically to check for excessive vibration or temperature. Operating conditions vary the frequency of inspection and lubrication. Motor lubrication instructions are found on the motor tag or nameplate. If not available, contact the motor manufacturer for instructions.

To re-lubricate the motor, complete the following steps:



Ensure that the power source is disconnected prior to lubricating the motor. Failure to do so may result in injury or death from electrical shock or moving parts.

  1. Turn the motor off. Make sure it cannot accidentally restart.
  2. Remove the relief plug and clean out any hardened grease.
  3. Add fresh grease through the fitting with a low-pressure grease gun.
  4. Run the motor for a few minutes to expel any excess grease through the relief vent.
  5. Stop the motor and replace the relief plug

2.5.3 Fan Bearing Lubrication

Fan bearings with grease fittings or grease line extensions should be lubricated with lithium-base grease that is free of chemical impurities. Improper lubrication can result in early bearing failure. To lubricate the fan bearings, complete the following steps:

  1. Lubricate the bearings while the unit is not running then disconnect the main power switch
  2. Connect a manual grease gun to the grease line or fitting.
  3. Add grease, preferably when the bearing is warm, while turning the fan wheel manually until a light bead of grease appears at the bearing grease seal.

2.5.4 Filters

To clean permanent filters, wash under a stream of hot water to remove dirt and lint. Follow with a wash of mild alkali solution to remove old filter oil. Rinse thoroughly and let dry. Recoat both sides of the filter with filter oil and let dry. Replace the filter element in the unit.



Always install filters with directional arrows pointing toward the fans.

2.6.0 Periodic Maintenance

All indicated maintenance procedures should be performed as scheduled. This prolongs the life of the unit and reduces the possibility of costly equipment failure and downtime. A checklist should be prepared listing the required service operations and the times they should to be performed.

2.6.1 Weekly Maintenance Checklist

  1. Check the compressor oil level. If low, allow the compressor to operate continually at full load for 3 to 4 hours. Check the oil level at 30-minute intervals. If the level remains low, add oil.
  2. Observe the oil pressure. The oil pressure gauge reading should be approximately 20 to 35 psi above the suction pressure gauge reading.
  3. Stop the compressor and check the shaft seal for excessive oil leakage. If found, check the seal with a refrigerant leak detector (open compressor only).
  4. Check the condition of the air filters and air-handling equipment. Clean or replace filters, as necessary.
  5. Check the general operating conditions, system pressures, refrigerant sight glass, and so forth.

2.6.2 Monthly Maintenance Checklist

(Repeat items 1 through 5 from the weekly checklist)

  1.  Lubricate the fan and motor bearings, as necessary. Obtain and follow the manufacturer’s lubricant specifications and bearing care instructions.
  2. Check the fan belt tension and alignment.
  3. Tighten all fan sheaves and pulleys. If found to be loose, check alignment before tightening.
  4. Check the condition of the condensing equipment. Observe the condition of the condenser coil in the air-cooled condenser. Clean, as necessary. Check the cooling tower water in the water-cooled condenser. If algae or scaling is evident, water treatment is needed. Clean the sump strainer screen of the cooling tower.

2.6.3 Annual Maintenance Checklist

(Repeat the items 1 through 9 from the monthly checklist)

  1. Drain all circuits of the water-condensing system. Inspect the condenser piping and clean any scale or sludge from the tubes of the condenser.
  2. If a cooling tower or evaporative condenser is used, flush the pumps and sump tank. Remove any rust or corrosion from the metal surfaces and repaint.
  3. Inspect all motor and fan shaft bearings for signs of wear. Check the shafts for proper end-play adjustment.
  4. Replace worn or frayed fan belts.
  5. Clean all water strainers.
  6. Check the condition of the ductwork.
  7. Check the condition of the electrical contacts of all contactors, starters, and controls. Remove the condensing unit control box cover and inspect the panel wiring. All electrical connections should be secure. Inspect the compressor and condenser fan motor contactors. If the contacts appear severely burned or pitted, replace the contactor. Do not clean the contacts. Inspect the condenser fan capacitors for visible damage.

2.7.0 Seasonal Shutdown-Startup

2.7.1 Shutdown

In preparation for seasonal shutdown, it is advisable to pump down the system and valve off the bulk of the refrigerant charge in the condenser. This action minimizes the quantity of refrigerant that might be lost due to any minor leak on the low-pressure side of the system, and, in the case of the open compressor, refrigerant that might leak through the shaft seal.

For hermetic compressor pump down, complete the following steps:

  1. Close the liquid line shutoff valve at the condenser and start the system. The compressor will stop when the suction pressure drops to the low-pressure control cutout setting.
  2. Open the compressor electrical disconnect switch to prevent the compressor from restarting, and then front-seat the compressor discharge and suction valves.

For open compressor pump down, complete the following steps:

  1. If the system is not equipped with gauges, install a pressure gauge in the backseat port of the compressor suction valve. Crack the valve off the backseat.
  2. Close the liquid line shutoff valve at the condenser.
  3. Manually open the liquid line solenoid valve(s). If the valves do not have manual opening devices, lower the setting of the system temperature controller so the valves are held open during the pump down.
  4. Install a jumper wire across the terminals of the low-pressure switch. Since the system suction pressure is to be pumped down below the cutout setting of the low-pressure switch, the jumper is necessary to keep the compressor running.
  5. Start the compressor. Watching the suction pressure gauge, stop the compressor by opening its electrical disconnect switch when the gauge reading reaches 2 psig.
  6. Front-seat the compressor discharge valve.



Do not allow the compressor to pump the suction pressure into a vacuum. A slight positive pressure is necessary to prevent air and moisture from being drawn into the system through minor leaks and through the now unmoving shaft seal.

  1. Remove the jumper wire from the low-pressure control.
  2. Remove the gauge from the port of the suction valve; replace the port plug and front-seat the valve.

The following steps are required for all systems:

  1. Use a refrigerant leak detector to check the condenser and liquid receiver for refrigerant leaks.
  2. Valve off the supply and return water connections of the water-cooled condenser. Allow the condenser to remain full of water during the off season. A drained condenser shell is more likely to rust and corrode than one full of water. If the condenser will be subjected to freezing temperatures, drain the water and refill it with an antifreeze solution.
  3. Drain the cooling tower or evaporative condenser, if used; flush the sump and paint any rusted or corroded areas.
  4. Open the system master disconnect switch and padlock it in the OPEN position.

2.7.2 Startup

The following steps are completed for seasonal start-up:

  1. Perform all annual maintenance on the air-handling system and other related equipment.
  2. If they are used, fill the water sumps of the cooling tower or evaporative condenser.
  3. Open the shutoff valves of the water-cooled condenser.
  4. Make certain the liquid line solenoid valve(s) is on automatic control.
  5. Open the liquid line shutoff valve.
  6. Back-seat the compressor suction and discharge valves.
  7. Close the system master electrical disconnect switch.
  8. Start the system.
  9. After the system has operated for 15 to 20 minutes, check the compressor oil level sight glass, oil pressure, and the liquid line sight glass. If satisfactory, readjust the system temperature controller to the proper temperature setting.

2.8.0 Safety Warnings

Most units used for comfort air conditioning operate using refrigerants that are not toxic except when decomposed by a flame. If liquefied refrigerant contacts an individual’s eyes, get that person to a doctor immediately.

Should the skin come in contact with the liquefied refrigerant, the skin is to be treated as though it had been frostbitten or frozen.

Do not adjust, clean, lubricate, or service any parts of equipment that are in motion. Ensure that moving parts, such as pulleys, belts, or flywheels, are fully enclosed with proper guards attached.

Before making repairs, open all electric switches controlling the equipment. Tag and lock the switches to prevent short circuits or accidental starting of equipment. When moisture and brine are on the floor, fatal grounding through the body is possible when exposed electrical connections can be reached or touched by personnel. De-energize electrical lines before repairing them, and ground all electrical tools.

Test Your Knowledge

3. Which of the following is another name for a floor-mounted air-conditioning unit?

A. Path-mounted unit
B. Package unit
C. Self-contained unit
D. Hallway unit

4. What type of system is used for air conditioning large tonnage capacities?

A. Heat pump
B. Oversized chiller
C. Strong cycle
D. Chilled-water

5. The lubrication of fan bearings should be done when the unit is running.

A. True
B. False


- To Table of Contents -


As an HVAC-R tech, you need to be able to recognize and have an understanding of the different types of air-conditioning cooling towers, compressors, and control elements. Also, it is necessary that you have an understanding of the basic maintenance requirements for cooling towers.

3.1.0 Cooling Towers

Cooling towers (Figures 21 and 22) are classified according to the method of moving air through the tower. These air moving methods include the following:

Figure 21 — Package tower with a remote, variable speed pump.

Figure 22 — Paralleled package towers.

3.1.1 Natural Draft

The natural draft cooling tower is designed to cool water by means of air moving through the tower at the low velocities prevalent in open spaces during the summer. Natural draft towers are constructed of cypress or redwood and have numerous wooden decks of splash bars installed at regular intervals from the bottom to the top. Warm water from the condenser is flooded or sprayed over the distributing deck and flows by gravity to the water-collecting basin.

A completely open space is required for the natural draft tower since its performance depends on existing air currents. Ordinarily, a roof is an excellent location. Louvers must be placed on all sides of a natural draft tower to reduce drift loss.

Important design considerations are the wind velocity and the height of the tower. A wind velocity of 3-miles per hour is generally used for a design of natural draft cooling towers. The natural draft cooling tower was once the standard design for cooling condenser water in refrigeration systems up to about 75 tons. It is now rarely selected unless low initial cost and minimum power requirements are primary considerations. The drift loss and space requirements are much greater than for other cooling tower designs.

3.1.2 Inducted Draft

An induced draft cooling tower is provided with a top-mounted fan that induces atmospheric air to flow up through the tower, as warm water falls downward. An induced draft tower may have only spray nozzles for water backup, or it may be filled with various slat and deck arrangements. There are several types of inducted draft cooling towers.

In a counter-flow induced draft tower (Figure 23), a top-mounted fan forces air into the bottom of the tower, which flows vertically upward as the water cascades down through the tower. The counter-flow tower is particularly well adapted to a restricted space as the discharge air is directed vertically upward, and if equipped with an inlet on each side, requires only minimum clearance for air intake area. The primary breakup of water may be either by pressure spray or by gravity from pressure-filled flumes.

Figure 23 — Counter-flow induced draft tower.

A parallel-flow induced draft tower (Figure 24) operates the same way as a counter-flow tower, except the top-mounted fan pulls the air in through the top of the tower and pushes it out the bottom.

Figure 24 — Parallel-flow induced draft tower.

The airflow goes in the same direction as the water. Comparing counter-flow and parallel-flow induced draft towers of equal capacity, the parallel-flow tower is somewhat wider, but the height is much less. Cooling towers must be braced against the wind. From a structural standpoint, therefore, it is much easier to design a parallel-flow than a counter-flow tower, as the low silhouette of the parallel-flow type offers  much less resistance to the force of the winds.

Mechanical equipment for counter-flow and parallel-flow towers is mounted on top of the tower and is readily accessible for inspection and maintenance. The water-distributing systems are completely open on top of the tower and can be inspected during operation. This makes it possible to adjust the float valves and clean stopped-up nozzles while the towers are operating.

The cross-flow induced draft tower (Figure 25) is a modified version of the parallel flow induced draft tower. The fan in a cross-flow cooling tower draws air through a single, horizontal opening at one end and discharges the air at the opposite end. The cooling tower is a packaged tower that is inexpensive to manufacture and is extremely popular for small installations.

Figure 25 — Cross-flow induced draft tower.

As a packaged cooling tower with piping and wiring in place, it is simple to install and may be placed wherever there is a clearance of 2 feet for the intake end and a space of 10 feet or more in front of the fan. The discharge end must not face the prevailing wind and should not be directed into a traffic area because drift loss may be objectionable.

In some situations, the cooling tower may need to be located indoors. For indoor installations, a counter-flow or cross-flow design is generally selected. Two connections to the outside are usually required—one for drawing outdoor air into the tower and the other for discharging it back outside. A centrifugal blower is often necessary for this application to overcome the static pressure of the ductwork. There are many options available for the point of air entrance and air discharge. This flexibility is often important in designing an indoor installation. Primary water breakup is by pressure spray and fill of various types.

The induced draft cooling tower for indoor installation is a completely assembled packaged unit. However, it is designed to allow partial disassembly, which permits it to pass through limited entrances. Indoor installations of cooling towers are becoming more popular. External space restrictions, architectural compatibility, and convenience for observation and maintenance all combine to favor an indoor location. The installation cost is somewhat higher than an outdoor location. Packaged towers are available in capacities to serve the cooling requirements of refrigeration plants in the 5- to 75-ton range.

3.1.3 Forced Draft

A forced draft cooling tower (Figure 26) uses a fan to force air into the tower. In the usual installation, the fan shaft is in a horizontal plane. The air is forced horizontally through the fill and upward to be discharged out of the top of the tower. Underflow cooling towers are an improved design of the forced draft tower that retains all the advantages of the efficient parallel-flow design. Air is forced into the center of the tower at the bottom. The air is then turned horizontally (both right and left) through fill chambers and is discharged vertically at both ends.

Figure 26 — Forced draft cooling tower.

By forcing the air to flow upward and outward through the fill and leave at the ends, operating noise is baffled and a desirable reduction of sound level is achieved. All sides of the underflow tower are smoothly encased with no louver openings. This blends with modern architecture and eliminates the necessity of masonry walls or other screening devices often necessary to conceal cooling towers of other types.

3.1.4 Cooling Tower Materials

 For many years, redwood has been the standard construction material for cooling towers. Other materials, such as cypress, and treated fir and pine, have also been used. Casings are constructed of laminated, waterproof plywood. In areas having a highly corrosive atmosphere, the use of casings and other noncorrosive materials at critical points is essential. During cooling tower construction, nails, bolts, and copper or aluminum nuts are usually standard practice.

Cooling towers of metal coated with plastic or bituminous materials that have air intake louvers and fill made of redwood have met with only limited success. The limited success is primarily because of the high maintenance cost as compared to wood towers.

Some cooling towers are constructed of metal coated with plastic or bituminous materials, and have air intake louvers and fill made of redwood. Due to their high maintenance costs, metal cooling towers have had only limited success.

Packaged towers with metal sides and wood fill are reasonably common. For limited periods of time, some manufacturers used sheet aluminum for siding. Plastic slats have been used for fill material but have not proved satisfactory in all cases.

Fire ordinances of a large city may require that no cooling towers can be constructed of wood. A cooling tower constructed of steel or some other fireproof casing and without fill will comply with the most restrictive ordinances.

3.1.5 Maintenance of Cooling Towers

 It has been found that cooling towers have been linked to the spread of Legionnaire's disease. Several precautionary measures are recommended to help eliminate this problem. These measures include placing cooling towers downwind and using chloride compounds as disinfectants during scheduled monthly maintenance.

An important part of cooling tower operation is water treatment. During the cooling tower water evaporation process, some solids are left behind. Recirculation of the water in the condenser cooling tower circuit, and in the accompanying evaporation, causes the concentration of solids to increase. This concentration must be controlled or scale and corrosion will result.

One method of control is to drain the system from time to time and then refill it with fresh water. However, this method is not recommended because after refilling the system, the dissolved solids again build up to a dangerous concentration.

A more common practice is to waste a certain amount of water continually from the system to the sewer. The water wasted is called blow-down. Blow-down is sometimes accomplished by wasting sump water through an overflow. A better practice is to bleed the required quantity of blow-down from the warm water that leaves the condenser on its way to the cooling tower. If serious corrosion and scaling difficulties are to be avoided, a mineral salt buildup (calcium bicarbonate concentration) of 10 grains per gallon is considered the maximum allowable concentration for untreated water in the sump.

For each ton of refrigeration, cooling towers will evaporate about 2 gallons of water every hour. A gallon of water weighs 8.3 pounds, and about 1,000 Btu is needed to evaporate 1 pound of water. Thus, to evaporate a gallon of water, 8.3 x 1,000 or 8,300 Btu is required.

In many instances, the makeup water contains dissolved salts in excess of 10 grains per gallon. It is obvious, then, that even 100 percent blow-down will not maintain a sump concentration of 10 grains. If the blow-down alone cannot maintain satisfactory control, then chemicals should be used.

Cooling tower makeup water is the sum of drift loss, evaporation, and blow-down. The drift loss for mechanical draft towers ranges from 0.1 percent of the total water being cooled and as much as 0.3 percent for better designed towers. In estimating makeup water for a cooling tower, the higher value of 0.3 percent for drift loss is suggested. If the drift loss is actually less than this value, the excess makeup water supplied is merely wasted down the overflow. This increases the amount of blow-down, which is favorable because it lowers the concentration of scale-forming compounds in the tower sump.

Redwood is a highly durable material but it is not immune to deterioration. The type of deterioration varies depending on the nature of the environmental conditions to which the wood is exposed. The principal types of deterioration are leaching, delignification, and microbiological attack.

The rate of heat transfer in the condenser will be materially reduced if the algae and slime present in the water are not chemically controlled. Condenser tubing, cooling tower piping, and metal surfaces in the water-circulating system must be protected from scale and corrosion.

Using too much of a chemical or using the wrong chemical is known as overtreatment. It can materially reduce the performance or the life of a cooling tower condenser circuit.

3.2.0 Compressors

A compressor is the machine used to withdraw the heat-laden refrigerant vapor from the evaporator, compress it from the evaporator pressure to the condensing pressure, and push it to the condenser. A compressor is merely a simple pump that compresses the refrigerant gas. Compressors may be divided into the following three types:

The function of compressing a refrigerant is the same in all three general types, but the mechanical means are considerably different. Rotary compressors are used in small sizes only, and their use is limited almost exclusively to domestic refrigerators and small water coolers. Centrifugal compressors are used in large refrigerating and air-conditioning systems (Figure 27).

Figure 27 — High-speed (36,000 rpm) single-stage centrifugal chiller.

3.2.1 Reciprocating Compressors

Reciprocating compressors are usually powered by electric motors, although gasoline, diesel, and turbine drivers are sometimes used. In terms of capacity, reciprocating compressors are made in fractional horsepower for small, self-contained air conditioners and refrigeration equipment, increasing in size to about 250 tons or more capacity in larger installations. The types of reciprocating compressors consist of furnished in open, semi-sealed, and sealed (hermetic). Open

The shaft of an open type of compressor is driven by an external motor. The shaft is equipped with a seal that prevents refrigerant and oil from leaking or moisture and air from entering the compressor as the shaft passes through the crankcase housing. Pistons are actuated by crankshafts or eccentric drive mechanisms mounted on the shaft. Discharge valves are usually mounted in a plate over the pistons. Suction valves are usually mounted either in the pistons, if suction vapors enter the cylinder through the side of the cylinder or through the crankcase, or in the valve plate over the pistons, if suction vapors enter the cylinder through the head and valve plate

Figure 28 shows a cross section of a typical open type of eccentric shaft compressor with suction valves in the valve plate of the head. Most belt-driven, open type of compressors under 3 horsepower use a splash feed lubrication, but in larger size compressors, forced feed systems having positive displacement oil pumps are more common. The oil pump is usually driven from the rear end of the main shaft. Oil from the crankcase is forced under pressure through a hole in the main shaft to the seal, main bearing, and rod bearing, and through a hole in the rod up to the piston pins. Hermetically sealed compressor units used in window air conditioners are quite common in commercial sizes (under 5 horsepower) and are even made by some manufacturers in large tonnage sizes.

Figure 28 — Cross section of an open type of reciprocating compressor. Semi-Sealed

Large tonnage units are always semi-sealed type compressors, but these compressors can also be made in smaller sizes. The primary difference between a fully sealed and a semi-sealed motor compressor is that in semi-sealed types, the valve plates, and in some units the oil pump, can be removed for repair or replacement. This type of construction is helpful in larger sizes that are so bulky they would cause considerable trouble and expense in shipping, removing, and replacing the unit as a whole. Figure 29 shows a small semi-sealed compressor.

Figure 29 — Small semi-sealed compressor.

Sealed or semi-sealed units eliminate the belt drive and crankshaft seal, both of which are among the chief causes of service calls. Sealed and semi-sealed compressors are made either vertical or horizontal. Although vertical type compressors (Figure 30) can be splash oiled, they usually have positive displacement oil pumps that use 10 to 30 psi of pressure to force oil to the main bearings, rod, or eccentric and pins.

Figure 30 — Vertical semi-sealed compressor.

Although oil pumps for forced feed lubrication are also used on horizontal hermetic compressors, oil circulation at low oil pressure may be provided by slingers, screw type of devices, and the like. Splash and other types of oil feed must not be considered inferior forced feed. With good design, they lubricate well.

It is most important to maintain the proper oil level, use a correct grade of oil, and keep the system clean and free of dirt and moisture. This is true for all compression refrigeration systems, especially those equipped with hermetically sealed units whose motor windings may be attacked by acids or other corrosive substances introduced into the system or formed by the chemical reaction of moisture, air, or other foreign substances. Hermetic

The term sealed or hermetic unit merely means that the motor rotor and compressor crankshaft of the refrigeration system are made in one piece, and the entire motor and compressor assembly are put into a gastight housing that is welded shut (Figure 31).

Figure 31 — Reciprocating hermetic compressor.

This method of assembly eliminates the need for certain parts found in the open unit. These parts are as follows:

The elimination of the preceding parts in the sealed unit similarly does away with the following service operations: replacing motor pulleys, replacing flywheels, replacing belts, aligning belts, and repairing or replacing seals. When it is realized there are major and minor operations that maintenance personnel must perform and the sealed unit dispenses with only five of these, it can be readily seen that servicing is still necessary.

3.2.2 Rotary

Rotary compressors are generally associated with refrigerators, water coolers, and similar small capacity equipment. However, they are available in larger sizes. A typical application of a large compressor is found in compound compressor systems where high capacity must be provided with a minimum of floor space.

In a rotary compressor (Figure 32), an eccentric rotor revolves within a housing in which the suction and discharge passages are separated by means of a sealing blade. When the rotating eccentric first passes this blade, the suction area is at a minimum. Further rotation enlarges the space and draws in the charge of refrigerant. As the eccentric again passes the blade, the gas charge is shut off at the inlet, compressed, and discharged from the compressor. There are variations of this basic design, some of which provide the rotor with blades to trap and compress the vapor.

Figure 32 — Cutaway view of a rotary compressor.

3.2.3 Centrifugal Compressors

Centrifugal compressors are used in large refrigeration and air-conditioning systems, handling large volumes of refrigerants at low-pressure differentials. Their operating principles are based on the use of centrifugal force as a means of compressing and discharging the vaporized refrigerant. Figure 33 is a cutaway view of one type of centrifugal compressor. In this application, one or two compression stages are used, and the condenser and evaporator are integral parts of the unit. The impeller wheel is the heart of this type of compressor.

Figure 33 — Cutaway view of one type of centrifugal compressor.

3.2.4 Scroll Compressors

A scroll compressor has two different offset spiral disks to compress the refrigerant vapor. The upper scroll is stationary, while the lower scroll is the driven scroll. Intake of refrigerant is at the outer edge of the driven scroll, and the discharge of the refrigerant is at the center of the stationary scroll. The driven scroll is rotated around the stationary or "fixed" scroll in an orbiting motion. During this movement, the refrigerant vapor is trapped between the two scrolls. As the driven scroll rotates, it compresses the refrigerant vapor through the discharge port. Scroll compressors have few moving parts and have a very smooth and quiet operation.

3.3.0 Controls

The controls used in air conditioning and refrigeration are generally the same— thermostats, humidistats, pressure and flow controllers, and motor overload protectors shown in Figure 34 are discussed in detail below.

Figure 34 — Packaged air-cooled chiller controls.

3.3.1 Thermostats

The thermostat (Figure 34, Item 6) is an adjustable temperature-sensitive device, which through the opening and closing of its contacts controls the operation of the cooling unit. The temperature-sensitive element may be a bimetallic strip or a confined vaporized liquid.

The thermostats used with air conditioners are similar to those used with heating equipment, except their action is reversed. The operating circuit is closed when the room temperature rises to the thermostat control point and remains closed until the cooling unit decreases the temperature enough. Also, cooling thermostats are not equipped with heat-anticipating coils.

Wall type of thermostats most common for heating and air conditioning in the home and on some commercial units use a bimetallic strip and a set of contacts, as shown in Figure 35. This type of thermostat operates on the principle that when two dissimilar metals, such as brass and steel, are bonded together, one tends to expand faster than the other does when heat is applied. This causes the strip to bend and close the controls.

Figure 35 — Bimetallic thermostat.

As an HVAC-R tech, you may be required to make an adjustment that sets the temperature difference between the cut-in and cutout temperatures. For example, if the system is set to cut in at 76°F and cut out at 84°F, then the differential is 8°F. This condition prevents the unit from cycling continually as it would if there were no differential.

3.3.2 Humidistats

A room "humidistat" may be defined as a humidity-sensitive device controlling the equipment that maintains a predetermined humidity of the space where it is installed. The contact of the humidistat is opened and closed by the expansion or contraction of natural blonde human hairs, which are one of the major elements of this control. It has been found that this type of human hair is most sensitive to the moisture content of the air surrounding them.

3.3.3 Pressure-Flow Controllers

The purpose of air conditioning controllers is to act as safety switches by securing the system, regardless of the position of the operating switches, when head pressure is too high or suction pressure is too low.

3.3.4 Refrigerant-Flow Controllers

The refrigerant-flow controllers are either of the capillary type or externally equalized expansion valve type and are usually of larger tonnage than those used for refrigerators.

3.3.5 Motor Overload Protectors

When the compressor is powered by an electric motor, either belt driven or as an integral part of the compressor assembly, the motor is usually protected by a heat-actuated overload device. This is in addition to the line power fuses (Figure 34, Item 1). The heat to actuate the overload device is supplied by the electrical energy to the motor, as well as the heat generated by the motor itself. If there is too much heat from either source of heat or a combination of the two, it causes the overload device to open, removing the motor from the line.

Figure 36 shows a thermal-element type of overload cutout relay. It is housed in the magnetic starter box (Figure 34, Item 2). When current overload occurs, the relay contacts open allowing the holding coil to release the starting mechanism, causing the motor to stop.

Figure 36 — Thermal overload relay.

An oil failure cutout switch (Figure 34, Item 5) is provided on many systems to protect the compressor against oil failure. The switch is connected to register pressure differential between the oil pump and the suction line.

Figure 37 shows a typical oil failure cutout switch. The switch contains two bellows, which work against each other, and springs for adjusting. Tubing from the oil pump is connected to the bottom bellows of the switch. Tubing from the suction line is connected to the upper bellows. When a predetermined pressure differential is not maintained, a pair of contacts in the switch is opened and breaks the circuit to the compressor motor. A heating element with a built-in delay is in the switch to provide for starting the compressor when oil pressure is low.

Figure 37 — Oil failure cutout switch.

The water-regulating valve used with a water-cooled condenser responds to a predetermined condensing pressure. A connection from the discharge side of the compressor to the valve transmits condensing pressure directly to a bellows inside the valve. High pressure opens the valve, allowing a greater flow of water; low pressure throttles the flow. Use of such a valve provides for a more economical use of water for condensing. Figure 38 shows a typical water-regulating valve. When condenser water is supplied by a cooling tower, water-regulating valves are not customarily used because the cooling tower fan and circulating pump are wired into the compressor motor control circuit.

Figure 38 — Water-regulating valve.

3.3.6 Step Controller

The step controller contains a shaft with a series of cams mounted on it. When the shaft cams start rotating, electrical switches begin to operate. Adjustment of the cams establishes the temperature at which each switch is to close and open (differential). The switches can also be adjusted to operate in almost any sequence. Figure 39, View A) shows the internal components and how you could rearrange the wires to change the sequence. The pressure sensor (Figure 39, View B) is connected to the step controller.

Figure 39 — Step controller and pressure-sensor configuration.

3.4.0 Troubleshooting

A troubleshooting chart (Table 1) is generally applicable to all types of air conditioners. Most manufacturers include more detailed and specific information in publications pertaining to their units. If you unpack the unit and find that it did not include a manual, write the manufacturer to have one sent as soon as possible.

Table 1 — Troubleshooting Chart for Air Conditioners.

Type of Unit Complaint Cause Possible Remedy
With open-type compressor Electric motor will not start Power failure Check circuit for power source
Compressor stuck Locate cause and repair
Belt too tight Adjust belt tension
Manual reset in starter open Determine cause of overload and repair. Reset overload cutout
Thermostat setting too high Lower thermostat setting
 Low voltage Check with voltmeter, then call power company
 Burned-out motor Repair or replace
Frozen compressor caused by locked or damaged mechanism Remove and repair compressor
Unit cycles on and off Intermittent power interruption Tighten connections or replace defective power supply parts
High-pressure cutout defective Replace high-pressure cutout
High-pressure cutout set too low. Overload opens after having been reset Raise cutout pressure. Check voltage and current drawn
Leaky liquid-line solenoid valve  Repair or replace
Dirty or iced evaporator Clean or defrost evaporator. Check filters and fan drive
Overcharge or refrigerant non-condensable gas Remove excess refrigerant or purge non-condensable gas Lack of refrigerant Repair refrigerant leak and recharge
Type of Unit Complaint Cause Possible Remedy
With open-type compressor (cont'd) Unit cycles on and off (cont) Restricted liquid-line Clean strainer
Faulty motor Repair or replace faulty motor
Coil frosts Filters dirty Clean filters
Not enough air over coil Clean or remove restriction from supply or return ducts or grilles
Defective expansion valve Replace valve
Unit runs but will not cool Unit not fully charged Recharge slightly, then check for leaks in the refrigerant circuit, then full charge
Leaky suction valve Remove compressor cylinder head and clean or replace valve plate
Expansion valve not set correctly Adjust expansion valve
Strainer clogged Remove, clean, and replace valve
Air in refrigerant circuit. Moisture in expansion valve orifice Purge unit of air. Clean orifice and install silica gel dryer
Flash gas in liquid line Add refrigerant
No air blows from supply grille Ice or dirt on evaporator Clean coil or defrost
Blower belt broken or loose Adjust belt tension, or replace belt
Blower bearing frozen Repair or replace bearing and lubricate as directed
Discharge pressure too high Improper operation of condenser Correct airflow. Clean coil surface
Air in system Purge
Overcharge of refrigerant Remove excess or purge
Discharge pressure too low Lack of refrigerant Repair leak and charge
Broken or leaky compressor discharge valves  Remove head, examine valves and replace those found to be operating improperly
Type of Unit Complaint Cause Possible Remedy
With open-type compressor (cont.) Suction pressure too high Overfeeding of expansion valve Regulate superheat setting expansion valve and check to see that remove bulb is properly attached to suction line
Expansion valve stuck in open position Repair or replace valve
Broken suction valves in compressor Remove head, examine valves and replaced those found to be inoperative
Suction pressure too low Lack of refrigerant Repair leak and charge
Clogged liquid line strainer Clean strainer
Expansion-valve power assembly has lost charge Replace expansion valve power assembly
Obstructed expansion valve  Clean valve and replace if necessary
Contact on control thermostat stuck on closed position Repair thermostat or replace if necessary
With hermetic motor-compressor combination  Compressor runs continuously; good refrigeration effect Air over condenser restricted Remove restriction or provide for more air circulation over the condenser
Compressor runs continuously; unit is too cold Thermostatic switch contacts badly burned Replace thermostatic switch
Thermostatic switch bulb has become loose Secure bulb in place
Thermostatic switch improperly adjusted Readjust thermostatic switch
Compressor runs continuously; little refrigeration effect Extremely dirty compressor Clean compressor
No air circulating over condenser Provide air circulation
Ambient temperature too high Provide ventilation or move to a cooler location Load too great Analyze load
Type of Unit Complaint Cause Possible Remedy
With hermetic motor-compressor combination (cont.) Compressor runs continuously; no refrigeration A restriction that prevents the refrigerant from entering the evaporator. A restriction is usually indicated by a slight refrigeration effect at the point of restriction Locate the possible points of restriction, and try jarring it with a plastic hammer, or heating to a temperature of about 110 degrees F. If the restriction does not open, replace the unit.
Compressor not pumping. A cool discharge line and a hot compressor housing, indicates this. The wattage is generally low. Replace the unit
Short of refrigerant See manufacturer’s instructions
Compressor short cycles, poor refrigeration effect Loose electrical connections Locate loose connections and make them secure
Defective thermostatic switch Replace thermostatic switch
Defective motor starter Replace defective motor starter or relay
Air restriction at evaporator Remove air restriction
Compressor short cycles, no refrigeration Dirty condenser Clean the condenser
Ambient temperature too high Provide ventilation or move to a cooler location
Defective wiring Repair or replace defective wiring
Thermostatic switch operating erratically Replace thermostatic switch
Relay erratic Replace relay
Compressor runs too frequently Poor air circulation around the condenser or ambient temperature too high Increase the air circulation around the condenser. In some localities the temperature is extremely high, and nothing can be done to correct the problem
Type of Unit Complaint Cause Possible Remedy
With hermetic motor-compressor combination (cont.) Compressor runs too frequently (cont) Load too great. Worn compressor. Generally accompanied by rattles and knocks Analyze end use. Replace unit or bring it to the shop for repairs
Compressor does not run Motor is not operating If the trouble is outside the sealed unit, it should be corrected; for example, wires should be repaired or replaced and thermostatic switches or relays should be replaced. If the trouble is inside the sealed unit, the sealed unit should be replaced.
Compressor will not run (Assume that the thermostatic switch and relay and the electric wiring and current supply are in good condition and operating normally) If the cabinet has been moved, some oil may be on top of the piston Wait an hour or so, and then attempt to start the motor by turning the current on and off many times. On some compressors, it may be necessary to wait 6 or 8 hours
Compressor may be stuck, or some parts may be broken Replace the unit
Connections may be broken on the inside of the unit, or the motor winding may be open Replace the unit. Sometimes after sealed units have been standing idle for a long time, the piston may be stuck in the cylinder wall. It is sometimes possible to start the compressor by turning on the current and bumping the outer housing with a rubber mallet.
Compressor is unusually hot Condenser is dirty, or there is a lack of air circulation Clean the condenser, increase the air circulation
Unusually heavy service or load If possible, decrease load. Perhaps another unit is required
Type of Unit Complaint Cause Possible Remedy
With hermetic motor-compressor combination (cont.) Compressor is unusually hot (cont.) Low voltage Feed wires too small could cause this. If the wires feeding the refrigerating unit become warm, it is an indication that they are too small and should be replaced with larger wires
A shortage of oil Add oil if possible; if this is not possible, the unit must be replaced. A shortage of refrigerant causes a shortage of oil in the crankcase of the compressor
No refrigeration after starting up after a long shutdown or on delivery Generally, during a long shutdown, an amount of liquid refrigerant will get into the crankcase of the compressor. When this happens, the compressor operation will cause no noticeable refrigeration effect until the entire liquid refrigerant has evaporated from the crankcase. Allow the compressor to operate until its internal heat driver the liquid refrigerant from the crankcase. Under some conditions, this may take as long as 24 hours. This time can be shortened by turning an electric heater on the compressor and raising the compressor temperature, not exceeding 110 degrees F.
Compressor is noisy Mountings have become worn or deteriorated. The walls against which the unit is placed may be of an extremely hand surface and may resound and amplify the slight noise from the compressor into the room. Replace the rubber mountings. Place a piece of soundabsorbing material on the wall against which the unit is placed or move the unit to a new location.
Shortage of oil and/or refrigerant Add oil and refrigerant if possible. If it is impossible, the unit must be replaced.
The sealed unit mechanism has become worn Replace the unit
  After each defrosting there is a long on cycle before refrigeration is again normal Slight shortage of refrigerant Add refrigerant if possible, if not, replace the unit.
Condenser is dirty Clean the condenser
Thermostatic switch bulb is loose Secure the bulb in place
There is a restriction between the receiver or condenser and/or the evaporator Attempt to remove the restriction by jarring with a plastic hammer or by heating the possible points of restriction to about 110 degrees F. If this does not correct the trouble, the unit must be replaced or brought to the shop for repairs.


Test Your Knowledge

6. What type of cooling tower requires a completely open space because its performance depends on existing air currents?

A. Induced draft
B. Natural draft
C. Forced draft
D. Single draft

7. Which of the following reciprocating compressors is driven by an external motor?

A. Open
B. Hermetic
C. Rotary
D. Semi-sealed


- To Table of Contents -


As part of the responsibilities as an HVAC-R tech, it is important that you have an understanding of the basic principles of operation, maintenance, and repair of automotive air conditioning

Vehicle air conditioning is the cooling (refrigeration) of air within a passenger compartment. Refrigeration is accomplished by using heat transfer, latent heat of vaporization, and the effects of pressure on boiling or condensation.

4.1.0 Effect of Pressure on Boiling or Condensation

The saturation temperature (the temperature where boiling or condensation occurs) of a liquid or vapor increases or decreases according to the pressure exerted on it.

In the fixed orifice tube refrigerant system, liquid refrigerant is stored in the condenser under high pressure (Figure 40). When the liquid refrigerant is released into the evaporator by the fixed orifice tube, the resulting decrease in pressure and partial boiling lowers its temperature to its new boiling point.

As the refrigerant flows through the evaporator, passenger compartment air passes over the outside surface of the evaporator coils. As it boils, the refrigerant absorbs heat from the air and cools the passenger compartment. The heat from the passenger compartment is absorbed by the boiling refrigerant and hidden in the vapor. The refrigeration cycle is now under way. The following functions must be done to complete the refrigeration cycle:

  1. Disposing of the heat in the vapor
  2. Converting the vapor back to liquid for reuse
  3. Returning of the liquid to the starting point in the refrigeration cycle

The compressor and condenser (Figure 40) perform these functions. The compressor pumps the refrigerant vapor (containing the hidden heat) out of the evaporator and suction accumulator drier, then forces it under high pressure into the condenser which is located in the outside air stream at the front of the vehicle. The increased pressure in the condenser raises the refrigerant condensation or saturation temperature to a point higher than that of the outside air. As the heat transfers from the hot vapor to the cooler air, the refrigerant condenses back to a liquid. The liquid under high pressure now returns through the liquid line to the fixed orifice tube for reuse.

Figure 40 — Air-conditioning refrigeration system-fixed orifice.

It may seem difficult to understand how heat can be transferred from a comparatively cooler vehicle passenger compartment to the hot outside air. The answer lies in the difference between the refrigerant pressure that exists in the evaporator and the pressure that exists in the condenser. In the evaporator, the compressor suction reduces the pressure and the boiling point below the temperature of the passenger compartment, thus, heat transfers from the passenger compartment to the boiling refrigerant. In the condenser, the compressor raises the condensation point above the temperature of the outside air, thus, the heat transfers from the condensing refrigerant to the outside air. The fixed orifice tube and the compressor simply create pressure conditions that permit the laws of nature to function.

4.2.0 Automotive Compressors

For automotive applications, there are three basic types of air-conditioning compressors in general use. These types of compressors include the following:

Each of these uses a reciprocating (back-and-forth motion) piston arrangement. Most automotive compressors are semi-hermetic.

Two-cylinder compressors (Figure 41) usually contain two pistons in a parallel V-type configuration. The pistons are attached to a connecting rod, which is driven by the crankshaft. The crankshaft is connected to the compressor clutch assembly, which is driven by an engine belt. Reed valves generally are used to control the intake and exhaust of the refrigerant gas during the pumping operation. These compressors are usually constructed of die cast aluminum.

Figure 41 — Two-cylinder reciprocating compressor.

In the swash plate or "wobble plate" compressor (Figure  42), the piston motion is parallel to the crankshaft. The pistons are connected to an angled swash plate using ball joints. Swash plate compressors consist of the following three types

Figure 42 — Five-cylinder swash plate compressor.

The five- and six-cylinder swash compressor has three cylinders at each end of its inner assembly. A swash plate of diagonal design is mounted on the compressor shaft. It actuates the pistons, forcing them to move back and forth in the cylinders as the shaft is rotated. Reed valves control suction and discharge; crossover passages feed refrigerant to both high- and low-service fittings at the rear end of the compressor. A gear-type oil pump in the rear head provides compressor lubrication.

The five-cylinder variable swash plate compressor is different from the other swash plate compressors. It uses a plate connected to a hinge pin that permits the swash plate to change its angle. The angle of the swash plate is controlled by a bellows valve that senses suction pressure. During high load conditions the swash plate angle is large, and during low load conditions, the swash plate angle is smaller. The displacement of the compressor is high at a large angle and low at a small angle.

A scotch-yoke compressor changes rotary motion into reciprocating motion. The basic mechanism of the scotch yoke contains four pistons mounted 90 degrees from each other. Opposed pistons are pressed into a yoke that rides on a slide block located on the shaft eccentric (Figure 43). Rotation of the shaft provides a reciprocating motion with no connecting rods. Refrigerant flows into the crankcase through the rear and is drained through the reeds attached to the piston tops during the suction stroke. Refrigerant is then discharged through the valve plate out the connector block at the rear. These compressors are shorter in length and larger in diameter than other compressors.

Figure 43 — Four-cylinder scotch-yoke mechanism.

4.2.1 Compressor Service Valves

Some air-conditioning systems have compressor service valves built into them. They serve as a point of attachment for test gauges or servicing hoses. Service valves have the following three position controls:

The position of this double-faced valve (Figure 44) is controlled by rotating the valve stem with a service valve wrench. Clockwise rotation seats the front face of the valve and shuts off all refrigerant flow in the system. This position isolates the compressor from the rest of the system.

Figure 44 — Service valve positions.

Counterclockwise rotation unseats the valve and opens the system to refrigerant flow (mid-position). Systematic checks are performed with a manifold gauge set with the service valve in mid-position.

Further counterclockwise rotation of the valve stem seats the rear face of the valve. This position opens the system to the flow of refrigerant but shuts off refrigerant to the test connector. The service valves are used for observing of operating pressures; isolating the compressor for repair or replacement; and discharging, evacuating, and charging the system.

Compressors used in automotive air-conditioning systems generally are equipped with an electromagnetic clutch that energizes and de-energizes to engage and disengage the compressor. The rotating coil and stationary coil are the two types of clutches that are used most often.

The rotating coil clutch has a magnetic coil mounted in the pulley that rotates with the pulley. It operates electrically through connections to a stationary brush assembly and rotating slip rings. The clutch permits the compressor to engage or disengage as required for adequate air conditioning. The stationary coil clutch has the magnetic coil mounted on the end of the compressor. Electrical connections are made directly to the coil leads.

The belt-driven pulley is always in rotation while the engine is running. The compressor is in rotation and operation only when the clutch engages it to the pulley.

Air-conditioning and refrigeration systems use various control devices, including those for the refrigerant, the capillary tube usually found on window units, the automatic expansion valves also found on window units and small package units, the thermal expansion valve, and various types of suction pressure-regulating valves and devices. A suction pressure-regulating valve is used on automotive air conditioning because the varying rpm of the compressor unit must maintain a constant pressure in the evaporator.

4.2.2 Suction Pressure-Regulating Valves

Suction pressure-regulating valves may be installed in the suction line at the outlet of the evaporator when a minimum temperature must be maintained. Suction pressure-regulating valves decrease the temperature difference, which would otherwise exist between the compartment temperature and the surface of the cooling coils. The amount of heat that can be transferred into the evaporating refrigerant is directly proportional to the temperature difference. Figure 45 shows an exploded view of a typical suction pressure-regulating valve, sometimes called a suction throttling valve in automotive air conditioners.

Figure 45 — Suction pressure regulating valve.

The following three types of suction pressure-regulating valves are in use:

These valves were developed by General Motors, and are adjustable in most cases.

The POA valve uses a sealed pressure element that maintains a constant pressure independent of the altitude of the vehicle. The following two basic types of metering devices are built into a single container:

These units combine the POA valve, receiver-drier, thermostatic expansion valve, and sight glass into a single unit.

The VIR assembly is mounted next to the evaporator, which eliminates the need for an external equalizer line between the thermostatic expansion valve and the outlet of the POA valve. The equalizer function is carried out by a drilled-hole (equalizer port) between the two-valve cavities in the VIR housing.

The thermostatic expansion valve is also eliminated. The diaphragm of the VIR expansion valve is exposed to the refrigerant vapor entering the VIR unit from the outlet of the evaporator. The sight glass is in the valve housing at the inlet end of the thermostatic valve cavity where it gives a liquid indication of the refrigerant level.

The VIR thermostatic expansion valve controls the flow of refrigerant to the evaporator by sensing the temperature and pressure of the refrigerant gas as it passes through the VIR unit on its way to the compressor. The POA valve controls the flow of refrigerant from the evaporator to maintain a constant evaporator pressure of 30 psi. The VIR and the POA valves are capsule type of valves. When found to be defective, you must replace the complete valve capsule.

The drier desiccant is in a bag in the receiver shell. It is replaceable by removing the shell and removing the old bag and installing a new bag of desiccant. Service procedures for the VIR system differ in some respects from the service procedures performed on conventional automotive air-conditioning systems.

4.3.0 Service Precautions

When tasked to service air-conditioning equipment observe the following precautions:

4.4.0 Diagnosis, Testing, and Servicing

Diagnosis is more than just following a series of interrelated steps to find the solution to a specific condition. It is a way of looking at systems that are not functioning the way they should and finding out why. Also, diagnosis includes knowing how the system should work and whether it is working correctly. All good diagnosticians use the same basic procedures. There are basic rules for diagnosis. Follow these rules when going through the system the first time to find the cause of the condition.

  1. Know the system; know how the parts go together. Also, know how the system operates and its limits, and what happens when something goes wrong. Sometimes this means comparing a system that is working properly with the one you are servicing.
  2. Know the history of the system. How old or new is the system? What kind of treatment has it had? Has it been serviced in the past in such a manner that might relate to the present condition? What is the service history? A clue in any of these areas might save a lot of diagnosis time.
  3. Know the probability of certain conditions developing. It is true that most conditions are caused by simple things, rather than by complex ones, and they occur in a fairly predictable pattern. Electrical problem conditions, for instance, usually occur at connections rather than in components. An engine "no-start" is more likely to be caused by a loose wire or some component out of adjustment than a sheared-off camshaft. Know the difference between impossible and improbable. Many good technicians have spent hours diagnosing a system because they thought certain failures were "impossible," only to find out the failures eventually were just "improbable" and actually had happened. Remember, new parts are just that—new. It does not mean they are good functioning parts.
  4. Do not cure the symptom and leave the cause. Recharging a refrigerant system may correct the condition of insufficient cooling, but it does not correct the original problem unless a cause is found. A properly working system does not lose refrigerant over time.
  5. Be sure to find the cause. Do not be fooled into thinking the cause of the problem has been found. Perform the proper tests then double-check the results. The system should have been checked for refrigerant leaks. If no leaks were found, perform a leak test with the system under extremely high pressure. If the system performed properly when new, it had to have a leak to be low in charge.
  6. No matter what form charts may take, they are simply a way of expressing the relationship between the basic logic and a physical system of components. It is a way of determining the cause of a condition in the shortest possible amount of time. Diagnosis charts combine many areas of diagnosis into one visual display that allows you to determine the following:

The fastest way to find a condition is to work with the tools that are available, which means working with proven diagnosis charts and the proper special tools for the system being worked on.

Servicing procedures for automotive air-conditioning units are similar to those used to service conventional air-conditioning systems. Discharging, evacuating, charging procedures, connections, and positions of valves on the gauge manifold set are shown in Figure 46.

Figure 46 — Procedures for observing operating pressures, charging, purging, and evacuating a unit.

Servicing procedures for the VIR system are also similar to those used when servicing conventional air-conditioning systems. However, the hookup of the manifold gauge set is to the VIR unit. The high-pressure fitting is located in the VIR inlet line. The low-pressure fitting is located in the VIR unit.

4.5.0 System Visual Inspection

When conducting a careful visual inspection of a unit’s refrigerant system, it is often possible to find out what could be causing a problem. This could include broken belts, obstructed condenser air passages, a loose clutch, loose or broken mounting brackets, disconnected or broken wires, and refrigerant leaks.

A refrigerant leak usually appears as an oily residue at the leakage point in the system. The oily residue will appear greasy because it has picked up dust or dirt particles from the surrounding air. Through time, this builds up and appears to be heavy, dirt-impregnated grease.

Another type of leak may appear at the internal Schrader type of air-conditioning charging valve core in the service gauge port valve fittings. If tightening the valve core does not stop the leak, it should be replaced with a new air-conditioning charging valve core.

A refrigerant leak can also be caused if a service gauge port valve cap is missing. When the valve cap is missing, dirt enters the area of the air-conditioning charging valve core. After the service hose is attached, the valve depressor in the end of the service hose forces the dirt into the valve seat area, and it destroys the sealing surface of the air-conditioning charging valve core. If you find a service gauge port valve cap is missing, you should clean the protected area of the charging valve core and install a new service gauge port valve cap.


The service gauge port valve cap must be installed finger tight. If tightened with pliers, the sealing surface of the service gauge port valve may be damaged.

4.6.0 Cleaning a Badly Contaminated Refrigerant System

A refrigerant system can become badly contaminated for a number of reasons.

A badly contaminated system contains water, carbon, and other decomposition products. When such a condition exists, the system must be flushed with a special flushing agent, using equipment designed especially for this purpose.

4.6.1 Flushing Agents

A refrigerant to be suitable as a flushing agent must remain in the liquid state during the flushing operation to wash the inside surfaces of the system components. Refrigerant vapor will not remove contaminant particles. They must be flushed with a liquid. Some refrigerants are better suited for this purpose than others.

For years, R-11 and R-113 refrigerants were used as the primary flushing agents however, both were phased out in the year 2000. At this time there are other refrigerants that are being used in their place. R-123 refrigerant has replaced R-11, and R-113 has been replaced by R-141b refrigerant.


Use extreme care and adhere to all safety precautions related to the use of refrigerants when flushing a system.

4.6.2 System Cleaning and Flushing

When it is necessary to flush a refrigerant system, the suction accumulator/drier must be removed and replaced, as it is impossible to clean. Remove the fixed orifice tube. If a new tube is available, replace the contaminated one; otherwise, wash it carefully in flushing refrigerant or mineral spirits and blow it dry. If it does not show signs of damage or deterioration, it may be reused. Install new O rings.

Any moisture in the evaporator will be removed during leak testing and system evacuation following the cleaning job. Perform the following steps of the cleaning procedure carefully:

  1. Check the hose connections at the flushing cylinder outlet and flushing nozzle to ensure they are secure.
  2. Ensure the flushing cylinder is filled with approximately 1 pint of R-141b and that the valve assembly on top of the cylinder is tightened securely.
  3. Connect a can of R-134a to the Schrader valve at the top of the charging cylinder. A refrigerant hose and a special, safety type of refrigerant dispensing valve are required for connecting the small can to the cylinder. Ensure all connections are secure.
  4. Connect a gauge manifold and a discharge system. Disconnect the gauge manifold.
  5. Remove and discard the suction accumulator/drier. Install a new accumulator/drier and connect it to the evaporator. Do not connect it to the suction line from the compressor. Ensure a protective cap is in place on the suction line connection.
  6. Replace the fixed orifice tube. Install a protective cap on the evaporator inlet tube as soon as the new orifice tube is in place. The liquid line will be connected later.
  7. Remove the compressor from the vehicle for cleaning and servicing or replacement, whichever is required. If the compressor is cleaned and serviced, add the specified amount of refrigerant oil before installing it on the mounting brackets in the vehicle. Install the shipping caps on the compressor connections. Install a new compressor on the mounting brackets in the vehicle.
  8. Back flush the condenser and the liquid line as follows:
    1. Remove two O rings from the condenser inlet tube spring lock coupling.
    2. Remove the discharge hose from the condenser and clamp a piece of (1/2- inch ID) heater hose to the condenser inlet line. Ensure the hose is long enough to insert the free end into a suitable waste container to catch the flushing refrigerant.
    3. Move the flushing equipment into position and open the valve on the can of R-134a (fully counterclockwise).
    4. Back flush the condenser and the liquid line by introducing flushing refrigerant into the supported end of the liquid line with the flushing nozzle. Hold the nozzle firmly against the open end of the liquid line.
    5. After the liquid line and condenser have been flushed, lay the charging cylinder on its side so the R-134a will not force more of the flushing refrigerant into the liquid line. Press the nozzle firmly to the liquid line and admit the R-134a to force all the flushing refrigerant from the liquid line and condenser.
    6. Remove the 1/2-inch hose and clamp from the condenser inlet connection.
    7. Stand the flushing cylinder upright and flush the compressor discharge hose. Secure it so the flushing refrigerant goes into the waste container.
    8. Close the dispensing valve of the R-134a can (fully clockwise). If there is any flushing refrigerant in the cylinder, it may be left there until the next flushing job. Put the flushing kit and R-134a can in a suitable storage location.
    9. Install the new lubricated O rings on the spring lock coupling male fittings on both the condenser inlet and the liquid lines. Assemble the couplings.
  9. Connect all refrigerant lines. All connections should be cleaned and new O rings should be used. Lubricate new O rings with clean refrigerant oil.
  10. Connect a charging station or manifold gauge set and charge the system with 1 pound of R-134a. (Do not evacuate the system until after it has been leak tested.)
  11. Leak-test all connections and components with a flame type of leak detector or an electronic leak detector. If no leaks are found, go to Step 12. If leaks are found, service as necessary; check the system and then go to Step 12.
  12. Evacuate and charge the system with a specified amount of R-134a. Operate the system to ensure it is cooling properly.

4.7.0 Safety Precautions

The use of safety when handling or using refrigerants can never be stressed enough. Routinely think of safety for yourself and co-workers.

Extreme care must be taken to prevent any liquid refrigerant from coming in contact with the skin and especially the eyes. A bottle of sterile mineral oil and a quantity of weak boric acid solution must always be kept nearby when servicing the air-conditioning system. Should any liquid refrigerant get into your eyes, immediately use a few drops of mineral oil to wash them out; then wash the eyes clean with the weak boric acid solution. Seek a doctor’s aid immediately even though irritation may have ceased. Always wear safety goggles when servicing any part of the refrigerant system.

To avoid a dangerous explosion, never weld, solder, steam clean, bake body finishes, or use any excessive amount of heat on or in the immediate area of any part of the refrigerant system or refrigerant supply tank while they are closed to the atmosphere, whether filled with refrigerant or not.

The liquid refrigerant evaporates so rapidly that the resulting refrigerant gas displaces the air surrounding the area where the refrigerant is released. To prevent possible suffocation in enclosed areas, always discharge the refrigerant into recycling/reclaiming equipment. Always maintain good ventilation surrounding the work area.

Even though R-12 refrigerant was banned in 1996, some systems you may be working on may still contain R-12. Therefore, it is important that you adhere to the following information.

Under normal conditions, R-12 gas is non-poisonous. However, the discharge of R-12 near an open flame can produce a very poisonous gas. This gas, which also attacks all bright metal surfaces, is generated when you use the flame type of leak detector. Avoid inhaling the fumes from the leak detector. Chances are you will have to replace the R- 12 with R-134a and modify system components accordingly. After exchanging R-12, it is important that it is properly disposed of according to federal, state, and local ordinances.

When admitting R-134a gas into the cooling unit, always keep the tank in an upright position. The tank cannot be on its side or upside down because liquid R-134a will enter the system and may damage the compressor.

4.8.0 Truck and Bus Air Conditioning

Many truck-tractors, long distance hauling trucks, and earthmoving equipment have cabs that are air-conditioned. Most of this air-conditioning equipment is the “hang on” type and is installed after the cab has been made.

Some truck air-conditioning units have two evaporators—one for the cab and one for the relief driver's quarters in back of the driver. Some systems use a remote condenser, mounted on the roof of the cab. This type of installation removes the condenser from in front of the radiator so the radiator can operate at full efficiency. This is especially important during long pulls in low gear.

The system is similar to the automobile air conditioner and is installed and serviced in the same general way.

The air conditioning of buses has progressed rapidly. Because of the large size of the unit, most bus air-conditioning systems use a separate gasoline engine with an automatic starting device to drive the compressor. The system is standard in construction except for the condensing unit. It is made as compact as possible and is installed in the bus, so it can be easily reached for servicing.

To aid in servicing the system, condensing units are often mounted on rails with flexible suction and liquid lines, which allow sliding of the condensing unit out of the bus.

Air-cooled condensers are used. Thermostatic expansion valve refrigerant controls are standard. Finned blower evaporators are also used.

The duct system usually runs between a false ceiling and the roof of the bus. The ducts, usually one on each side of the bus, have grilles at the passenger seats. The passengers may control the grille by opening and closing.

4.9.0 Certification

In accordance with the Clean Air Act (CAA), the Environmental Protection Agency (EPA) has established that all technicians who maintain or repair air-conditioning or refrigeration equipment must be certified. Technicians who operate recycling, reclaiming, and recovery equipment must also be certified. Certification is administered by organizations with certification programs that are approved by the EPA.

As an HVAC-R tech, it is important that you understand if you are not certified, you cannot service any Heating Ventilation Air Conditioning and Refrigeration (HVA/R) units that require you to use or remove refrigerants. Certification requirements are divided into the following two areas:

4.9.1 Automotive Air-Conditioning

Certification In today’s world, people spend more and more time in their vehicles, causing the air conditioning to be used continuously during the hotter months of the year. This higher usage requires that automotive air-conditioning be serviced or repaired more than other types of air-conditioning systems.

To become a certified technician, you must meet the following EPA requirements:

A certification card is issued to the applicant after being tested and meeting the EPA requirements.

4.9.2 Heating, Ventilating, Air Conditioning, and Refrigeration Certification

The certification requirements for standard types of air-conditioning systems are the same as those for automotive air-conditioning certification. Unlike the automotive certification program, standard air-conditioning certification is divided into levels corresponding to the type of service the technician performs. There are four types of certification:

Individuals will be required to take a proctored, closed-book test.

These tests are offered by organizations approved by the EPA for the specific type of certification required by the technician. Technicians can only work on air-conditioning systems that they have been certified to service.

Test Your Knowledge

8. What type of swash plate compressor uses a plate connected to a hinge pin that permits the swash plate to change its angle?

A. Six-cylinder
B. Scotch-yoke
C. Two-cylinder
D. Five-cylinder variable

9. What type of suction pressure-regulating valve uses a sealed pressure element to maintain a constant pressure independent of the altitude of the vehicle?

A. Isolated system pressure (ISP) valve
B. Pilot operated absolute (POA) valve
C. Suction throttling valve (STV)
D. Evaporator pressure regulators (EPR)

10. Which of the following refrigerants has replaced R-113 as a primary flushing agent used for cleaning a system?

A. R-141b
B. R-502
C. R-22
D. R-123


- To Table of Contents -


As an HVAC-R tech, it is important that you understand the basic types of ductwork systems, and the components of those systems used for distributing conditioned air.

Distributed air must be clean, provide the proper amount of ventilation, and absorb enough heat to cool the conditioned spaces. To deliver air to the conditioned space, air carriers are required, which are called ducts. Ducts work on the principle of air pressure difference. If a pressure difference exists, air will flow from an area of high pressure to an area of low pressure. The larger this difference, the faster the air will flow to the low-pressure area.

5.1.0 Classification of Ducts

The three common classifications of ducts are:

Conditioned air ducts carry conditioned air from the system and distribute it to the conditioned area. Recirculating air ducts take air from the conditioned space and distribute it back into the system. Fresh air ducts bring fresh air into the system from outside the conditioned space.

Ducts commonly used for carrying air are round, square, or rectangular in shape. The most efficient duct is a round duct, based on the volume of air handled per perimeter distance. In other words, less material is needed for the same capacity as a square or rectangular duct because circular ducts cause less turbulence and allow more flow.

Square or rectangular duct fits better to building construction. It fits above ceilings and into walls and is much easier to install between joists and studs.

5.2.0 Types of Duct Systems

There are several types of supply duct systems (Figure 47) that deliver air to room(s) and then return the air from the room(s) to the cooling (evaporator) system. These supply systems can be grouped into the following four types:

  1. Individual round pipe system
  2. Extended plenum system
  3. Reducing trunk system
  4. Combination (of two or more systems) Return air systems are normally of three types—single return, multiple return (Figure 47), or a combination of the two systems.

Figure 47 — Supply duct systems.

5.3.0 Construction

Ducts may be made of metal, wood, ceramic, and plastic. Most commonly used is sheet steel coated with zinc (galvanized steel). Sheet metal brakes and forming machines are used in fabricating ducts. Elbows and other connections, such as branches, are designed using geometric principles. Some types of duct connections used in constructing duct systems are shown in Figure 48.

Figure 48 — Typical duct connections

When sheet metal ducts heat and cool, they expand and contract. To absorb this movement, fabric joints are often used. Fabric joints should also be used where the duct connects to the air conditioner. Many ducts are insulated to lower noise and reduce heat transfer. The insulation can be on the inside or the outside of the duct. Adhesives or metal clips are commonly used to fasten the insulation to the duct

5.4.0 Components

To enable a duct system to circulate air at the proper velocity and volume to the proper conditioned areas, you can use different components within the duct system, such as diffusers, grilles, and dampers

5.4.1 Diffusers, Grilles and Registers

Room openings to ducts have several devices that control the airflow and keep large objects out of the duct. These devices consist of the following:

Diffusers deliver fan-shaped airflow into a room. In certain types of diffusers duct air will mix with some room air. Grilles control the distance, height, and spread of air-throw, as well as amount of air.

Grilles cause some resistance to airflow. Grille cross-section pieces block about 30 percent of the air. Because of this reason and to reduce noise, cross sections are usually enlarged at the grille. Grilles have many different designs, such as fixed vanes which force air in one direction, or adjustable, which forces air in different directions.

Registers are used to deliver a concentrated air stream into a room, and many have one-way or two-way adjustable air stream deflectors.

5.4.2 Dampers

One way of getting even air distribution is through the use of duct dampers. Dampers balance airflow or can shut off or open certain ducts for zone control. Some are located in the grille, and some are in the duct itself. The following three types of dampers, shown in Figure 49, are used in air-conditioning ductwork:

Figure 49 — Three types of duct dampers.

5.4.3 Fire Dampers

Automatic fire dampers should be installed in all vertical ducts. Ducts, especially vertical ducts, will carry fumes and flames from fires. Fire dampers must be inspected and tested at least once a year to be sure they are in proper working order. The following two types of fire dampers are fail-safe units:

Fire dampers are usually held open by a fusible link. Heat will melt the link and the damper will be closed by gravity, weights, or springs (Figure 50).

Figure 50 — Fire damper in OPEN position.

5.4.4 Fans

Air movement is usually produced by some type of forced airflow. Fans are normally located in the inlet of the air conditioner. Air is moved by creating either a positive pressure or negative pressure in the ductwork. The two most popular types of fans are the axial flow (propeller) or radial flow (squirrel cage) (Figure  51).

Figure 51 — Principal types of fans.

The axial-flow fan is usually direct-driven by mounting the fan blades on the motor shaft. The radial-flow fan is normally belt-driven but can also be direct-driven.

5.5.0 Balancing the System

Balancing a system basically means sizing the ducts and adjusting the dampers to ensure each room receives the correct amount of air. To balance a system, follow these steps:

  1. Inspect the complete system. Locate all ducts, openings, and dampers.
  2. Open all dampers in the ducts and at the grilles.
  3. Check the velocities at each outlet.
  4. Measure the "free" grille area.
  5. Calculate the volume at each outlet. Velocity x Area = Volume
  6. Area in square inches divided by 144 multiplied by feet per minute equals cubic feet/minute.
  7. Total the cubic feet/minute.
  8. Determine the floor areas of each room. Add to determine total area.
  9. Determine the cubic feet per minute (cfm) for each room. The area of the room divided by the total floor area, multiplied by the total cfm, equals cfm for the room.
  10. Adjust duct dampers and grille dampers to obtain these values.
  11. Recheck all outlet grilles.

In some cases, it may be necessary to overcome excess duct resistance by installing an air duct booster. These are fans used to increase airflow when a duct is too small, too long, or has too many elbows.

Test Your Knowledge

11. What classification of air ducts takes air from the conditioned space and then distributes it back into the air conditioner system?

A. Fresh-air
B. Recirculating air
C. Full-capacity air
D. Conditioned air

12. Which of the following duct components is used to deliver a concentrated air stream into a room?

A. Grille
B. Diffuser
C. Damper
D. Register

13. A radial flow fan can be belt-driven or direct-drive.

A. True
B. False



Air conditioning is the simultaneous control of temperature, humidity, air movement, and the quality of air in a conditioned space or building. This course introduced the principles of air conditioning and the operation of basic air-conditioning systems. It also described how to recognize the characteristics and procedures required to install, operate, and maintain air-conditioning systems. Finally, this course described how to recognize air-conditioning system components and controls along with their application, and the types of duct systems that can be used.

Review Questions

1. What is the term given to the net effects of temperature, humidity, and air motion?

A. Comfort zone
B. Effective temperature
C. Relative humidity
D. Dew point

2.  Electricity can be used to purify the air in some large air-conditioning systems.

A. True
B. False

3. Window-mounted air-conditioning units usually range from 4,000 to ______ Btu per hour in capacity.

A. 16,000
B. 28,000
C. 32,000
D. 36,000

4. Floor-mounted air-conditioning units that range in size from 24,000 to 360,000 Btu per hour are also referred to as what type of unit?

A. Complete
B. Controlled
C. Major
D. Package

5.  Flooded shell-and-tube coolers use less refrigeration than other systems of equal size.

A. True
B. False

6. What type of condenser is used for practically all water-cooled refrigeration systems?

A. Shell-and-tube
B. Tube-within-a-tube
C. Shell-and-coil
D. Coil-and-tube

7. Cooling towers have been linked to the spread of what disease?

A. Pneumonia
B. Emphysema
C. Legionnaire’s
D. Cystic fibrosis

8. How much Btu is required to evaporate 3 gallons of water?

A. 16,400
B. 18,700
C. 21,800
D. 24,900

9. What type of compressor is used in large air-conditioning systems that handle large volumes of refrigerants at low-pressure differentials?

A. Centrifugal
B. Rotary
C. Hermetic
D. Scroll

10. What air-conditioning control is an adjustable temperature-sensitive device that has contacts that open and close to control the operation of the air conditioner?

A. Step controller
B. Motor overload protector
C. Thermostat
D. Humidistat

11. What type of automotive air-conditioning compressor changes rotary motion into reciprocating motion, and contains four pistons mounted 90 degrees from each other?

A. Scotch-yoke
B. Swash plate
C. Two-cylinder reciprocating
D. Five-cylinder variable

12. What is the first step to be followed when performing diagnosis of an air conditioning system?

A. Know the probability of certain conditions developing
B. Know how to cure the symptoms
C. Know the history of the system
D. Know the system and how the parts work together

13. According to the EPA, what refrigerant can now be used to replace R-12 refrigerant in air-conditioning systems?

A. R-141b
B. R-134a
C. R-123
D. R-113

14.  When servicing HVA/R units, certification is not required for using or removing refrigerants.

A. True
B. False

15. What type of standard air-conditioning certification must a technician have to service or dispose of low-pressure appliances?

A. Type I
B. Type II
C. Type III
D. Type IV

16.  Conditioned air ducts bring outside air into the air-conditioning system, then distributes it to the conditioned area.

A. True
B. False

17. What air-conditioning duct is the most efficient?

A. Rectangular
B. Round
C. Triangular
D. Square

18. Which of the following duct components controls the distance, height, and spread of air-throw, as well as the amount of air, but can also cause some resistance to airflow?

A. Diffuser
B. Register
C. Grille
D. Damper

19.  Automatic fire dampers that are installed in vertical ducts must be inspected at least once a year to ensure they are working properly.

A. True
B. False

20. What type of air conditioner fan is also known as a squirrel cage fan?

A. Axial flow
B. Straight stream
C. Uneven draw
D. Radial flow

21. What is the fourth step that a technician should follow when balancing the system?

A. Measure the “free” grille area.
B. Check the velocities at each outlet.
C. Open all dampers in the ducts and at the grilles.
D. Determine the cubic feet per minute for each room.


- To Table of Contents -

Copyright © David L. Heiserman
All Rights Reserved