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The boilerman is concerned primarily with the FIRE-TUBE type of boiler. However, the WATER- TUBE type of boiler may occasionally be used at some activities. The information in this chapter primarily concerns the different designs and construction features of fire-tube boilers.

The basis for identifying the two types is as follows:

  • Water-tube boilers are those in which the products ofcombustion surround the tubes through which the water flows.
  • Fire-tube boilers are those in which the products of combustion pass through the tubes and the water surrounds them.


Water-tube boilers may be classified in a number of ways. For our purpose, they are classified as either straight tube or bent tube. These classes are discussed separately in succeeding sections. To avoid confusion, make sure you study carefully each illustration referred to throughout the discussion.

Straight Tube

The straight-tube class of water-tube boilers includes three types:

  1. Sectional-header cross drum
  2. Box-header cross drum
  3. Box-header longitudinal drum

In the sectional-header cross drum boiler with vertical headers, the headers are steel boxes into which the tubes are rolled. Feedwater enters and passes down through the downcomers (pipes) into the rear sectional headers from which the tubes are supplied. The water is heated and some of it changes into steam as it flows through the tubes to the front headers. The steam-water mixture returns to the steam drum through the circulating tubes and is discharged in front of the steam-drum baffle that helps to separate the water and steam.

Steam is removed from the top of the drum through the dry pipe. This pipe extends along the length of the drum and has holes or slots in the top half for steam to enter.

Headers, the distinguishing feature of this boiler. are usually made of forged steel and are connected to the drums with tubes. Headers may be vertical or at right angles to the tubes. The tubes are rolled and flared into the header. A handhold is located opposite the ends of each tube to facilitate inspection and cleaning. Its purpose is to collect sediment that is removed by blowing down the boiler.

Baffles are usually arranged so gases are directed across the tubes three times before being discharged from the boiler below the drum.

Box-header cross drum boilers are shallow boxes made of two plates—a tube-sheet plate that is bent to form the sides of the box, and a plate containing the handholds that is riveted to the tube-sheet plate. Some are designed so that the front plate can be removed for access to tubes. Tubes enter at right angles to the box header and are expanded and flared in the same manner as the sectional-header boiler. The boiler is usually built with the drum in front. It is supported by lugs fastened to the box headers. This boiler has either cross or longitudinal baffling arranged to divide the boiler into three passes. Water enters the bottom of the drum, flows through connecting tubes to the box header, through the tubes to the rear box header, and back to the drum.

Box-header longitudinal drum boilers have either a horizontal or inclined drum. Box headers are fastened directly to the drum when the drum is inclined. When the drum is horizontal, the front box header is connected to it at an angle greater than 90 degrees. The rear box header is connected to the drum by tubes. Longitudinal or cross baffles can be used with either type.

Bent Tube

Bent tube boilers usually have three drums. The drums are usually of the same diameter and positioned at different levels with each other. The uppermost or highest positioned drum is referred to as the STEAM DRUM, while the middle drum is referred to as the WATER DRUM, and the lowest, the MUD DRUM. Tube banks connect the drums. The tubes are bent at the ends to enter the drums radially.

Water enters the top rear drum, passes through the tubes to the bottom drum, and then moves up through the tubes to the top front drum. A mixture of steam and water is discharged into this drum. The steam returns to the top rear drum through the upper row of tubes, while the water travels through the tubes in the lower rear drum by tubes extending across the drum and enters a small collecting header above the front drum.

Many types of baffle arrangements are used with bent-tube boilers. Usually, they are installed so that the inclined tubes between the lower drum and the top front drum absorb 70 to 80 percent of the heat. The water-tube boilers discussed above offer a number of worthwhile advantages. For one thing, they afford flexibility in starting up. They also have a high productive capacity ranging from 100.000 to 1,000,000 pounds of steam per hour. In case of tube failure, there is little danger of a disastrous explosion of the water-tube boiler. The furnace not only can carry a high overload, it can also be modified for tiring by oil or coal. Still another advantage is that it is easy to get into sections inside the furnace to clean and repair them. There are also several disadvantages common to water-tube boilers. One of the main drawbacks of water-tube boilers is their high construction cost. The large assortment of tubes required of this boiler and the excessive weight per unit weight of steam generated are other unfavorable factors.


There are four types of fire-tube boilers—the Scotch marine boiler. the vertical-tube boiler, the horizontal return tubular boiler. and the firebox boiler. These four types of boilers are discussed in this section.

Scotch Marine Boiler

Figure 1-1 is a portable Scotch marine tire-tube boiler. The portable unit can be moved easily and requires only a minimal amount of foundation work. As a complete self-contained unit. its design includes automatic controls. a steel boiler. and burner equipment. These features are a big advantage because no disassembly is required when you must move the boiler into the field for an emergency.

Figure 1-1.—Scotch marine type of fire-tube boiler.

The Scotch marine boiler has a two-pass (or more) arrangement of tubes that run horizontally to allow the heat inside the tubes to travel back and forth. It also has an internally fired furnace with a cylindrical combustion chamber. Oil is the primary fuel used to fire the boiler; however. it can also be fired with wood, coal, or gas. A major advantage of the Scotch marine boiler is that it requires less space than a water-tube boiler and can be placed in a room that has a low ceiling.

The Scotch marine boiler also has disadvantages. The shell of the boiler runs from 6 to 8 feet in diameter, a detail of construction that makes a large amount of reinforcing necessary. The fixed dimensions of the internal surface cause some difficulty in cleaning the sections below the combustion chamber. Another drawback is the limited capacity and pressure of the Scotch marine boiler.

An important safety device sometimes used is the fusible plug that provides added protection against low-water conditions. In case of a low-water condition. the fusible plug core melts, allowing steam to escape, and a loud noise is emitted which provides a warning to the operator. On the Scotch boiler the plug is located in the crown sheet, but sometimes it is placed in the upper back of the combustion chamber. Fusible plugs are discussed in more detail later in this chapter.

Access for cleaning, inspection, and repair of the boiler watersides is provided through a manhole in the top of the boiler shell and a handhold in the water leg. The manhole opening is large enough for a man to enter the boiler shell for inspection, cleaning, and repairs. On such occasions, always ensure that all valves are secured, locked, tagged, and that the person in charge knows you are going to enter the boiler. Additionally, always have a person located outside of the boiler standing by to aid you in case of an incident occurring that would require you to need assistance. The handholds are openings large enough to permit hand entry for cleaning, inspection, and repairs to tubes and headers. Figure 1-2 shows a horizontal fire-tube boiler used in low-pressure applications. Personnel in the boilerman rating are assigned to operate and maintain this type of boiler more often than any other type of boiler.

Figure 1-2.—Horizontal fire-tube boiler used in low-pressure applications.

Vertical-Tube Boiler

In some fire-tube boilers, the tubes run vertically, as opposed to the horizontal arrangement in the Scotch boiler. The vertical-tube boiler sits in an upright position, as shown in figure 1-3. Therefore, the products of combustion (gases) make a single pass, traveling straight up through the tubes and out the stack. The vertical fire-tube boiler is similar to the horizontal fire-tube boiler in that it is a portable, self-contained unit requiring a minimum of floor space. Handholds are also provided for cleaning and repairing. Though self-supporting in its setting (no brickwork or foundation being necessary), it must be level. The vertical fire-tube boiler has the same disadvantages as that of the horizontal-tube design—limited capacity and furnace volume.

Figure 1-3.—Cutaway view of a vertical fire-tube boiler.

Before selecting a vertical fire-tube boiler, you must know how much overhead space is in the building where it will be used. Since this boiler sits in an upright position, a room with a high ceiling is necessary for its installation.

The blowdown pipe of the vertical tire-tube boiler is attached to the lowest part of the water leg. and the feedwater inlet opens through the top of the shell. The boiler fusible plug is installed either (1) in the bottom tube sheet or crown sheet or (2) on the outside row of tubes, one third of the height of the tube from the bottom.

Horizontal Return Tubular Boiler

In addition to operating portable boilers, such as the Scotch marine and vertical fire-tube boilers. the boilerman must also be able to operate stationary boilers, both in the plant and in the field. A stationary boiler can be defined as one having a permanent foundation and not easily moved or relocated. A popular type of stationary fire-tube boiler is the horizontal return tubular (HRT) boiler shown in figure 1-4.

Figure 1-4.—Horizontal return tubular (HRT) fire-tube boiler.

The initial cost of the HRT boiler is relatively low and installing it is not too difficult. The boiler setting can be readily changed to meet different fuel requirements—coal, oil, wood, or gas. Tube replacement is also a comparatively easy task since all tubes in the HRT boiler are the same in size, length, and diameter.

The gas flows in the HRT boiler from the firebox to the rear of the boiler. It then returns through the tubes to the front where it is discharged to the breaching and out the stack.

The HRT boiler has a pitch of 1 to 2 inches to the rear to allow sediment to settle toward the rear near the bottom blowdown connection. The fusible plug is located 2 inches above the top row of tubes. Boilers over 40 inches in diameter require a manhole in the upperpart of the shell. Those over 48 inches in diameter must have a manhole in the lower, as well as in the upper, part of the shell. Do not fail to familiarize yourself with the location of these and other essential parts of the HRT boiler. The knowledge you acquire will definitely help in the performance of your duties with boilers.

Firebox Boiler

Another type of fire-tube boiler is the firebox boiler that is usually used for stationary purposes. A split section of a small firebox boiler is shown in figure 1-5. Gases in the firebox boiler make two passes through the tubes. Firebox boilers require no setting except possibly an ash pit for coal fuel. As a result, they can be quickly installed and placed in service. Gases travel from the firebox through a group of tubes to a reversing chamber. They return through a second set of tubes to the flue connection on the front of the boiler and are then discharged up the stack.

Figure 1-5.—Split section of a small firebox boiler.

Q6. What are the two types of boilers?

Q7. What are the four types office-tube boilers?

Q8. What is the primary factor that allows the firebox boiler to be quickly installed and placed into service?

David L. Heiserman, Editor

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Revised: June 06, 2015