INTRODUCTION

Fire-tube boilers are so named because the products of combustion pass through tubes that are surrounded by water. They may be either internally fired (Fig. 2.3) or externally fired (see Fig. 2.5). Internally fired boilers are those in which the grate and combustion chamber are enclosed within the boiler shell. Externally fired boilers are those in which the setting, including furnace and grates, is separate and distinct from the boiler shell. Fire-tube boilers are classified as vertical tubular or horizontal tubular.

The vertical fire-tube boiler consists of a cylindrical shell with an enclosed firebox (Figs. 2.3 and 2.4). Here tubes extend from the crown sheet (firebox) to the upper tube sheet. Holes are drilled in each sheet to receive the tubes, which are then rolled to produce a tight fit, and the ends are beaded over.

In the vertical exposed-tube boiler (see Fig. 2.3), the upper tube sheet and tube ends are above the normal water level, extending into the steam space. This type of construction reduces the moisture carry-over and slightly superheats the steam leaving the boiler. However, the upper tube ends, not being protected by water, may become overheated and leak at the point where they are expanded into the tube sheet by tube expanders during fabrication. The furnace is water-cooled and is formed by an extension of the outer and inner shells that is riveted to

Figure 2.3 Sectional view of vertical fire-tube boiler, exposed-tube type.

the lower tube sheet. The upper tube sheet is riveted directly to the shell. When the boiler is operated, water is carried some distance below the top of the tube sheet, and the area above the water level is steam space. This original design is seldom used today.

In submerged-tube boilers (see Fig. 2.4), the tubes are rolled into the upper tube sheet, which is below the water level. The outer shell extends above the top of the tube sheet. A cone-shaped section of the plate is riveted to the sheet so that the space above the tube sheet pro-vides a smoke outlet. Space between the inner and outer sheets com-prises the steam space. This design permits carrying the water level above the upper tube sheet, thus preventing overheating of the tube ends. This design is also seldom used today.

Since vertical boilers are portable, they have been used to power hoisting devices and operate fire engines and tractors, as well as for stationary practice, and still do in some parts of the world. They range

Figure 2.4 Sectional view of vertical fire-tube boiler, submerged-tube type.

in size from 6 to 75 bhp; tube sizes range from 2 to 3 in in diameter;

pressures to 100 psi; diameters from 3 to 5 ft; and height from 5 to 10 ft. With the exposed-tube arrangement, 10 to 15°F of superheat may be obtained.

Horizontal fire-tube boilers are of many varieties, the most common being the horizontal-return tubular (HRT) boiler (Fig. 2.5). This boiler has a long cylindrical shell supported by the furnace sidewalls and is set on saddles equipped with rollers to permit movement of the boiler as it expands and contracts. It also may be suspended from hangers (Fig. 2.6) and supported by overhead beams. Here the boiler is free to move independently of the setting. Expansion and contraction do not greatly affect the brick setting, and thus maintenance is reduced.

Figure 2.5 Horizontal return tubular boiler and setting.

In the original designs of this boiler, the required boiler shell length was secured by riveting (see Fig. 2.5) several plates together.

The seam running the length of the shell is called a longitudinal joint and is of butt-strap construction. Note that this joint is above

Figure 2.6 Horizontal return tubular boiler and setting, overhanging front.

Figure 2.7 Horizontal four-pass fire-tube package boiler designed for natural gas firing.

(Cleaver-Brooks, a Division of Aqua-Chem, Inc.)

the fire line to avoid overheating. The circumferential joint is a lap joint.

Today’s design of a return tubular boiler (Fig. 2.7) has its plates joined by fusion welding. This type of construction is superior to that of a riveted boiler because there are no joints to overheat. As a result, the life of the boiler is lengthened, maintenance is reduced, and at the same time higher rates of firing are permitted. Welded construction is used in modern boiler design.

The boiler setting of Fig. 2.6 includes grates (or stoker), bridge wall, and combustion space. The products of combustion are made to pass from the grate, over the bridge wall (and under the shell), to the rear end of the boiler. Gases return through the tubes to the front end of the boiler, where they exit to the breeching or stack. The shell is bricked in slightly below the top row of tubes to prevent overheat-ing of the longitudinal joint and to keep the hot gases from comoverheat-ing into contact with the portion of the boilerplate that is above the waterline.

The conventional HRT boiler is set to slope from front to rear. A blowoff line is connected to the underside of the shell at the rear end of the boiler to permit drainage and removal of water impurities. It is extended through the setting, where blowoff valves are attached. The line is protected from the heat by a brick lining or protective sleeve.

Safety valves and the water column are located as shown in Fig. 2.5. A dry pipe is frequently installed in the top of the drum to separate the moisture from the steam before the steam passes to the steam outlet.

Still another type of HRT boiler is the horizontal four-pass forced-draft packaged unit (see Fig. 2.7), which can be fired with natural gas or fuel oil. In heavy oil-fired models, the burner has a retractable nozzle for ease in cleaning and replacing. It is this type of design that is the most common fire-tube boiler found in today’s plants.

The four-pass design can be described as follows: Inside the fire-tube boiler (see Fig. 2.7) the hot gases travel from the burner down through the furnace during the combustion process, and this is con-sidered the first gas pass. The rear head of the boiler seals the flue gas in the lower portion, and the flue gas is directed to the second-pass tubes, where the flue gas flows back toward the front of the boiler.

The front head of the boiler seals the flue gas from escaping and directs the flue gas to the third-pass tubes, which causes the flow to move to the rear of the boiler. The flue gas is then directed through the fourth-pass tubes, where the flue gas moves to the boiler front and then exits to the stack.

Such units are available in sizes of 15 to 800 bhp (approximately 1000 to 28,000 lb/h) with pressures of 15 to 350 psi. Some units are designed for nearly 50,000 lb/h. These units are compact, requiring a minimum of space and headroom, are automatic in operation, have a low initial cost, and do not need a tall stack. For these reasons, they find application and acceptance in many locations. Because of their compactness, however, they are not readily accessible for inspection and repairs. Larger fire-tube boilers tend to be less expensive and use simpler controls than water-tube units; however, the large shells of these fire-tube boilers limit them to pressures less than 350 psi.

Fire-tube boilers serve in most industrial plants where saturated steam demand is less than 50,000 lb/h and pressure requirements are less than 350 psig. With few exceptions, nearly all fire-tube boilers made today are packaged designs that can be installed and in operation in a short period of time.

The ability of the fire-tube boiler to have a compact plant arrange-ment is illustrated in Fig. 2.8. Two four-pass fire-tube boilers are shown on their foundations with all associated piping and controls. Low NO_x emissions are also critical from fire-tube boilers, as well as maintaining

high boiler efficiency. As a method for reducing NO_xlevels to as low as 20 ppm, these types of boilers can be designed using the combustion air to draw flue gas from the fourth pass.

Fire-tube boilers were designed originally for hand firing of coal, wood, and other solid fuels; however, today nearly all are designed for oil or natural gas firing and are generally similar in design to the unit shown in Fig. 2.7. Solid fuel firing can be accommodated if there is enough space underneath the boiler to add firing equipment and the required furnace volume to handle the combustion of the fuel. The burn-ing of solid fuels also requires environmental control equipment for particulate removal and possibly for SO₂ removal depending on local site requirements. These added complexities and costs basically have eliminated fire-tube boilers for consideration when firing solid fuels.