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Firetube Boilers

Learning Outcome When you complete this module you will be able to:

Describe the design, characteristics and applications of HRT, locomotive, firebox, Scotch and packaged firetube boilers.

Learning Objectives Here is what you will be able to do when you complete each objective:

1. Sketch and describe a Horizontal Return Tubular boiler. 2. Sketch and describe a Locomotive type boiler.

3. Sketch and describe a Firebox boiler.

4. Sketch and describe a Scotch boiler.

5. Sketch and describe a packaged firetube boiler.

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INTRODUCTION The module “Boiler Design and Terminology” presented the basic firetube boiler design, as well as some specifics of firetube boilers. That module discussed the differences between internally- and externally-fired boilers, the HRT boiler, and the four-pass packaged boiler. This module will review and enhance the material on the HRT and four-pass firetube boilers, and introduce other firetube designs. HORIZONTAL RETURN TUBULAR (HRT) BOILER The following points highlight the construction details of the boiler illustrated in Fig. 2: Shell and Head-Plates • joined by rivets; nowadays joined by fusion

welding.

Stayed Surfaces • diagonal and longitudinal stays attached by rivets.

Tubes • sizes vary between 76 mm and 102 mm in

diameter, and the tubes are expanded at each end into the tubesheets.

Combustion Zone • externally-fired, furnace outside the shell and

surrounded by brickwork; combustion gases make two passes as they travel from the furnace to chimney; thus, a two-pass design.

Nozzles • for safety valve and steam outlets, are

attached by rivets with flanges fitted to the curvature of the boiler; nowadays are welded in place.

Access Openings • manhole and handholes, rivet assembly.

Water Level • several mm above top row of tubes; the

feedwater enters through an internal pipe and discharges below water level toward rear of boiler.

Steam Outlet • sometimes a dry pipe is used to prevent

moisture from carrying over with the steam; Fig. 2 shows a dry pipe below a steam outlet attached to the boiler shell with rivets.

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Figure 1

Internal Dry Pipe Detail Capacity • 454 to 6800 kg/h at pressures up to 1700 kPa.

Application • small industrial plants and heating plants.

Advantages • low cost.

• ease of cleaning. • large water capacity.

Disadvantages • limited steaming capacity and operating pressure.

• high cost of brickwork setting. • foundation required.

Figure 2 HRT Boiler

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LOCOMOTIVE TYPE BOILER (Dry Bottom Type) A locomotive type boiler is shown in Fig. 3. The details of its construction follow.

Figure 3 Locomotive Boiler

Shell and Head • attached by rivets; internally-fired with a brick setting surrounding the firebox or furnace; combustion gases travel from the firebox through the tubes to the smoke box and chimney; thus, a single-pass boiler.

Stayed Surfaces • elaborate staying is required to support the top

of the firebox (crown sheet) and the sides of the firebox.

Diagonal Stays • used to support the upper sections of the end

plates from the shell; they are attached with rivets.

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Stay Bolts • used to stay flat surfaces that are close

together; they are often called screw stays and are used in water legs of the firebox.

Radial Stays • are used to stay the crown sheet; sometimes

staybolts are used.

Crown Sheets • in earlier constructions, the crown sheet was flat and girder stays were used.

Figure 4 Crown Sheet Stays

Girder Stays • modern practice is to use curved crown sheets

and radial stays that have different radii of curvature, as illustrated in Fig. 4.

Figure 5 Girder Stays

Tubes • attached at each end by expanding into the

tubesheets.

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Water Level • is several mm above the crown sheet, and in

order to provide more steam space, a steam dome is attached to the top of the barrel.

Capacity • up to 6800 kg/h at 2400 kPa pressure.

Application • heating plants, small industrial plants.

Advantages • low cost.

• rugged construction.

Disadvantages • large amount of staying required • poor water circulation. • poor access for cleaning, repair, and

inspection. • high maintenance costs. • leakage from rivet joints.

WET BOTTOM LOCOMOTIVE BOILER The construction is basically the same as the dry bottom type. A waterleg surrounds the firebox or furnace on four sides, increasing the overall heating surface and eliminating the brick setting. Rugged construction makes this unit suitable for portable service. Having the firebox built into the boiler (internally-fired) increases the heat transfer capabilities. This type of boiler is used in the oil field industry today.

Figure 6 Locomotive Boiler

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FIREBOX BOILER The firebox boiler is an externally-fired, horizontal, firetube boiler that is similar to the HRT boiler in that it is a two-pass boiler. The shell is made in two sections and there are two groups of firetubes. The combustion gases travel from the firebox, through the tubes in the lower shell section to the rear of the boiler, and then reverse through the tubes in the upper shell section to the uptake at the front of the boiler.

Figure 7 Firebox Boiler

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Usually the firebox is surrounded by a brick setting enclosed in a steel casing, as shown in Fig. 8. Some designs feature a waterleg around the firebox. See Fig. 9.

Figure 8 Firebox Boiler Showing Gas Passes

Figure 9 Internally-Fired Firebox Boiler

Applications • used as heating boilers; the waterleg type is usually used for this service.

• for higher pressure service, the brick setting type is preferred up to 1720 kPa and 6800 kg/h steam capacity.

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Advantages • low first cost.

• good efficiency. • compact.

Disadvantages • some areas are inaccessible for cleaning and inspection.

SCOTCH BOILER This is a horizontal, internally-fired, return tubular, firetube boiler. It is usually a two-pass design, with the combustion gases travelling first through a large furnace tube or flue to the rear of the boiler and then reversing through a set of firetubes to the uptake at the front of the boiler. If the rear of the boiler is brick-lined, as in Fig. 10, the boiler is classed as a dryback Scotch boiler.

Figure 10 Scotch Marine Dryback Boiler

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Figure 11 Dryback, Two-Pass, Corrugated Furnace, Scotch Boiler

If a waterleg surrounds the rear chamber, the boiler is classed as a wetback type. The wetback type (Fig. 12) was designed for marine service, but versions of it are found in stationary plants.

Figure 12 Wetback Scotch Boiler

Each type of Scotch boiler incorporates a corrugated furnace (Fig. 13 and Fig. 14) for the following reasons:

1. It allows for differential expansion between the furnace tube and fire tubes, thus reducing stress on the tubesheets.

2. The strength of the furnace is increased, while allowing the wall to be

thinner.

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3. Heat transfer to the water increases because of the thinner walls and increased turbulence of combustion gases.

In both types, the large furnace tube is surrounded by water and provides additional heating surface. Early type boilers used longitudinal stays above the tubes, rather than diagonal stays.

Figure 13 Corrugated Furnace

Figure 14 Wetback Scotch Boiler

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Application • dryback types are used in heating plants,

while wetback types are used in marine services.

• design pressures up to 1700 kPa and capacities to 6800 kg/h of steam.

Advantages • low cost.

• compact. • simple foundations. • more efficient boiler.

Disadvantages access difficult for cleaning and inspection of certain areas.

PACKAGED FIRETUBE BOILER Packaged firetube boilers represent the majority of firetube boilers being manufactured today. They are basically a two-pass, three-pass, or four-pass Scotch boiler, and can be either dryback or wetback in design. The boiler unit is engineered, built, tested, shop assembled, mounted on a steel base with controls and auxiliaries included in one “package”, and shipped to the customer ready for installation. 1. Three-Pass Design

Fig. 15 shows an internal furnace with a firebox having a flat bottom.

Figure 15 Three-Pass Firetube Boiler

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Referring to Figs. 15, 16 and 17, (which show boilers with an internal furnace tube) the combustion takes place in the firebox or furnace tube. The combustion gases then travel through the firebox to a rear reversing chamber where they reverse and return to the front through a set of short firetubes. They then pass to the outlet at the rear through an upper set of longer tubes. The fuel burned in this type of boiler is either oil or gas, and a forced draft fan is mounted on the boiler front.

Figure 16 Three-Pass Wetback Firetube Boiler

Figure 17 Three-Pass Dryback Firetube Boiler

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2. Four-Pass Design

A four-pass design is used to increase the distance the combustion gases travel from the furnace to stack, and hence increase efficiency. See Fig. 18. In these boilers, flue gas velocity is maintained by a decrease in the number of tubes, so that each succeeding pass has a smaller cross-sectional area. However, more fan power is required due to the additional pass.

Side View

End View Figure 18

Four-Pass Design

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Details of the four-pass design are:

Construction • shell and head assembled by welding.

Stayed Surfaces • diagonal and longitudinal stays, welded.

Tubes • large furnace tube or 1st pass, with smaller diameter tubes in return tubes.

Internal • furnace surrounded by water.

Nozzles • attached by threading or welding.

Applications • heating plants, small industrial plants.

Capacity • up to 12 000 kg/h and for pressures less than

1725 kPa.

Advantages • more efficient heat transfer, due to the fact the combustion gases have more opportunity to give up their heat to the boiler water.

• cost of manufacturing less.

Disadvantages • limited steam capacities and pressures.

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References and Reference Material

For more information on this topic, the following are recommended: Babcock & Wilcox. Steam/its generation and use. 39th ed. New York: Babcock & Wilcox; 1978.