Fire Tube Boilers

Fire Tube Boilers

Oil- and gas-fired fire tube package boilers are widely used in cogeneration plants (Figure 3.43). They generate low-pressure saturated steam, and steam capacity is limited to about 20−35 t/h as larger sizes would involve large-diameter shells and may be uneconomical. The burner is located in the large central pipe called the Morrison pipe, which is a cor- rugated pipe to handle the differential thermal expansion between the first pass and the other passes that are fixed to the tube sheets. The number of passes can be three or even four to improve the efficiency. An economizer may also be added if required. Figure 3.44 shows a fire tube boiler with a superheater. Note that it is somewhat cumbersome to add

a superheater in a fire tube boiler. In a water tube boiler, the superheater can be located after any number of evaporator tubes. However, in a fire tube boiler, it can only be at the end of the second pass or between the boiler and economizer. The flue gas temperature at the exit of the third pass may not be significant to add a superheater, and at part loads, the gas temperature will drop off significantly. In Chapter 4, a fire tube waste heat boiler with a superheater and an economizer beyond the evaporator is shown. However, the steam temperature will be low.

Some boiler suppliers use grooved tubes (Figure 3.45) to improve the tube-side heat transfer coefficient and thus either reduce the number of passes or improve the performance.

FIGURE 3.43

Fire tube boilers in a process plant. (Courtesy of Cleaver Brooks Inc., Thomasville, GA.)

158 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

CL of boiler

CL of furnace

FD fan FD fan

FIGURE 3.44

Fire tube boiler with a superheater. (Courtesy of Thermodyne Technologies, Chennai, India.)

FIGURE 3.45

Grooved tubes used in fire tube boilers to improve energy transfer. (Courtesy of Cleaver Brooks Inc., Thomasville, GA.)

Fire tube boilers are classified as dry back or wet back depending on how the turn- around section of the first pass is built (Figure 3.46). Wet back boilers do not use refrac- tory in the turnaround section thus decreasing the maintenance costs and improving the boiler life; however, they are slightly more expensive. As discussed in Chapter 4 on waste heat boilers, using smaller-diameter tubes in fire tube boilers helps reduce weight and length of the boiler.

Steam Generators 159

Insulation

C.L. vessel Furnace

Surner (a)

Insulation

C.L. vessel Furnace

Surner (b)

FIGURE 3.46

(a) Dry back and (b) wet back boilers.

Oil and gaseous fuels are generally fired in package fire tube boilers. Solid fuels such as wood chips have also been fired. The boiler capacity is limited to about 35 t/h of steam as it becomes very expensive to build these boilers beyond this capacity. The heat transfer coefficient when flue gas flows inside the tubes is generally lower than when it flows out- side the tubes. Besides, extended surfaces cannot be used to make the boiler compact as in water tube boilers, though grooved tubes help to some extent.

NO x control methods such as FGR or low NO x burners have been used with fire tube boilers. Due to the large amount of water inventory compared to an equivalent water tube boiler, fire tube boilers take a little more time to start up. Steam purity is poorer than what

160 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

you get in water tube boilers unless one specifically uses an elevated drum design with chevron separator. Steam purity in a typical fire tube can be in the range of 3–15 ppm due to the use of simple devices for steam–water separation.

Sizing procedures for fire tube boilers are similar to that of water tube boilers.