Table 2.1 shows a few aspects of fire tube and water tube waste heat boilers. Generally water tube boilers are suitable for large gas flows exceeding millions of pounds per hour and can handle high steam pressures and temperatures. Fire tube boilers are suitable for low steam pressures, generally below 500psig. Table 2.2 shows the effect of pressure on tube thickness in both types of boilers, and one can see why fire tube boilers are not suggested for high steam pressure applications.

In water tube boilers, extended surfaces can be used to make them compact if the gas stream is clean, as discussed in Q8.21. Flue gas pressure drop will also be lower than for an equivalent fire tube boiler owing to the compactness of the design. Water tube boilers can be smaller and weigh less, particularly if the gas flow is large, exceeding 100,000 lb/h. Superheaters can be used in both types. In a water tube boiler they can be located in an optimum gas temperature zone. A shield screen section or a large convection section precedes the superheater. In a fire tube boiler, the superheater has to be located at either the gas inlet or exit, making the design less flexible and vulnerable to slagging or corrosion. If the waste gas is slagging in nature, a water tube boiler is desired because the surfaces can be cleaned by using retractable soot blowers. In general, the type of boiler to

Be used for a particular case is determined by the experience of the manufacturer. Sometimes a combination of fire and water tube boilers is used to suit special needs.