FIRE TUBE BOILERS
Packaged fire tube boilers (Fig. 3.3) generate low pressure saturated steam, generally below 300psig. Above this pressure, the thickness of the corrugated central furnace (referred to as Morrison pipe) becomes larger and it is difficult to make the corrugations. The corrugations help to reduce the thickness of the furnace, which operates at a high metal temperature because it contains the flame. The corrugations also help to handle the thermal expansion differences between the furnace and the smaller tubes in the second and third passes, which operate at lower tube wall temperatures. Note that the tube sheets are fixed at the ends of the tubes, and without this flexibility large stresses would be introduced into the tube sheets and the tubes. The thickness of a tube subjected to external pressure is higher than that subjected to internal pressure, as shown in Table 2.2. Fire tube boilers are typically rated in boiler horsepower (BHP); Q5.08 shows how one can relate BHP to steam generation. Often these boilers do not need an economizer, because the exit gas temperature, due to the low pressure of steam, is around 400- 450°F. However, an economizer is used when high efficiency is desired.
The number of passes on the tube side depends upon the supplier. Typically three to four passes are used. In the wetback design the turnaround section is immersed in the water, so the hot gases leaving the furnace do not contact the refractory as in the dryback design, which is less expensive to build. However, the wetback design has fewer problems with refractory maintenance than the dryback design. Wet or water-cooled rear doors are also available that minimize refractory maintenance concerns in dryback boilers. The typical gas temperature at the furnace exit is about 2000-2200°F, hence the turnaround section with refractory often requires maintenance.
Oil and gaseous fuels are generally fired in packaged fire tube boilers. Solid fuels such as wood shavings have also been fired. The boiler capacity has been limited to about 80,000 lb/h, because it becomes more expensive to build these boilers as shop-assembled units as the capacity increases. The heat transfer coefficient with gas flowing inside the tubes is generally less than when it flows outside the tubes; hence fire tube boilers are large compared to water tube designs. They are considered economical below 50,000 lb/h of steam. It is generally difficult to install a superheater in these designs. NOx control methods such as flue gas recirculation or the use of low-NOx burners have also been used with these boilers. Due to the large amount of water inventory compared to equivalent water tube designs, these boilers take a little longer to start up. Steam purity is generally poor, because the steam is mainly used in heating applications where steam purity is not a concern and therefore no drum internals are used. Often single-shell fire tube boilers such as those shown in Fig. 3.3 generate steam with 3-15 ppm purity. Elevated drums have been used on fire tube boilers to obtain steam with a very high purity if required. The design would be similar to the elevated drum waste heat boiler discussed in Chapter 2.
When it comes to generating superheated steam, a water tube boiler has more options, because the superheater can be placed within a bank of tubes or in the radiant section or beyond the convection section as discussed above. However, in the case of a fire tube boiler, the options are limited; a possible location is between the tube passes, but the gas temperatures there are either too high or too low, making it difficult to design a reasonable superheater. Therefore, packaged fire tube boilers generally generate saturated steam.
The water inventory in a fire tube boiler is generally larger, thus requiring a longer start-up period. Heating surfaces can be cleaned by using retractable or rotary blowers at any location in a water tube boiler, whereas in a fire tube, access for cleaning is available either at the turnaround section or at the tube sheet ends.