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Designing and maintaining steam coil air preheaters for reliability and effectiveness
If engineered well and drained properly, a simple finned-tube heat exchanger can help maximize a fossil-fueled power plant's combustion efficiency, capacity, and air pollution reduction. Use the guidelines in this article either to return a disabled steam coil air preheater to service or to improve the performance of a unit that may have been wasting steam and money for years.
Steam coil air preheaters (SCAPs) are found in most fossil-fueled utility and large industrial power plants in North America. Their primary function is to heat combustion air before it enters a rotary regenerative air heater (POWER, April 2006, p. 72) or a traditional tubular air heater. Whether a rotary regenerative air heater (RRAH) or a traditional tubular heater is downstream, the SCAP provides corrosion protection for the heater and maintains its cold end average exit gas temperature above a minimum. Improvements in boiler efficiency and heat rate and in unit capacity from heating of combustion air have been documented for decades.
Smaller industrial boilers fired by hog fuel or used for chemical recovery may use a SCAP as the primary preheater of combustion air. Increased use of alternative fuel sources such as hog fuel, biomass, tire-derived fuels, and refuse-derived fuels have made it more important to optimize the use of SCAPs.
Stricter emissions standards also have changed the utilization of SCAPs. With plants' increased use of selective catalytic and noncatalytic reduction (SCR and SNCR) systems and other emissions-reduction equipment, SCAPs now are needed year-round"rather than just during the ozone season"to maintain heaters' outlet gas temperature.
Location, location, location
Figure 1 is a very simplified schematic of a SCAP. Most units use vertical finned-tube coils embedded in perimeter entry walls to connect to the suction side of a forced-draft (FD) fan or fan room "the source of combustion air. Some SCAPs, however, are designed to be installed on the discharge side of the FD fan, between the fan's ductwork/transitions and the duct connection to the inlet (cold end) of an RRAH. As opposed to being bolted in place by duct flanges (Figure 2), units used in such a configuration are usually drawer-mounted, for easier access and removal (Figure 3).
Placing a SCAP ahead of...