May 15, 2006

Designing and maintaining steam coil air preheaters for reliability and effectiveness

James R. Smith, Armstrong Heat Transfer Group

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

1. Simple in concept, complex in operation. A rough schematic of a typical steam coil air preheater. For simplicity’s sake, the system shown depicts single coils in walls and ducts. In an actual system, the steam supply and condensate return lines typically feed 6 to 40 coils per forced-draft fan. Source: Armstrong Heat Transfer Group

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).

2. Bolted in place. A wall-mounted steam coil air preheater with common receiver drainage. Courtesy: Armstrong Heat Transfer Group

3. Easier to replace and maintain. A pre-piped, duct-mounted, drawer-type steam coil air preheater with individual coil trapping. Courtesy: Armstrong Heat Transfer Group

Placing a SCAP ahead of the cold end (inlet side) of a RRAH, however, can invite maintenance problems and shorten unit life. For example, if a SCAP is placed directly below an RRAH, corrosive sediment and crud dislodged during basket washdown will fall onto coil-finned surfaces and accumulate there (Figure 4). Clogging and reduced performance will be the undesirable outcome. Often (but unintentionally), the SCAP’s coils serve as a convenient platform for maintenance workers tasked with cleaning and inspecting the RRAH.

4. Look out below. Because this steam coil air preheater (SCAP) was mounted directly below a rotary regenerative air heater, corrosive sediment and crud dislodged during RRAH basket washdown fell onto the SCAP’s finned-coil surfaces, damaging them. Courtesy: Armstrong Heat Transfer Group

Another undesirable consequence of placing a poorly constructed, specified, or drained SCAP just upstream of the cold side of an RRAH is the development of steam and/or condensate leaks in the air stream. This can lead to evaporative cooling of the RRAH’s surface, which “steals” valuable energy from the unit. All three of these SCAP-related problems—increased maintenance requirements, shortened life, and higher-than-necessary energy consumption—have been reported by large fossil fuel–fired power plants in the U.S. and Canada.