plant auxiliary systems 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 ei- ther 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. By James R. Smith, Armstrong Heat Transfer Group

team coil air preheaters (SCAPs) are sitions and the duct connection to the inlet there (Figure 4). Clogging and reduced per- found in most fossil-fueled utility and (cold end) of an RRAH. As opposed to being formance will be the undesirable outcome. Slarge industrial power plants in North bolted in place by duct flanges (Figure 2), Often (but unintentionally), the SCAP’s coils America. Their primary function is to heat units used in such a configuration are usu- serve as a convenient platform for mainte- combustion air before it enters a rotary re- ally drawer-mounted, for easier access and nance workers tasked with cleaning and in- generative air heater (POWER, April 2006, p. removal (Figure 3). specting the RRAH. 72) or a traditional tubular air heater. Wheth- Placing a SCAP ahead of the cold end Another undesirable consequence of er a rotary regenerative air heater (RRAH) (inlet side) of a RRAH, however, can invite placing a poorly constructed, specified, or or a traditional tubular heater is downstream, maintenance problems and shorten unit life. drained SCAP just upstream of the cold side the SCAP provides corrosion protection for For example, if a SCAP is placed directly of an RRAH is the development of steam the heater and maintains its cold end average below a RRAH, corrosive sediment and crud and/or condensate leaks in the air stream. exit gas temperature above a minimum. Im- dislodged during basket washdown will fall This can lead to evaporative cooling of the provements in boiler efficiency and heat rate onto coil-finned surfaces and accumulate RRAH’s surface, which “steals” valuable en- and in unit capacity from heating of combus- tion air have been documented for decades. 1. Simple in concept, complex in operation. A rough schematic of a typical Smaller industrial boilers fired by hog steam coil air preheater. For simplicity’s sake, the system shown depicts single coils in walls fuel or used for chemical recovery may use 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 a SCAP as the primary preheater of com- bustion air. Increased use of alternative fuel From boiler and/or SCR system To boiler sources such as hog fuel, biomass, tire-de- rived fuels, and refuse-derived fuels have Hot end Turbine-generator made it more important to optimize the use Rotary regenerative Air heater Superheated of SCAPs. steam Condensate from Stricter emissions standards also have Cold end air preheat coils changed the utilization of SCAPs. With protection Condenser temperature plants’ increased use of selective catalytic Desuperheater and noncatalytic reduction (SNCR and SCR) To scrubber, etc. TCV Desuperheater TCV systems and other emissions-reduction equip- In-duct air ment, SCAPs now are needed year-round— preheater coils rather than just during the ozone season—to maintain heaters’ outlet gas temperature. Forced-draft fan Wall air preheater Location, location, location coils Figure 1 is a very simplified schematic of a SCAP. Most units use vertical finned-tube To condenser coils embedded in perimeter entry walls to connect to the suction side of a forced-draft Condensate Transfer (FD) fan or fan room —the source of com- receiver pump bustion air. Some SCAPs, however, are de- Condensate signed to be installed on the discharge side of Notes: SCR = selective catalytic reduction; receiver the FD fan, between the fan’s ductwork/tran- TCV = turbine control valve

 www.powermag.com POWER | May 2006 plant auxiliary systems ergy from the unit. All three of these SCAP- ing a unit’s heat rate and maximizing its ca- into the category of “troublesome” equip- related problems—increased maintenance pacity requires creative modifications of the ment not worth keeping in service. Accord- requirements, shortened life, and higher- entire SCAP “loop”—comprising all compo- ingly, they often receive little or no attention than-necessary energy consumption—have nents and controls from the steam source to during outage planning for design upgrade or been reported by large fossil fuel–fired pow- the condensate return. correction. er plants in the U.S. and Canada. It’s surprising that so many power plants have had so many problems with SCAPs; Where and why SCAPs go bad Problem children after all, they are fairly basic finned-tube Among the common system causes of steam Utility boilers obviously can and do remain units for exchanging heat between steam coil air preheater problems are the following. functional without the help of a SCAP. How- and air. EPRI has documented that, since the Desuperheater stations. Because they ever, the lack of a SCAP may reduce a boil- 1980s, many SCAPs had been shut down, provide pressure let-down and steam tem- er’s combustion efficiency enough to make removed, or partially blanked out of service perature control of a SCAP system, desuper- its unit only marginally profitable. Minimiz- due to maintenance, repair, and/or perfor- heater stations must be operated accurately mance-related (leakage) issues early in their and continuously. At a minimum, failing to 2. Bolted in place. A wall-mounted operating life. Often, problems appeared regulate the quality and amount of desuper- steam coil air preheater with common re- within only a few years of plant start-up and heating water and associated feedback sys- ceiver drainage. Courtesy: Armstrong Heat commissioning. tems drastically reduces SCAP performance. Transfer Group At the other extreme, SCAP coils at some At worst, failing to desuperheat steam for plants have been on the job for up to two de- optimum SCAP condensing requirements cades, but they may have been performing stresses the unit’s coil materials at key weld well below design parameters and wasting joints, with catastrophic consequences. steam for years. Often, such inefficiencies Among them is the passing of live, uncon- and losses go undetected, either due to cut- densed steam to the condensate system, backs in maintenance budgets or because overheating it. Desuperheating to a level of other maintenance items are considered a no more than 100 degrees F above saturation higher priority. temperature is a good guideline for SCAP A recent cursory inspection by the author’s design. company of just a few U.S. fossil fuel–fired Scheduled maintenance of desuperheater utility plants revealed that hundreds of SCAP station components does more than just coils had been isolated because they failed improve SCAP performance. The attention prematurely or required excessive mainte- indirectly helps improve critical pressure nance. A similar trend has been noted across differentials across the SCAP system, im- the U.S. and Canada, regardless of fuel type proving drainage. One large Midwest power or plant location. As maintenance budgets plant experienced measurable reductions in and staffs have shrunk, SCAPs have fallen wasted steam flow through targeted main-

3. Easier to replace and maintain. 4. Look out below. Because this steam coil air preheater (SCAP) was mounted di- A pre-piped, duct-mounted, drawer-type rectly below a rotary regenerative air heater, corrosive sediment and crud dislodged during steam coil air preheater with individual coil RRAH basket washdown fell onto the SCAP’s finned-coil surfaces, damaging them. Courtesy: trapping. Courtesy: Armstrong Heat Transfer Armstrong Heat Transfer Group Group

May 2006 | POWER  plant auxiliary systems tenance of desuperheater stations during vestment funded by the plant’s O&M budget 3. Evaluate the superheated steam sup- scheduled outages. can result in notable improvements. Justify- ply and the trapping ability of the Coil issues. A SCAP that underperforms ing modifications of a SCAP system on the pressure-reduction station. Because or fails prematurely may be a “victim of cir- basis of the plant environmental performance failed traps and blow-through cause un- cumstances.” Possibilities include airside gains they would produce is a common and necessary energy losses and operational fouling, damage during handling or removal, acceptable practice. problems, it’s a good idea to test traps reg- dirty finned tubes, changes in steam condi- ularly—at least once a year. At that time, tions, and poor positioning of the SCAP rela- Solving SCAP problems all piping insulation should be inspected tive to the air heater it serves. A total systems assessment—including sup- to ensure that it is in good condition. Condensate drainage and noncon- ply and drainage piping design and opera- densables venting issues. Poor drainage tional control details—is recommended as Options for modifying and venting practices retard efficient heat a first step. A project that is limited to coil steam supply transfer in SCAP tubes. These problems can replacement has little chance of eliminating Consider slightly increasing the steam pres- easily be remedied either by improving the most common SCAP problems. sure to the SCAP (after consulting its manu- design and specifications of materials, rees- The steam-source review portion of the facturer). Doing so may allow fin spacing to tablishing original steam conditions, or fol- systems assessment should include the fol- be widened, possibly reducing the number lowing industry best practices for venting, lowing three steps: of rows needed, airside pressure drops, and draining, and maintaining auxiliary compo- airside fouling of the finned coils. nents (more on these later). 1. Confirm the steam source conditions In the case of fouling of wall-type SCAPs, Original design issues/reviewing and setpoint criteria. Usually, the source realize that the finned surface in the first/face changes to existing installations. Through is superheated steam extracted from the row of a SCAP acts as a filter. Unless fin discussions among a plant’s engineering staff, turbine. But often, for cold start-ups, the spacing is widened sufficiently (for example, a consultant, and a qualified representative of steam source is an auxiliary or start-up from 11 to 12 fins per inch down to 7 to 8 the equipment vendor, contracts for upgrad- boiler. Because the type of unit service fins per inch, to allow particulate matter to ing a SCAP can be integrated with planned (baseloaded or cycling) affects SCAP uti- “pass through”), nearly all coil first-row sur- unit modifications. The resulting overall im- lization, large variations in the pressure of faces will eventually foul. Pressure-washing provement in plant heat balance helps ensure the steam supplied to a SCAP may affect these surfaces at least once a year (when the that all aspects and components of the SCAP its performance, drainage, and life. If the coils are off-line and cool) is highly recom- are viewed holistically. characteristics of the steam source have mended. Every SCAP system design should include changed significantly since the SCAP was However, widening the fin spacing on re- not just the coils, but options for condensate designed and placed in service, an overall placement coils will reduce the heat transfer return and steam conditioning and control, system modification may be in order. surface and the temperature of the air leav- provisions for steam trapping and venting, 2. Evaluate the desuperheating stations. ing the coil (at the same steam pressure). and—potentially—the use of flash recovery Are they working at the proper flow rates As a result, it will be necessary to slightly (see below) as well. Valuable extraction- and with properly treated water supply? modify steam supply pressure following any sourced steam can be saved by a thorough Are the controls operating properly? Are desired change in fin spacing. But efficiency system overview from extraction point to gauges and transmitters calibrated and re- improvements—including a smaller reduc- condenser supply point. Often, a modest in- porting accurate data to the control room? tion in desuperheating pressure and a re-

Best practice recommendations for steam coil air preheater construction Tubes. Should have a minimum outer diameter (OD) of 1 inch Fin/tube bonds. Embedding (kefinning) fins partially into and be made of 12-gauge carbon steel or 14-gauge stainless steel. tube walls eliminates fin/bond release at high temperatures or Larger (1.5-inch OD) tubes may be needed to allow for the specific throttling conditions. Avoid surface/edge-wound and plate edge/ volume of steam at lower pressure during modulation, or when collar-expanded types. Extruded fin designs, typically found in encountering partial load cycles. air-cooled fluid coolers, often require thin tube cores, which are Rows. There should no more than two rows of tubes per drained subject to leakage under and along the extruded fin surface. The header, to prevent short-circuiting of the steam/condensate path leaks often are hard to pinpoint, and sometimes the entire extru- and to improve pass-through of washdown debris during cleaning. sion can loosen in high-cycling, high-thermal-fluid temperature This constraint is especially important for units intended for use applications. Welded-fin designs like those found on economizers in cold climates. are typically not required for SCAP service. Fins. Fin spacing should be no more than 8 to 10 per inch, to Tube supports. Add multiple structural supports and angle-over minimize reduce airside fouling. Fin pitches can be reduced to 7 coil faces to minimize distortion and to improve rigidity, han- to 8 fins per inch by raising the steam pressure applied to coils. dling, and washdown support. Avoid excessive superheating by limiting temperatures to 100F Mounting. Use drawer-type designs where possible to speed re- above the saturation temperature. moval/replacement and inspection. Add a “dummy” spacer section Fin thickness. The standard minimum thickness is 0.02 inches, with an access plate in duct-mounted designs, and access plates but insist on 0.03 inches for fins expected to be forcefully cleaned (whatever the design), to enable real-time inspection of coils and or washed down or handled roughly. tracking of debris “dropout.”

 POWER | May 2006 plant auxiliary systems duction in fouling and airside pressure drop vidual coils are drained to a common receiv- 5. Uniform airflow required. A typi- (which shows up as a saving in horsepower er/accumulator (Figure 5) with liquid seal cal “master trapped” steam coil air preheater of the FD fan)—will result. level control. This practice—called “master condensate collection system. Courtesy: trapping”—requires that airflow over the en- Armstrong Heat Transfer Group Construction, drainage, tire face of the SCAP be absolutely uniform. and venting However, because uniform airflow over the Follow industry minimum best practices for finned surfaces is highly unlikely, the con- unit design. Insist on compliance with ASME densing rate on the coil faces probably will Section VIII, Division 1, as code/inspection vary. Among the factors affecting airflow criteria. SCAPs my not necessarily require uniformity are fouling of the unit’s air-side the “U” stamp, but the added assurances of surfaces, the geometry of the air inlet side an inspector’s review and tight supplier qual- duct, and even the location of the forced- ity control/quality assurance procedures can draft fan relative to the coils. add longevity in the key areas of welding Individual trapping of coils offsets the procedures and materials selection. (See box negative effects of poor airflow uniformity. for a list of SCAP construction recommenda- Facilities with properly sized, piped, and tions, delineated by component.) maintained trapping systems typically suf- Bear in mind that units intended for use as fer fewer coil problems—because by design nonsteam, air-cooled condensate/fluid cool- they enable full utilization of the supplied ers may not meet construction standards for steam by the coil and prevent steam from SCAPs. Although such units meet other in- passing through to downstream piping/drain- reducing 100-psig condensate to 15 psig and dustry standards (such as API-661 for rolled age systems. using it in a backpressure system). tube/header joints, box headers, and the like), A continuous venting device installed Several utility plants now are considering these designs may unnecessarily raise costs on the receiver will ensure the release of SCAP flash volumes in additional, low-pres- and confuse materials/construction criteria. gases such as O2, CO2, and N2 that reduce sure air preheater “booster” coils. Other ideas One way to minimize SCAP maintenance heat transfer. Ideally, each coil or coil bank include reducing parasitic load extraction for and repair costs is to use drawer-type units, should be equipped with an automatic air better support of “auxiliaries” such as HVAC where possible, to facilitate coil removal, venting device to preclude variations in the systems, hot water heating loops, showers, replacement, and inspection. Maintaining condensing rate across sections. Venting only and low- and medium-pressure docks and spare coils for these designs allows for quick at start-up is insufficient. Often, this practice door heaters. The direct paybacks of these change-outs and off-line repairs, minimizing allows the system to fill with noncondens- approaches are gains in unit capacity and downtime. ables that plate heat transfer surfaces, reduce improvements in turbine-generator heat rate. If a SCAP’s drainage and venting systems efficiency, and lead to long-term fluid-side The indirect benefit is to offset the environ- are not designed and maintained properly, corrosion issues. mental impact of generation needed to drive valuable heat in the steam designed for re- If a common receiver is used, ensure that the loads represented by these “necessary” lease in the unit may pass through it into the its level control is adjusted and operating parasitic requirements. Using flash steam condensate system. This would rob the tur- properly to prevent condensate from blow- also lowers the temperature of condensate, bine of energy, increase the generating unit’s ing through to downstream accumulators or increasing the efficiency of the condenser heat rate, and reduce its output. Steam trap the condenser. If blowthrough occurs, more and further lowering overall cycle heat rate. selection is important. Typically, using car- energy literally goes “down the drain” than bon or forged-steel inverted buckets and/or does work inside the SCAP. Condensate Closing the loop floats and thermostatic steam traps that are overtemperature alarms may indicate a con- A thorough assessment of the entire SCAP sized for a SCAP’s minimum and maximum trol problem, such as steam blowing through loop (from steam supply, through return, to operating pressures and flows extends the coils to the return system or the failure of a the condenser and feedwater heater) is highly life of the unit’s coils. Slow-cycling or ther- header drip trap that drains to the common recommended. At many power plants, such a mostatic trapping devices should never be accumulator. comprehensive, in-depth evaluation has led utilized on SCAP coils. For example, a Mid- Finally, ensure that the pump used to re- to measurable improvements in boiler heat west utility recently reported earlier failures move condensate from the accumulator has rate, recovery of “lost” kilowatts, extension of coils that incorporate thermostatic traps enough head to handle high-temperature of SCAP longevity, and reduced pollutant compared with those in which inverted buck- condensate and that its controls are function- emissions. ets originally were installed. ing properly. A steam-powered, pneumatic, If a steam coil air preheater system is de- Traps should be mounted as close to (but or mechanical pump may be more suitable signed properly, its renewed use also will always below) coils as possible without re- than a centrifugal pump for this application. provide measurable and acceptable returns ducing the size of outlet piping. The traps’ on investments in maintaining it. The SCAP, discharge level should always be below Flash steam recovery an often-forgotten but nonetheless key plant the lowest outlet coil level. Use “tees” and The electric power industry is warming to auxiliary system, could be reborn as a major “crosses” rather than elbows and reducers. a concept for maximizing SCAP utilization contributor to efficient fossil-fueled power Properly sized strainers should be installed that has proven itself in the food-process- generation. n ahead of steam traps with valved blowdown ing and chemical manufacturing sectors for —James R. Smith is product manager connections and should be tested annually years. The idea is to exploit the tremendous in the Power/Utilities Group of for clear flow and proper operation. potential energy in the volumes of steam Armstrong Heat Transfer Group (Granby, SCAP coils often are designed without generated by hot condensate by “flashing” Quebec, Canada). He can be reached at traps. In such cases, as many as 30 to 40 indi- the steam to a lower pressure (for example, 330-837-0440 or [email protected].

May 2006 | POWER