CCJ3 Air Inlet Filters Final

COMBINED CYCLE JOURNAL, Second Quarter 2004 1

GT INLET AIR SYSTEMS

Selecting gas-turbine inlet air systems for new, retrofit applications

By Steven Howes, Altair Filter Technology Inc

Gas-turbine (GT) inlet air systems usually are supplied by the engine OEM (origi-nal equipment manufacturer) as part of a complete turbine package. Competi-

tive pressures often dictate that intake and filtration packages be of a standard design, irrespec-tive of where the plant will be located. While this approach may be cost-effective and viable for some users, for others the specif-ics of their respective site environ-ments are such that a standard solution is not appropriate and may lead to operational problems.

Caveat emptor certainly pre-vails here. To guard against mis-application of design alternatives, users must be aware of the intake solutions available and how they perform under various environ-mental conditions. Bear in mind that while inlet air systems may appear similar, the technologies employed and the resulting perfor-mance characteristics can be very different.

Understanding these issues is important to the asset owner purchasing a new inlet air system, and even more critical to the engineer responsible for ret-rofitting an existing system or component. Greater knowledge of how various air intake components, such as filtration systems and power augmentation technologies, interact with each other and the local environment enable better deci-sions. Those, in turn, should result in increased plant output and avail-ability, and a reduction in intake-related maintenance issues.

Filtration basicsThe main purpose of any filtration system is to protect the turbine from harmful airborne contaminants. Typically, contaminants cause dam-age in the following ways:

Erosion occurs when relatively

large solid particles (bigger than 10 microns), and on occasion droplets, impact compressor blades at high velocities. Over time the damage can be sig-nificant enough to change the aerodynamic charac-teristics of the compressor. In extreme situations, where very poor intake systems are installed, cata-strophic damage can occur.

Corrosion. Even relatively small quantities of a corrosive substance entering a turbine can cause substantial damage in a short period of time. For example, one of the most common contaminants associated with turbine corrosion is salt. This can be any metal alkali salt, but sodium chloride and potassium chloride are the most common. A typical OEM input limit for the total amount of sodium and potassium in the air stream is 0.01 ppm (parts per million).

Fouling. Some contaminants are predisposed to collect on compressor blades. Reasons include: They are sticky in nature, have a low melting point, or

perhaps because another con-taminant, such as oil mist, has precoated the blades. What-ever the reason, the build up over time of such fouling will impair compressor perfor-mance.

Plugging, a process similar to fouling, refers to the block-age of blade cooling passages. Plugged cooling paths eventu-ally force blades to operate at a higher than desired tempera-ture. Blade failure is a possibil-ity over time.

The majority of inlet air fil-tration systems will protect a turbine sufficiently to prevent

catastrophic failure. But the performance of sys-tems subjected to harmful contaminants can vary significantly. Many have poor efficiencies against small and sub-micron particles, particularly when the filters are first installed. This frequently results in accelerated fouling of the compressor blades, and operators are left with a choice of accepting a reduc-tion in turbine output, or doing more maintenance.

Filtration efficiency can be difficult to under-stand given the many different testing methods

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Filter life

Average lifetime efficiency

Maximum efficiencyMinimum efficiency

1. Efficiency of a filter can change over its lifetime, illustrating the importance of carefully comparing alternative offerings when making purchasing decisions

Avoid these five common errors in the design of GT inlet air systems Using low first cost as the basis for design, thereby sacrificing aerodynamics and inhibiting gas-turbine (GT) performance. Forgetting that a poorly designed inlet air cooling system adversely impacts filter and/or compressor performance. Specifying incorrect materials of construction for the inlet housing, possibly causing compressor fouling or damage. Selecting an inappropriate type of filter or media for the intended service conditions, increasing O&M costs. Installing improper weather protection, or not at all when needed.

2 COMBINED CYCLE JOURNAL, Second Quarter 2004


available. But it is equally difficult to pick out one method as being superior to all others. The guide-lines offered below may assist in your evaluations of alternative filter offerings.

First, mass efficiency (also known as weight or gravimetric arrestance) is of little value to the tur-bine engineer. Reason: These tests typically use test dusts with a relatively large average particle

size (greater than 5 microns, for example). This makes it difficult to distinguish between a filter that has good small particle performance, and one whose performance is poor.

When comparing the efficiencies of filters from different manufacturers, be sure your compari-son is on a like-for-like basis. In particular, pay attention to the air velocity used as the basis for

Rating filter performance

To understand filters and their application, it is necessary to rate them for pressure drop, dust holding capacity, and efficiency. These factors are often abused in industry when only data favorable to a particular filter is presented in sales literature. To fairly evaluate filter effectiveness, sev-eral test procedures have been adopted by filter manufacturers and users. For the kinds of dust concentrations encountered in most industrial work areas, including power, the standard most often used for measuring filter efficiency is ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) Standard 52-76. It addresses multiple test methods, the most impor-tant being:

1. Weight arrestance, 2. Dust spot efficiency, and 3. Hot DOP (Di-octyl Phthalate), also known as MIL STD 282 (1958). The weight arrestance test is simple. It

involves feeding a synthetic dust to a filter and ratioing the weight of dust exiting the filter to the weight of dust originally fed into the filter. Since small particles have little mass, this method offers almost no way of revealing small-particle collec-tion efficiency. The method is used for low- and medium- efficiency media filters. During testing, pressure drop is observed until it reaches a final value (usually 1 in. H2O). The number of grams of dust, less that logged on a special high efficiency filter behind the test filter, yields the filters final dust holding capacity at 1-in.-H2O pressure drop. Collection efficiency also depends on the weight of dust collected on the test filter and the final filter. Since dust weight is not logged until the test filter becomes loaded, this method yields only an aver-age efficiencyone averaged over the entire test run. It gives no indication as to how long it took for the filter to build to the final rated efficiency nor does it offer any clue about starting efficiency.

Dust spot test. Where small-particle efficiency is critical (most industrial oil smokes generate par-ticles in the 0.01 to 2 micron size range), the dust spot test is often used. Here standard ambient air is passed through the test filter and the air stream has special test filters in front of and behind the test fil-ter to monitor the presence of airborne particulates. Over time, both filters become soiled and are mea-sured optically for relative soiling. These results are then translated into a filter efficiency rating.

Hot DOP test. A common method for measur-ing the efficiency of high-efficiency filter media is the hot DOP test. It involves boiling DOP and injecting the vapor into the air stream in front of the test filter. As the vapor condenses back to ambi-ent temperature, it forms very uniform droplets of about 0.3 micron diameter. Use of light scattering instrumentation allows measurement of upstream and downstream particle concentrations. HEPA (high efficiency particulate air) filters, usually rated for efficiencies in excess of 99.9% on 0.3 micron size particles, are tested using this method.

To understand how misleading efficiency tests can be, the table shows how certain filters would respond to the three tests described above. For example, assume a standard filter rated 65% under ASHRAE 52-76 (dust spot). Locate the 60-80% range in the center column. Now, look left and right. Note that this same filter could be rated 95% efficient if measured by the weight arrestance method or 35-40% efficient if measured by the hot DOP method. The only types of filters that will show high efficiency on the sub-micron particles generated in the hot DOP test are HEPA filters and electrostatic precipitators.

Comparing filter efficiency using different tests ASHRAE weight ASHRAE dust MIL-STD 282, arrestance method, % spot method, % hot DOP method, % 70-80 15-30 0 80-90 20-35 0 90-95 40-60 15-25 95 60-80 35-40 Not applicable 80-90 50-55 Not applicable 90-98 75-90 Not applicable Not applicable 95-99.999* *HEPA filters test at -100% efficiency using the arrestance and dust spot meth-ods

Finally, note that every filter configuration can be tested individually for efficiency and pressure drop. Standard filters generally have been tested for effi-ciency by particle size. Most standard filter com-binations have been tested for pressure drops at nominal air flow ratings. When units are specified as being ducted for source capture, blower speeds are preset at the factory to match specified field conditions. Detailed specifications for all standard unit configurations are listed by their respective manufacturers.



the efficiency measurement. Is it comparable to your operating conditions? If the actual in-service air velocity is, say, 50% greater than the test velocity you should anticipate significantly reduced performance.

Additionally, is the efficiency quoted a maximum, minimum or an average lifetime value? Many filters have a relatively low effi-ciency against small particulates early in their service lives but their performance improves as the element becomes progressively loaded with contaminants (Fig 1). In certain situ-ations it is possible for a filter to never reach its quoted maximum efficiency before it is replaced.

For details on the various tests used for determining filter effi-ciency, see the accompanying side-bar and companion table.

Pressure drop. Another major consideration when selecting a filtration system is its pressure drop. Intake pressure lossthat is, resistance to air flowwill have a direct impact on turbine efficiency and power output. A common mistake is to only con-sider the clean or startup pres-sure drop. Of real value to plant operations personnel is an under-standing of how quickly pressure drop will increase over time. To a large extent, this depends on the type of contaminants in the local environment.

A given filter and filtration sys-tem will have inherent character-istics that can be tested to provide an indication of relative perfor-mance. A typical test of a filters dust holding capacity involves

recording the increase in pres-sure drop as dust-laden air is passed through the filter. Using a standard dust test such as ASHRAE 52-76 (Amer-ican Society of Heating, Refrig-erating and Air Conditioning Engineers), different filters can be subjected to the same test in a comparative manner. Although not foolproof, this technique is recognized as giv-ing a good indication of real-world performance.

The best filter will have a shallow dust retention curve, which means it has a high specific dust holding capacity (Fig 2). If users change filters at a predetermined pressure drop, they will maximize the time between filter change-outs, reducing maintenance and replacement-filter costs. Alternatively, if the filters are changed at a fixed time peri-od, such as during the annu-al plant shutdown, the lower average pressure loss over the life of the filter will allow greater power production and higher revenuesprovided, of course, that the plants maxi-mum capability can be dis-

patched. Running wet. Another factor to consider when

Relating filter efficiencies and standards Eurovent EN 779 Type class class Efficiency, % Measured by StandardsCoarse EU1 G1



reviewing the pressure-drop characteristics of alternative offerings is the ability of a filtration system to operate efficiently when wet. The differ-ence in performance between a system designed to run wet and one that is not, can be significant (Fig 3). Running wet is not that unusual, given the high levels of precipitation in many areas and the increased use of inlet air cooling systems to boost GT output.

Testing of various filter types has shown con-siderable variation in performance going from dry

to wet conditions. In some instances, the pressure drop through a filter subjected to a relatively small amount of water spray can increase by several hun-dred percent. This usually happens to filters with cellulose fibers, which swell when wet. The high delta p may remain for several hours after the water spray has been stopped because the cellulose fibers retain moisture.

Lab results are confirmed by field experience. Many users report that early morning fog (a com-mon cause of filters running wet) results both in

User perspective

Many plant managers and their key staff will tell you that they didnt give much thought to air filters the first time their companies purchased gas turbines (GT), leaving specifica-tion and selection to the turbine manufacturer and/or the engineer/constructor. Knowledge of turbine inlet-air systems generally is acquired on the job when performance issues arise.

Gabe Fleck, an electrical engineer for Associ-ated Electric Cooperative Inc, Springfield, Mo, and chairman of the 501D5/D5A Users group, says fil-tration is a learning process. Fleck says AECI has no filtration problems per se, but that doesnt mean there isnt room for improvement in the specifica-tion/verification process. The company recently experienced a miscommunication with the ven-dor selected to supply replacement prefilters that resulted in receiving high-efficiency filters instead of low-efficiency filters. The mistake was discovered after repeated unit trips on high differential pressure suggested a laboratory review of the product. The bottom line: The filters supplied were 85% efficient based on a dust spot test rather than 85% efficient based on an arrestance test. These and other filter test procedures that you should become familiar with are detailed in a companion sidebar.

This was not the only snafu encountered by AECI regarding filters. In another case a new GT was to be supplied by the turbine manufacturer complete with G4 prefilters and F8 final filters. What the utility received was G3 prefilters and F7 final filters. The message seems clear: To avoid potential operating problems, sample a prefilter and final filter from your next order and send them to a lab to be sure you received what was specified.

Based on his experience, Fleck suggests that when ordering filters you should specify a dust spot or arrestance efficiency plus the maximum delta p across the filter and a minimum dust holding capacity at the final delta p. One of Flecks current projects is to determine if the addition of a roughing filter in front of the prefilters would be beneficial.

If you ask Alan Pearce his opinion on that it would be an unqualified yes. But then Pearce, an engineer assigned to the combined-cycle units at Alabama Power Cos Barry Generating Station, has experience in a climate that differs from Flecks.

GT inlet air systems are especially sensitive to local environmental conditions as most industry veterans have learned.

Pearce says Barrys experience with sheets of a polyester filtration medium hung by clips in front of the prefilters has been excellent. The medium catches bugs and big airborne debris, allowing prefilters to run a month longer and reduce their replacement to three times annually from four. The pre-prefilters for the sites four 7FA gas turbines (GE Energy, Atlanta) are changed online monthly and disposed of by incineration.

The Barry units have stainless steel inlet-air housings supplied by Braden Manufacturing LLC, Tulsa, Okla, that feature evaporative coolers behind the filters. One of the changes Barry made to the inlet air system was to redesign the frame struc-ture supporting the prefilters. The OEMs design required too many man-hours to change the 340 2-ft-square prefilters for each GT.

The original design required maintenance per-sonnel to unscrew and replace two wingnuts for each prefilter. Damaged threads, dropped wing-nuts, bent studs, and other annoyances were elimi-nated by a galvanized steel structure that supports a grid of hinged framesone per prefiltereach equipped with a simple locking device. The down-stream side of the prefilter frames has a gasket to ensure a tight seal between the prefilters and final filters. A work crew can replace prefilters on all four GTs during a normal weekend shutdown. Work is done off-line to prevent dust re-entrainment and early fouling of final filters. Used prefilters are incin-erated.

A compelling feature of the new prefilter grid structure is that it permits the use of unframed coalescing media which is half the cost of framed prefilters. Pearce figures the modified prefilter structure paid for itself in about a year, considering the saving in manpower and material.

Final filters are changed out every two to three years. The Barry staff uses a visual inspection and a delta p target for scheduling filter replacement. Philosophy is to maintain high performance by changing all filters before they reach their maximum capacity.

Bob Schwieger



an increase in pressure drop during the fogs occur-rence and also for several hours after the fog has cleared.

The importance of air velocity through the filter, noted earlier, deserves greater scrutiny. A filters operating velocity generally is defined as the volumetric air flow divided by the filter face area. Hence, a filter house with an air flow of 500,000 ft3/min and a filter face area of 1250 ft2 would have a filter velocity of 400 ft/min. Performance data for a filter normally is given at a nominal velocity, which is chosen to be representative of the typical in-service velocity.

The testing of filter characteristics are conduct-ed at the nominal velocity and quoted in sales lit-erature. When the actual in-service velocity is the same, or close to, the nominal velocity, the filtration system will perform as expected. But, for a variety of reasons (typically, poor aerodynamic design of the filter house and/or intake structure), some sys-tems operate at velocities well above the nominal. Result is an increase in pressure drop, a reduction in specific dust holding capacity, and usually a reduction in filtration efficiency.

To avoid this situation, all intake housings should be analyzed for proper aerodynamics using com-putational fluid dynamics. Generic CFD analyses generally are available for new intakes. But for ret-rofits to correct design deficiencies or an improper design for actual conditions, have a knowledgeable party conduct a dedicated analysis. Pay particu-lar attention to the velocity profile through the intake housing, spe-cifically the filtration system. A well-designed system will have a relatively uniform velocity profile across the entire filter bank. This velocity should be similar to the nominal velocity against which performance data are quoted.

How plant location impacts system design

At the most basic level, the choice of filtration sys-tem comes down to either a static (barrier) or a pulse (self-cleaning) design. Pulse systems usually consist of some form of weather protection followed by a deep-pleated cylindrical and/or conical filter. The filters are periodically cleaned by means of a reverse pulse of air.

Static systems typically consist of weather pro-tection, prefilters (designed to capture coarse con-

taminants), and high-efficiency filters. There are many different types of static filters, but prefilters are normally pleated panels, while high-efficiency elements tend to be bags or so-called mini-pleat filters. Unlike a pulse system, a static filter does not self-clean, and when the filters reach a certain pressure drop they must be replaced. Prefilters are changed more frequently than high-efficiency fil-ters, which can last anywhere from 8000 to 24,000 hours depending on the environment (see User perspective sidebar).

Pulse jet systems have higher capital costs than static systems, as well as higher lifetime main-tenance costs. They were originally developed to protect turbines operating in the Middle East that were subjected to frequent sand-storms. And in dry areas, with high concentrations of airborne parti-cles, they are very effective.

Over the last decade or so pulse filters have gained acceptance as a more general filtration solu-tion for GT applications, but the system does have some shortcomings. In addition to the higher life-cycle cost noted above, pulse jets may work poorly in areas with low or medium levels of airborne particles. This is because a pulse filter relies on a buildup of dust (known as a cake) on the surface of the media to both improve filtration efficiency. In areas with low or perhaps medium levels of dust, a cake does not form, and many contaminants, particularly small and sub-micron particles, pen-etrate the filter media. It is extremely difficult to remove particles that have penetrated the media, and because pulse elements are not designed to operate as depth filters, the pressure drop can increase rapidly. This is common in urban environ-ments when the contamination is from automobile and truck exhaust and oily in nature rather than dusty.

A pulse filter also is prone to pressure loss and other problems in high-moisture environments when the fabric has cellulose fibres woven into its structure. Also, if the contaminants captured on the filter media swell in humid conditions, the pressure drop will increase further and the cake can become difficult to remove.

For most situations, with the possible exception of some desert and cold-climate environments, a static system generally is the filtration solution with the lowest life-cycle cost (table can help guide your selec-

Filtration system selection guide

Environment Dust level Weather protection Filtration system Hot and dry High Weather hood PulseHot and dry Low/medium Weather hood StaticHumid All Weather hood StaticCoastal All Weather Static, downstream hood/separator separatorHigh rainfall All Separator StaticIce and snow High Snow hood PulseIce and snow Low/medium Snow hood Static, anti-icing

The importance of proper filter selection is obvious if you think of a gas turbine as a huge vacu-um cleaner.


GT INLET AIR SYSTEMStion). Nevertheless, to ensure optimum perform-ance, considerable care is required when specifying a static system for a particular environment.

For example, consider a costal location where, on a windy day, the concentration of salt in the atmosphere can be in the 0.05 to 0.5 ppm rangeor about five to 50 times the amount GT manufac-turers allow into their enginesfor as far as five miles inland. Many standard static systems are unable to cope with airborne salt (which is in the form of a fine aerosol) and it will pass directly into the compressor.

This situation can be neutralized with technol-ogy borrowed from the offshore oil and gas indus-try, which deals with atmospheric salt all the time. Perhaps the best solution within 500 yards of shore is to specify an inlet housing with effec-tive weather protection, plus a marine separator ahead of filter elements capable of coalescing any salt aerosols that penetrate the weather protection. A down-stream separator also should be installed to remove any coa-lesced droplets that become re-entrained in the air stream to prevent their entry into the compressor.

Problems with static systems usually can be traced to inad-equate weather protection. The standard weather hoods supplied with filtration systems often are suitable only for light rain. For locales subject to heavy rains, the solution is a marine separator, which is capable of removing from 10 to 20 times more water than a standard hood (Fig 4). For heavy snow, the only real solution is to have an extended-area 90-deg hood.

Marrying filtration and coolingToday, power augmentation systems are installed on most gas turbines for merchant and utility ser-vice. Here, the focus is on the three main types of inlet cooling systems: fogging, chiller coils, and evaporative coolers. The engineer challenged with system selection should factor into his or her analy-sis the interaction of the inlet cooling system with other components in the intake housing, including filter media. Remember that all three systems can introduce free water into the intake.

Fogging. With a fogging system, there is no secret as to how the water gets into the air intake structure: It purposely injects micron-size droplets directly into the air stream. These droplets typical-ly have an average particle size of about 10 microns and, under ambient conditions, would take around one second to evaporate.

Thus, at an in-duct air velocity of 600 ft/min, an average-size water droplet will travel 10 ft before it evaporates. For this example, the first point to consider is that anything within 10 ft of the fog-ging system will get wet. Second, this is a theoreti-

cal situation. Reality is that some of the very large droplets will take much longer to evaporate, others will fall out of the air stream and still others will impinge on the duct wall. Over time, the mean water-droplet size increases because of normal fog-ging-nozzle wear.

The real estate saying location, location, loca-tion applies to the positioning of a fogging system as well. If it is being retrofitted, the obvious posi-tion is upstream of the filters. However, if there is not sufficient space between the nozzles and the filtration system, the prefilters and high-efficiency filters will run wet and must be selected to accom-modate moisture.

Filters may run wet even if adequate space is available because of the high humidity of the air downstream of the fogging system. Suggestion:

Avoid filters with cellulose fibres, as noted earlier, and ones with cardboard frames, which lose their mechanical integrity when wet.

If the fogging system is placed downstream of the fil-ters, ensure against small parts coming loose and being ingested by the compressor. Also, provide sufficient drain-age from low points in the ductwork where water can accumulate to prevent both corrosion and turbine inges-tion. Finally, design the fog-ging system to prevent the for-mation of large droplets which

could enter the compressor and erode blades.In a chiller system, no water is introduced

directly into the system; the coolant removes heat from the air via finned-tube heat transfer surface. However, under some conditions, a chiller system has the potential to cause the formation of free water in the air stream. Reason: Even at moderate-ly low relative humidity, it is commonplace for the downstream temperature of air passing through the chiller to fall below its dew point. When this occurs, the air is no longer able to support the amount of water vapor in it and condensation occurs.

The amount of condensation can be considerable. For example, a chiller coil that cools a 650-lb/sec stream of air with a relative humidity of 50% from 95F to 50F will produce condensate at the rate of approximately 12 lb/secthats more than 5000 gal/hr.

Clearly, this amount of condensation must be controlled, particularly because carryover from the chiller coil can consist of large droplets that would erode compressor blades. If a chiller sys-tem is in your future, be sure the drainage system within the coil is well-designed and of adequate capacity. You should also plan to use filters that are well suited for running wet because this will be a fact of life.

Orientation of finned tubes is important, too. Vertical fins significantly reduce the amount of

Face velocity, ft/min0 250 500 750 1000 1250

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Typical weather hood or louver

Leading marine separator

4. Marine separators are signifi-cantly more efficient at removing large amounts of entrained water than a typi-cal weather hood or louver


GT INLET AIR SYSTEMSentrainment coming off the back of a coil module because water runs down the fins under gravity. However, from the viewpoint of header connections it often is more convenient to orient the tubes verti-cally and the fins horizontally.

Finally, pay close attention to air velocity: The higher the velocity the more likely that entrain-ment will occur. A rule of thumb is that, below a face velocity of about 300 ft/min very little entrainment occurs, particularly if there is good in-coil drain-age. It is not essential that the air flow through the chiller be at a low velocity, but rather that the oper-ating staff is aware that as the velocity increases, more elaborate solutions are required to deal with the carryover.

For example, you can protect the compressor against ingestion of large water droplets by install-ing a mist/drift eliminator downstream of the coils. Careful selection of a mist eliminator is prudent because the effectiveness of any given design depends on air velocity.

Location of the chiller coil relative to the filtra-tion system is an important consideration as it is for fogging systems and evaporative coolers. If upstream of the filters in an area with a high or medium level of contamination, the fins will quickly foul, reducing thermal efficiency. One defense is to install guard filters upstream of the coils. These are merely prefilters (normally panels because of space constraints) that provide a nominal level of protection for the coils.

Evaporative coolers, like fogging systems,

introduce water into the intake duct. In theory, if the system is functioning correctly, water is retained within the confines of the evaporative cooling module. However, in practice it is likely that some carryover will occur. Thats why many systems are fitted with downstream drift or mist eliminators. But despite best efforts, carryover sometimes passes through the mist eliminator for one or more of these reasons: operating the system beyond its design air velocity, degradation of cool-ing media, excessive water flow rate, and poorly designed mist eliminator.

As with other GT inlet air cooling systems, posi-tioning of the evaporative module is critical. Placing the unit in front of the filters means they will oper-ate wet and both the coolers cascading water and media will collect much of the ingested contamina-tion. Depending on the cleanliness of the ambient air, this may or may not constitute a problem. Of importance is where contaminants captured by the wetted media are goingthat is, is it being retained by the media or flushed away by the cooling water? Either way, plant operators must ensure that the buildup of contamination does not adversely impact the performance of the evaporation module.

If the cooler is placed downstream from the filtration system, it is essential that the issue of water carryover be addressed. There have been various reports of evaporative coolers being held responsible for compressor blade erosion. Also, all parts must be secure to prevent foreign-object dam-age to the compressor. CCJ

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CCJ3 Air Inlet Filters Final