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A Review of Heat Exchanger Fouling in the Context of Aircraft Air-conditioning Systems, And the Potential for Electrostatic Filtering

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Applied Thermal Engineering 29 (2009) 2596–2609 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Review A review of heat exchanger fouling in the context of aircraft air-conditioning systems, and the potential for electrostatic filtering S. Wright a,*, G. Andrews a, H. Sabir b a b Energy & Resources Research Institute (ERRI, SPEME), Faculty of Engineering, University of Leeds, Leeds LS2 9JT, UK Faculty of Engineering,
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  Review A review of heat exchanger fouling in the context of aircraft air-conditioningsystems, and the potential for electrostatic filtering S. Wright a, * , G. Andrews a , H. Sabir b a Energy & Resources Research Institute (ERRI, SPEME), Faculty of Engineering, University of Leeds, Leeds LS2 9JT, UK  b Faculty of Engineering, Kingston University, Roehampton Vale, London SW15 3DW, UK  a r t i c l e i n f o  Article history: Received 9 October 2007Accepted 6 January 2009Available online 13 January 2009 Keywords: AircraftFoulingElectrostatic filteringHeat exchangersAircraft packs a b s t r a c t This paper presents afocusedliterature reviewtounderstand thecommonproblemof fouling of air-con-ditioning heat exchangers aboard aircraft, with the academic consideration to employ electrostatic pre-cipitation to remove airborne particulate matter.Particulate matter suspended in air, is carried through the matrices of aircraft environmental coolingsystems. The deposition and build up of such contaminants affects the thermal performance of coolingsystems and leads to component failure, expensive repairs and loss of service of an aircraft.Although there have been many publications of material pertaining to heat exchangers andfouling, very little publications specifically to aircraft air-conditioning systems or failures havebeen published. Nonetheless, the literature review indicates that sizes and distribution of partic-ulate matter including Reynolds numbers and rates of deposition have been established in previ-ous papers.The novel approach to this industrial problem has been to evaluate the operational problem of air-craft air-conditioning systems, identify local factors, and to consider the use of means of protectionemployed in other non-aerospace industries. It is believed the application of electrostatic precipitationcould potentially aid prevention of fouling particulate matter on aircraft air-conditioning heatexchangers. Ó 2009 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25971.1. Fresh cabin air requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25972. Sources of airborne pollutants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25972.1. Particulate matter (PM 10 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25982.2. Particulate matter sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25982.3. Particulate matter (PM 10 ) at London Heathrow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25993. Heat exchanger findings in other non-aircraft industries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25993.1. Domestic air conditioning fouling – a similar problem to aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26023.2. Fouling factors and distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26033.3. Fouling factors – a diverse approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26033.4. Fouling and a cleaning routine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26064. Electrostatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26064.1. Electrostatic attraction/precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26064.2. Electrostatic precipitation in transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26074.3. Precipitation in air-conditioners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26074.4. Electrostatic precipitation potential application to aircraft heat exchanger pack fouling in the ram air stream . . . . . . . . . . . . . . . . . . . 26075. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2608References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2608 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.applthermaleng.2009.01.002 * Corresponding author. Tel.: +44 113 3432497. E-mail address: s.j.wright@leeds.ac.uk(S. Wright).Applied Thermal Engineering 29 (2009) 2596–2609 Contents lists available atScienceDirect Applied Thermal Engineering journal homepage:www.elsevier.com/locate/apthermeng  1. Introduction Large commercial aircraft employ a system known as an envi-ronmental control system ( ECS  ) to provide conditions in flight thatallow passengers to travel in relative comfort. The ECS providesboth cooling or hot air to pressurise the passenger cabin, whichcontrols temperature, fresh air ventilation and cabin altitude (viapressurisation, by moderating the outflow valves).The source of air used by the ECS is typicallytakenfromthe gasturbine compressor, knownas bleed air  (betweenstations P2.5 andP3 in a gas turbine), and is both high pressure and temperature.Prior to allowing hot bleed air into the passenger cabin, the airmustfirstbecooledtoanacceptablevalue.Thisisperformedusinga unit known as an aircraft pack. The pack is comprised of severalunits, including a number of heat exchangers cooled by ambient ram air  , air that is ducted from outside the aircraft and post cool-ing, is vented back to atmosphere. The pack heat exchangers arethe main units of interest, and comprise typically of a fin and plateassemblies, co-flow single pass units. The hot bleed air (from thegas turbine) is cooled to measured value by the pack whichin turnis controlled by the ECS. The external ram cooling airflow passinginto the pack heat exchangers contains debris, which is depositedon the forward face of the fin and plate single pass exchanger.Themajorityofdebrisisbelievedtobedepositedontheoutermostsurface – namely as the air passes further into the pack exchanger,the level of contamination decreases. Note, this review is not con-cerned with passenger air quality.Major aircraft manufacturers and a UK airline have recentlysuggestedthatair-conditioningsystemoverheatposesregularfail-ures under given operational conditions, which has a considerableeffect on aircraft technical dispatch reliability of aircraft.One specific airline operates their fleets currently in a singleclasslayoutofthepassengercabin,whichisknownashighpassen-ger density configuration. This relies on the air-conditioning sys-tem running constantly in ‘‘Hi Flow” mode to provides sufficientventilationfortheadditionalpassengers.Suchanairlineshighpas-senger density must meet the legislated requirements to provide aminimum quantity of fresh conditioned air to each passenger. 1.1. Fresh cabin air requirements Until 1996, both the Federal Aviation Authority (FAA, USA) and Joint Aviation Authority (JAA, Europe) had the same basic require-ment for cabinventilation rates. FAR 25.831 and JAR 25.831[1]re-quired a minimum supply of 10 cubic feet per minute (cfm) of fresh air per flight crew member, which ‘‘must be free from harm-ful or hazardous concentrations of gases or vapours.”The resulting high flow operation causes the air-conditioningsystemtooperateat 120%of the‘‘normal”manufacturersflowrate[1]. High flow operation of levels greater than ‘‘normal operation”may be required to provide additional levels of ventilation duringabnormal situations/operations. One such operation would besmoke in the passenger cabin, where additional air is required toprevent asphyxiation. Another abnormal operation could be highdensity single economy class configurations.The higher than average air-conditioning system failures areobservedin theaircraft pack, causedby overheating whilst the air-craft is either on the ground, or during take-off/landing.During these specific operations, large volumes of ambient ramair do not flow over the primary and secondary heat exchangers,ratheramechanicalfanoperatestoassistinthetransferofheatbe-tween the system and ambient.The overheating in the system is due to particulate matteraccretion in the air-conditioning pack heat exchanger matrix.When excessive levels of particulate are deposited in the matrix,the thermal performance of the heat exchanger decrease: the packreacheshigherthanacceptableoperationaltemperaturesandmustbe isolated. This system failure results in the flight crew ‘‘shuttingdown” the overheating pack to prevent further damage and thepossibility of a fire. It is worth noting that the ECS unit and packsare typically located on narrow and wide bodied aircraft, belowthewater linenormallyclosetothewingboxsection. Acentrefueltank is normally in close vicinity to this system.Rectification can only be achieved by removing the pack fromthe aircraft, returning the unit back to the srcinal equipmentmanufacturer (OEM) for deep clean to remove operationalcontaminants.Fig. 1[2]shows hot bleed air from the engine compressors ismeteredthrougha bleedair valve and being passed to the primaryheat exchanger. After primary cooling, the air passes to the air cy-cle machine and latterly into the secondary heat exchanger beforebeing distributed into the cabin air system.Ambientairprovidescoolingatboththeprimaryandsecondaryexchangers.In this paper, a review of the known pollutants at airfields isconsidered, along with non-aviation industry publications in deal-ing with the field of particulate matter fouling applied to heatexchangers. The potential of electrostatic filtering will be finallyconsidered as a solution to prevent airborne particulate foulingon the aircraft pack heat exchangers. 2. Sources of airborne pollutants DepartmentforTransport (DoT) UK published a white paper[3], theorising air transportation development over the next 30years. The paper and content is of specific importance to aca-demic study, as it is believed that increased levels of growth willlead to higher levels of airborne debris, thus potentially effectingthe operations of aircraft heat exchangers. The numbers of air-craft operating at a given period of time will not be correlatedto the levels of technical dispatch reliability, but it would be Fig. 1. Airbus air conditioning schematic VACBI[2]. S. Wright et al./Applied Thermal Engineering 29 (2009) 2596–2609 2597  expected that increased levels of operation will lead to higherrates of failure.  2.1. Particulate matter (PM  10 ) TheDepartmentfortheEnvironment,FoodandRuralAffairsUKdefines particulate matter being ‘‘classified according to its sizeand this classification is used in concentration measurements.For example, PM 10 is – to a good approximation – the concentra-tion of particles that are approximately equal to 10 l m”[4].Airborne particulate matter is made up of a collection of solidand/orliquidmaterialsofvarioussizesthatrangefromafewnano-metres in diameter (about the size of a virus) to around 100 l m(about the thickness of a human hair). It consists of both primarycomponents, which are released directly from the source into theatmosphere, and secondary components, which are formed in theatmosphere by chemical reactions. Particulate matter comes fromboth human made and natural sources. It contains a range of chemical compounds and the identity of these compounds isdependant on the source materials (as perTable 1).Measurements of the concentration of particulate matter in airare made by recording the mass of particulate matter in one cubicmetre of air, using the units micrograms per cubic metre, l gm À 3 .The particulate matter[5]is described by the Department of Transport (DoT) publication ‘‘Project for sustainable developmentat London Heathrow” as one of the main particulate contributiontoairbornefouling, andthereforeparticularattentionwillbe giveninthisreviewtoitsparticlesize,distribution,sourcesandeffects.Itmakes reference to dispersion modelling close to airfields and thetypes of measured emissions found close to the airfield (LondonHeathrow). London Heathrow International Airport, located inWest London adjacent to the M25. Pollutants monitored includedconcentrations of NO 2 (gas) and particulate matter (includingPM 10 ).The inclusion of particulate matter is of specific interest to thisreview, as it is believed that such materials are likely to accrue onheat exchanger surfaces withthe potential to affect the heat trans-ferability.Thepaper[5]doesnotincludedetailsoffuturepollutantmodels or content that would enable one to calculate/generate theemissions that would be required should this task need evaluationand further consideration. The methodology the DoT employedwas via the appointment of panel members (selected by DoT) pro-viding written details of recommendations on how to set up de-tailed ‘‘Bottom up inventory.” This approach is considered tooqualitative/subjective, therefore considered to be limited to thisacademic study.  2.2. Particulate matter sources The mean UK limit for particulate matter PM 10 is 40 l gm À 3 ,across the UK as a whole: The report stated that the maximumva-lue measured was 27.3 l gm À 3 with an estimated mean averagearound the London Heathrow vicinity of 24–25 l gm À 3 . Measure-ments of smaller particles namely PM 2.5 were estimated to be be-tween 17 and 21 l gm À 3 . Sources of PM 10 pollution are known toinclude:  Aircraft from both ground and flight operations including oper-ation of Auxiliary Power Units (APU’s), operations of tyres andbrakes and ground engine tests.  Airside vehicles – both supportive and service vehicles (air startequipment, ground power units).  Road vehicles, at both landside and airside (airside is the withinthesecurityperimeterof theairfield, landsideareall other areasnot subject to strict security controls).  Airport car parks, bus stations and taxi cues.  Airportbuildingheatingandventilationairconditioning(HVAC)(including boiler plant).  Airport fire training exercises.Further sources of emissions are later discussed including criticalphases of flight, namely take-off and landing, however quantifiedvalues of emission types are not given. A general approach trendis suggested in accordance with the International Civil AviationOperation documentation, that at a distance of 10 nautical miles(20km) from the threshold (of the runway) the aircraft will beapproximately at 3000ft. At this time a moderate flap setting islikely to be made resulting in the deceleration of the aircraft. Atapproximately 6.5 nautical miles (13km) the undercarriage isextended and the aircraft is then approximately at 2000ft. Thefinal flap setting is made at 5 nautical miles (10km) at an approx-imate altitude of 1500ft.This account is particularly important to this study as the oper-ational failure observed by commercial airlines of the air-condi-tioning systems is not in flight, but whilst the aircraft is on theground, typically after the aircraft has flown a sector and then is  Table 1 Sources of particulate matter[5]. Primary components Sources Sodium chloride Sea saltElemental carbon Black carbon (soot) is formed during high temperature combustion of fossil fuels such as coal, natural gas and oil (diesel and petrol) andbiomass fuels such as wood chipsTrace metals These metals are present at very low concentrations and include lead, cadmium, nickel, chromium, zinc and manganese. They aregeneratedbymetallurgicalprocesses,suchassteelmaking,orbyimpuritiesfoundinoradditivesmixedintofuelsusedbyindustry.Metalsin particles are also derived from mechanical abrasion processes, e.g. during vehicle motion and break and tyre wearMineral component These minerals are found in coarse dusts from quarrying, construction and demolition work and from wind-driven dusts. They includealuminium, silicon, iron and calcium Secondary components Sources Sulphate Formed by the oxidation of sulphur dioxide (SO 2 ) in the atmosphere to form sulphuric acid, which can react with ammonia (NH 3 ) to giveammonium sulphateNitrate Formedbytheoxidationofnitrogenoxides(NO  x )whichconsistsofnitricoxide(nitrogenmonoxide,NO)andnitrogendioxide(NO 2 )intheatmosphere to form nitric acid, which can react with NH 3 to give ammonium nitrate. Also present as sodium nitrateWater Somecomponentsoftheaerosolformofparticulatematter,suchasammoniumsulphatesandammoniumnitrates,takeupwaterfromtheatmosphere Primary and secondarycomponentsSources Organic carbon Primary organic carbon comes from traffic or industrial combustion sources. Secondary organic carbon comes from the oxidation of volatile organic compounds (VOCs). There may be several hundred individual components. Some of these trace organic compounds, suchas certain polycyclic aromatic hydrocarbons, are highly toxic2598 S. Wright et al./Applied Thermal Engineering 29 (2009) 2596–2609  performing ground operations. It is believed that the initial and fi-nal phases of flight where the aircraft is known to fly at reducedspeeds, reducesthemassairflowthroughtheair-conditioningheatexchangers and coupled with this is the ambient air containinghigher values of particulate matter, namely PM 10 as shown inFig. 2.  2.3. Particulate matter (PM  10 ) at London Heathrow Fig. 2clearly shows two areas of higher than expected levels of PM 10 in West London – as indicated by the brightly colour shadedregionsrepresentinghighPMconcentration: The sites are believedto be London Heathrow (West London) and RAF Northolt whichdisplayed very high levels of PM 10 pollution located in North WestLondon. It should also be noted that aircraft take-off and/or ap-proach is likely to track the aircraft across central London, whichisalsohighinPM 10 aerosolmaterial–thusfurtheraffectingthede-posit of debris on the heat exchangers.Auxiliary Power Unit, APU, emissions are briefly discussed andtheir use is significantly less complex than the emission of aircraftas APU operation is typically whilst the aircraft is on ground with-out ground services. The APU may be used in early and late phasesof flight namely take-off and landing, but generally the use of APUin flight in not necessary.Eq.(1)shows the relationship for total APU emissions whichsuggests that the means of operation for a given APU coupled withthe rate of fuel flowand an emission index of the respective mode. APU emissions ¼ X mod e ¼ 4mod e ¼ 1 Time mod e  Fuel Flow mod e  Emisions Index mod e ð 1 Þ The mode conditions contained within formula(1)are defined as:1. no load,2. electric,3. full ECS+electric,4. MES+electric.where ECS=environmental condition services, MES=main enginestart.The report[5]noted that mode 2 and 3 produced similar levelsof NO  x and PM 10 values, modes 4 and 1 were distinct. The reporthoweverdoesnotincludereferencetoparticulatemattercomposi-tion or of size distribution of average particulate size. This is be-cause the mandated measurement for particulates at present isonlymandatedfor PM 10 . Theinclusionof asizedistributionof par-ticulates including analysis of the volatile organic content wouldbe beneficial to this field.The report states that APU emissions are higher than moderngas turbine main engines. This statement suggests that APU usewill have a significantly higher impact on the reliability of air-con-ditioning heat exchangers than modern gas turbines due to thePM 10 levels produced. Additional APU use being restricted to verylow altitude use which also suggest slower aircraft operatingspeedsthuslowermassairflowthoughtherespectiveheatexchan-ger matrixes.Airside traffic contributes to PM 10 being measured, but due tothe lack of exact data values for distances travelled the report isunabletogivenanexactbreakdownof vehicletypeandPM 10 massproduction.This will not hinder the investigation, as values of PM 10 havebeenmonitoredat varioussites bothinsideandoutsidethe airportvicinity, thus trends of PM 10 pollution are known fromobservations. 3. Heat exchanger findings in other non-aircraft industries Publications on aircraft heat exchanger fouling is very limited.The approach employed is to review other non-aircraft industrialexperiencesfrompapers/publicationtobetterunderstandnon-avi-ation industry knowledge.General Electric Water Company has considered the commer-cial effects of heat exchanger efficiency[6], and as such has devel-oped commercial software CHeX, which contains a predictivealgorithm to estimate the heat exchanger efficiency. The publica-tion suggests the field of heat exchanger fouling has a significantfinancial effect on US industry, ca. $10 billion per annum in1985. Although the period of data is now significantly out of date,the inclusion and consideration of such a unit is useful in under-standingthecommercialvaluefoulinghastoplayinbettermanag-ing heat exchanger assets. CHeX software has various inputs andcan address fouling detection, fouling prediction and fouling diag- Fig. 2. 2004 PM 10 concentrations London[5]. S. Wright et al./Applied Thermal Engineering 29 (2009) 2596–2609 2599
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