ASHRAE HVAC APPLICATIONS IP CH 12-2015 AIRCRAFT.pdf

1、12.1CHAPTER 12AIRCRAFTDesign Conditions. 12.1Typical Flight 12.11Air Quality. 12.13Regulations 12.14NVIRONMENTAL control system (ECS) is a generic termEused in the aircraft industry for the systems and equipment asso-ciated with ventilation, heating, cooling, humidity/contaminationcontrol, and press

2、urization in the occupied compartments, cargocompartments, and electronic equipment bays. The term ECS oftenencompasses other functions such as windshield defog, airfoil anti-ice, oxygen systems, and other pneumatic demands. The regulatoryor design requirements of these related functions are not cov

3、ered inthis chapter.1. DESIGN CONDITIONSDesign conditions for aircraft applications differ in several waysfrom other HVAC applications. Commercial transport aircraft oftenoperate in a physical environment that is not survivable by the unpro-tected. In flight, the ambient air may be extremely cold an

4、d dry, andcan contain high levels of ozone. On the ground, the ambient air maybe hot, humid, and contain many pollutants such as particulatematter, aerosols, and hydrocarbons. These conditions changequickly from ground operations to flight. A hot-day, high-humidityground condition usually dictates t

5、he thermal capacity of the air-conditioning equipment, and flight conditions determine the supplyair compressors capacity. Maximum heating requirements can bedetermined by either cold-day ground or flight operations.In addition to essential safety requirements, the ECS should pro-vide a comfortable

6、cabin environment for the passengers and crew.This presents a unique challenge because of the high-density seatingof the passengers. Furthermore, aircraft systems must be light-weight, accessible for quick inspection and servicing, highly reli-able, able to withstand aircraft vibratory and maneuver

7、loads, andable to compensate for various possible system failures.Ambient Temperature, Humidity, and PressureFigure 1 shows typical design ambient temperature profiles forhot, standard, and cold days. The ambient temperatures used for thedesign of a particular aircraft may be higher or lower than th

8、oseshown in Figure 1, depending on the regions in which the aircraft isto be operated. The design ambient moisture content at various alti-tudes that is recommended for commercial aircraft is shown inFigure 2. However, operation at moisture levels exceeding 200 gr/lbof dry air is possible in some re

9、gions. The variation in ambient pres-sure with altitude is shown in Figure 3. Refer to the psychrometricchart for higher altitudes for cabin humidity calculations. Figure 4shows a psychrometric chart for 8000 ft altitude.Heating/Air Conditioning Load DeterminationThe cooling and heating loads for a

10、particular aircraft model aredetermined by a heat transfer study of the several elements that com-prise the air-conditioning load. Heat transfer involves the followingfactors:Convection between the boundary layer and the outer aircraft skinRadiation between the outer aircraft skin and the external e

11、nviron-mentThe preparation of this chapter is assigned to TC 9.3, Transportation AirConditioning.Fig. 1 Ambient Temperature ProfilesFig. 2 Design Humidity Ratio12.2 2015 ASHRAE HandbookHVAC ApplicationsSolar radiation through windows, on the fuselage and reflectedfrom the ground.Conduction through c

12、abin walls and the aircraft structureConvection between the interior cabin surface and the cabin airConvection and radiation between the cabin and occupantsConvection and radiation from internal sources of heat (e.g., elec-trical equipment)Latent heat from vapor cycle systemsAmbient Air Temperature

13、in FlightDuring flight, very cold ambient air adjacent to the outer surfaceof the aircraft increases in temperature through ram effects, and maybe calculated from the following equations:TAW= T+ r(TT T)TT= TorTAW= Tr = Pr1/3wherePr = Prandtl number for air (e.g., Pr = 0.73 at 432RT= ambient static t

14、emperature, RTT= ambient total temperature, Rk = ratio of specific heat; for air, k = 1.4M = airplane Mach numberFig. 3 Cabin Pressure Versus AltitudeFig. 4 Psychrometric Chart for Cabin Altitude of 8000 ft1k 12- M2+1 rk 12- M2+Aircraft 12.3r = recovery factor for turbulent boundary layer (i.e., fra

15、ction of total temperature recovered in boundary layer as air molecules rest on the surface)TAW= recovery temperature (or adiabatic wall temperature), RExample 1. The International Civil Aviation Organization (ICAO) coldday at 30,000 to 40,000 ft altitude has a static temperature of 85F(375R) and a

16、Prandtl number of 0.739. If an airplane is traveling at0.8 Mach, what would the external temperature be at the airplanes skin?Solution: Iteration is usually required. First guess for r 0.9:Pr = 0.728 at 0.9(423 375) + 375 = 418Rr = Pr1/3= (0.728)1/3= 0.8996Air Speed and Mach NumberThe airplane airsp

17、eed is related to the airplane Mach number bythe local speed of sound:u= Mwherek = ratio of specific heats; 1.4 for airR = gas constant; 1716 ft2/s2RM = airplane Mach numberu= airplane airspeed, fpsAmbient Pressure in FlightThe static pressure over most of the fuselage (the structurearound the cabin

18、) is essentially equal to the ambient pressure at theappropriate altitude.Ps= Pinf+ CpwherePs= pressure surrounding the fuselage, lb/ft2CP= pressure coefficient, dimensionless; approximately zero for passenger section of fuselage= free-stream or ambient air density, slug/ft3External Heat Transfer Co

19、efficient in FlightThe fact that the fuselage is essentially at free-stream static pres-sure implies that a flat-plate analogy can be used to determine theexternal heat transfer coefficient at any point on the fuselage:Rex= q = hA(T TAW)whereh = external heat transfer coefficient, Btu/sft2FRex= loca

20、l Reynolds number, dimensionlessx = distance along the fuselage from nose to point of interest, 10.21 1010(T*)3/2cp= constant-pressure specific heat; for air, 0.24 Btu/lbFw= ambient air (weight) density at film temperature T*, lb/ft3 = absolute viscosity of air at T*; 1.021 109(T*)3/2734.7/(T* + 216

21、) lbm/ftsA = outside surface area, ft2T = outer skin temperature, Rq = convective heat loss from outer skin, Btu/huinf= airplane airspeed, ft/hExternal Heat Transfer Coefficient on GroundThe dominant means of convective heat transfer depends onwind speed, fuselage temperature, and other factors. The

22、 (free con-vection) heat transfer coefficient for a large, horizontal cylinder instill air is entirely buoyancy-driven and is represented as follows:Gr = for 109 GrPr 1012:hfree= whereg = gravitational acceleration, 32.2 ft/s2k = thermal conductivity of air, Btuft/hft2R = kinematic viscosity, ft2/sd

23、 = fuselage diameter, fthfree= free-convection heat transfer coefficient, Btu/hft2R = expansion coefficient of air = 1/Tf, where Tf = (Tskin+ Tinf)/2, RT = Tskin TinfTskin= skin temperature, RTinf= ambient temperature, RGr = Grashof numberPr = Prandtl numberA relatively light breeze introduces a sig

24、nificant amount of heatloss from the same horizontal cylinder. The forced-convection heattransfer coefficient for a cylinder may be extrapolated from the fol-lowing:Re = for 4 104 Re 4 105hforced= where V is wind speed in ft/s, and is evaluated at Tf = (Tskin+ Tinf)/2.Example 2. One approximation of

25、 the fuselage is a cylinder in cross-flow.The fuselage is 12 ft in diameter and 120 ft long, in a 14 fps crosswindand a film temperature of 575R. The surface temperature varies withthe paint color and the degree of solar heating. For instance, a typicalwhite paint could be 30F higher than the ambien

26、t air temperature, sothe heat transfer from the fuselage would beFree convection:Gr = = 8.05 1010for 109 GrPr 1012.TTT1k 12- M2+375 11.4 12- 0 . 82+423R= =TAWTr+ TTT375 0.8996 423 375+418.2R 41.5F=kRT12-u2h wcpuinf0.185 log10Rex2.584Pr23=note: 107Rex1094000, soC = 0.193 and n = 0.618, which leads to

27、These two correction factors have been combined see Equation(37) in Chapter 9 of the 2013 ASHRAE HandbookFundamentalsto produce a simpler relationship that applies to the full velocityrange (0 1 means that concentrations in the breathing zone are lowerthan in a perfectly mixed system; VE 1 means the

28、y are higher.There is a distinction between VE for bleed air and VE for totalventilation. For bleed air, the inlet concentration cinis the concen-tration of gases in the supply air to the entire system (i.e., bleed airconcentration). The local concentration will be larger than the inletconcentration

29、 only if the contaminant is generated within the cabin.For total ventilation, VE uses the cinat the nozzle (i.e., supply mix-ture concentration) and includes contaminants from the recircula-tion system. The practical use of this VE applies to particulate levelsin the cabin, because the recirculated

30、air is equivalent to bleed air inthis regard.Contaminant concentrations in the cabin can be converted toflows delivered to the breathing zone Qlocalusing the following rela-tionship:Qlocal= Fig. 7 Cabin Air Velocities from CFD, fpm(Lin et al. 2005)cmixedcinclocalcin-QlocalQcabin-Table 2 FAA-Specifie

31、d Bleed Airflow per PersonCabin PressureAltitude, ftRequired Flow per Person at 75F, cfmpsia lb/ft214.696 2116 0 7.414.432 2078 500 7.514.173 2041 1000 7.713.917 2004 1500 7.813.664 1968 2000 8.013.416 1932 2500 8.113.171 1897 3000 8.312.930 1862 3500 8.412.692 1828 4000 8.612.458 1794 4500 8.712.22

32、8 1761 5000 8.912.001 1728 5500 9.111.777 1696 6000 9.211.557 1664 6500 9.411.340 1633 7000 9.611.126 1602 7500 9.810.916 1572 8000 10.0qgenclocalcin-12.8 2015 ASHRAE HandbookHVAC ApplicationsSubstituteqgen= Qsupplied (cmixed cin)whereqgen=CO2generation rate, 0.0105 scfm/personclocal=local CO2concen

33、tration by volumecin=inlet CO2concentrationQsupplied= flow to cabin or zone, cfmQlocal= flow delivered to breathing zone, cfmSome consideration can be given to distribution effectiveness(DE), where flows to higher-occupant-density sections of the cabin(such as coach) are used to set minimum flows to

34、 the cabin, andlower-density sections (such as first class) may subsequently beoverventilated:DE = whereQzone/nzone= flow per person in zoneQcabin/ncabin= average flow per person for entire cabinDistribution effectiveness accounts for a system that provides auniform flow per length of cabin yet has

35、varying seating densitiesalong the length. For bleed air distribution, this effectiveness is tem-pered somewhat by occupant diversity D (see ASHRAE Standard62.1), because underventilated zones feed into the same recircula-tion flow. For total flow (bleed + recirculated) and for systems with-out reci

36、rculation, however, occupant diversity does not apply.System ventilation efficiency (SVE) is a measure of how wellmixed the recirculated air is with the bleed air before it enters thecabin. The SVE can be determined from the concentration varia-tions in the ducts leaving the mix manifold (see Figure

37、 11), forinstance. The SVE is similar to VE in formulation:SVE = wherecall zones= average concentration of all supply ductsczone= concentration in individual supply ductcamb= ambient reference concentration = Cfr(bleed air concentration)Dilution Ventilation and TLVContaminants that are present in th

38、e supply air and are also gen-erated within the cabin require increasing dilution flows to avoidreaching Threshold Limit Values (TLVs) (ACGIH). For example,suppose carbon monoxide is present in the atmosphere at 210 ppband that each person generates 0.168 mL CO per minute, or 5.93 106cfm (Owens and

39、Rossano 1969). The amount of bleed airrequired to stay below the EPA guideline of 9000 ppb will depend onthe ambient CO levels, the human generation rate, and the CO con-tribution of the ventilation system:Qreq= whereCTLV= allowable concentrationCsystem= concentration rise from systemCsupply= concen

40、tration in supply (air entering cabin)Cfr= concentration in bleed airqgen= CO generated per personExample 3. If the ventilation system does not contribute carbon monoxideto the supply air, then the required ventilation rate to stay below thethreshold isCTLV= 9000 ppb = 0.000009qgen= 5.93 106cfmCsyst

41、em= 0Cfr = 210 ppb = 2.1 107If, however, the ventilation system produces a 1000 ppb rise in car-bon monoxide, then the required ventilation isCTLV= 9000 ppb = 0.000009qgen= 5.93 106cfmCsystem= 1000 ppb = 0.000001Cfr= 210 ppb = 2.1 107It is important to note that, under certain circumstances, qgenand

42、Csystemmay change sign as contaminant sources become contaminantsinks. This simplified approach shown here is more conservative, andcould overpredict contaminant levels in real situations.Air ExchangeHigh occupant density ventilation systems have higher airexchange rates than most buildings (i.e., o

43、ffices). The typical air-plane may have an air exchange rate of 10 to 20 air changes per hour(ach), whereas an office might have 1 ach. The air is not replaced ina mixed system at every air exchange. Actually, the ratio Q/V (airexchange rate) is more like the inverse of decay time constant . Anairpl

44、ane cabin can be approximated as a partially mixed volume (avolume with ventilation effectiveness) as long as the contaminantsources are uniformly distributed throughout the volume. For awell-mixed volume, contaminant in equals contaminant out pluscontaminant accumulated in the volume, orQcin= Qcout

45、+ VAccounting for ventilation effectiveness, the concentration leav-ing the volume coutis related to the concentration within the volumec and the concentration entering the volume cinby the ventilationeffectiveness VE:VE = = cin+ VE(c cin)Substituting,Qcin= Qwhich leads toQlocalQsuppliedcmixedcinclo

46、calcin-=QzonenzoneQcabinncabin-call zonescambczonecamb-qgenCTLVCsystem Cfr-qgenCTLVCsupply-=QreqqgenCTLVCsystem Cfr-5.93 1060.000009 0 2.1 107-=0.67 cfm/person=QreqqgenCTLVCsystem Cfr-5.93 1060.000009 0.000001 2.1 107-=0.76 cfm/person=dcdt-cmixedcambclocalcamb-coutcambclocalcamb- cout=cmVE ccin+Vdcd

47、t-+ccincincoeQ VEV- t=Aircraft 12.9Although air exchange rates are occasionally used as require-ments on the ventilation system, in the case of cabin ventilation,there is no basis for setting one. Air exchange rate can be a surro-gate (only for similarly sized volumes) for temperature uniformity,air quality, or smoke clearance. The flow-per-person specificat