The Vapor Compression Cycle in Aircraft Air-Conditioning Systems

Transcription

1 FLECS_N_Vaor-Cycle Aircraft Design and Systems Grou (AERO Deartment of Automotive and Aeronautical Engineering Hamburg University of Alied Sciences (HAW Berliner or 9 D Hamburg e Vaor Comression Cycle in Aircraft Air-Conditioning Systems Cristian Müller Dieter Scolz (finalized ecnical Note 1

2 able of Contents age 1 Introduction Inner Structure of a Vaor Comression Cycle Validation of te Vaor Comression Cycle Control Asects of te Vaor Comression Macine Dynamic Simulation of a VCM Air-Conditioning System Comarison of an Air-Conditioning System Consisting of an ACM and a VCM Pack Model Alternative VCM Configurations References

3 1 Introduction e vaor ression cycle (VCC is te rincile of Vaor Comression Refrigeration Systems (VCRS known in aviation as te Vaor Cycle Macines (VCM. Conventional itioning systems eat ums and refrigeration systems tat are able to cool (or eat for eat ums and deumidify in a defined volume (e.g. a living sace an interior of a veicle a freezer etc. work on tis rincile. e vaor ression cycle is made ossible because te refrigerant is a fluid tat exibits secific roerties wen it is laced under varying ressures and temeratures. A tyical vaor ression cycle system is a closed loo system and includes a ressor a enser an exansion device and an orator. e various onents are connected via a uit (usually coer tubing. A refrigerant continuously circulates troug te four onents via te uit and will cange state as defined by its roerties suc as temerature and ressure wile flowing troug eac of te four onents. e main oerations of a vaor ression cycle are ression of te refrigerant by te ressor eat rejection by te refrigerant in te enser trottling of te refrigerant in te exansion device and eat absortion by te refrigerant in te orator. Refrigerant in te majority of eat excangers is a two-ase vaor-liquid mixture at te required ensing and orating temeratures and ressures. Some common tyes of refrigerant include R22 R134a and R410a. In following R134a is considered as refrigerant. In te vaor ression cycle te refrigerant nominally enters te ressor as a sligtly suereated vaor (its temerature is greater tan te saturated temerature at te local ressure and is ressed to a iger ressure. e ressor includes a motor (usually an electric motor and rovides te energy to create a ressure difference between te suction line and te discarge line and to force te refrigerant to flow from te lower to te iger ressure. e ressure and temerature of te refrigerant increases during te ression ste. e ressure of te refrigerant as it enters te ressor is referred to as te suction ressure and te ressure of te refrigerant as it leaves te ressor is referred to as te ead or discarge ressure. e refrigerant leaves te ressor as igly suereated vaor and enters te enser. A tyical -cooled enser rises single or arallel uits formed into a serentine-like sae so tat a lurality of rows of uit is formed arallel to eac oter. Altoug te resent document makes reference to -cooled ensers te invention also alies to oter tyes of ensers (for examle watercooled. Metal fins or oter aids are usually attaced to te outer surface of te serentine-saed uit in order to increase te transfer of eat between te refrigerant assing troug te enser and te ambient. A fan mounted roximate te enser for uming outdoor ambient troug te rows of uit also increase te transfer of eat. 3

4 As refrigerant enters a enser te suereated vaor first becomes saturated vaor in te first section of te enser and te saturated vaor undergoes a ase cange in te remainder of te enser at aroximately constant ressure. Heat is rejected from te refrigerant as it asses troug te enser and te refrigerant nominally exits te enser as sligtly sub-cooled liquid (its temerature is lower tan te saturated temerature at te local ressure. e exansion (or metering device reduces te ressure of te liquid refrigerant tereby turning it into a saturated liquid-vaor mixture at a lower temerature before te refrigerant enters te orator. is exansion is also referred as te trottling rocess. e exansion device is tyically a caillary tube or fixed orifice in small caacity or low-cost itioning systems and a termal exansion valve (XV or EV or electronic exansion valve (EXV in larger units. e XV as a temerature-sensing bulb on te suction line. It uses tat temerature information along wit te ressure of te refrigerant in te orator to modulate (oen and close te valve to try to maintain roer ressor inlet itions. e temerature of te refrigerant dros below te temerature of te indoor ambient as te refrigerant asses troug te exansion device. e refrigerant enters te orator as a low quality saturated mixture ( Quality is defined as te mass fraction of vaor x in te liquid-vaor mixture. A direct exansion orator ysically resembles te serentine-saed uit of te enser. Ideally te refrigerant letely boils by absorbing energy from te defined volume to be cooled (e.g. te interior of a refrigerator. In order to absorb eat from tis volume of te temerature of te refrigerant must be lower tan tat of te volume to be cooled. Nominally te refrigerant leaves te orator as sligtly suereated gas at te suction ressure of te ressor and reenters te ressor tereby leting te vaor ression cycle (It sould be noted tat te enser and te orator are tyes of eat excangers. An ressor driven by an electric motor is usually ositioned in front of te orator; a searate fan/motor combination is also usually ositioned next to te enser. Inside te orator te eat transfer is from te ressed ambient to te refrigerant flowing troug te orator. For te enser te eat transfer is from te refrigerant flowing troug te enser to te ambient. 4

5 2 Inner Structure of a Vaor Comression Cycle Figure 2.1: Caracteristic mas of vaor ression cycle model e algoritm of vaor ression cycle (VCC is based on te caracteristic mas sown in Figure 2.1. Using caracteristic mas ave te advantage tat te VCC model can be used easily for oter refrigerants. e enser R134a flow is cooled by a cold flow ( m & cold cold. inlet. Inside te Evaorator a ot flow ( m & ot ot inlet is cooled down to a target temerature ottarget. 1. Evaorator (see Figure 2.1: Cooling: Q& m& ot c ( ot inlet ot target Mass flow of te refrigerant: Q& m& R134a 1 4 5

6 6 Saturation emerature: ( 1 ( ( inlet ot target ot inlet ot sat + η Outlet Parameter: sat R134a R134a inlet target ot target ot inlet cold suereating suereating c c x Q 4 1 ''( '( ''( ( '( ( ''( + + & 2. Comressor (see Figure 2.1: Isentroic Comression: ( isentroic isentroic R1134a isentroic γ R134a R134a γ γ Outlet Parameter: ( ( 1 2 R134a isentroic isentroic R134a R134a isentroic m P c c + & η 3. Condenser (see Figure 2.1: Subcooling: inlet cold inlet cold - ( + η Outlet Parameter: cold inlet cold cold R134a inlet cold cold R134a cold sat cold c m Q m Q c m m 4 3 ( ( ''( ( & & & & & & +

7 Derivation Entaly: inlet Derivation Cold Air Mass Flow: m& cold target m& mm & & m& m& c R134a m& ( ( '' ( cold cold target inlet m& m& cold cold inlet cold inlet e secific entaly is calculated witout knowledgee of te calculated secific inlet entaly inlet. In general bot entalies aren t equal. erefore te derivation can be used to adjust te cold flow. e discussed algoritm is a non-closed cycle and can be used as dynamic vaor cycle model wit el of a set of different controllers (see section Figure 2.1: - -s resectiv vely - diagrams of R134a refrigerant 7

8 3 Validation of te Vaor Comression Cycle e VCC algoritm discussed above can be validated wit el of an examle given in [Baer 2006] (see able 3.1. able 3.1: Validation Exam le [Baer 2006] e examle sown in able 13 is related wit water cooling of te R134a Flow inside te orator and an ambient temerature ambient coldinlet of 18 C. Ensuring a sufficient cooling in te case of cooling an flow of & 8 kg/s as to be assumed. e mass flow of te ot is fixed to m& m ot 0.5 kg/s m cold able 3.2: Set of efficiencies Efficiencies: Heat ransfer Efficiency Evaorator η : 0.90 Isentroic Efficiency Comressor η : 0.78 Heat ransfer Efficiency Condenser η : 0.80 e algoritm can be validatedd using te set of efficiencies sown in able 3.2. e results of te simulation are sown in Figure

9 Figure 3.1: Validation of te VCC algoritm wit el of an examle given in [Baer 2006] 9

10 4 Control Asects of te Vaor Comression Macine e general control structure of an craft ECS is sown in Figure 4.1. Firgure 4.1: Control configurationn of an environmental control system Vaor Cycle Macine (VCM Comressor Controller: is controller controls te ressor ressure ensuring a closed refrigerant loo ( Q & Q & + P. e controller is caracterized by a resonse time τ vcm. e ressor controller is a art of te inner structure of te VCM model. Using te VCM model as an craft -itioning ack additionally tree oter controllers ave to be defined. Ram Air Fan Rotational Seed Controller: is controller controls te rotational seed of te ram fan. In general as iger te rotational seed as iger te ram mass flow. e ram mass flow and te ram inlet temerature define te cooling caacity of te R134a flow inside te enser. As actuating variable te value is used. e controller is caracterized by a resonse time τ n fan. Flow Controller: is controller controls te volume flow of te ot bleed to a target value V & target. e control variable is te oening angle β FCV of flow control valve (FCV. e iger te rotational seed te iger te ram mass flow. e ram mass flow and te ram inlet temerature define te cooling caacity of te R134a flow inside te enser. As actuating variable te value is used. e controller is caracterized by a resonse time 10

11 τ FCV. Knowing te volume flow V & target te mass flow m& FCV troug te flow control valve can be calculated using Equation 8. cabin m& FCV Vt arg et ( Air Comressor Rotational Seed Controller: is controller controls te rotational seed of te ressor assuring a ressure cabin Pa and a temerature 50 C K. e controller is caracterized by a resonse time τ. 11

12 5 Dynamic Simulation of a VCM Air-Conditioning System Wit el of te discussed algoritm and control asects a VCM -itioning system can be build u (see Figure 5.1. Additionally to te VCM model a VCM -itioning system requires a ram cannel and an ressor. Figure 5.1: Arrangement for te vaor ression system inside -itioning system e dynamic beavior of te VCM -itioning system is caracterized by a set of efficiencies (see able 5.1 and a caracteristic ma of te ressor (see Figure 5.2. able 5.1: Set of efficiencies Efficiencies: Heat ransfer Efficiency Evaorator η : 0.80 Isentroic Efficiency Comressor η : 0.80 Heat ransfer Efficiency Condenser η : 0.80 e target value V & target of te flow controller is fixed to 0.31 m³/s. e target temerature of te bleed is given as inut variable of te simulation (see Figure 5.3. e resonse times 12

13 of te different controllers are τ vcm τ n fan 20 s and τ FCV τ 5 s. ree different test cases (1: ISA Cold Day 2: ISA Standard Day 3: ISA Hot Day are discussed. Figure 5.2: Caracteristic m as of te ressor Figure 5.3: e target temerature of te bleed 13

14 1. ISA Cold Day: Ambient Pressure: Pa Ambient emerature: K -23 C 2. ISA Standard Day: Ambient Pressure: Pa Ambient emerature: K 15 C 3. ISA Hot Day: Ambient Pressure: Pa Ambient emerature: K 38 C Results: Figure 5.4: e dynamic beavior of te VCM -itioning system under different ambient itions 14

15 Figure 5.5: e dynamic beavior of te VCM -itioning system under different ambient itions Comments: In te case tat te system is controlled to a ressure < te VCM ressor ower consumtion P is set to zero (see Figure 50g and Figure 50i ISA Cold Day. e variable can be used to ceck if te refrigerant loo is closed. In te case ISA Cold Day and ISA Standard Day te relative derivation is smaller tan 5 % (see Figure 5.6 and terefore te loo is fully or almost closed. In te case ISA Hot Day is significant iger tan 5 %. erefore te loo is only artially closed. Under tis ambient itions a cascade refrigeration cycle sould be used (see Figure

16 Figure 5.6: e relative derivation under different ambient itions is cycle will leads to a decrease in te temerature at te oint 3 meaning increasing cooling caacity. Additionally tis configuration will minimize te ower consumtion of te ressor. Wit tis cycle it is not an obligation to use te same refrigerant. It te refrigerants are different te aroriate -s diagram as to be used. Figure 5.7: A cascade refrigeration cycle 16

17 6 Comarison of an Air-Conditioning System Consisting of an ACM and a VCM Pack Model In tis section te erformance of a -itioning consisting of an ACM ack is ared wit a system consisting of a VCM ack. e arameterization of te overall system e.g. te trim system or te cabin model is already mentioned in Section 3.2 of ecnical Note FLECS_W3.2_5.1_5.2_5.3_N (confidential [Müller 2008]. e dimension of te ram inlet is given in able 12 of Müller Inside te VCM systems te area of te ram inlet is fixed to te maximum value (ζ 1. e temerature of te VCM model sows a steady state beavior (see Figure 5.4. e temerature is an inut value of te simulation and is fixed by te ack controller (see Figure 11 of Müller o are bot systems also te ACM ack model as to be defined in a steady state configuration. A steady state model can be described by mass flow source. e temerature is fixed by te ack controller. e mass flow can be calculated wit el of Equation 1 ( V & target 0.34 m³/s. Inside te ECS system te rotational seed of te recirculation fan is fixed to /min assuring a ratio m & / m 1. e floor temerature inside te cabin model is always recirculation ack 10 C smaller tan te average temerature of te cabin ( floor 0.5 ( zone1 + zone2 10 C. Resonse time controller: VCM System: VCM Comressor Controller: 20 s Ram Air Fan Rotational Seed Controller: 20 s Flow Controller: 5 s Air Comressor Rotational Seed Controller: 5 s Global Parameter: On ground: In fligt: Bleed temerature: rim ressure: rim temerature (ACM: rim temerature (VCM: ISA standard day ( ambient 15 C ambient Pa ISA itions 200 C K cabin Pa 200 C K 17

18 Aircraft Mission: ime: 0 s 1200 s e craft is on ground. e boarding starts. e target temerature of te cabin is 21 C. In 20 minutes 200 assengers enter te cabin (see Figure 5.5 assuming a constant flow of assengers. e eat load wic flows into te cabin increases significantly. ime: 1200 s 2250 s 1200 s: e boarding is leted. e craft starts. In 1050 s te craft climbs to a fligt altitude of ft (climb rate 2000 ft/min (see Figure 5.4. e ambient itions are described by an ISA ition. Knowing te ambient temerature ambient te skin temerature skin can be calculated ( skin ambient ( Ma² Ma: Mac number. e target temerature of te cabin zone 1 is 22 C K te target temerature of te cabin zone 2 is 24 C K. e cabin ressure is fixed to Pa. ime: 2250 s 5850 s Cruise: e target temerature of te cabin zone 1 is 22 C K te target temerature of te cabin zone 2 is 24 C K. e cabin ressure is fixed to Pa. ime: 2250 s 6900 s Descent: In 1050 s te craft wit a rate of 2000 ft/min. e target temerature of te cabin zone 1 is 22 C K te target temerature of te cabin zone 2 is 24 C K. e rofiles of te craft altitude cabin ressure Mac number and skin temerature are sown in Figure 5.4. In fligt te minor loss coefficient of te ram inlet is fixed to 1.5 (see Equation 6 of Müller e simulated temerature rofiles in zone 1 and zone 2 are sown in Figure 5.4a. te trim oening angle is sown in Figure 5.4b. e ower consumtion of te ACM system can be calculated using a entaly equation (see Equation 2 and Figure 5.4c. P ACM m& c (2 ack ( bleed ack e overall ower consumtion of te VCM system is te sum of te ower consumtion of te ram fan P fan te ressor P and te VCM Comressor P (see Equation 3 and Figure 5.4c. P P + P + P (3 VCM fan 18

19 Figure 6.1: ime deendant ambient itions General benefits of te cycle system are: low weigt low maintenance costs ig reliability low costs roblem-free wit leakages eating and cooling caability delivery of sub-freezing (enables to meet ig dynamic cooling demands as medium ensures automatically environmental atibility oeration over wide temerature ranges General benefits of te vaor cycle system are: ig termodynamic efficiency low ower inut more electric system can oerate from ground electrical ower low noise level. 19

20 General ECS selection criteria are: An cycle system is best at o ig required cabin mass flows o low ower and bleed enalties. A vaor cycle system is best at o low required cabin mass flows o ig ower and bleed enalties. Quantitative results of fuel enalties due to selected ECS systems need to be calculated. Furtermore ECS systems can be ared based on Direct Oerating Costs defined for craft systems (DOCsys (Scolz

21 7 Alternative VCM Configurations e standard VCM arrangement was already sown as Figure 5.1 and is reeated ere as Figure 7.1 called Configuration 1. Alternative VCM Configurations 2 3 and 4 are sown as Figure 7.2 Figure 7.3 and Figure 7.4. Figure 7.1: VCM integrated into an craft ECS in a standard way (Configuration 1 Figure 7.2: VCM integrated into an craft ECS in an alternative way (Configuration 2 21

22 Figure 7.3: VCM integrated into an craft ECS in an alternative way (Configuration 3 Figure 7.4: VCM integrated into an craft ECS in an alternative way (Configuration 4 22

23 References Baer 2006 BAEHR Hans Dieter: ermodynamik. Berlin : Sringer Müller 2008 Scolz 1998 MÜLLER Cristian: FLECS - Funktionale Modellbibliotek des Environmental Control Systems : Alications. ecnical Note. Aircraft Design and Systems Grou (AERO Deartment of Automotive and Aeronautical Engineering Hamburg University of Alied Sciences (HAW DocumentID: FLECS_W3.2_5.1_5.2_5.3_N SCHOLZ Dieter: DOCsys - A Metod to Evaluate Aircraft Systems. In: SCHMI D. (Ed.: Bewertung von Flugzeugen (Workso: DGLR Facausscuß S2 - Luftfartsysteme Müncen 26./27. Oktober Bonn : Deutsce Gesellscaft für Luft- und Raumfart Download from: tt://l2.dglr.de tt:// tt:// 1998_SCHOLZ_DOCsys_Eine_Metode_zur_Bewertung_von_Flugz eugsystemen.df 23