2.1 The ideal vapour-compression cycle

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1 Refrigeration (Kylteknik) course # E v Vapour-compression refrigeration processes Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory / Värme- och strömningsteknik tel. (02 215)3223 ; ron.zevenhoven@abo.fi Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 ÅA Refrigeration / Kylteknik 2.1 The ideal vapour-compression cycle Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

2 Entropy balance : Q L Q H S gen TL TH Reversible : Q L Q H TL TH Q L TL Q T H H Reversed Carnot cycle /1 1-2 and 3-4: reversible and isothermal (~ heat) 2-3 and 4-1: Isentropic (~ work) maximum thermal efficiency η th = 1 Q H /Q L if reversible η th = 1-T H /T L in T,s diagram: liquid-vapour saturation dome Picture: ÇB98 Condensation / evaporation of a fluid can be done at almost any temperature/pressure combination, unlike freezing / melting, and involves greater heat effects (ΔH vaporisation >> ΔH melting ) for example: water The Carnot power cycle can be executed in reverse within the saturation dome of a refrigerant fluid Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Reversed Carnot cycle /2 1-2 and 3-4: reversible and isothermal 2-3 and 4-1: isentropic maximum thermal efficiency η th = 1 Q H /Q L if reversible η th = 1-T H /T L liquid-vapour saturation dome Picture: ÇB98 The (reversed) Carnot cycle is the most efficient cycle operating between two temperature levels. But: process 2-3 involves compression of a two-phase mixture, and process 4-1 involves expansion of wet refrigerant Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

3 Ideal vapour-compression cycle /1 Operating the Carnot cycle outside the saturation region no isothermal conditions, for heat absorption and rejection Picture: ÇB98 Q H = 2 3 Tds Q L = 4 1 Tds Expansion step (3-4) can be simplified by using a throttling valve (or a capillary tube) This results in a process with 3 reversible steps, and 1 irreversible step Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 T,s diagram (here for H 2 O) LINES OF CONSTANT ENTHALPY IN THE SATURATION REGION isenthalpic lines Pictures: SEHB06 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

4 Ideal vapour-compression cycle /2 Step 4-1: boiling of refrigerant at low p and T Step 1-2: compression of saturated vapour to high p and T Step 2-3: high pressure superheated gas is cooled to saturated liquid at high T, high p Step 3-4: expansion to low p, also T down (due to some evaporation) Note: sub-cooling a bit beyond (3) reduces the risk of flashing in the evaporator Picture: ÇB98 For each step: (Q in -Q out ) + (W in -W out ) +. m refr (h in -h out ) = 0 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Pressure levels A freezer at -18 C in a room at 21 C Operation pressures for evaporator and condensor are the vapour pressures for T cold and T hot for the refrigerant Reversible if cold reservoir T low = T cold, hot reservoir T high = T hot For R-134a, p sat = C, C 0 F = -18 C 70 F = 21 C 250 F = 121 C R-134a Reversible: T refrigerant = T reservoir T high = 21 C, T low = -18 C COP R = 1 / (T high /T low -1) = 6.6 Picture: T06 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

5 Example: ideal vapour-compression cycle /1 A vapour-compression refrigeration cycle uses refrigerant R-134a at pressure levels p 1 = 1.4 bar and p 2 = 8 bar, respectively, with mass flow ṁ = 0.05 kg/s. Calculate: The rate of heat removal Q L and compressor power input W in The rate of heat rejection Q H and the COP R of the refrigerator Source & picture: ÇB98 Answer: data for R-134a gives T low = C, T high = 31.3 C, for (1) h 1 = h g = kj/kg; s 1 = s g = kj/(kg.k); for (2) s 2 = s 1 gives h 2 = kj/kg, for (3) h 3 = h f = kj/kg, s 3 = kj/(kg.k); for (4) h 3 h 4, Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 R134a data: saturation pressure leforsaturatedr-134a-pressure.pdf Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

6 R134a data: saturation temperature 100 C leforsaturatedr-134a-temperature.pdf Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 R134a data: superheated vapour 1.6 MPa rsuperheatedvaporofr-134a.pdf Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

7 Example: ideal vapour-compression cycle /2 Answer (cont.):. Q L = m (h 1 -h 4 ) = 7.13 kw. W in = m (h 2 -h 1 ) = 1.80 kw. Q H = Q L + W in = 8.93 kw. COP R = Q L / W in = 3.96 = (h 1 -h 4 )/(h 2 -h 1 ) Source & picture: ÇB98 Comment: Replacing the throttling valve (3 4) by an isentropic turbine (3 4s) gives, with h 4s = kj/kg a turbine power output of 0.34 kw, reducing the net power input W in to 1.46 kw. The removal of heat from the refrigerated space Q L increases from kw to m (h 1 h 4s ) = 7.46 kw. COP R increases from 3.96 to 5.11, an increase of 29%. Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 ÅA Refrigeration / Kylteknik 2.2 Household refrigerators Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

8 Household refrigerator /1 Four Main Components: Compressor, which increases the pressure of the refrigerant vapour, pushing it through the system, and increasing the vapour's temperature above that of the surrounding kitchen. Condenser, usually behind the refrigerator, where the refrigerant vapour condenses to a liquid. Expansion valve, which causes a sudden drop in refrigerant pressure, causing it to boil; also called a "metering" valve, since it passes only as much liquid as can be completely vaporised in the evaporator. evaporator, where the latent heat of refrigerant vaporisation is absorbed from the cold box. Picture & text: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Household refrigerator /2 Picture: ÇB98 Picture: T06 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

9 Irreversible heat transfer COP R,rev = 6.6 A freezer at -18 C in a room at 21 C Heat transfer TO the refrigerant in evaporator and FROM the refrigerant in condensor requires a temperature difference R-134a T hot T cold T cold 1 C or T hot 1 C gives COP by 2-4 % T surr T hot T cold T cold space ΔT, say, ΔT = 10 C T cold = -28 C (p sat = 0.93 bar), T hot = + 31 C (p sat = 7.93 bar) for the refrigerant 0 F = -18 C 70 F = 21 C 250 F = 121 C Irreversible, real: T refrigerant T reservoir ; if ΔT =10 C T cold = -28 C, T hot = +31 C COP R = 1 / (T hot /T cold -1) = 4.2 Picture: T06 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Temperature rise ( lift ) for heat transfer (here: air cooling) Picture: HTW08 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku

10 ÅA Refrigeration / Kylteknik 2.3 Pressure - enthalpy diagrams Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Pressure, enthalpy diagrams In a p, h diagram 1. the vapour-compression refrigeration cycle gives straight lines for 3 of the 4 steps, and 2. the heat transferred (Q H, Q L ) is proportional to the length of the lines COP COP R HP QL Win Q W H in h h h h h h h h h1 p1 and h3 p3 for the ideal case also possible: sub-cooling s s isentropic Picture: ÇB98 The corresponding Carnot cycle Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

11 p,h diagram R-134a Picture: ÇB98 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 p,h diagram R-134a Picture:Ö96 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

12 p,h diagram R-717 (NH 3 ) Picture:Ö96 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 p,h diagram R-22 Picture:Ö96 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

13 p,h diagram R-12 Picture:Ö96 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 p,h diagram R-744 (CO 2 ) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

14 p,h diagram R-407c ÅA VST heat pump Note sloping lines for boiling / condensation A zeotropic blend of difluoromethane (R-32), pentafluoroethane (R-125), and 1,1,1,2-tetrafluoroethane (R-134a) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 ÅA Refrigeration / Kylteknik 2.4 The real vapour-compression cycle Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

15 Real vapour-compression cycle /1 In a real refrigerator quite a few irreversibilities reduce the efficiency: Fluid friction (gives heat ) Heat exhange with the surroundings Picture: ÇB98 The real process differs a bit from the ideal process: To ensure complete vaporisation, the refrigerant is slightly overheated at the evaporator inlet (8) A (long) line between evaporator and compressor gives fluid friction and heat exchange with surroundings (8 1) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Real vapour-compression cycle /2 More differences compared to the ideal process: The compression is not isentropic: s > 0 (1 2) or s < 0 (1 2 ) by cooling, decreasing the volume! Picture: ÇB98 There will be some pressure drop between compressor and condensor, in the condensor, between condensor and throttling device (2/2 4 5) and in the evaporator The saturated liquid will be sub-cooled before going to the throttling device, located near the evaporator. Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

16 Example: real vapour-compression cycle /1 A vapour-compression refrigeration cycle uses refrigerant R-134a with mass flow ṁ = 0.05 kg/s. Vapour enters the compressor at -10 C, 1.4 bar and leaves it at 50 C, 8 bar. The vapour enters the condenser at 7.2 bar and is cooled to 26 C. The throttling valve reduces the pressure to 1.5 bar. Calculate: The heat removal Q L and the compressor power W in The adiabatic (isentropic) efficiency of the compressor The COP R value Picture: ÇB98 Neglect the heat tranfer and pressure drops in connecting lines Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Example: real vapour-compression cycle /2 At p 1,T 1 : h 1 = kj/kg At p 2,T 2 : h 2 = kj/kg At p 3,T 3 : h 3 h f = kj/kg h 4 h 3 Picture: ÇB98 Q L = ṁ (h 1 -h 4 ) = 7.88 kw W in = ṁ (h 2 -h 1 ) = 2.05 kw Adiabatic eff. of compressor η c = (h 2s h 1 )/(h 2 -h 1 ) p 2s = 8 bar, s 2s = s 1, h 2s = kj/kg gives η c = Finally, COP R = Q L /W in = 7.88 kw / 2.05 kw = 3.84 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

17 ÅA Refrigeration / Kylteknik 2.5 Refrigerants for vapour-compression refrigerators Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Refrigerants, freezing mixtures In a refrigeration process, energy is converted into transferred heat, using a heat carrier. The heat carrier medium will take up the heat at a low temperature (and pressure) and gives it off at higher temperature (and pressure) at another location A refrigerant (sv: köldmedie, kylmedel) participates in the process by a phase transition and/or pressure changes. It can also be electricity! A cooling or freezing mixture (sv: köldblandning) can carry or store heat, which can involve a phase transition, but little or no pressure changes. Coolant for an engine: not a refrigerant... Picture: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

18 Refrigerants for vapourcompression (v-c) systems /1 T critical > T process, maximum and T melt < T process, minimum Reasonable pressure levels p sat at T boil and T condens Large Δh vaporisation/condensation ( latent heat ) per unit volume Safe handling, non-toxic, no smell Low cost Chemically stable Should not be problematic when contacting water, oil, air when contacting metals, rubber or other polymers at high temperatures (non-flammable!) for the environment: ozone layer depletion, the enhanced greenhouse effect Picture: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Refrigerants for v-c systems /2 Most important: the temperature levels of the cold and hot spaces with which the refrigerant exchanges heat Temperatures at the condensor ranges from -20 C (cold winter air) to +85 C (heat pumps) At the lowest temperature the refrigerant should have enough pressure to allow for 1) transport to the evaporator (and compressor), 2) proper operation of the throttling device and 3) avoid air leakage into the system in practice a bit > 1 bar At the highest temperature the pressure should not be so high that expensive pressure vessels and tubing elements are needed in practice below 20 bar, preferably. Picture: Picture: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

19 Refrigerants for v-c systems /3 R-codes Used / found in refrigeration systems (see also D03, TW00): CFCs (chloro fluoro carbons), HCFCs (hydro chloro fluoro carbons), HFCs (hydro fluoro carbons) mostly CFCs: R-11 in water chillers in building air conditioning, R-12 in domestic refrigerators, in automotive air conditioning, R-22 in air conditioning, in industrial refrigeration, R-134a replaces R-12, R-502 (R-115 / R-22 mix) in supermarket refrigeration Ammonia primarily in food refrigeration; other inorganics (R-7xx) Hydrocarbons (C 3, C 2, C 2=...) (R-6xx) (Non-)Azeotropic mixtures R-4xx and R-5xx, respectively Inorganics R-7yy, yy = molar mass (g/mol): NH 3 R-717, CO 2 R-744 making a return; used in aircraft Air also used in aircraft; and also: Water Not used any longer: ethyl ether, MeCl, SO 2 Halogenated hydrocarbon R-code: rightmost digit = no. of F, 10-digit = 1+no. of H, 100-digit = -1+no. of C, 1000-digit = no. of double bonds, a indicates isomer unbalance, the rest is Cl, B = no. of Br. Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Picture: Examples R- codes R-11 F=1, H+1 = 1, C-1 = 0, rest is Cl CFCl 3 R-134a: F = 4, H = 2, C = 2, a: assymmetric C 2 H 2 F 4 CF 3 -CFH 2 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku

20 Refrigerant vapour pressure Picture: S90 Vapour pressures of gases and refrigerants Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Some refrigerant data Gas Refrigerant T boil C * Gas Refrigerant T boil C * (C 2 H 5 ) 2 O R CCl 3 F R SO 2 R CCl 2 F 2 R CH 3 Cl R CHClF 2 R CH 2 Cl 2 R C 2 Cl 3 F 3 R NH 3 R C 2 Cl 2 F 4 R CO 2 R C 2 ClF 5 R CH 4 R CF 3 CH 2 F R-134a -26 C 2 H 6 R i-c 4 H 10 R-600a -12 CHClF 2 + C 2 ClF 5 ** hydrocarbon mix * for pressure = 1 bar ** azeotrope R HC-12a -33 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

21 Refrigerants for v-c systems /4 Boiling temperatures for 1 bar and 20 bar Ammonia: -33 C and +50 C R12: -30 C and +70 C R11: +25 C and +140 C R114: +5 C and +120 C R134a: -26 C and + 68 C Heat of vaporisation and density at 0 C: Ammonia: 1260 kj/kg, 3.45 kg/m kj/m 3 R22: 207 kj/kg, kg/m kj/m 3 (volumetric heat of vaporisation) Picture: Pure_Brand_New_R134a_Refrigerant_In_DOT_Or_Normal_Cylinders.jpg Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Greenhouse gases (GHGs) Greenhouse gases (GHGs), most importantly carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) trap the outgoing solar radiation that is emitted by the earth s surface, which leads to global warming Note that water causes ⅔ of the greenhouse effect; the changing amounts of other GHGs cause an enhanced greenhouse effect Other GHGs and their global warming potential (GWP, CO 2 = 1 by definition) CH 4 (~22), N 2 O (~300) HFCs (hydro fluoro carbons) ( ) PFCs (per fluoro carbons) (7400) SF 6 (23900) Source: ZK01 Picture: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

22 Ozone depleting substances (ODS) ODS substances do not have a direct global warming effect but influence the formation/ destruction of tropospheric/ stratospheric ozone Most important: CO, NOx, non-methane VOCs (volatile organic compounds) Class I ODS (Ozone Depleting Potential, ODP ) Carbon tetrachloride, methyl chloroform, halons C n F x Cl y Br z CFCs are replaced by non-ods (but GHG!) compounds: HFCs, PFCs, SF 6 Class II ODS (ODP << 1) HCFCs (hydrogenated chloro fluoro carbons) ODP = (definition) 1 for CFC-11 (R-11) Source: ZK01 Picture: wc.notes/1.atmosphere/ozone_depletion.2.htm Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Refigerant use in Finland Most important: CFCs R11, R12; HFC R134a (R-22 belongs to HCFC group) Finnish decision 1990: use of CFC forbidden except in special cases EU legislation: production and import/expert of CFCs forbidden as of 1995, as a well as putting CFC containing products on the market HCFC use (mainly R-22) is phased out Alternatives should be found for HFCs also (mainly R-134a and R- 400-types): Kigali agreement Oct e.g. CFCs, HCFCs and HFCs are hazardous wastes Special regulations as to the handling of CFCcontaining coolers, freezers, and isolation materials (R-11 in poly urethane foam!) In the future, more use of iso-butane (R-600a), propane, propene, CO 2 and ammonia End-of-life refrigerator handling at Ekokem Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Pictures: Sources: Ö96, D03, SKL06/12

23 Refrigerant properties LT = C; MT = C; HT = C Table: HTW08 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku Refrigerant selection and COP (compared to R22; air conditioning with evaporator at T = 5 C) Picture: HTW08 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku

24 ÅA Refrigeration / Kylteknik 2.6 Special vapour-compression refrigeration systems Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Cascade vapour-compression system In industry, efficiency may be more important than simplicity Sometimes the temperature range is too wide for a single v-c cycle use a cascade cycle (with several refrigerants) Two cycles, a bottoming cycle and a topping cycle are connected via a heat exchanger For the heat exchanger without heat losses or kinetic / m potential energy effects, and mass streams m A, m B : A (h h ) m B (h m h) m A B h h h h ; COPR Q W m Picture: ÇB98 m B (h h ) (h h ) m (h h ) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 L net,in One figure if the same refrigerant used in both cycles. A. B

25 Example: 2-stage vapour-compression system Consider the system in the Figure: a cascade v-c refrigerator operating between 1.4 and 8 bar with R-134a as refrigerant. The heat exchanger operates at 3.2 bar for both streams. (In practice p and T are a bit higher in the. bottom cycle.) Mass stream m A = 0.05 kg/s. Calculate. mass stream m B, the heat stream Q L taken from the refrigerated space. compressor power W in the COP R for the process Q L m B ( h 1 h m 4 A ) 7.13 kw; COP ( h R 5 h 8 Q W L ) m W B in m ( h 2 W h ) m in,top 3 W in,bottom h h m B ( h1 h4 ) ( h h ) m ( h m h h ( h h ) net, in A 6 5 B 2 1 B 5 2 A kg/s; h ) m 7.13 kw 1.60 kw ( h 4.46 Picture: ÇB98 h ) 1.60 kw Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku / m 6 A 5 B stage v-c refrigeration with sub-cooler Pictures: HTW08 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku

26 2-stage compression refrigeration In a cascade system using one refrigerant, a mixing chamber (flash chamber) can be used instead of a heat exchanger Picture: ÇB98 Referred to as multistage compression refrigeration systems Saturated vapour from the flash chamber is fed to the high pressuire compressor, saturated liquid is fed to the low pressure expansion valve Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Example: 2-stage compression refrigeration /1 Consider the system in the Figure: a cascade c-v refrigerator operating between 1.4 and 8 bar with R-134a as refrigerant. The refrigerant leaves the condenser as saturated liquid and is throttled to a flash chamber at 3.2 bar. The vapour product is mixed with the refrigerant leaving the low pressure condenser. Assuming that both compressors are isentropic and that the refrigerant leaves the evaporator as saturated vapour: (continues) Picture: ÇB98 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

27 Example: 2-stage compression refrigeration /2 Calculate The mass fraction, x, ( quality ) of the refrigerant that is evaporated when throttled to the flash chamber The amount of heat that is removed from the refrigerated space and the compressor work per unit mass refrigerant flowing through the condenser, q L and w, and The COP R for the system; using the given T,s plot Picture: ÇB98 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Example: 2-stage compression refrigeration /3 The mass fraction, x, of refrigerant evaporated as it is throttled to the flash chamber equals x 6 = (h 6 -h f )/ (h g -h f ) =(h 6 -h 7 )/(h 3 -h 7 ) = The amount of heat removed from the refrigerated space per unit mass equals q L = Q L / m = (1-x 6 ) (h 1 -h 8 ) = kj/kg Enthalpy h 9 follows from h 9 = x 6 h 3 +(1-x 6 ) h 2 = kj/kg With s 9 = kj/(kg K) = s 4 (at 8 bar) it follows from the data tables for R-134a that h 4 = kj/kg Compressor work w in = (1-x 6 ) (h 2 -h 1 )+(h 4 -h 9 ) = 31.8 kj/kg COPR = q L /w in = / 31.8 = 4.56 Picture: ÇB98 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

28 Multi-purpose refrigeration with a single compressor Picture: ÇB98 Refrigeration at more than one temperature (as in an ordinary household refrigerator + freezer) can be accomplished with one compressor by throttling in two steps Using one throttle valve and one cold temperature would give ice in the refrigerator section. Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Trans-critical CO 2 cycle Evaporation at -10 C, ~26 bar, gas cooling at C, at ~100 bar Picture: HTW08 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku

29 ÅA Refrigeration / Kylteknik 2.7 Real vapour-compression cycles and p,h diagrams Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Real v-c refrigeration process A real vapourcompression refrigeration process in a p, h diagram: 1s = throttle valve in 2s = throttle valve out 2i = evaporator in 2u = evaporator out 2k = compressor in 1k = compressor out 1i = condenser in 1u = condenser out Includes pressure drop over connection lines 2u-2k and 1k-1i; heat exchange with surroundings and in the compressor Picture: Ö96 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

30 A commercial v-c refrigerator Picture: D03 Using a water-cooled condensor and a heat exchanger Temperature, pressure and heat of vaporisation can be optimised Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Vapour-compression refrigeration process with superheat / subcooling Picture: D03 Heat exchange between evaporator outlet and condensor outlet can improve the COP R value. Superheating by increased compressor pressure gives no improved efficiency but only results in larger condensor equipment Subcooling also ensures 100% liquid to the throttling valve and gives either more heat extracted from the refrigerated space, or a smaller required refrigerant mass flow Less attractive if the suction line to the compressor is long, especially when using ammonia as refrigerant Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

31 Two-stage compression refrigeration Picture: Ö96 compressor Especially suitable for wide temperature ranges while still using one refrigerant at acceptable vapour pressures (a one-stage +10 C/-30 C unit can reach -65 C with two stages or -100 C with three) With minimum and maximum pressures p 1 and p 2 it can be shown that the optimum intermediate pressure level p m = (p 1 p 2 ) Disadvantages are lower efficiency, higher power input, increased temperature of refrigerant vapor from first compressor Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Cascade v-c systems /1 A two-stage cascade uses two different refrigerants and heat exchange Allows for a lower temperature than with a single-stage system Typically -150 C can be reached Compressor work decreases COP improves Condenser B of system I is cooled by evaporator C of system 2 Picture: D03 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

32 Cascade v-c systems /2 Cascade systems are commonly used for CO 2 or natural gas liquefaction Pictures: D03 Linde-Hampson system Intercooled compression Picture: ÇB98 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 ÅA Refrigeration / Kylteknik 2.8 Final remarks Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

33 Defrosting, purging air Defrosting is necessary from time to time to remove ice (from air humidity) An effective method is to use hot refrigerant gas from compressor; otherwise warm air, water or electricity can be used Picture: D03 Air leaking into the system lowers the efficiency (usually being immiscible with the refrigerant it acts as an insulator at heat transfer surfaces, making the condensor smaller ) Manual or automatic purging methods can remove this air Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Tons of refrigeration For refrigerators used for producing ice, one way to express the capacity is as tons of refrigeration 1 ton of refrigeration = heat needed to freeze 1 short ton (= 2000 lbm = 907kg) water at 0 C to ice at 0 C in 24 hours 1 ton of refrigeration = 211 kj/min = 200 BTU/min = 3.52 kw heat removal from the refrigerated space See also A11 p. 99 Picture: Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

34 Heat exchanger irreversibilities (vs91) A simple steady-state heat transfer process; heat is transported from medium 1 to medium 2 by conduction through a material that separates them. Temperature T 1 > T 2 Thermodynamic analysis Energy balance Q Q Entropy balance Q Q S gen T T S gen Q T Q T T T T T. Q 1. Q 2 T = T 1 T = T 2 This shows that S gen is large for large temperature differences (T 1 -T 2 ) and low temperatures T 1 and T 2. Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 ÅA Refrigeration / Kylteknik 2.9 Vapour-compression cycle heat pumps Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

35 Heat pumps using v-c cycle A heat pump vapour-compression system with reversing valve for summer / cooling (a) or winter / heating operation (b) NOTE: COP HP = COP R +1 Pictures: KJ05 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Heat pumps in Finland (2013/2014) Total capacity (2013/2014): HPs using 4 TWh year around buildings GSHP = ground-source HP Source / picture: (accessed: ) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72

36 Heat pumps in Finland ( ) Heat pumps: to be continued Total capacity (2015): ~ HPs using ~ 5 TWh year around buildings Source / picture: (accessed: ) Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Sources #2 A11: R. C. Arora Refrigeration and air conditioning, 2nd. Ed. PHI Learning Private Limited, New Delhi (2011) CB98: Y.A. Çengel, M.A. Boles Thermodynamics. An Engineering Approach, McGraw-Hill (1998) D03: İ. Dinçer Refrigeration systems and applications Wiley (2003) HTW08: G.F. Hundy, A.R. Trott, T.C. Welsh Refrigeration and air conditioning 4 th ed. Butterworth-Heinemann (2008) KJ05: D. Kaminski, M. Jensen Introduction to Thermal and Fluids Engineering, Wiley (2005) SEHB06: P.S. Schmidt, O. Ezekoye, J. R Howell, D. Baker Thermodynamics: An Integrated Learning System (Text + Web) Wiley (2006) S90: A.L. Stolk Koudetechniek A1, Delft University of Technology (1990) SKL06/12: Suomen Kylmäliikkeiden Liitto (2006, 2012) T06: S.R. Turns Thermal Fluid Sciences, Cambridge Univ. Press (2006) TW00: A.R. Trott, T.C. Welsh Refrigeration and Air-Conditioning 3rd Ed. Butterworths-Heineman (2000) ZK01: R. Zevenhoven, P. Kilpinen Control of pollutants in flue gases and fuel gases Picaset (Espoo), 2001 (Chapter 9) Ö96: G. Öhman Kylteknik, Åbo Akademi University (1996) See also: Martinez, I. Lectures on Thermodynamics lecture 18 (English or Spanish) updated and based on Termodinámica básica y aplicada", Ed. Dossat, Madrid (1992) ISBN Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, Turku /72 Picture: