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  1. 1. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-493 Effects of refrigerant properties on refrigerant performance comparison: A review Rotchana Prapainop,1,* K O Suen2 1 Department of Mechanical Engineering, Kasetsart University, Chonburi, Thailand 2 Department of Mechanical Engineering, University College London, London, United Kingdom The effects of thermo-physical properties and changes due to compatibility issues, and “new system”derived parameters of refrigerants on system - replacement of old systems with new ones designedperformance comparison are reviewed here. The specifically for the alternative refrigerant (MPROG:properties reviewed are density, viscosity, thermal BNCR35).conductivity, molar heat capacity, critical For new systems, components can be designed andtemperatures, specific refrigerating effect and sized to suit individual refrigerants to achieve themolecular weight. It was noted that to obtain a high required system capacities at specified refrigerantCOP, combinations of high values of latent heat, temperatures (AREP , 1994). For examples,liquid thermal conductivity and vapour density and refrigerants with relatively poorer heat transferlow values of liquid viscosities and molecular weight characteristics will require a larger heat exchanger areaare required. Critical temperature and vapour (Calm and Domanski, 2004) or better designed heatspecific heat are important properties when exchangers (Lee and Yoo, 2000, Domanski et al.,considering trade-offs between capacity and COP. 2005) to achieve the same capacity. On the other hand,The oil existance in the system could reduce or refrigerants with a high volumetric refrigerating effectincrease heat transfer or/and pressure drop (or volumetric refrigerating capacity, kJ/m3) cancomparing to those ignore the presence of oil employ smaller compressors (Hwang et al., 2005). Independing on the amount of oil and solubility. such cases, the capital cost per unit capacity and the COP will probably be one of the main interests for theIntroduction comparison. In both the drop-in and retrofit scenarios, Among available refrigerants, R-22 has been the same original performance and/or capacity may notwidely used for many years. It possesses many be attained due to the use of the existing heatdesirable physical and thermodynamic properties and exchangers and compressor, unless the two refrigerantscan be employed in a wide range of applications and have very similar properties. For example, whentemperatures with good system performance. It is also retrofitting a R22 system to R407C or R290, lowersafe in terms of toxicity and flammability. cooling capacities were obtained (Devotta et al., 2005a,Nevertheless, in response to Montreal Protocol (UNEP, 2005b, Purkayastha and Bansal, 1998), due to their2006), R22, as the last remaining ozone depleting lower volumetric refrigerating effects.HCFC, will face the eventual phase-out in probably This paper is to review relevantless than 5 to 10 years time (UNEP, 2006). Many properties/parameters for the refrigerant performancealternative refrigerants have been developed to replace comparison. First, properties for refrigerant only,R22 as well as many of those already phased out. The including density, viscosity, thermal conductivity,choice of alternative refrigerants is vast and it is not molar heat capacity, critical temperatures, specificalways easy to make the right decision, though many of refrigerating effect and molecular weight, arethe new refrigerants are expected to deliver the same or reviewed. Derived parameters such as volumetriceven better energy performance than those being refrigerating capacity, heat transfer coefficient andphased out. pressure drop are reviewed in the following section. Several scenarios of refrigerant substitution Finally, the refrigerant-oil properties/parameters arecan be adopted; these are “drop-in”, “retrofit”, and reviewed.“new” systems. “Drop-in” - where the old refrigerant istaken out and the system charged with the alternative Effects of key refrigerant properties on systemrefrigerant and occasionally with some minor performanceadjustments to the control settings, “retrofit” - where Density or Specific volumethe old refrigerant is replaced with an alternative Density () or specific volume ( = 1/) of arefrigerant often accompanied by oil and material refrigerant influences pressure drop throughout the 486 | P a g e
  2. 2. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-493refrigerant circuit as well as the compressor capacity. Critical temperatureThe lower the vapour density, the larger will be the In general, as the critical temperature of apressure drop in the evaporator and the condenser for a refrigerant increases the volumetric refrigeratinggiven mass flux, and thus for instance, R290 has a capacity (refrigerating effect per specific volume,larger pressure drop compared to R404A, R410A and kJ/m3) decreases; this is due to the fact that lowerR22 (Spatz and Motta, 2004). For a given compressor vapour densities (at a given evaporating temperature)size and speed, a lower vapour density will also result are usually associated with refrigerants with a highin a lower capacity. As shown in Hwang and critical temperature (McLinden and Didion, 1987,Radermacher (2002), the saturated vapour density of Didion, 1999). The implication is that for a givenR410A is approximately 40% higher than that of R22 compressor (speed and size), the reduction of suction(and R410A’s latent heat is also 7% higher), for the density will lead to a decrease in the mass flow ratesame compressor, it will provide much higher capacity and hence the cooling capacity. On the other hand,than R22. They needed to use a smaller compressor for when design a new system, a refrigerant with a highR410A to obtain similar capacity as R22. Tcrit will require a larger compressor swept volume rate to provide the same capacity. The COP, however, Viscosity increases for refrigerants with a high critical Higher values of liquid viscosity increase temperature, due to reduced flash gas losses. Therefore,pressure drop in both evaporators and condensers a trade-off between high capacity and high efficiency(Domanski and Didion, 1987). As a result, suction has to be considered (McLinden and Didion, 1987,pressure at the compressor inlet decreases, discharge Didion, 1999).pressure increases and the mass flow rate of refrigerantalso decreases, followed by a reduction in system Specific refrigerating effectcapacity. The compressor work (kW) also decreases, The specific refrigerating effect determinesthough at a slower rate than capacity. Higher liquid the mass flow rate of refrigerant required to produce aviscosities also give rise to reduced HTCs. given capacity. The larger the specific refrigeratingHydrocarbon refrigerants have lower liquid viscosities effect, the smaller the required mass flow rate. It,than R22, hence higher HTCs in the evaporator and therefore, also determines the size of the componentscondenser (Chang, et al., 2000). (Gosney, 1982). R290, having a higher specific refrigerating effect (or a high latent heat), thus requires Thermal conductivity a smaller size of the compressor than R12 for In general, a higher liquid thermal providing the same capacity (Kumar and Rajagopal,conductivity would give rise to a higher HTC. With an 2007).equal mass flux or an equal heat flux, evaporation HTCof R290, R600 and R600/R290 are higher when Molecular weightcompared to R134a as these refrigerants have higher Large values of enthalpy of evaporation arethermal conductivities than R134a (Wen and Ho, found for substances with light molecules (Gosney,2005). 1982), and the energy losses across a compressor’s valves are high when the molecules are heavy Molar heat capacity or Specific heat (Woollatt, 1982). Therefore a low value of molecular The magnitude of the molar heat capacity at weight is preferred for having high efficiency (Devotta,constant volume has a strong influence on the slope of 1995).the saturated liquid and vapour lines as shown on the To sum up, in general, to obtain high COP, theT-s diagram, Figure 1 (Didion, 1999). The slope of the following combination of properties is desirable, assaturated vapour line can be positive or negative. In shown in Table 1. Note that for critical temperature andgeneral, refrigerants with more complex molecules are vapour specific heat, a trade-off between capacity andlikely to have a higher molar heat capacity, and more COP should be considered.likely that their saturated vapour line will bend to give Domanski and Didion (1987) and Hogberg anda negative slope at the lower pressure/temperature and Vamling (1996) showed that each individuala positive slope at the higher pressure. When the heat refrigerant property affects the system performancecapacity is too low, the compression work will be high with different sensitivities or by different extents. Aswith increased discharge temperature and reduced shown in Table 2, the normal boiling point, the criticalthermal efficiency. On the other hand, for a high heat temperature, the liquid thermal conductivity andcapacity, wet compression may be encountered viscosity, the vapour density and the evaporative HTC(McLinden and Didion, 1987). Therefore, a trade-off in appeared to have the most impact on systemthe value of the heat capacity is needed. performance. Evaporator pressure drop has only a slight influence whereas the vapour-phase thermal 487 | P a g e
  3. 3. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-493conductivity and viscosity, together with the half of that R22, which is approximately the value onecondensation HTC and condenser pressure drop, were would experience in drop-in tests.found to have the least impacts. Table 2 includes some examples of the influence of variations of HTC and pressure drop on capacity andEffects of derived parameters on system COP based on the sensitivity studies carried byperformance Domanski and Didion (1987) and Hogberg and Volumetric (or volumic) refrigerating capacity Vamling (1996).(VRC = hrefrig/suc, kJ/m3) When comparing the HTC and pressure drop of In general, fluids with a higher vapour different refrigerants, one could base the evaluation onpressure would also have higher values of volumetric the same mass fluxes (Lee et al., 2006) or the samerefrigerating capacity (VRC) and thus require a smaller specified heat fluxes (Choi et al., 2000); the lattercompressor displacement rate (Jung et al., 2000 and usually corresponds to same capacities. However, theseGranryd, 2001) to deliver the similar capacity. To two comparison approaches could lead to differentreplace or retrofit an existing refrigerant without possible conclusions regarding which refrigerant has achanging the compressor, VRC of alternative fluids higher HTC or a lower pressure drop. Chang et al.should be similar to that of the replaced fluid (Saleh (2000) compared pure and mixture hydrocarbon withand Wendland, 2006). However, whether the retrofit R22 experimentally in a heat pump system. At theCOP will increase or decrease depends on many other same mass fluxes, HTCs of the hydrocarbons are muchfactors, such as the retrofit refrigerant temperatures, greater than R22 whereas when compared under thepressure ratio and compressor efficiencies. same capacities, hydrocarbons show slightly lower values of HTC than R22 due to the fact thePressure ratio hydrocarbons have lower mass fluxes under the same In general, when pressure ratio increases, both capacity.isentropic and volumetric efficiencies of areciprocating compressor decrease (Bansal et al., 1992, The oil effects The oil effects on thermo-physical properties,Urchueguia et al., 2004, Peterson and Thorsell, 1990), heat transfer coefficients and pressure drops, and heatinfluencing the COP and mass flow rate, respectively. exchanger/system performance are reviewed as follow.Heat transfer coefficient (HTC) and pressure drop The results of refrigerant performance Effect of oil on thermo-physical properties,comparison when only considering the HTC and pressure dropthermodynamics may be different from those when Lubricant oils have much higher boilingtaking into account the heat transfer and pressure drop points (Tb) and molecular weights (MW) than most ofin the system. Spatz and Motta (2004) showed that, the the common refrigerants and therefore the Tb and MWCOPs of a system using R410A and R290, based on of refrigerant-oil mixture will be higher than thethermodynamic analysis, are slightly lower than that of refrigerants. The presence of miscible oil in aR22 when all are operating under the same capacity. refrigerant increases its critical temperature, liquid However, when including heat transfer and viscosity, surface tension, specific heat and thermalpressure drop effects of the individual refrigerants in conductivity; on the other hand, the oil presencethe heat exchangers, the reverse was observed. At the reduces its critical pressure and liquid density (Thome,same mass flux, R410A has a higher HTC and a lower u.d, Wei et al., 2008). Oil is essentially non-volatile under typical refrigeration operating condition, andP than R22. The lower impact of a given P on therefore it mainly affects the liquid-phase properties.saturation temperatures coupled with an improved heat Thome (u.d.) commented that vapour pressure oftransfer for R410A reduces the temperature lift oil is extremely small compared to that of thebetween Tr,dew,cond and Tr,dew,evap, thus lowering refrigerant, resulting in practically very little oilthe compressor power requirement. On the other hand, entering the vapour phase with the refrigerant. Heatthough at the same mass flux, the HTC of R290 is transfer and pressure drop characteristics ofhigher than R22, R290 also has a higher pressure drop. evaporators are very sensitive to the presence of oilHowever at a given capacity, R290 has a much lower (Youbi-Idrissi, et al., 2003, 2004). However, it ismass flux than R22, and hence its HTC and pressure difficult to predict accurately the HTC due to oil’sdrop are comparable to R22, resulting in the COP of presence. In some cases, a small quantity of oil (e.g.R290 only marginally better than R22. Simulation 0.1 % oil by mass in a R410A/POE mixture) couldwork of Devotta et al. (2005b) also found the pressure improve HTCcond and HTCevap by up to 10% and 5%,drop of R290 in both HXs are lower than that of R22 respectively, while a larger oil mass fraction, say 5%,when specifying a refrigerant mass flow rate of R290 488 | P a g e
  4. 4. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-493the HTC decreases more severely by 25% (Lottin et al., conductivity and viscosity, the vapour density and the2003b). evaporative HTC appeared to have the significant In general, a reduction in HTC and an increase in impact on system performance.two-phase pressure drop, for both evaporator and In general, the presence of oil could decrease HTCcondenser, could be expected when oil is present and pressure drop, nevertheless it depends on the(Thome, u.d., Bassi and Bansal, 2003, Shen and Groll, amount of oil and the solubility. The consequence of2005a, b). In addition, the effect of oil on local HTC the change in HTC and pressure drop can affect thevaries with vapour quality, for example a 5% by mass system R134a/oil, at a vapour quality (x) of 0.5, couldreduce the HTCevap by 20%, and at x = 0.9, the NomenclaturesHTCevap could drop by 80% (Zurcher et al., 1997, cp isobaricThome, u.d.). specific heat [kJkg 1K1] Effect of oil on heat exchanger/system COP coefficient ofperformance performance (= A heat exchanger performance simulated by Qcool / Wcom ) [---]Rajakphaksa (2003) using R407C with an oil contentof 0.5% – 2.5% of the refrigerant mass in the HX heat exchangerevaporator revealed a 2% reduction in overall heat HTC heat transfertransfer coefficient due to both the reduction in coefficientrefrigerant mass flow rate and an about 25% increase [Wm2K1]in two-phase pressure drop. These caused about a 5% k thermalreduction in evaporator capacity. conductivity Experimentally, Lottin et al. (2003a) found that [Wm2K1]typical content of 1 – 5% of oil by mass in R410A MW molecular weightreduces the system COP by about 4 – 24%. [kg/kmol]Nevertheless, the effect of oil on the system T temperature [C orperformance is negligible when the lubricant does not K]exceed 0.5% of the total refrigerant weight in the hrefrig refrigerating effectsystem. For a particular refrigerant (e.g. R407C) using [kJkg1]different oils with the same viscosity grade, the P pressure drop [kPa]evaporator capacity decreases when refrigerant-oil VRC volumetricsolubility increases (Youbi-Idriss et al., 2003) due to refrigeratingthe reduction in the enthalpy change across the capacity or effectevaporator. (= hrefrig  suc )Conclusions [kJ/m3] In general, to obtain a high COP,  density [kgm3]combinations of high values of latent heat, liquid  dynamic (orthermal conductivity and vapour density and low absolute) viscosityvalues of liquid viscosities and molecular weight are [Pa s]required. Trade-offs between capacity and COP should  specific volumebe considered for critical temperature and vapour [m3kg1]specific heat. Subscripts It was noted that the evaluation of c, cond condenser orproperties/parameters impact on the system condensation orperformance also depends on whether the same conductionequipments are used or not. For new system, a dew dew (saturatedrefrigerant with a higher volumetric refrigerating vapour state)capacity (VRC) could use a smaller compressor for a e, evap evaporator,given capacity. For drop-in or retrofit, the VRC of evaporationalternative fluids should be similar to that of the lat latentreplaced fluid to obtain a similar capacity without r refrigerantchanging the compressor. From sensitivity point of view, the normal boilingpoint, the critical temperature, the liquid thermal 489 | P a g e
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  7. 7. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-49363. Woollatt, D. 1982. Estimating valve losses evaporator: experimental validation of a when dynamic effects are important. refrigerant/oil mixture enthalpy calculation64. Proceeding of Purdue compressor technology model. International Journal of Refrigeration, conference. Purdue university. 27:215-224.65. Youbi-Idrissi, M., Bonjour, J., Marvillet, C. 67. IIF-IIR Bulletin. and Meunier, F. 2003. Impact of refrigerant-, oil solubility on an evaporator performance O., Thome, J.R. and Favrat, D. 1997. Flow working with R407C. International Journal of boiling and pressure drop measurements for Refrigeration, 26: 284-292. R134a/oil mixtures. Part 2: Evaporation in a66. Youbi-Idrissi, M., Bonjour, J., Marvillet, C. Plain tube. HVAC&R Research, 3(1): 54-64. and Meunier, F. 2004. Oil presence in an Temperature excess superheat low cv,vap,mol high cv,vap,mol wet compression Entropy Figure 1 Effect of vapour heat capacity on the shape of the two-phase region or “vapour dome” on a temperature-entropy diagram (as adapted from Didion, 1999) Table 1 Summary of desirable values of refrigerant properties for high COP Refrigerant properties Recommended values High Low Latent heat x Critical temperature x Liquid thermal conductivity x Vapour density x Vapour specific heat x Liquid viscosity x Molecular weight x 492 | P a g e
  8. 8. Rotchana Prapainop, K O Suen / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 4, July-August 2012, pp.486-493Table 2 Variations in system performance caused by changes in thermo-physical properties andderived parameters in sensitivities analysisProperties Sensitivities ReferencesTcrit, Tb Tcrit and Tb  2 % => Qheat  11-14 %, COP  0.3-1.8 Hogberg and Vamling (1996) % (vary depending on refrigerants); Tcrit and Tb  2 % => Qheat  11-14 %, COP  0.7-2.3 % (vary depending on refrigerants)cp cp,vap  5 % => Qheat  1-4 %, COP  0.5-1.2 %, Hogberg and Vamling (1996) cp,vap  5 % => Qheat  1-4 %, COP  0.5-1.2 % liq  15% => Qheat  0.6-0.7 %, COP  0.4-0.5 % Hogberg and Vamling (1996) vap  15% => Qheat and COP ~ no significant change liq  50 % => Qcool  4 %, W  1% Domanski and Didion (1987)k kliq  10 % => Qheat  0.5-0.6 %, COP  0.5-0.6 % Hogberg and Vamling (1996) kvap  15 % => Qheat and COP ~ no significant change kliq  50 % => Qcool  3 %, Wcom 0.8 % Domanski and Didion (1987) kliq  50 % => Qcool  7 %, Wcom  1.6 % liq  10 % => Qcool  0.5 %, Wcom ~ no change Domanski and Didion (1987) vap  10 % => Qcool  0.3 %, Wcom  6 % vap  10 % => Qcool  0.5 %, Wcom  7 %HTCevap HTCevap  30 % => Qheat  2.2 %, COP  1.8 % HTCevap  30 % => Qheat 4 %, COP  2.5 % Hogberg and Vamling (1996) HTCevap  50 % => Qcool 2.5 %, Wcom  1 % Domanski and Didion (1987) HTCevap  50 % => Qcool  6 %, Wcom  2.5 %HTCcond HTCcond  30 % or  30 % => Qheat and COP ~ no significant change Hogberg and Vamling (1996) HTCcond  50 % => Qcool  0.5 %, Wcom  0.5 % Domanski and Didion (1987) HTCcond  50 % => Qcool  2 %, Wcom  1.5 %(P/z)evap (P/z)evap  20 % => Qheat  0.6 %, COP  0.2 % (P/z)evap  20 % => Qheat  0.6 %, COP  0.1 % Hogberg and Vamling (1996) (P/z)evap  100 % => Qcool  2 %, Wcom  0.8 % Domanski and Didion (1987) (P/z)evap  50 % => Qcool  1 %, Wcom  0.3 %(P/z)cond (P/z)cond change from 50 to 100 %, Qcool and Wcom ~ no significant change. Domanski and Didion (1987)Notes: 1) The numbers shown in the table are derived from the graphical results in the published literature.2. Domanski and Didion (1987) studied only one refrigerant (R22), while Hogberg and Vamling (1996)studied R134a, R600a, R152a and R22/R142b. 493 | P a g e