Department of Mechanical Engineering, Kasetsart University, Si Racha Campus, Chonburi, Thailand *
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1 Influence of heat transfer fluid conditions in a condenser on refrigerant performance comparison in retrofit (Part 1: condenser) Influence of Heat Transfer Fluid Conditions in a Condenser on Refrigerant Performance Comparison in Retrofit (Part 1: Condenser) Rotchana Prapainop * 1 K O Suen 2 Abstract The paper investigates whether or not changes in heat transfer fluid conditions in a condenser, which represents the changes in ambient conditions or the condensing temperature, influence the refrigerant performance. A mathematical model is developed to analyze how the performance varies with heat transfer fluid conditions and relates its sensitivity to refrigerant properties. Simplified models are formulated as a function of density, thermal conductivity, viscosity, specific heat and pressure. Keywords: condenser, heat transfer fluid, temperatures, refrigerant performance, comparison Nomenclature c p isobaric specific heat [kj.kg -1.K -1 ] COP coefficient of performance (=Q cool /W com ) [---] HCFC hydrochlorofluorocarbon HTF heat transfer fluid k thermal conductivity [W.m -1.K -1 ] 1 Department of Mechanical Engineering, Kasetsart University, Si Racha Campus, Chonburi, Thailand * Corresponding author sfengrcp@src.ku.ac.th Tel. +66(0) ext Department of Mechanical Engineering, University College London, United Kingdom 113
2 L length [m] ṁ mass flow rate [kg.s -1 ] P pressure [kpa] Q capacity [kw] SC degree of sub-cooling [ C or K] SH degree of superheating (for evaporator), or degree of de- superheating (for condenser) [ C or K] T temperature [ C or K] V sw compressor swept volume rate [m 3 /s] W compressor work [kw] the difference h enthalpy difference [kj.kg -1 ] h refrig refrigerating effect [kj.kg -1 ] T w the difference between inlet and outlet HTF temperatures of the entire heat exchanger [ C] Greek symbols η efficiency [%] density [kg.m -3 ] μ dynamic (or absolute) viscosity [Pa.s] Subscripts cond condenser or condensation or conduction cool cooling com compressor dew dew (saturated vapour state) dis discharge evap evaporator, evaporation in inlet isen isentropic I liquid out outlet r refrigerant ratio ratio ref reference SC sub-cooling SH superheating suc suction v vapour vol volumetric w water or heat transfer fluid 1. Introduction R-22 has been a widely used refrigerant for many years. It possesses many desirable physical and thermodynamic properties and can be employed in a wide range of applications and temperatures with good system performance. It is also safe in terms of toxicity and flammability. Nevertheless, in accordance with the Montreal Protocol [1], R22, as the last remaining ozone depleting HCFC, will face eventual phase-out in probably less than 5 to 10 years [1]. Many alternative 114 ว ศ ว ก ร ร ม ส า ร ม ก.
3 refrigerants have been developed to replace R22 as well as others already phased out. One of several options for replacement is to retrofit - where the old refrigerant is replaced with an alternative refrigerant, often accompanied by oil and material changes due to compatibility issues [2]. When the HTF conditions vary, the difference in cooling capacities of different refrigerants may vary as shown in the experimental study of Park and Jung [3] for R22 and R290 capacity comparison (at various HTF conditions). Many experimental studies ([4],[5],[6],[7],[8],[9],[10]) showed the trend lines of compressor work against the variation of T w,in,cond. However, they did not relate the effect of refrigerant properties to performance comparison. This paper formulates a relationship between HTF conditions, refrigerant properties, and performance. Part 1 focuses on the variation of inlet HTF temperatures in a condenser, which represents the changes in ambient conditions or the condensing temperature. 2. Methodology and assumptions Simulations are carried out for 12 refrigerants [11], based on the same R22 system, for a range of T w,in,cond. between 14.5 to 37.7ºC at a fixed T w,in,evap of 12.2ºC; T w,in,evap = 12.2ºC and T w,in,cond = 29ºC are set as reference temperatures. Each refrigerant will have a different reference value of T r,dew,evap and T r,dew,cond. Over the range of HTF conditions considered, SH evap ṁ w,evap and ṁ w,cond are assu med constant; the η vol and η isen are also assumed fixed in order to isolate their influence from that of condition variations. The refrigerant charge is kept the same as in the reference condition; in other words, the SC cond is no longer a constant when the HTF conditions vary. The simulation results are reported in the following sections. Section 3 provides some comparisons with available published experimental data, and also gives observations made from the trends among refrigerants. In Section 4, the simulated results are presented in two alternative forms to assist the formulation of the new model by regression analysis. The model enables us to relate the sensitivity of performance, with respect to variation in HTF conditions, to refrigerant properties. 3. Variation of retrofit performance and conditions in response to a change in T w,in,cond At various values of T w,in,cond, the cooling capacity (Q evap or Q cool ), compressor work input (W com ) and COP are shown in Figures 1-3, respectively. The corresponding variations in operating conditions (i.e. T r,dew,cond and P cond ) are 115
4 shown in Figure 4 and Figure 5. T r,dew,evap and P evap (data not included) are found to increase very slightly (no more than 1.1ºC and 3%, resp.) with increasing T w,in,cond for the entire range of T w,in,cond considered. As expected, when T w,in,cond increases, the cooling capacity decreases, the work input increases, and, as a result, the COP decreases; at the same time, T r,dew,cond and P cond also increase. Since the change in P evap is relatively insignificant, the increase in W com is mainly caused by the increase in P cond (or P cond /P evap ). All these trends agree well with the experimental results of other researchers ([4],[5],[6],[7],[8], [9],[10]). In addition, the experimental work of Jabaraj, et al. (2007, 2006) for a window air conditioner showed that the trend lines for the variations of the discharge temperature over a range of condenser inlet air temperatures for different refrigerants (e.g. R407C/R290/R600a mixtures and R22) are nearly parallel to each other. A similar observation can be made regarding the simulated trend lines of T r,dew,cond (Figure 4) among the refrigerants. However, this observation is not apparent in the other four parameters. Figure 1 shows, for R22, that when T w,in,cond is increased by 15ºC, the Q evap is reduced by about 9%; this reduction is caused by a 11% reduction in h refrig combined with a 3% in crease in ṁ r based on the simulation. This agree Figure 1 Cooling capacity versus T w,in,evap for different refrigerants Figure 2 Compression work input at various T w,in,evap for different refrigerants Figure 3 COP at verious T w,in,evap for different refrigerants 116 ว ศ ว ก ร ร ม ส า ร ม ก.
5 Figure 4 Condensing temperature at various T w,in,cond for different refrigerants Figure 5 Condensing pressure at various T w,in,cond for different refrigerants s well with the simulation of Jabaraj, et al. [5], which showed, though for a different system, that for the same 15ºC change in T w,in,cond, the same percentage changes in h refrig and ṁ r were observed. When increasing T w,in,cond from 29 to 37.7ºC (Figure 5), the simulation shows that P cond for R22 increases by 22%, which agrees fairly well with the experimental trends of Mei, et al. [9] that showed an 18% increase. The discrepancies between their work and the current simulation are probably due to differences in the system characteristics and operating conditions. In addition, the simulation assumes constant η vol and η isen whereas in the experiments, these efficiencies vary with conditions. When T w,in,cond increases, the difference in Q evap between R32 and R410A becomes larger; on the other hand, the difference between R125 and R22 becomes smaller (Figure 1). Likewise (Figure 3), when T w,in,cond increases, the COP trend lines among refrigerants become closer, and when T w,in,cond is reduced, significant differences in COP among refrigerants are observed. It is observed that a high pressure refrigerant, such as R32 and R410A, (Figure 5) will experience more significant change in W com (Figure 2) when T w,in,cond is varied. Though individual refrigerants exhibit different sensitivities with respect to variation in T w,in,cond, the relative changes in the values of the cooling capacity and COP (Figure 1 and Figure 3) are not sufficient to affect the rankings of refrigerant performance, within the range of HTF conditions considered. 117
6 4. Simplified sensitivity model for the performance with respect to T w,in,cond The results shown in Figure 4 can also be presented based on the change relative to the reference value [11], i.e. T r,dew,cond against T w,in,cond, as shown in Figure 6 where, for instance, T w,in,cond = T w,in,cond T w,in,cond,ref. For completeness, T r,dew,evap is also included in Figure 6. It can be seen that all refrigerants have nearly the same trend line, suggesting a single curve-fitted model can be used to represent all the studied refrigerants when expressed in terms of changes from the reference values. Based on observation, a linear function is used to represent the trend of the curve. The regression coefficients generated from Minitab 15.0 are displayed in Table 1. Figure 6 T r,dew,cond and T r,dew,evap at various T w,in,cond for different refrigerants Table 1 Regression coefficients for a linear model, y = ax+b, using data from Figure 6 y a b R 2 T r,dew,cond T r,dew,evap Note: y = ax+b where x is the difference between T w,in,cond and the reference value; a and b are curve-fitted coefficients; y is the change of the interested parameters (e.g. T r,dew,cond or T r,dew,evap ) relative to the reference value. The coefficient of determination (R 2 ) for T r,dew,cond is very close to 1 (i.e. very good fit) since as previously seen in Figure 4 the trend lines of T r,dew,cond for individual refrigera nts have similar gradients. Though, R 2 for T r,dew,evap is 0.92, suggesting a relatively large variation in their sensitivities against T w,in,cond among the refrigerants, the overall agreement of the linear model is considered acceptable and no further refinement of this function is performed. Having predicted T r,dew,evap and T w,in,cond by the proposed equation, the refrigerant temperatures (T r,dew,cond and T r,dew,evap ) for a number of refrigerants in a given system, when the HTF inlet temperatures are varied, can be estimated. It should be noted that the reference retrofit temperatures (T r,dew,cond,ref and T r,dew,evap,ref ) for individual refrigerants are different [11]. Subsequently, the corresponding pressures can be obtained, allowing the retrofit compressor work to be estimated. 118 ว ศ ว ก ร ร ม ส า ร ม ก.
7 If the SC cond is known, together with the two temperatures (T r,dew,cond and T r,dew,evap ), the cooling capacity and COP can also be determined. However, since the refrigerant quantity is kept constant, when HTF conditions are varied, the SC cond becomes an unknown. Hence, it is not directly possible to determine Q evap using only T r,dew,evap and T r,dew,cond. Therefore, an additional model is developed to directly relate Q evap with T w,in,cond, eliminating the need to calculate SC cond. Figure 7 The ratio of cooling capacity versus the ratio of T w,in,cond for different refrigerants refrigerants are rather different. The trend lines among refrigerants are nearly linear but with different slopes, and hence each individual refrigerant will have its own regression coefficients, computed using the Minitab 15.0 and shown in Table 2. Using a linear relationship y = ax + b to express the variation in Q evap ratio (i.e. Q evap / Q evap,ref ) as a function of T w,in,cond ratio (i.e. T w,in,cond, / T w,in,cond,ref ), both x and y are equal to 1 at the reference point, giving a = 1- b. Therefore, it is possible to relate/formulate refrigerant properties to either a or b alone. b is selected for this purpose. The last column in the Table 2 is the b values predicted by using a function of refrigerant properties, which will be presented below. To assist the development of a relationship between Q evap and T w,in,cond among refrigerants, the results in Figure 1 are normalized by dividing by the reference values for individual refrigerants, as shown in Figure 7. In Figure 7, Q evap ratio = Q evap /Q evap,ref and T w,in,cond ratio = T w,in,cond /T w,in,cond,ref. It is seen that the sensitivities of Q evap ratio to T w,in,cond ratio for individual 119
8 Table 2 The regression coefficients for individual refrigerants and the predicted b Curve-fitted coefficients of a function y = ax + b Predicted b using Eq.1 a from b from regression regression R R134a R R600a R R R R407C R417A R422D R427A R410A Note: For the linear function y = ax + b in this table, y is Q evap ratio (Q evap /Q evap,ref ), x is T w,in,cond ratio (T w,in,cond /T w,in,cond,ref ). When T w,in,cond varies, the relative change of Q evap depends on a combination of several parameters, including the change of the mass flow rate and of the refrigerating effect. The former relates to the P evap and ρ v. The latter depends on the change of P cond and SC cond, both of them have a strong relationship to refrigerant charge (Prapainop, 2010). Let s say, a variation of T w,in,cond will lead to a new balanced conditions. If this happens at an assumed fixed SC cond (same as the reference value), the system will require a new charge quantity. However, for each individual refrigerant, the charge quantity is fixed at the previously specified conditions, and hence SC cond must vary. In other words, a variation in T w,in,cond could be interpreted as either a slightly under-or over-charge condition. Based on the analysis in Prapainop (2010), the key properties identified to relate the charge sensitivity with the retrofit performance are ρ 1, ρ v, k 1, k v, μ 1, μ v, c p,1, c p,v, P cond, P evap. The functional relationship is used to correlate the coefficient b with the properties as suggested in Prapainop (2010), shown in Eq. 1. (1) where the subscripts l and v refer to liquid and vapour, resp., and P ratio = P cond /P evap. It is noted that the properties are obtained at the condensing bubble point at reference condition for individual refrigerants. The values of constants c 1 to c 6 for Eq. 1 are shown in Table 4, using the relevant properties in Table 3, and obtained by Minitab ว ศ ว ก ร ร ม ส า ร ม ก.
9 Table 3 The relevant derived refrigerant properties and predicted index b Refrigerant properties of liquid to vapour Predicted from Eq.1 ρ 1 /ρ v k 1 /k v μ 1 /μ v c p,1 /c p,v P cond /P evap b Qcond b Qevap b Wcom b Pcond R R134a R R600a R R R R407C R417A R422D R427A R410A Table 4 Constants c 1 -c 6 for Eq. 1 c 1 c 2 c 3 c 4 c 5 c 6 R Table 2 shows that the predicted coefficients b from the properties function (Eq. 1) agree well with the values generated by regression. Having obtained a value for b and subsequently a, the proposed function (Eq. 1), together with the linear function, can be used to relate relevant refrigerant properties to the sensitivity of Q evap to T w,in,cond A refrigerant with a high value of b, in other words, a combination of low values of ρ l /ρ v, k l /k v and c p,l /c p,v and high values of µ l /µ v and P ratio, will lead to a high sensitivity of Q evap to a change in T w,in,cond. As shown in Table 2, it can be observed that low values of ρ l /ρ v, k l /k v and c p,l /c p,v coupled with a moderate P ratio of R125 results in the highest sensitivity among the compared refrigerants, despite its low µ l /µ v The constants c 1 to c 6 in Eq. 1 also provide some insight into the significance and impact of individual properties groups. As seen in Table 4, k l /k v has the greatest influence, as c 3 has the larger numerical values among the constants, 121
10 followed by µ l /µ v. Therefore, Q evap for R32, which has a high value of k l /k v and a low value of µ l /µ v, is the least sensitive to a change in T w,in,cond, despite its low values of ρ l /ρ v and c p,l /c p,v, and high P ratio. Having established a linear relationship, Q evap /Q evap,ref = a (T w,in,cond /T w,in,cond,ref ) + b, the Q evap at a new T w,in,cond can be calculated by multiplying the ratio Q evap /Q evap,ref by Q evap,ref of individual refrigerants. Similarly, using the corresponding coefficients a and b from Table 1 and a known reference temperatures (T r,dew,cond,ref, T r,dew,evap,ref ), the two linear functions of temperature difference, T r,dew,cond = a ( T w,in,cond ) +b and T r,dew,evap can be used to predict the new refrigerant temperatures when T w,in,cond is varied. Sub-sequently, the corresponding pressures, and hence the compressor work and the COP can be computed. 5. Conclusion 1. For all the refrigerants studied, it is observed that the changes in refrigerant temperatures with respect to change in HTF inlet temperatures follow the same trend that can be expressed as a linear function. 2. The model developed can be used to estimate the refrigerant temperatures and capacity at a new balance point, based on which the correspondent pressures can be determined, and the work input and COP obtained. 3. The sensitivity of cooling capacity to T w,in,cond for individual refrigerants has a strong dependence on key refrigerant properties, including ρ 1, ρ v, k 1, k v, μ 1, μ v, c p,1, c p,v, P cond, P evap. 4. The cooling capacity and COP rankings of refrigerants is unlikely to be affected by a moderate variation in HTF conditions, though their variations between refrigerants do change with HTF conditions. 6. References [1] UNEP. Handbook for the Montreal Protocol on Substances that Deplete the Ozone Layer, Seventh edition [cited 2007 November,1]; Available from: MP_Handbook/index.shtml. [2] BNCR35: Overview of new and alternative refrigerants. [cited 2008 January, 31 ]; Available from: w w w. m t p r o g. c o m / A p p r o v e d BriefingNotes/PDF/MTP_BNCR35 _2008January17.pdf [3] Park, K.J. and Jung D Performance of R290 and R1270 for R22 applications with evaporator and condenser temperature variation. Journal of Mechanical Science and Technology. 22: p ว ศ ว ก ร ร ม ส า ร ม ก.
11 [4] Jabaraj. D.B., et al., Experimental investigation of HFC407C/HC290/ HC600a mixture in a window air conditioner. Energy Conversion and Management. 47: p [5] Jabaraj, D.B., et al Evolving an optimal composition of HFC407C/ HC290/HC600a mixture as an alternative to HCFC22 in window air conditioners. International Journal of Thermal Sciences. 46(3): p [6] Devotta S., Padalkar A.S. and Sane N.K Performance assessment of HCFC-22 window air conditioner retrofitted with R-407C. Applied Thermal Engineering. 25: p [7] Devotta S., Padalkar A.S. and Sane N.K Performance assessment of HC- 290 as a drop-in substitute to HCFC-22 in a window air conditioner. International Journal of Refrigeration. 28(4): p [8] Ravikumar T.S. and Lal D.M Onroad performance analysis of R134a/ R600a/R290 refrigerant mixture in an automobile air-conditioning system with mineral oil as lubricant. Energy conversion and management. 50: p [9] Mei V.C., et al Performance tests of R22 and R32/R125/R134a mixture for baseline air conditioning and liquid over-feeding operations. ASHARE Transactions. 101 (part 2): p [10] Weiss P., Barreau M. and Macaudiere S Field tests using HCFC-22 replacements. in 19th International Congress of Refrigeration Proceedings. [11] Prapainop R. 2010, Development of an assessment method for refrigerant performance comparison. University College London: London. 123
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