Performance Comparison in Retrofit

Size: px

Start display at page:

Download "Performance Comparison in Retrofit"

Bernard Lamb
5 years ago
Views:

1 Influence of Heat Transfer Fluid Conditions in an Evaporator on Refrigerant Performance Comparison in Retrofit (Part 2: Evaporator) Influence of Heat Transfer Fluid Conditions in an Evaporator on Refrigerant Performance Comparison in Retrofit (Part 2: Evaporator) Rotchana Prapainop *1 K O Suen 2 Abstract The paper studied an impact of a variation of heat transfer fluid conditions in an evaporator, which reflects to variations in cooling load and application temperature, on the refrigerant performance comparison in retrofit situation. The study adopts the same methodology as that described in Part 1 (Influence of heat transfer fluid conditions in a condenser on refrigerant performance comparison in retrofit (Part 1: condenser)). A mathematical model is formulated to analyze how the performance varies with heat transfer fluid conditions in the evaporator and relates its sensitivity to refrigerant properties. Applications of the models developed (from both Part 1 and Part 2) are illustrated at the end of the paper. Keywords: evaporator, 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 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 124 ว ศ ว ก ร ร ม ส า ร ม ก.

2 . k thermal conductivity [W.m -1.K -1 ] 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 desuperheating (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 [%] ρ 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 125 ฉบ บท 81 ป ท 25 กรกฎาคม - ก นยายน 2555

3 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 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]. It has been shown by many researchers that for a given refrigerant capacity or/and compression work vary with evaporator inlet heat transfer fluid temperatures (T w,in,evap ) ([3],[4],[5]). Nevertheless, how different refrigerants response to this variation and whether or not the refrigerant performance comparison is affected is unclear. This paper formulates a relationship between HTF conditions and the performance comparison relating to refrigerant properties. In Part 2, the variation of HTF conditions in an evaporator, which reflects variations in cooling load and application temperature, is studied. The illustration of applications of the models developed (Part 1 and Part 2) is also presented in Section Methodology and assumptions The same methodology as described in Part 1 is applied here. Simulations are carried out [6], for 12 refrigerants, based on the same R22 system, for a range of T w,in,evap between 8.6 and 17.1 C at a fixed T w,in,cond of 29 C; T w,in,evap = 12.2 C and T w,in,cond = 29 C are set as the reference temperatures. Correspondingly, each individual 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 assumed constant ; the η vol and η isen are also assumed fixed in o rder 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 presented in Sections 3 and 4. Section 3 provides some comparisons with available published experimental data, and also allows observations made from the trends among refrigerants. In Section 4, some of the simulated results are presented in alternative forms to assist the formulation of a 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. 126 ว ศ ว ก ร ร ม ส า ร ม ก.

4 3. Variation of retrofit performance and conditions in response to a change in T w,in,evap The variations of the cooling capacity (Q evap or Q cool ), the compressor work input (W com ) and the COP with the inlet water temperature to the evaporator (T w,in,evap ) are shown in Figure 1 to Figure 3, respectively. Figure 4 and Figure 5 show the variations of T r,dew,evap and P evap with respect to T w,in,evap. Since the change on the condenser side is relatively small, the values of T r,dew,cond and P cond, in response to the change of T w,in,evap, are not included. It can be observed that Q evap increases with increasing T w,in,evap (Figure 1), while W com only increases slightly (Figure 2), and thus the corresponding COP increases (Figure 3). These trends agree well with experimental results of Camporese et al. [3] and Granryd [4] which showed both Q evap and COP increase with respect to an increase in T w,in,evap. Using their results, it was found that their W com also increased slightly in response to an increase in T w,in,evap. Figure 4 shows that T r,dew,evap changes significantly with T w,in,evap, leading to a large change in P evap (Figure 5); this observation also agrees well with the experimental results of Lee and Su [5]. Figure 1 Cooling capacity versus T w,in,evap for different refrigerants Figure 2 Compression work at various T w,in,evap for different refrigerants (Note: Figure symbols in Figure 2 are the same as those in Figure 1) Figure 3 COP versus T w,in,evap for different refrigerants 127 ฉบ บท 81 ป ท 25 กรกฎาคม - ก นยายน 2555

5 Figure 4 Refrigerant evaporation temperatures at various T w,in,evap for different refrigerants (Note: Figure symbols in Figure 4 are the same as those in Figure 1) Figure 5 Evaporating pressures at various T w,in,evap for different refrigerants ṁ r increase, but simultaneously the specific compression work decreases due to reduced pressure ratio (P cond /P evap ). Among the refrigerants tested, it is noted that the trend lines for refrigerant temperatures, T r,dew,evap in Figure 4 are nearly parallel. This suggests a single model can be generated to represent all of them. 4. Simplified model for performance sensitivity to change in T w,in,evap Adopting the same approach as in Part 1, the results in Figure 4 are plotted in terms of T r,dew,evap against T w,in,evap as shown in Figure 6, where T r,dew,evap = T r,dew,evap -T r,dew,evap,ref and T w,in,evap = T w,in,evap - T w,in,evap,ref. The T r,dew,cond is also included in the figure for completeness. Almost identical linear trend When T w,in,evap is increased, assuming a constant SH evap, P evap and hence the suction vapor density increase, resulting in a rise in refrigerant mass flow rate. Since the change in the specific refrigerating effect is small due to a nearly unchanged P cond, the increase of Q evap is mainly caused by an increase in P evap. On the other hand, W com is mostly unaffected because when T w,in,evap increases, the P evap and thus the Figure 6 Change in refrigerant temperature with respect to reference value versus the corresponding change in T w,in,evap 128 ว ศ ว ก ร ร ม ส า ร ม ก.

6 lines are observed for all refrigerants. The regression models for the change of the evaporator and condenser refrigerant temperatures with respect to the change in T w,in,evap, using data in Figure 6 are presented in Table 1. Table 1 The curve-fitted coefficients a and b of a linear function, y = ax + b y a b R 2 T r,dew,evap T r,dew,cond Note: x is T w,in,evap and y is either T r,dew,cond or T r,dew,evap all with respect to the reference values Using the models T r,dew,cond = a. ( T w,in,evap ) + b and T r,dew,evap = a.(t w,in,evap ) + b, with the coefficients in Table 1, the new refrigerant temperatures T r,dew,evap and T r,dew,cond can be estimated for any T w,in,evap. The corresponding pressures and suction vapor density can also be obtained accordingly. As before, the new balance conditions allow the refrigerant. mass flow rate (ṁ r = ρ suc.v sw.η vol ), cooling capacity, work input, and COP to be determined. Analysis of the data reveals that when T w,in,evap is varied from the reference value, the Q evap ratio can be approximated by the P evap ratio, i.e. Q evap /Q evap,ref P evap /P evap,ref. Therefore, alternatively, Q evap can be indirectly estimated by this approximation, once the new evaporation temperature T r,dew,evap has been obtained from the above linear function. Unlike for T w,in,cond, it appears that a separate linear function of Q evap ratio (Q evap /Q evap,ref ) against T w,in,evap ratio (T w,in,evap / T w,in,evap,ref ) may be not needed when T w,in,evap is varied. However, in order to assess the combined effect from the simultaneous variations of both the T w,in,evap and T w,in,cond, which is quite possible in practice, the same form of linear function is developed. Using the same format of graphical presentation, Figure 7 shows the Q evap ratio (Q evap /Q evap,ref ) against T w,in,evap ratio (T w,in,evap /T w,in,evap,ref ) based on the data previously shown in Figure 1. It is seen that the Q evap ratio of R600a and R134a has a higher sensitivity towards the variation in T w,in,evap ratio when compared to the others. This is because the P evap ratio (i.e. P evap / Figure 7 The ratio of cooling capacity at various ratios of T w,in,evap for different refrigerants 129 ฉบ บท 81 ป ท 25 กรกฎาคม - ก นยายน 2555

7 P evap,ref ) of R600a and R134a are higher, resulting in relatively larger changes in their refrigerant mass flow rates. 5. Application of the functions and illustration When both T w,in,cond and T w,in,evap vary at the same time, the resulting changes in refrigerant temperatures and capacity can be determined by simply superimposing the previously developed functions of temperature difference or ratio [6], as shown below. It is noted that some information from Part 1 of this article is referred to in the discussion. T r,dew,evap = f1+f2 (1) T r,dew,cond = f3+f4 (2) and (3) where f 1 to f 6 are summarised below based on data from Table 1 in Part 1 and Table 1 in Part 2. f1 = T r,dew,evap = ( T w,in,cond ) f2 = T r,dew,evap = ( T w,in,evap ) f3 = T r,dew,cond = ( T w,in,cond ) f4 = T r,dew,cond = ( T w,in,evap ) where b is obtained from Eq. 1 (Part 1) for individual refrigerants To illustrate, increase T w,in,cond and T w,in,evap by 5.8 C and 2.4 C from their reference value (T w,in,cond,ref = 29 C and T w,in,evapref = 12.2 C). For all refrigerants, this will give f1 = +0.4 C and f 2 = +2.0 C, resulting in the total T r,dew,evap = +2.4 C. Similarly, f 3 = +5.5 C and f 4 = +0.2 C, resulting in a total change in T r,dew,cond = +5.7 C. The new balanced refrigerant temperatures can then be calculated using the refrigerant reference temperatures simulated earlier. Functions f 5 and f 6 represent the relative changes in Q evap caused by changes in T w,in,cond and T w,in,evap. For the same increase of 5.8 C in T w,in,cond, the temperature ratio used in f 5 is 1.2. Unlike f 1 to f 4 which take the same values for all the refrigerants considered, f 5 and f 6 are different for individual refrigerants. For instance, for R125, f 5 is 0.94 and f 6 is 1.06, giving overall Q evap /Q evap,ref = 1.00 in Eq. 3. As for R134a, f 5 is 0.96 and f 6 is 1.08, resulting in a Q evap /Q evap,ref of These examples illustrate that when T w,in,cond increases Q evap tends to decrease. 130 ว ศ ว ก ร ร ม ส า ร ม ก.

8 However at the same time when T w,in,evap increases, Q evap tends to increase. The overall impact on capacity depends on their relative differences of these two functions. To illustrate when T w,in,cond increases and T w,in,evap reduces, a T w,in,cond ratio of 1.2 and a T win,evap ratio of 0.8 are used. One can deduce that both changes would lead to an overall reduction in Q evap. For instance, for R125, f 5 is still 0.94 and f 6 is now 0.94, providing an overall Q evap /Q evap,ref = 0.88 when f 5 and f 6 are combined. The results show that, for fixed η vol and η isen, when the change in HTF temperatures ( T w,in,cond or T w,in,evap ) is the same for all the refrigerants, they will all experience approximately the same change in refrigerant temperatures ( T r,dew,cond or T r,dew,evap ). The implication is that the analysis of the performance sensitivity to HTF conditions is also applicable to the influence of refrigerant temperature on the performance. Say, if one compares refrigerants in the same system at various refrigerant temperatures, the T w,in,cond ratio in f 5 can be approximated and replaced by the T r,dew,cond ratio, to allow us to understand how, for a given refrigerant, Q evap ratio changes with variation in refrigerant temperature in the condenser. The proposed model can be used, for instance, to explain the experimental results of Park and Jung [7]. They tested R22 and R290 in the same system at various refrigerant temperatures. At T r,dew,evap of 7 C and T r,dew,cond of 45 C, they found that the Q evap of R22 and R290 are 3.75 and 3.50 kw, respectively. At T r,dew,evap of 21 C and T r,dew,cond of 28 C, the Q evap of R22 and R290 become 1.45 and 1.50 kw, respectively. If using the former conditions as the reference, the experimental Q evap ratio (Q evap /Q evap,ref ) for R22 and R290 are 0.39 and 0.43, respectively. By using the model, from Table 2 in Part 1, it is seen that the coefficient b, which is derived from refrigerant properties in Eq. 1 in Part 1, for R22 is 1.16 and for R290 is Hence, R290 will experience a relatively larger increase in Q evap than R22, though the reduction in their T r,dew,cond is the same. Applying the T r,dew,cond ratio of 0.62 (= 28 C/45 C) into f 5, it is found that Q evap ratio of R22 is 1.06 and of R290 is For the effect of T r,dew,evap, the pressures at the two temperatures can be calculated giving a P evap ratio (P evap /P evap,ref ) of 0.38 for R22 and 0.40 for R290, indicating that the relative increase of Q evap with respect to the change in T r,dew,evap of R290 will be larger than that of R22. Overall, the predicted Q evap ratio from the combined effects of varying T r,dew,cond and T r,dew,evap for R22 is 0.40 (= 1.06x0.38) and for R290 is 0.43 (= 1.08x0.40). The Q evap ratios predicted match very well to those from the experiments of Park and Jung. It is clear that the proposed model can 131 ฉบ บท 81 ป ท 25 กรกฎาคม - ก นยายน 2555

9 be used to demonstrate the interaction between the HTF temperatures (T w,in,cond and T w,in,evap ) and the performance, and to correlate the sensitivities among refrigerants with relevant properties. The procedures involved are considered relatively simple compared to many of the full simulation procedures published. 6. Conclusion 1. For all the refrigerants studied, it is observed that changes in refrigerant temperatures with respect to change in HTF inlet temperatures follow the same trend that can be expressed as a linear relationship. It is noted that T w,in,cond mainly affects T r,dew,cond whereas T w,in,evap mainly influences T r,dew,evap. 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 be obtained. 3. The sensitivity of cooling capacity to T w,in,cond and T w,in,evap 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. With respect to cooling capacity and COP, the ranking of refrigerants is unlikely to be affected by the moderate variation in HTF conditions, though the differences between refrigerants do change with HTF conditions. 7. References 1. UNEP. Handbook for the Montreal Protocol on Substances that Deplete the Ozone Layer, Seventh edition [cited 2007 November, 1]; Available from: Publications/MP_Handbook/index.shtml. 2. BNCR35: Overview of new and alternative refrigerants. [cited 2008 January, 31]; Available from: Briefing Notes/PDF/MTP_BNCR35_2008 January17.pdf 3. Camporese, R., et al Experimental evaluation of refrigerant mixtures as substitutes for CFC12 and R502. International Journal of Refrigeration. 20(1): p Granryd, E Hydrocarbons as refrigerants, an overview. International Journal of Refrigeration. 24(1): p Lee, Y.S. and C.C. Su Experimental studies of isobutene (R600a) as the refrigerant in domestic refrigeration system. Applied Thermal Engineering. 22: p ว ศ ว ก ร ร ม ส า ร ม ก.

10 6. Prapainop R Development of an assessment method for refrigerant performance comparison, Ph.D. thesis. University College London: London. 7. Park K.J. and Jung D Performance of R290 and R1270 for R22 applications with evaporator and condenser tem-perature variation. Journal of Mechanical Science and Technology. 22: p ฉบ บท 81 ป ท 25 กรกฎาคม - ก นยายน 2555

Department of Mechanical Engineering, Kasetsart University, Si Racha Campus, Chonburi, Thailand *

Department of Mechanical Engineering, Kasetsart University, Si Racha Campus, Chonburi, Thailand * 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