1、471 1 Coefficient of Performance of Fluorinated Ether and Fluorinated Ether Mixtures Ismail Kul, Ph.D. Darryl D. DesMarteau, Ph.D. Adolph Beyerlein, Ph.D. Member ASHRAE ABSTRACT The coejcient ofperformance (COP) is estimatedfrom thermodynamic literature data using methods of Morrison andMcLinden (I
2、986) and reportedfor the ether R-E125 and the ether mixtures R-E218(50%)/R-E143a(50%), R-E218/ R- I34a/R-I 61, R-E125(75%)/R- I61 (25%), R-EI25/R-32/ R-l52a, and R-E125/R-32/R-I34a. The chemical formulae for the ethers R-E218, R-E143a, and R-E125 are CF30CF2CF3, CF30CH3, and CF30CF2H, respectively.
3、The binary mixtures are azeotropic mixtures with the azeo- tropic compositions given in mole fraction, and the ternary mixtures are equimolar mixtures. The calculated COP of the mixtures rangedfrom 80% to 90% of thatfor R-22 except for the azeotropic mixtures R-E125(75%)/R-l61(25%) and R- E218(50%)/
4、R-E143A, whose calculated COPS are lower. The calculations show that R-EI25 and mixtures containing R-E2I8 would require superheat on the suction line to elim- inate “wet compression.“ INTRODUCTION The search for R-22 alternatives is complicated because it is difficult to match the wide spread betwe
5、en the boiling point (-40.1“C -40.2“F) and critical temperature (96C 204.8“F) of R-22. The leading class of compounds that provide us refrigerant alternatives that do not harm the strato- spheric ozone are the hydrofluorocarbons (HFC). Some of the HFC compounds (such as fluoroethane R-1611) with the
6、 highest critical temperatures and low boiling points that approach the boiling point of R-22 have a low fluorine content and tend to be flammable. Because of the difficulties in identifying an ideal HFC alternative, the search for R-22 alternatives has been extended to mixtures (Calm and Didion 199
7、8). This has led to the discovery of a number of mixture alternatives, and two of the leading HFC mixture alternatives that are currently being marketed, are R-410A (McLinden et al. 1998) and R-407C (Nagel and Bier 1995). However when comparing the cycle performance calculations of R-41 OA and R-407
8、C with R-22 some shortcomings are evident and the search for R-22 alter- natives continues. The low critical temperature of R-41 OA (72.5“C 162.5“F) results in higher volumetric capacity but reduced efficiency. Higher heat capacities of the more complex alternatives (particularly fluorinated propane
9、 deriv- atives) also lower rehgerant efficiencies. Methods for improving efficiencies by addition of a liquid-line/suction- line heat exchanger have been investigated by Domanski et al. (1 994a). Liquid-line/suction-line heat exchanger will super- heat the suction gas and may at first appear to incr
10、ease the cooling capaciy or refrigerating effect. However, this must be balanced against the lowered volume capacity and increased compression work associated with the superheated vapor. Domanski et al. (1994a) have found that the overall effect on refrigeration capacity and efficiency is usually sm
11、all and may be either positive or negative depending on a combination of factors with heat capacity being the most influential property. In view of the limitations of HFC refrigerant alternatives, the authors have measured the vapor pressure, liquid density, and critical properties of fluorinated et
12、hers, fluorinated sulfur compounds (CF,SF5 and CF3SCF3) and mixtures of these compounds (Ku1 2001; Beyerlein et al 1998). Although the heat capacity of these compounds is high, these compounds yield mixtures with high critical temperatures for improved refrigerant efficiency and at the same time hav
13、e boiling points approaching that of R-22. Ismail Ku1 is an assistant professor of chemistry at Widener University, Chester, Penn. Darryl D. DesMarteau is the Tobey-Beaudrot professor of chemistry and Adolph Beyerlein is retired chair and professor emeritus, Chemistry Department, Clemson University,
14、 Clemson, S.C. 02004 ASHRAE. 189 R-El25(33.3%)/R-32(33.3%)/R-134a(33.3%) P -40.4 40.7 86.2 187.2 H Figure 1 A pressure-enthalpy diagram illustrating an ideal refrigeration cycle. The measured data, which is used to estimate the coe6 cient of performance (COP) on six ether mixtures that are Iisted in
15、 Table i, are reported in the Ph.D. thesis of one of the authors (Ku1 2001). The COPs for the mixtures are estimated using the methods of Morrison and McLinden (1985, 1986) and the Camahan-Starling-DeSantis (CSD) equation of state (Carnahan and Starling 1969; DeSantis et al. 1976). It is the purpose
16、 of this paper to report the results of these investiga- tions. ESTIMATION OF COEFFICIENT OF PERFORMANCE In this section we briefly review the methods of Morrison and McLinden (1985, 1986) that are used for calculating the enthalpies needed to estimate the COP. The COPs of the vari- ous refrigerants
17、 in Table 1 are estimated for the ideal cycle sketched in Figure 1. The cycle is ideal in the sense that it does not account for the irreversibility in a real cycle. Briefly, the cycle includes a reversible evaporation at constant pressure represented by line ab in Figure I in which the liquid is va
18、por- ized to gas phase. The evaporation absorbs heat from the low- temperature heat reservoir (refrigeration or cooling effect) whose temperature should be above the normal boiling point of the refiigerant in order for the refrigerator to operate above atmospheric pressure. After this expansion ther
19、e is a reversible adiabatic, i.e., isentropic compression, of the vapor, repre- sented by line bc in Figure 1, which increases the temperature of the vapor to a temperature above the condensation temper- ature. The refrigerant is then released into a condenser repre- sented by line cde in Figure 1 i
20、n which all of the vapor is condensed to a liquid and in the process heat is transported from the refrigerant to the condenser heat reservoir. Following the condensation there is a Joule-Thomson expansion (constant enthalpy) represented by line ea in Figure 1, which brings the temperature of the sys
21、tem back to the Iow-temper- ature heat reservoir. The coefficient of performance (COP) is calculated by using the following equation, where H represents the molar enthalpy, qab is the heat absorbed during the evaporation cycle ab, and W is the work done for the complete cycle. Implicit in Equation 1
22、, we assume the isobaric condensa- tion and isobaric evaporation are also isothermal processes, which implies the equation is exact for pure chemicals and azeotropk mixtures. However, it also useful for “near azeo- tropic” mixtures that exhibit a minimal temperature glide (less than 1C to 2C l.SF to
23、 3.6”FI) during the evaporation portion of the cycle. The enthalpy change represented by line ab in Figure 1 can be obtained from the heat of vaporization calculated from vapor pressure measurements. The enthalpy change Hc-Hbfor the compression portion of the cycle is estimated using the methods of
24、Morrison and McLinden (1985, 1986). An advan- tage of their method is that it allows one to extrapolate the enthalpy of vaporization obtained from vapor pressure 190 ASHRAE Transactions: Research measurements to H, ofthe superheated vapor using the follow- ing Camahan-Starling-DeSantis (CSD) equatio
25、n of state (Carnahan and Starling 1969; DeSantis et al. 1976; Morrison and McLinden 1985): LV = 1 +y+y2-y3 a b - where y = - . (2) RT (1 -y)3 RT( Y+ b) 4v In the above equation R is the gas constant, V is the molar volume, P is the pressure, Tis the absolute temperature, and a and b are parameters d
26、ependent on temperature. The CSD equation is a two-parameter equation of state that predicts PVT behavior with remarkable accuracy (DeSan- tis et al. 1976; McLinden et al. 1989) and is useful for systems where a minimal amount of measured data is available. Using this equation of state the enthalpy
27、and entropy can be expressed in closed form in terms of the CSD parameters a and b. The CSD parameters (a and b) are adjusted at a specified temperature so that the following quadratic form is mini- mized: The quantities al, o, and op, are weighting factors, the subscripts 1 and v refer to the satur
28、ated liquid density and satu- rated vapor density, respectively, and P is the vapor pressure. During the minimization process, all of the weighting factors were set to unity. The subscript c designates quantities calcu- lated from the CSD equation (Equation 2). The molar volume of the vapor V, is es
29、timated to about 2% accuracy from the measured critical density, critical temperature, critical pres- sure, and the boiling point using the modified corresponding states calculation of Beyerlein et al. (1993). The molar volume of the liquid Vl and the vapor pressure P are measured. The Vv,c, and P,
30、are estimated by a combination of the Cama- han-Starling-DeSantis equation and the Maxwell equal area rule. This rule is applied via the condition, where P(a,bl notes that the calculated pressure is a function of the CSD parameters a and b. The VVc and Vl,c are determined as solutions to the CSD equ
31、ation and Equation 4 for P = PSap After obtaining CSD parameters a and b, vapor and liquid densities, and saturated pressures over a wide temperature range that spans the temperature range of the cycle, the param- eters a and b are fit into the following temperature-dependent equations: b = bo+ b,T+
32、 b2?. (6) ASHRAE Transactions: Research Thus one can calculate the thermodynamic functions as explicit functions of temperature using a combination of Equations 2 to 6. In order to calculate the thermodynamic quantities such as entropy and enthalpy, a reference state is needed. The refer- ence state
33、 is chosen to be the liquid state at its saturation pres- sure at -40C (-4O“F), which is lower than the operating temperatures for the cycle. The main equations for calculat- ing the enthalpy and entropy, are given in the following equa- tions with respect to a chosen reference state, H(K T)-HL(Vref
34、 Trey) = WV, T)-ff“(T)I “. where the superscript o denotes the ideal gas state and H(KT)- P(T) and S(KT)-sO(V,T,J are calculated using the CSD equa- tion of state (Equation 2). As seen in Equations 7 and 8, enthalpy and entropy are dependent on heat of vaporization, ideal gas heat capacities, as wel
35、l as CSD parameters at selected temperatures. The heat of vaporization data are obtained from vapor pressure data that are reported in the thesis of Ku1 (2001) and in Kul, DesMart- eau, and Beyerlein (2000a, 2000b, 2001). The ideal gas molar heat capacities of the pure components were estimated as a
36、 function of temperature using the Benson et al. method (1 969) except the heat capacity data for the CF2H2, which was obtained from Hozumi et al. (1 996). The ideal gas molar heat capacity contributions of the mixtures were calculated using the ideal mixing nile, where XI is the mole fraction of co
37、mponent 1 and the subscripts 1 and 2 correspond to components 1 and 2, respec- tively. Finally, the COP values were calculated for two operating conditions. For the lower temperature operating condition, the evaporation temperature is -2 1C (-5.8“F) and condensa- tion temperature of this condition i
38、s 36C (963F). For the higher temperature operating conditions, the evaporation temperature is -2C (28.4“F) and the condensation tempera- ture is 50C (122F). All of the vapor pressure data and liquid density data up to the critical temperature, which were needed for the COP calculation, were measured
39、 (Ku1 2001; Kul, DesMarteau, Beyerlein 2000, 2001)and are reported in the Ph.D. thesis ofKu1 (2001) and in Kul, DesMarteau, and Beyer- lein (2000a, 2000b, 2001). The vapor densiy is calculated 191 Table 2. The Results for COP Along with the Ideal Gas Heat Capacity at 25“C* I CP COP COP, l I I l I Mi
40、xture R-E2 18(5O%)/ R-E143a(50%) R-E218/R-134a/R-161 R-E 125 J/(K-Mol) Btu/(R-lb) Low Temp High Temp Low Temp High Temp 125.2 0.197 2.75 2.98 0.78 O. 73 100.3 0.203 2.85 3.09 0.81 0.76 106.7 O. 187 2.96 3.22 0.84 0.79 r R-22 I 56.1 I 0.111 I 3.51 1 4.07 1 1.00 I 1.00 1 R-El25(75%)/R-l61(25%) R-E125/
41、R-32/R-l52a R-El25m-32iR-134a R-4 1 OA R-407C 94.4 0.198 2.43 2.73 0.69 0.67 71.4 0.201 3.16 3.73 0.90 0.92 77.9 0.192 3.27 3.751 0.93 0.92 61 0.143 3. 142 3.492 0.93 0.89 83 0.194 3.222 3.692 0.95 0.94 from the boiling point and critical density using the modified corresponding states method. (Beye
42、rlein et al. 1993). I R-22 56.1 RESULTS AND DISCUSSION I .o0 0.111 3.38 3.92 1 .o0 The mixtures and pure chemicals selected for calcula- tion of COP values for an ideal cycle are listed in Table 2 along with the results for the COP estimates, heat of vapor- ization at the normal boiling point, and t
43、he ideal gas heat capacity. These mixtures were selected for study because their boiling point and critical temperature suggested a high potential as R-22 alternatives. For comparison purposes we have also included in Table 2 the COP for R-22, R-4 1 OA, and R-407C, which are calculated using CYCLE D
44、 (Domanski et al. 2003). If the parameters of the CSD equation are calcu- lated from the saturated density and pressure data of REFPROP7 (Lemmon et al. 2002), the CSD equation predicts COP values that are in excellent agreement with those calculated from CYCLE-D for R-410A, R-407C, and R-22. This pr
45、ovides strong support for the accuracy of the CSD equation. For our mixtures, however, in place of REFROP7 data, modified corresponding states methods (Beyerlein et al. 1993) are used to obtain the saturated vapor densities for calculating the CSD parameters. The COP values for R-22 are 3.5 1 and 4.
46、07, respectively, for the low and high temperature cycles when the parameters of the CSD equation are calculated from saturated vapor densities estimated using modified corre- sponding states methods. These values are in good agreement with the values, 3.38 and 3.92, calculated using CYCLE-D. This p
47、rovides confidence the modified corresponding states methods combined with the CSD equation are useful for eval- uating refigerant performance when more detailed equation of state data are lacking. For consistency, the COP relative to that of R-22 should be estimated by using COP values calculated b
48、y the same methods. Therefore, the COP relative to that of R-22 of refrig- erants investigated in this work using modified corresponding state methods to obtain the saturated vapor density are calcu- lated using the COP values 3.5 l and 4.07 for R-22. Similarly, consistency requires that the COP rel
49、ative to that of R-22 for R-410A and R-407C should be calculated using the COP for R-22 estimated with CYCLE-D. The binary mixtures R-E218(5O%)/R-E143a(50%) (Ku1 et al. 2000b) and R-E125(75%)/R-161(25%) (Ku1 et al. 2001) are azeotropic mixtures. The ternary mixture R-E218(33.3%)/ R-l34a(33.3%)/R-i61(33.3%) is a near azeotropic mixture that exhibits less than a two-degree temperature glide during the evaporation portion of the cycle. The ternary mixtures R- E I 25 (3 3.3 %)/R-32(33.3%)/R- 1 43a(33.3%) and R- El25(33.3%)/R-32(33.3%)/R-134a(
