DuPont
™
Suva
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REFRIGERANTS
Technical Information
The PH diagram is a common method of graphically present-ing the thermodynamic properties of a refrigerant. From this diagram, approximate thermodynamic properties can be quickly determined. If accuracy is essential, detailed thermo-dynamic tables must be used.
The PH diagram can be used to determine approximately any of the following theoretical values:
1. Condenser Pressure. 2. Evaporator Pressure.
3. Heat Content (enthalpy) of liquid.
4. Heat Content (enthalpy) of saturated vapor. 5. Latent heat of vaporization.
6. Refrigerating effect.
7. Weight of refrigerant circulated per ton of refrigeration. 8. Specifi c volume of vapor leaving evaporator.
9. Entropy of vapor.
10. Temperature of vapor after compression.
12. Horsepower requirement per ton of refrigeration. 13. Heat removed in condenser.
14. Volume of vapor handled per ton of refrigeration. 15. Specifi c heat of the vapor at constant pressure (Cp). 16. Specifi c heat of the vapor at constant volume (Cv).
The use of a PH diagram is illustrated by means of the follow-ing examples usfollow-ing Suva® R-134a refrigerant.
For simplifi cation, it is assumed that the temperature of the liquid leaving the condenser and entering the evaporator is 86°F, while the temperature of liquid and vapor in the evapora-tor and the vapor entering the compressor is 5°F. It has been assumed that the other refrigerant properties do not change from leaving the condenser to entering the evaporator, or from leaving the evaporator to entering the compressor.
NOTE: The above stated conditions can be plotted on the PH diagram at the end of this paper.
Using the Pressure Enthalpy Diagram
The PH Diagram
Table 1
Examples �f Therm�dynamic Pr�perties ... �u�a� 134a (R-134a)
Pressure, psia Volume, cu. ft./lb. Density, lb./cu. ft. Enthalpy, BTU/lb. Entropy, BTU/(lb.) (°R) Temp., °F Liquid Vapor Liquid Vapor Liquid Vapor Liquid Latent Vapor Liquid Vapor
–40 7.42 7.42 0.0113 5.7904 88.31 0.1727 0.0 97.2 97.2 0.0000 0.2316 –20 12.89 12.89 0.0116 3.4471 86.30 0.2901 6.0 94.2 100.2 0.0139 0.2281 0 21.16 21.16 0.0119 2.1580 84.23 0.4634 12.1 91.0 103.1 0.0275 0.2255
20 33.13 33.13 0.0122 1.4090 82.08 0.7097 18.4 87.7 106.0 0.0408 0.2235 40 49.77 49.77 0.0125 0.9523 79.82 1.0501 24.8 84.1 108.8 0.0538 0.2220 60 72.17 72.17 0.0129 0.6622 77.43 1.5102 31.4 80.2 111.5 0.0666 0.2208
80 101.49 101.49 0.0134 0.4709 74.89 2.1234 38.1 75.9 114.0 0.0792 0.2199 100 139.00 139.00 0.0139 0.3408 72.13 2.9347 45.1 71.2 116.3 0.0918 0.2190 120 186.02 186.02 0.0145 0.2494 69.10 4.0089 52.4 65.9 118.3 0.1043 0.2181
For additional thermodynamic properties refer to DuPont publication T-134a – ENG.
Figure 1. DuP�nt� �u�a� Pressure Enthalpy (PH) Diagram (IP) Values
1. Condenser Pressure: The 86°F point of the saturated liquid line gives a pressure of 112 psia, which is read on the left hand side of the diagram.
2. Evaporator Pressure: The 5°F point of the saturated liquid line gives a pressure of 24 psia, which is read on the left hand side of the diagram.
3. Heat Content (Enthalpy) of Liquid: The 86°F point on the saturated liquid line gives a heat content of the liquid leaving the condenser (or entering the evapora-tor) or 40.2 BTU/lb., which can be read on the horizontal marginal scales.
4. Heat Content (Enthalpy) of Saturated Vapor: The 5°F point on the saturated vapor line gives a heat content of the vapor leaving the evaporator (or entering the com-pressor) or 103.9 BTU/lb., which can be read on the horizontal marginal scales.
5. Latent Heat of Vaporization: The latent heat of vapor-ization may be defi ned as the difference in heat content of saturated vapor and saturated liquid at the same tem-perature. At the evaporator temperature of 5°F, the heat content of the saturated liquid is 13.7 BTU/lb. Thus, the latent heat of vaporization at 5°F is 90.2 BTU/lb. (103.9 – 13.7 = 90.2).
6. Refrigerating Effect: The theoretical refrigerating ef-fect may be simply determined by subtracting the heat content of the liquid entering the evaporator at 86°F from the heat content of the vapor leaving the evaporator at 5°F. Thus the theoretical refrigerating effect is 63.7 BTU/ lb. (103.9 – 4.2 = 63.7).
7. Weight of Refrigerant Circulated per Ton of Refriger-ation: The standard ton of refrigeration may be defi ned as the removal of 200 BTU of heat per minute. Thus 3.1 pounds of Suva® R-134a per minute per ton of refrigera-tion must be circulated in this case.
8. Specifi c Volume of Vapor Leaving Evaporator: The specifi c volume of the vapor leaving the evaporator (or entering the compressor) can be determined by referring to the constant volume lines intersecting the saturated vapor line nearest the 5°F point. The specifi c volume of the vapor thus determined is 1.93 cu. ft./lb.
9. Entropy of Vapor: The entropy of the vapor can be determined by referring to the constant entropy line intersecting the saturated vapor lines nearest the 5°F point. This gives a vapor of 0.225 BTU/lb/°R.
10. Temperature of Vapor after Compression: If the com-pression is assumed to be isentropic (heat of compres-sion added at constant entropy), the theoretical tem-perature at the point in the superheat region where the constant entropy line of 0.225 BTU/lb/°R intersects the condenser pressure line of 112 psi is the proper value. Thus, the theoretical temperature of the vapor after compression is 98°F, which is estimated by referring to the constant temperature lines nearest the specifi c point on the diagram.
11. Heat Equivalent of Compression Power: The theoreti-cal heat equivalent of compression power (isentropic compression) can be obtained by subtracting the heat content of the vapor entering from the heat content of the vapor leaving the compressor. The heat content of the vapor leaving the compressor can be read from the horizontal marginal scale for the point determined imme-diately above on the 98°F constant temperature line. This gives 117 BTU/lb. for the heat content of vapor leaving the compressor, and thus a heat equivalent of compres-sion power of 13.1 BTU/lb. (117.0 – 103.9 = 13.1).
12. Horsepower Requirement per Ton of Refrigeration: The theoretical required horsepower per ton of refrigera-tion now can easily be calculated for this system.
Required H.P./Ton =
13. Heat Removed in Condenser: The heat removed in the condenser may be determined by the difference in the heat content of the superheated vapor entering the condenser and the saturated liquid (zero degrees of sub-cooling) leaving the condenser. This gives 76.8 BTU/lb. of heat removed in the condenser (117 – 40.2 = 76.8). 200 BTU/min./ton
63.7 BTU/lb = 3.1 lbs./min./ton
76.8
Heat of Compression (BTU/lb. x 200 (BTU/min/ton) x 778 (ft.-lb./BTU) Refrigereation Effect (BTU/lb.) x 33,000 (ft/-lb./min./H.P.)
Heat of Compression x 4.715 Refrigeration Effect =
13.1 x 4.715 63.7
14. Volume of Vapor Handled per Ton of Refrigeration: This value can be determined by multiplying the number of pounds of refrigerant circulated per minute per ton of refrigeration by the specifi c volume of the vapor entering the compressor (or leaving the evaporator). This gives 5.98 cu. ft./min./ton (3.1 x 1.98 = 5.98), which is the theoretical piston displacement or volumetric displace-ment in cubic feet per minute per ton of refrigeration.
15. Specifi c Heat of the Vapor at Constant Pressure (Cp): Find the enthalpy of the vapor at temperatures just below and just above that desired, but at the same pressure. Find the difference in the enthalpy and divide by the dif-ference in temperatures. To fi nd the Cp for Suva® R-134a at 150°F and 100 psia.
Enthalpy at 160°F and 100 psia = 133.2 BTU/lb. Enthalpy at 140°F and 100 psia = 128.5 BTU/lb.
Enthalpy difference = 4.7 BTU/lb.
= 0.235 BTU/lb./°F = 0.235 BTU/lb./°F4.7 BTU/lb. = 0.235 BTU/lb./°F = 0.235 BTU/lb./°F
20°F Cv =
4.4 BTU/lb. 0.093 BTU/lb. 4.493 BTU/lb.
= 0.22 BTU/lb./°F 4.493 BTU/lb.
20°F
16. Specifi c Heat of the Vapor at Constant Volume (Cv): The procedure is similar to that used for fi nding Cp ex-cept that the enthalpy must be correct for the product of the volume times the change in pressure.
To fi nd the Cv for Suva® R-134a at 150°F and a volume of 1.00 cu. ft./lb.
Enthalpy of vapor at 160°F and 1.00 cu. ft./lb. = 134.7 Enthalpy of vapor at 140°F and 1.00 cu. ft./lb. = 130.3
Enthalpy difference = 4.4
Pressure at 160°F and 1.00 cu. ft./lb. = 61.5 psia Pressure at 140°F and 1.00 cu. ft./lb. = 61.0 psia
Pressure difference = 61.5 psia
J = 0.18505 (Conversion factor from Work Units to Heat Units (psia) (cu. ft. lb.) to heat units (BTU/lb.)
5
HFC-134a
Pressure-Enthalpy Diagram
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