DuPont™ Suva® REFRIGERANTS
Technical Information
Using the Pressure Enthalpy Diagram The PH Diagram This paper is for general purpose information only. For detailed engineering data always refer to specific engineering publications.
The PH diagram is a common method of graphically presenting the thermodynamic properties of a refrigerant. From this diagram, approximate thermodynamic properties can be quickly determined. If accuracy is essential, detailed thermodynamic 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. Specific volume of vapor leaving evaporator. 9. Entropy of vapor. 10. Temperature of vapor after compression. 11. Heat equivalent of compression power.
12. Horsepower requirement per ton of refrigeration. 13. Heat removed in condenser. 14. Volume of vapor handled per ton of refrigeration. 15. Specific heat of the vapor at constant pressure (Cp). 16. Specific heat of the vapor at constant volume (Cv). The use of a PH diagram is illustrated by means of the following examples using Suva® R-134a refrigerant. For simplification, 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 evaporator 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.
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Table 1 Examples of Thermodynamic Properties ... Suva® 134a (R-134a) Temp., °F
Pressure, psia Liquid Vapor
Volume, cu. ft./lb. Liquid Vapor
Density, lb./cu. ft. Liquid Vapor
Liquid
Enthalpy, BTU/lb. Latent Vapor
Entropy, BTU/(lb.) (°R) Liquid Vapor
–40 –20 0
7.42 12.89 21.16
7.42 12.89 21.16
0.0113 0.0116 0.0119
5.7904 3.4471 2.1580
88.31 86.30 84.23
0.1727 0.2901 0.4634
0.0 6.0 12.1
97.2 94.2 91.0
97.2 100.2 103.1
0.0000 0.0139 0.0275
0.2316 0.2281 0.2255
20 40 60
33.13 49.77 72.17
33.13 49.77 72.17
0.0122 0.0125 0.0129
1.4090 0.9523 0.6622
82.08 79.82 77.43
0.7097 1.0501 1.5102
18.4 24.8 31.4
87.7 84.1 80.2
106.0 108.8 111.5
0.0408 0.0538 0.0666
0.2235 0.2220 0.2208
80 100 120
101.49 139.00 186.02
101.49 139.00 186.02
0.0134 0.0139 0.0145
0.4709 0.3408 0.2494
74.89 72.13 69.10
2.1234 2.9347 4.0089
38.1 45.1 52.4
75.9 71.2 65.9
114.0 116.3 118.3
0.0792 0.0918 0.1043
0.2199 0.2190 0.2181
For additional thermodynamic properties refer to DuPont publication T-134a – ENG.
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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 evaporator) 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 compressor) or 103.9 BTU/lb., which can be read on the horizontal marginal scales.
5.
Latent Heat of Vaporization: The latent heat of vaporization may be defined as the difference in heat content of saturated vapor and saturated liquid at the same temperature. 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 effect 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.
63.7 BTU/lb
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 compression is assumed to be isentropic (heat of compression added at constant entropy), the theoretical temperature 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 specific point on the diagram. 11. Heat Equivalent of Compression Power: The theoretical 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 immediately 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 compression 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 refrigeration now can easily be calculated for this system. Required H.P./Ton = Heat of Compression (BTU/lb. x 200 (BTU/min/ton) x 778 (ft.-lb./BTU)
Weight of Refrigerant Circulated per Ton of Refrigeration: The standard ton of refrigeration may be defined as the removal of 200 BTU of heat per minute. Thus 3.1 pounds of Suva® R-134a per minute per ton of refrigeration must be circulated in this case. 200 BTU/min./ton
8.
9.
Refrigereation Effect (BTU/lb.) x 33,000 (ft/-lb./min./H.P.) =
= = 3.1 lbs./min./ton
Heat of Compression x 4.715 Refrigeration Effect 13.1 x 4.715 63.7
= 0.970
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 subcooling) leaving the condenser. This gives 76.8 BTU/lb. of heat removed in the condenser (117 – 40.2 = 76.8).
Specific Volume of Vapor Leaving Evaporator: The specific 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 specific volume of the vapor thus determined is 1.93 cu. ft./lb.
Heat of rejection ratio is therefore
76.8 63.7
= 1.2
The condenser load is 20% greater than the evaporator load.
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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 specific 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 displacement in cubic feet per minute per ton of refrigeration.
16. Specific Heat of the Vapor at Constant Volume (Cv): The procedure is similar to that used for finding Cp except that the enthalpy must be correct for the product of the volume times the change in pressure. To find 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
15. Specific 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 difference in temperatures. To find the Cp for Suva® R-134a at 150°F and 100 psia.
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
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. 4.7 BTU/lb. 20°F
(Conversion factor from Work Units to Heat Units (psia) (cu. ft. lb.) to heat units (BTU/lb.)
(1.00 cu. ft./lb.) x (0.5 psia) x (0.18505 J factor) = 0.093 BTU/lb 4.4 BTU/lb. 0.093 BTU/lb.
= 0.235 BTU/lb./°F
4.493 BTU/lb.
Cv =
4.493 BTU/lb. 20°F
= 0.22 BTU/lb./°F
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(Engineering Units)
Pressure-Enthalpy Diagram
HFC-134a
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