TRG-TRC003-EN 33
period four
Pressure–Enthalpy Chart
notes
The temperature of the refrigerant entering the expansion device (*) is 110°F
[43.3°C] and its pressure is 280 psia [1.93 MPa]. (The refrigerant condensed at
121.5°F [49.7°C] and was subcooled to 110°F [43.3°C].) The enthalpy of the
refrigerant at this condition is 42.4 Btu/lb [98.6 kJ/kg]. As mentioned previously,
there is no change in enthalpy during the expansion process—it is the same at
both * and $.
The refrigerant leaves the expansion device ($) at evaporator conditions,
85 psia [0.59 MPa] and 41.2°F [5.1°C]. At this pressure, the enthalpy of saturated
liquid is 21.8 Btu/lb [50.7 kJ/kg] and the enthalpy of saturated vapor is
108.2 Btu/lb [251.7 kJ/kg]. Because there is no change of enthalpy during the
expansion process, the mixture of liquid and vapor leaving the expansion
device must have the same enthalpy as the liquid entering the expansion
device. This is true if 76.2% of the refrigerant is liquid and 23.8% of the
refrigerant is vapor. This is determined as shown below:
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Figure 50
% of Refrigerant Vapor at A
h
A
h
saturated liquid
–
h
saturated vapor
h
saturated liquid
–
=
% of Refrigerant Vapor at
A
42.4 Btu/lb 21.8 Btu/lb–
108.2 Btu/lb 21.8 Btu/lb–
23.8%==
% of Refrigerant Vapor at
A
98.6 kJ/kg 50.7 kJ/kg–
251.7 kJ/kg 50.7 kJ/kg–
23.8%==
