26 TRG-TRC003-EN
notes
period four
Pressure–Enthalpy Chart
During this period we will again analyze the basic vapor-compression
refrigeration cycle. However, this time we will use a graphic tool called the
pressure–enthalpy chart.
The pressure–enthalpy (P-
h
) chart plots the properties of a refrigerant—
refrigerant pressure on the vertical axis and enthalpy on the horizontal axis.
Enthalpy is a measure of heat quantity, both sensible and latent, per pound
[kg] of refrigerant. It is typically expressed in terms of Btu/lb [kJ/kg].
The right-hand side of the chart indicates the conditions at which the refrigerant
will be in the vapor phase. The left-hand side of the chart indicates the
conditions at which the refrigerant will be in the liquid phase. In the middle of
the chart is an envelope (curve). The left-hand boundary of the envelope
indicates the saturated liquid condition. The right-hand boundary indicates the
saturated vapor condition. If the enthalpy of the refrigerant lies inside the

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