TRG-TRC003-EN 29
period four
Pressure–Enthalpy Chart
notes
At the inlet to the evaporator, the refrigerant is at a pressure of 85 psia
[0.59 MPa] and a temperature of 41.2°F [5.1°C], and is a mixture of liquid and
vapor (mostly liquid). This cool, low-pressure refrigerant enters the evaporator
($) where it absorbs heat from the relatively warm air that is being cooled. This
transfer of heat boils the liquid refrigerant inside the evaporator and
superheated refrigerant vapor is drawn to the compressor (&).
The change in enthalpy from A to C that occurs inside the evaporator is called
the refrigeration effect. This is the amount of heat that each pound [kg] of
liquid refrigerant will absorb when it evaporates.
Compressors are designed to compress vapor. Liquid refrigerant can cause
damage if drawn into the compressor. In some refrigeration systems additional
heat is added to the saturated vapor (%) in the evaporator to ensure that no
liquid is present at the compressor inlet. This additional amount of heat, above
saturation, is called superheat. This superheated vapor (&) is generally 8°F to
12°F [4.4°C to 6.7°C] above the saturated vapor condition when it enters the
compressor. In this example, the refrigerant vapor is superheated 10°F [5.6°C],
from 41.2°F [5.1°C] to 51.2°F [10.7°C].
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Figure 45
