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TRG-TRC005-EN 29
period four
Expansion Devices
notes
portion of the liquid to boil off, or flash, and has the effect of cooling the
remaining liquid refrigerant to the desired evaporator temperature. The
resulting pressure of the refrigerant is 85 psia [0.59 MPa], which corresponds to
41°F [5°C].
Inside the evaporator tubes, as heat is transferred to the liquid refrigerant it
boils until only vapor remains (A). From this point, the vapor continues to
absorb heat as it passes through the final lengths of coil tubing, superheating
the vapor.
Continuing with this example, the refrigerant enters the evaporator coil at
85 psia [0.59 MPa] and 41°F [5°C]. Assuming that the pressure drop through the
coil tubes is 6 psi [0.04 MPa], the refrigerant vapor leaves the coil at 79 psia
[0.54 MPa]. The temperature gauge at the outlet of the evaporator coil indicates
that the superheated refrigerant vapor leaves at 49°F [9.4°C].
A table of refrigerant properties, for Refrigerant-22 in this example, would show
that the 79 psia [0.54 MPa] pressure corresponds to a 37°F [2.8°C] evaporating
temperature. The 12°F [6.7°C] temperature difference between this evaporating
temperature and the temperature measured at the outlet of the evaporator is
the amount of additional heat, or superheat, absorbed by the refrigerant vapor
in the final lengths of coil tubing. Notice that superheating the refrigerant vapor
changed only its temperature—not its pressure.
These same properties—the evaporator pressure and the final refrigerant vapor
temperature—are measured and used by the thermostatic expansion valve to
control the quantity of liquid refrigerant entering the evaporator.
Superheat
TXV
TXV
49ºF
49ºF
[9.4ºC]
[9.4ºC]
79
79
psia
psia
[0.54
[0.54
MPa
MPa
]
]
290
290
psia
psia
, 109ºF
, 109ºF
[2
[2
MPa
MPa
, 42.8ºC]
, 42.8ºC]
85
85
psia
psia
, 41ºF
, 41ºF
[0.59
[0.59
MPa
MPa
, 5ºC]
, 5ºC]
Figure 44