Trane trg-trc005-en manual Txv

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.

Superheat

	85 psia, 41ºF
	[0.59 MPa, 5ºC]
	TXV
79 psia	290 psia, 109ºF
	[2 MPa, 42.8ºC]

[0.54 MPa]

49ºF

[9.4ºC]

Figure 44

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.

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