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28 TRG-TRC005-EN
notes
period four
Expansion Devices
On the other hand, if too much liquid refrigerant enters the evaporator, not all of
it will be vaporized. As a result, some liquid refrigerant gets into the
compressor suction line. Since the compressor is designed to compress vapor
and not liquid, liquid refrigerant can cause excess wear and damage to the
compressor.
The TXV meters refrigerant by measuring the condition of the refrigerant vapor
leaving the evaporator. Superheat is the additional heat absorbed by the
refrigerant in the evaporator after it has completely vaporized. It provides a
safety factor by preventing liquid refrigerant from entering the compressor.
In the example above, subcooled liquid refrigerant enters the TXV at a
condensing pressure of 290 psia [2 MPa] and a temperature of 109°F [42.8°C].
(The refrigerant condensed at 124.3°F [51.3°C] and was subcooled to 109°F
[42.8°C].) Passing through the TXV causes a large pressure drop, reducing the
refrigerant pressure to that of the evaporator. This pressure drop causes a small
TXV Control
liquid/vapor
liquid/vapor
mixture
mixture
liquid
liquid
refrigerant
refrigerant
Figure 42
Superheat
superheated
superheated
vapor
vapor
TXV
TXV
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]
A
liquid
liquid
refrigerant
refrigerant
Figure 43