32 TRG-TRC003-EN
notes
period four
Pressure–Enthalpy Chart
The primary purpose of the expansion device is to drop the pressure of the
liquid refrigerant to equal the evaporator pressure. At this lower pressure, the
refrigerant is now inside the saturation envelope where it exists as a mixture of
liquid and vapor.
The high-pressure liquid refrigerant (*) flows through the expansion device,
causing a large pressure drop. This pressure drop reduces pressure and
temperature of the refrigerant to that of the evaporator ($). At the lower
pressure, the temperature of the refrigerant is higher than its boiling point. This
causes a small portion of the liquid to boil, or flash. Because heat is required to
boil this small portion of refrigerant, boiling refrigerant absorbs heat from the
remaining liquid refrigerant, cooling it to the evaporator temperature. Notice
that there is no change in enthalpy during the expansion process.
The purpose of subcooling the liquid refrigerant in the condenser is to avoid
flashing the refrigerant before it reaches the expansion device. If a valve is used
as the expansion device, the presence of refrigerant vapor can cause improper
operation and premature failure.
H[SDQVLRQ
H[SDQVLRQ
GHYLFH
GHYLFH
SUHVVXUH
SUHVVXUH
HQWKDOS\
HQWKDOS\
([SDQVLRQ'HYLFH
'
*
&
%
(
)
$
Figure 49