adjustment), maintain a superheat ranging
approximately from 4° to 12 °F at the cooling coil
outlet. The proper superheat adjustment varies,
of course, with the design and the service
operating conditions of the valve, and the design
of a particular plant. Increased spring pressure
increases the degree of superheat at the coil outlet;
decreased pressure on the other hand, has the
opposite effect. Many thermostatic expansion
valves are initially adjusted by the manufacturer
to maintain a predetermined degree of superheat,
and no provisions are made for further ad-
justments in service.
When the expansion valves are adjusted to give
a high degree of superheat at the coil outlet, or
when a valve is stuck shut, the amount of
refrigerant admitted to the cooling coil is reduced.
With an insufficient amount of refrigerant, the
coil is starved and operates at a reduced capac-
ity. Compressor lubricating oil carried with the
refrigerant tends to collect at the bottom of the
cooling coils, thus robbing the compressor
crankcase, and providing a condition whereby
slugs of lubricating oil are drawn back to the com-
pressor. If an expansion valve is adjusted for too
low a degree of superheat, or if the valve is stuck
open, the liquid refrigerant may flood from the
cooling coils back to the compressor. Should the
liquid refrigerant collect at a low point in the suc-
tion line or coil, and be drawn back to the com-
pressor intermittently in slugs, there will be danger
of injury to the moving parts of the compressor.
In general, the expansion valves for air con-
ditioning and water cooling plants (high
temperature installations) are adjusted for higher
superheat than the expansion valves for cold
storage refrigeration and ships service store
equipment (low temperature installations).
If it is impossible to adjust expansion valves
to the desired settings, or if it is suspected that
the expansion valve assembly is defective and re-
quires replacement, make appropriate tests. (First
make sure that the liquid strainers are clean, that
the solenoid valves are operative, and that the
system is sufficiently charged with refrigerant.)
The major equipment required for expansion
valve tests is as follows:
1. A service drum of R-12, or a supply of
clean dry air at 70 to 100 psig. The service drum
is used to supply gas under pressure. The gas
used does not have to be the same as that
employed in the thermal element of the valve be-
2. A high pressure and a low pressure gage.
The low pressure gage should be accurate and in
good condition so that the pointer does not have
any appreciable lost motion. The high pressure
gage, while not absolutely necessary, is useful in
showing the pressure on the inlet side of the valve.
Normally, refrigeration plants are provided with
suitable replacement and test pressure gages.
The procedure for testing is as follows:
1. Connect the valve inlet to the gas supply
with the high pressure gage attached to indicate
the gas pressure to the valve, and the low pressure
gage loosely connected to the expansion valve
outlet. The low pressure gage must be connected
up loosely so as to provide a small amount of
leakage through the connection.
2. Insert the expansion valve thermal element
in a bath of crushed ice. Do not perform this test
with a container full of water in which only a small
amount of crushed ice is floating.
3. Open the valve on the service drum or in
the air supply line. Make certain that the gas
supply is sufficient to build up the pressure to at
least 70 psi on the high pressure gage.
4. Adjust the expansion valve, if it is desired
to adjust for 10°F superheat, the pressure on the
outlet gage should be 22.5 psig. This pressure is
equivalent to the pressure of an R-12 evaporating
temperature of 22°F. Since the ice maintains the
bulb at 32°F, the valve adjustment is for 10°F
superheat (difference between 32 and 22). For a
5 °F superheat adjustment, the valve should be
adjusted to give a pressure of approximately
26.1 psig. Allow for a small amount of leakage
through the low pressure gage connection while
this adjustment is being made.
5. To determine if the valve operates
smoothly, tap the valve body lightly with a small
weight. The low pressure gage needle should not
jump more than 1 psi.
6. Tighten the low pressure gage connection
and stop the leakage at the joints. Determine if
the expansion valve seats tightly. If the valve is
in good condition, the pressure will increase a few
pounds and then either stop or build up very
ENGINEMAN 1 & C