SERVICING
28
Checking expansion valve operation
1. Remove the remote bulb of the expansion valve from
the suction line.
2. Start the system and cool the bulb in a container of
ice water, closing the valve. As you cool the bulb,
the suction pressure should fall and the suction
temperature will rise.
3. Next warm the bulb in your hand. As you warm the
bulb, the suction pressure should rise and the suction
temperature will fall.
4. If a temperature or pressure change is noticed, the
expansion valve is operating. If no change is noticed,
the valve is restricted, the power element is faulty, or
the equalizer tube is plugged.
5. Capture the charge, replace the valve and drier and
evacuate.
Fixed Orifice Restriction Devices
The fixed orifice restriction device (flowrator) used in
conjunction with the indoor coil is a predetermined bore
(I.D.).
It is designed to control the rate of liquid refrigerant flow
into an evaporator coil.
The amount of refrigerant that flows through the fixed
orifice restriction device is regulated by the pressure
difference between the high and low sides of the system.
In the cooling cycle when the outdoor air temperature rises,
the high side condensing pressure rises. At the same time,
the cooling load on the indoor coil increases, causing the
low side pressure to rise, but at a slower rate.
Since the high side pressure rises faster when the
temperature increases, more refrigerant flows to the
evaporator, increasing the cooling capacity of the system.
When the outdoor temperature falls, the reverse takes
place. The condensing pressure falls, and the cooling loads
on the indoor coil decreases, causing less refrigerant flow.
If a restriction should become evident, proceed as follows:
1. Recover refrigerant charge.
2.
Remove the orifice assembly and clean or replace.
3. Replace liquid line drier, evacuate and recharge.
Checking Equalization Time
During the “OFF” cycle, the high side pressure bleeds to
the low side through the fixed orifice restriction device.
Check equalization time as follows:
1. Attach a gauge manifold to the suction and liquid line
dill valves.
2. Start the system and allow the pressures to stabilize.
3. Stop the system and check the time it takes for the
high and low pressure gauge readings to equalize.
If it takes more than seven (7) minutes to equalize, the
restriction device is inoperative. Replace, install a liquid line
drier, evacuate and recharge.
Checking Restricted Liquid Line
When the system is operating, the liquid line is warm to the
touch. If the liquid line is restricted, a definite temperature
drop will be noticed at the point of restriction. In severe
cases, frost will form at the restriction and extend down the
line in the direction of the flow.
Discharge and suction pressures will be low, giving the
appearance of an undercharged unit. However, the unit will
have normal to high subcooling.
Locate the restriction, replace the restricted part, replace
drier, evacuate and recharge.
Refrigerant Overcharge
An overcharge of refrigerant is normally indicated by
excessively high head pressure and/or liquid return to the
compressor.
If high head pressure is not indicated, an overcharge or a
system containing non-condensables could be the problem.
If overcharging is indicated:
1. Start the system.
2. Remove small quantities of gas from the suction
line dill valve until the head pressure is reduced to
normal.
3. Observe the system while running a cooling
performance test, if a shortage of refrigerant
is indicated, then the system contains non-
condensables.
Non-Condensables
Check for non-condensables:
1. Shut down the system and allow the pressures to
equalize for a minimum of 15 minutes.
2. Take a pressure reading.
3. Compare this pressure to the temperature of the
coldest coil since this is where most of the refrigerant
will be. If the pressure indicates a higher temperature
than that of the coil temperature, non-condensables
are present.
To remove the non-condensables:
1. Remove the refrigerant charge.
2. Replace and/or install liquid line drier.
3. Evacuate and recharge.
Compressor Burnout
When a compressor burns out, high temperature develops
causing the refrigerant, oil and motor insulation to
decompose forming acids and sludge.
If a compressor is suspected of being burned-out, attach
a refrigerant hose to the liquid line dill valve and properly
remove and dispose of the refrigerant.
