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SERVICING
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 Restrictor Devices
The fixed orifice restrictor 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 restrictor 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.
A strainer is placed on the entering side of the tube to
prevent any foreign material from becoming lodged inside
the fixed orifice restriction device.
If a restriction should become evident, proceed as follows:
1. Recover refrigerant charge.
2. Remove the orifice or tube strainer assembly and
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
restrictor 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.
Overcharge Of Refrigerant
An overcharge of refrigerant is normally indicated by an
excessively high head pressure.
An evaporator coil, using an TXV expansion valve metering
device, will basically modulate and control a flooded
evaporator and prevent liquid refrigerant return to the
compressor.
An evaporator coil, using a fixed orifice restrictor device
(flowrator) metering device, could allow liquid refrigerant
to return to the compressor under extreme overcharge
conditions.
Also with a fixed orifice restrictor device (flowrator)
metering device, extreme cases of insufficient indoor air
can cause icing of the indoor coil and liquid refrigerant
return to the compressor, but the head pressure would be
lower.
There are other causes for high head pressure.
If other causes check out normal, an overcharge or a
system containing non-condensables would be indicated.
If this is observed:
1. Start the system.
2. Remove and capture small quantities of refrigerant
as from the suction line access fitting 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.
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