SERVICING
32
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.
If a restriction is located, replace the restricted part, replace
drier, evacuate and recharge.
Refrigerant Overcharge
An overcharge of refrigerant is normally indicated by an
excessively high head pressure.
An evaporator coil, using an 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 system 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.
Non-Condensables
If non-condensables are suspected, shut down the system
and allow the pressures to equalize. Wait at least 15
minutes. Compare the 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.
Non-condensables are removed from the system by first
removing the refrigerant charge, replacing and/or installing
liquid line drier, evacuating and recharging.
