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SERVICING
Superheat Formula = Suct. Line Temp. - Sat. Suct. Temp.
Ambient Condenser
Inlet Temp
(°F Drybulb)
100
95
90
85
80
75
70
65
60
SYSTEM SUPERHEAT
EXAMPLE:
a. Suction Pressure = 143
b. Corresponding Temp. °F. = 50
c. Thermometer on Suction Line = 59°F.
To obtain the degrees temperature of superheat, subtract
50.0 from 59.0°F.
The difference is 9° Superheat. The 9° Superheat would
fall in the ± range of allowable superheat.
S-111 FIXED ORIFICE RESTRICTION DEVICES
The fixed orifice restriction device (flowrator) used in con-
junction 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 tempera-
ture 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
Return Air Temp. (°F Drybulb)
65
70
75
80
---
---
---
10
---
---
10
10
---
---
12
15
---
10
13
17
---
10
15
21
10
13
17
25
10
17
20
28
13
19
26
32
17
25
30
33
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
85
line drier, evacuate and recharge.
10
10
18
S-112 CHECKING RESTRICTED LIQUID LINE
20
When the system is operating, the liquid line is warm to the
26
29
touch. If the liquid line is restricted, a definite temperature
32
drop will be noticed at the point of restriction. In severe
35
cases, frost will form at the restriction and extend down the
37
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.
S-113 OVERCHARGE OF REFRIGERANT
An overcharge of refrigerant is normally indicated by an
excessively high head pressure.
An evaporator coil, using an expansion valve metering de-
vice, 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 which may
be found in the "Service Problem Analysis Guide."
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.
S-114 NON-CONDENSABLES
If non-condensables are suspected, shut down the sys-
tem 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.
47
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