SERVICING
26
If these symptoms are observed:
1. Check for a restricted liquid line or drier. A restriction
will be indicated by a temperature drop across the
drier.
2. Check the operation of the power element of the
valve as described in Checking Expansion Valve
Operation.
Superheat
The expansion valves are factory adjusted to maintain
12 to 15 degrees superheat of the suction gas. Before
checking the superheat or replacing the valve, perform
all the procedures outlined under Air Flow, Refrigerant
Charge, Expansion Valve - Overfeeding, Underfeeding.
These are the most common causes for evaporator
malfunction.
Checking Superheat
Refrigerant gas is considered superheated when its
temperature is higher than the saturation temperature
corresponding to its pressure. The degree of superheat
equals the degrees of temperature increase above
the saturation temperature at existing pressure. See
Temperature - Pressure Chart on following page.
CAUTION
To prevent personal injury, carefully connect and
disconnect manifold gauge hoses. Escaping liquid
refrigerant can cause burns. Do not vent refrigerant to
atmosphere. Recover during system repair or final unit
disposal.
1. Run system at least 10 minutes to allow pressure to
stabilize.
2. Temporarily install thermometer on suction (large)
line near compressor with adequate contact and
insulate for best possible reading.
3. Refer to the superheat table provided for proper
system superheat. Add charge to lower superheat or
recover charge to raise superheat.
Superheat Formula = Suct. Line Temp. - Sat. Suct.
Temp.
EXAMPLE:
A. Suction Pressure = 143
B. Corresponding Temp. °F. = 50
C. Thermometer on Suction Line = 61°F.
To obtain the degrees temperature of superheat, subtract
50.0 from 61.0°F.
The difference is 11° Superheat. The 11° Superheat would
fall in the ± range of allowable superheat.
100
-
-
-
10
10
95
-
-
10
10
10
90
-
-
12
15
18
85
-
10
13
17
20
80
-
10
15
21
26
75
10
13
17
25
29
70
10
17
20
28
32
65
13
19
26
32
35
60
17
25
30
33
37
Ambient Condenser
Inlet Temp (°F
Drybulb)
65
70
75
Return Air Temp. (°F Drybulb)
80
85
Superheat And Subcooling Adjustment On TXV
Applications
Single Speed Application (*PC1460)
1. Purge gauge lines. Connect service gauge manifold
to access fittings. Run system at least 10 minutes to
allow pressure to stabilize.
2. Temporarily install thermometer on liquid (small) line
near liquid line access fitting with adequate contact
and insulate for best possible reading.
3. Check subcooling and superheat. Systems with TXV
application should have a subcooling of 10 + 2°F and
superheat of 15 to 18ºF.
A. If subcooling and superheat are low, adjust TXV
to 15 - 18ºF then check subcooling.
B. If subcooling is low and superheat is high, add
charge to raise subcooling to 10 ± 2ºF then
check superheat.
C. If subcooling and superheat are high, adjust TXV
valve to 15 - 18ºF then check subcooling.
D. If subcooling is high and superheat is low, adjust
TXV valve to 15 to 18ºF superheat and remove
charge to lower the subcooling to 10 ± 2ºF.
The TXV should NOT be adjusted at light load conditions
55º to 60ºF, under such conditions only the subcooling
can be evaluated. This is because suction pressure is
dependent on indoor airflow, and wet bulb temperature.
NOTE: Do NOT adjust charge based on suction
pressure unless there is a gross undercharge.
4. Disconnect manifold set. Installation is complete.
Subcooling = Sat. Liquid Temp. - Liquid Line Temp.
