SERVICING
47
S-106 OVERFEEDING
Overfeeding by the expansion valve results in high suction
pressure, cold suction line, and possible liquid slugging of
the compressor.
If these symptoms are observed:
1. Check for an overcharged unit by referring to the cooling
performance charts in the servicing section.
2. Check the operation of the power element in the valve as
explained in S-110 Checking Expansion Valve Opera-
tion.
3. Check for restricted or plugged equalizer tube.
S-107 UNDERFEEDING
Underfeeding by the expansion valve results in low system
capacity and low suction pressures.
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 S-110 Checking Expansion Valve Opera-
tion.
S-108 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 proce-
dures outlined under Air Flow, Refrigerant Charge, Expan-
sion Valve - Overfeeding, Underfeeding. These are the most
common causes for evaporator malfunction.
CHECKING SUPERHEAT
Refrigerant gas is considered superheated when its tempera-
ture 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.
SUPERHEAT AND SUBCOOLING ADJUSTMENT
ON TXV APPLICATIONS
1. Run system at least 10 minutes to allow pressure to
stabilize.
2. Temporarily install thermometer on liquid (small) line
near pressure switches with adequate contact and
insulate for best possible reading.
3. Check subcooling and superheat. Systems with TXV
application should have a subcooling and superheat of
12 - 15 ºF.
a.
If subcooling and superheat are low,
adjust
TXV
to 9 ± 3ºF then check subcooling.
b.
If subcooling is low and superheat is high,
add
charge to raise subcooling to 10ºF then check
superheat.
c.
If subcooling and superheat are high,
adjust
TXV
valve to 12 - 15 ºF then check subcooling.
d.
If subcooling is high and superheat is low,
adjust
TXV valve to 12 - 15 ºF superheat and
remove
charge to lower the subcooling to 2 - 15 ºF.
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 the indoor coil match, 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.
