Service Manual

SERVICING
47
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.
S-109 CHECKING SUBCOOLING
Refrigerant liquid is considered subcooled when its tem-
perature is lower than the saturation temperature corre-
sponding to its pressure. The degree of subcooling equals
the degrees of temperature decrease below the saturation
temperature at the existing pressure.
1. Attach an accurate thermometer or preferably a thermo-
couple type temperature tester to the liquid line close to
the pressure switch.
2. Install a high side pressure gauge on the high side
(liquid) service valve at the front of the unit.
3. Record the gauge pressure and the temperature of the
line.
4. Compare the hi-pressure reading to the "Required Liquid
Line Temperature" chart on the following page. Find the
hi-pressure value on the left column. Follow that line right
to the column under the design subcooling value. Where
the two intersect is the required liquid line temperature.
Alternately you can convert the liquid line pressure
gauge reading to temperature by finding the gauge
reading in Temperature - Pressure Chart and reading to
the left, find the temperature in the °F. Column.
5. The difference between the thermometer reading and
pressure to temperature conversion is the amount of
subcooling.
Add charge to raise subcooling. Recover charge to lower
subcooling.
Subcooling Formula = Sat. Liquid Temp. - Liquid Line Temp.
EXAMPLE:
a. Liquid Line Pressure = 417
b. Corresponding Temp. °F. = 120°
c. Thermometer on Liquid line = 109°F.
To obtain the amount of subcooling subtract 109°F from
120°F.
The difference is 11° subcooling. See the specification
sheet or technical information manual for the design
subcooling range for your unit.
S-110 CHECKING EXPANSION VALVE OPERA-
TION
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.