Checking Subcooling; Checking Expansion Valve Operation; Checking Restricted Liquid Line - Daikin ComfortNet DX18TC Service Instructions Manual

SERVICING
2. For best results, temporarily install a thermometer on the
liquid line at the liquid line service valve and 4-6" from the
compressor on the suction line. Ensure the thermometer
makes adequate contact and is insulated for best pos-
sible readings. Use liquid line temperature to determine
sub-cooling and vapor temperature to determine super-
heat.
NOTE: An optional method is to locate the thermometer
at the suction line service valve. Ensure the thermometer
makes adequate contact and is insulated for best
possible readings.
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.
S-109 CHECKING SUBCOOLING
Refrigerant liquid is considered subcooled when its temperature is
lower than the saturation temperature corresponding 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 as it
leaves the condensing unit.
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. Review the technical information manual or specification
sheet for the model being serviced to obtain the design
subcooling.
5. Compare the hi-pressure reading to the "Required Liquid
Line Temperature" chart (page 108). 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.
6. 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.
3. Next warm the bulb in your hand. As you warm the bulb,
the suction pressure should rise and the suction tempera-
ture will fall.
4. If a temperature or pressure change is noticed, the
expansion valve is operating. If no change is noticed, the
valve is restricted, the power element is faulty, or the
equalizer tube is plugged.
5. Capture the charge, replace the valve and drier, evacuate
and recharge.
S-112 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.
Locate the restriction, replace the restricted part, replace
drier, evacuate and recharge.
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