SERVICING
28
2. After the system will take all it will take, close the
valve on the high side of the charging manifold.
3. Start the system and charge the balance of the
refrigerant through the low side.
Refrigerant Charge Check
Units with Fixed Orifice Devices
After completing airflow measurements and adjustments
the unit’s refrigerant charge must be checked. All package
units with fixed orifice devices are charged using the super
heat method at the compressor suction line.
After superheat is verified, it is recommended to check unit
sub-cooling at the condenser coil liquid line out.
Superheat
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.
Procedure:
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 15 -20 minutes to allow pressure
to stabilize.
2. Install a low side pressure gauge on the suction line
access fitting.
3. Temporarily install thermometer on suction (large)
line near compressor with adequate contact and
insulate for best possible reading.
4. Record the gauge pressure corresponding
temperature and the temperature of the suction line.
5. Refer to the superheat table for proper system
superheat. Add charge to lower superheat recover
charge to raise 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
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 Formula = Suct. Line Temp. - Sat. Suct.
Temp.
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.
Procedure:
1. Attach an accurate thermometer or preferably a
thermocouple type temperature tester to the liquid
line close to the pressure switch.
2. Install a high side pressure gauge on the liquid
access fitting.
3. Record the gauge pressure and the temperature of
the line.
4.
The difference between the thermometer reading and
pressure to temperature conversion is the amount of
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.
See R410A Pressure vs. Temperature Chart
