Refrigeration Maintenance
106
Refrigerant Charge
Testing the Refrigerant Charge with
an Empty Truck
If the unit has an insufficient charge of refrigerant,
the evaporators will be “starved” and the box
temperature will rise even though the unit is
operating. The suction pressure will drop as the
refrigerant charge decreases. If the unit has an
overcharge of refrigerant, the unit may not cool
properly and the suction and discharge pressure
may be high. The charge may be determined by
checking the refrigerant level in the receiver tank
sight glass with the following conditions
established:
1. Place a test box over each evaporator.
2. Place a thermometer lead (P/N 204-135) in
each box near the evaporator return air
opening.
3. Install a gauge manifold.
4. Run both evaporators in Cool until the air in
each box is at 32 F (0 C). By allowing a box to
leak a small amount, you will be able to
maintain 32 F (0 C).
5. The suction pressure should be 12 to 18 psi
(84 to 124 kPa).
6. The discharge pressure should be at least
275 psi (1896 kPa).
If the discharge pressure is below this, it can
be raised by covering a portion of the
condenser grille with a piece of cardboard to
block the condenser airflow.
7. Under these conditions, the ball in the receiver
tank sight glass should be floating.
Testing the Refrigerant Charge with
a Loaded Truck
1. Install a gauge manifold.
2. Run both evaporators in Cool.
3. Build up and maintain a head pressure of 275
psi (1896 kPa).
If the pressure is below this, it can be raised by
covering the condenser grille with a piece of
cardboard to block the condenser airflow.
4. Cool the compartments to lowest temperatures
required.
5. Check the suction pressure. It should be
12 to 18 psi (84 to 124 kPa).
6. Under these conditions, the ball in the receiver
tank sight glass should be floating. If there is
no indication of refrigerant in the receiver tank
sight glass, the unit is low on refrigerant.
NOTE: If the ball floats, there is enough
refrigerant in the unit for that load at that
particular thermostat setting. This test does
not determine if the unit contains a full
charge of refrigerant.
Testing for an Overcharge
Use the following procedure to identify a unit
with an excessive refrigerant charge:
1. Install a calibrated gauge manifold on the
compressor.
2. Operate the condensing unit in high speed
with both evaporators in Cool to stabilize
system pressures and reduce the box
temperature to approximately 60 F (16 C) or
colder.
3. Observe the discharge pressure and cover the
condenser to increase the discharge pressure
approximately 75 to 100 psi (517 to 689 kPa)
above the observed pressure.
NOTE: If the ball and liquid level in the
receiver sight glass drop during step 3, the
unit is not overcharged and it is not
necessary to complete the procedure.
4. Remove the condenser cover to rapidly reduce
the discharge pressure.
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