Thermo King MD-MT, Operator'S Manual

Page: 106 / 192

Refrigeration Maintenance
106
Refrigerant Charge
Testing the Refrigerant Charge with
an Empty Truck
If the unit has an insufficient charge of refrigerant,
the evaporator will be “starved” and the
compartment temperature will rise even though
the unit is operating. The suction pressure will
drop as the refrigerant charge decreases. The
charge may be determined by inspection of the
refrigeration through the receiver tank sight glass
with the following conditions established:
1. Place a test box over the evaporator.
2. Place a thermometer (TK No. 204-135) test
lead in the box near the evaporator return air
opening.
3. Install the gauge manifold.
4. Run both evaporators on Cool until the air in
the box indicates 0 F (-18 C). By allowing the
box to leak a small amount, you will be able to
maintain 0 F (-18 C).
5. Under these conditions, the ball in the receiver
tank sight glass should be floating.
a. Discharge Pressure Reading:
R-404A Systems—The discharge or head
pressure gauge should read 275 psig
(1896 kPa).
NOTE: If the pressure is below this, it can be
raised by covering a portion of the condenser
coil with a piece of cardboard.
b. Suction Pressure Reading:
R-404A Systems—The compound gauge
should be indicating 13 to 18 psig (90 to
124 kPa) gauge pressure.
NOTE: If there is any doubt about the unit
gauge, check the calibration.
Testing the Refrigerant Charge with
a Loaded Truck
1. Install a gauge manifold (optional).
2. Run both evaporators on the Cool cycle.
3. Cover at least three quarters of the condenser
to drive any excess refrigerant from the
condenser into the receiver tank.
4. As the head pressure is rising, check the
receiver tank sight glass. The ball 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 sufficient
refrigerant in the unit for that load at that
particular box temperature. This test does
not determine if the unit contains a full
charge of refrigerant.
Testing for an Overcharge of
Refrigerant
Use the following procedure to identify a system
with an excessive refrigerant charge:
1. Install a calibrated gauge manifold on the
compressor.
2. Operate both evaporators in High Speed Cool
to stabilize system pressures and reduce the
compartment temperature to approximately
16 C or colder.
3. Observe the discharge pressure and cover the
condenser to increase the discharge pressure
75 to 100 psig (520 to 690 kPa) above the
observed pressure.
NOTE: If the ball and liquid level in the
receiver sight glass decline (or liquid level in
liquid line sight glass declines) during this
step, the unit is not overcharged. You do not
need to continue this test.
4. Remove the condenser cover to rapidly reduce
the discharge pressure.
5. Observe the receiver tank sight glass and the
unit’s discharge pressure.