Service Manual

Part Number: 000015431 Rev 02 6/20 163
Freeze Cycle Operation All Models
The head pressure control valve is non adjustable.
At ambient temperatures of approximately 70°F (21°C)
or above, refrigerant flows through the valve from the
condenser to the receiver inlet. At temperatures below
this (or at higher temperatures if it is raining), the head
pressure control dome’s nitrogen charge closes the
condenser port and opens the bypass port from the
compressor discharge line.
In this modulating mode, the valve maintains minimum
head pressure by building up liquid in the condenser and
bypassing discharge gas directly to the receiver.
Harvest Cycle Operation
During the Harvest cycle, the harvest valve opens and
allows refrigerant from the top of the receiver tank to
enter the evaporator. The refrigerants change of state
(from vapor to liquid) releases the heat necessary for the
Harvest cycle.
Opening the harvest valve causes a drop in discharge
pressure.
The discharge pressure will drop below the condenser
fan cycling control setpoint and the condenser fan motor
cycles off (at ambient temperatures above 110°F (43°C) the
condenser fan motor remains energized).
The warm discharge gas adds heat to the receiver in
the harvest cycle. Without this additional heat the head
pressure would continue to drop as liquid refrigerant boils
off in the receiver.
Example: A service technician removes refrigerant vapor
from a cylinder by boiling off the liquid refrigerant. A
refrigeration effect is created as the refrigerant changes
state from a liquid to a vapor. The cylinder cools and the
refrigerant pressure drops. To maximize flow and maintain
pressure the technician places the cylinder in warm water.