Manitowoc MD0420A, Technician'S Handbook

Page: 97 / 132

Part Number 040005626 5/17 97
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
Remote Condenser Models
The head pressure control cycles into full bypass due
to the pressure drop when the harvest valve opens.
Refrigerant flows from the compressor to the
evaporator through the harvest valve and the head
pressure valve is out of the circuit.
Diagnostics
FREEZE CYCLE - REMOTE CONDENSER
1. Determine if the coil is clean.
2. Determine the air temperature entering the
condenser.
3. Determine if the head pressure is high or low in
relationship to the outside temperature. (Refer to
the proper “Cycle Times/24-Hour Ice Production/
Refrigerant Pressure Charts”).
4. Determine the temperature of the liquid line
entering the receiver by feeling it. This line is
normally warm; “body temperature.”
5. Using the information gathered, refer to the chart.