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Refrigeration System Section 7
7-10
Part No. 80-1214-3
HEADMASTER CONTROL VALVE
Function
The headmaster control valve maintains the correct
discharge pressure and liquid line temperature.
Manitowoc Ice Machine systems require headmaster
control valves with special settings. Replace defective
headmaster control valves only with “original” Manitowoc
replacement parts.
FREEZE CYCLE OPERATION
Air-Cooled Ice Machines
During low ambient conditions, it is normal for the head
pressure control valve to hunt (head pressure may
fluctuate up and down) and generally settles out within
the first 6 minutes of the freeze cycle.
At ambient temperatures of approximately 70°F (21.1°C)
or above, refrigerant flows through the condenser to the
receiver inlet. At temperatures below this (or at higher
temperatures if it is raining), the head pressure controls
nitrogen dome charge starts to modulate (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 liquid line.
Air-Cooled Condensing Units
FREEZE CYCLE
1. Determine the air temperature entering the
condenser.
2. Determine if the head pressure is high or low in
relationship to the outside temperature. (Refer to the
proper “Operational Pressure Chart”) If the air
temperature is below 70°F (21.1°C), the head
pressure control will modulate to maintain the
correct liquid line temperature and head pressure.
3. Determine the temperature of the liquid line entering
the receiver by feeling it. This line is normally warm;
“body temperature.”
4. Using the information gathered, refer to the chart
below.
NOTE: A headmaster that will not bypass, will function
properly with condenser air temperatures of
approximately 70°F (21.1°C) or above. When the
temperature drops below 70°F (21.1°C), the headmaster
fails to bypass and the ice machine malfunctions.
Rinsing the condenser with cool water during the freeze
cycle will simulate lower ambient conditions.
Freeze Cycle Headmaster Control Valve Failure List
Symptom
Probable
Cause
Corrective
Measure
Valve not
maintaining
pressures
Non-approved
valve
Install a Manitowoc
Headmaster control
valve with proper
setting
Discharge pressure
extremely high;
Liquid line entering
receiver feels hot
Valve stuck in
bypass
Replace valve
Discharge pressure
low; Liquid line
entering receiver
feels extremely cold
Valve not
bypassing
Replace valve
Discharge pressure
low; Liquid line
entering receiver
feels hot
Ice machine
low on charge
Locate and repair
refrigerant leak
Revised 8/2003