Service manual

Section 7 Refrigeration System
Part No. 80-1206-3 7-23
HEADMASTER CONTROL VALVE
Function
The headmaster control valve maintains the correct
discharge pressure and liquid line temperature in the
freeze and harvest cycles.
Manitowoc QuietQube® systems require headmaster
control valves with special settings. Replace defective
headmaster control valves only with “original” Manitowoc
replacement parts.
Freeze Cycle Operation
AIR-COOLED MODELS
During low ambient conditions, it is normal for the head
pressure control valve to hunt (head pressure may
fluctuate up and down) on CVD condensing units.
Hunting varies by model an ambient, but 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 valve from 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.
WATER-COOLED MODELS
The water regulating valve setting is higher than the
head pressure control valve set point. All refrigerant is
directed through the condenser and the water regulating
valve maintains the correct head pressure.
Harvest Cycle Operation
During the harvest cycle the cool vapor 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 in the evaporator) releases the heat
necessary for the harvest cycle.
Opening the cool vapor valve causes a drop in
discharge pressure.
AIR-COOLED MODELS
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).
WATER-COOLED MODELS
The discharge pressure will drop below the water
regulating valve setpoint and the water regulating valve
will close.
ALL CVD MODELS
The head pressure control valve cycles into a complete
bypass position (stopping all refrigerant flow through the
condenser).
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.
A head pressure control valve that will not completely
bypass in the harvest cycle will result in longer harvest
cycles with lower than normal suction pressure and
safety limit #2 trips.
Refer to the “Cycle Times/24 Hour Ice Production/
Refrigerant Pressure Charts” for the model being worked
on, to obtain the correct harvest cycle pressure range.
Important
Head pressure settings will vary depending on the
QuietQube® model being worked on.