Manitowoc QD0202A, Service Manual

Page: 142 / 175

Refrigeration System Section 7
7-30
Part No. 80-1100-3
HEADMASTER CONTROL VALVE
Manitowoc remote systems require headmaster control
valves with special settings. Replace defective
headmaster control valves only with “original” Manitowoc
replacement parts.
Operation
The R404A headmaster control valve is non adjustable.
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 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.
Diagnosing
1. Determine the air temperature entering the remote
condenser.
2. Determine if the head pressure is high or low in
relationship to the outside temperature. (Refer to the
proper “Operational Pressure Chart” later in this
section.) If the air temperature is below 70°F
(21.1°C), the head pressure should be modulating
about 225 PSIG.
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. Lower
ambient conditions can be simulated by rinsing the
condenser with cool water during the freeze cycle.
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 warm to hot Ice machine low on charge See “Low on Charge Verification” on
Page 7-31