Manitowoc QuietQube Indigo Series Technician's Handbook page 199

Freeze Cycle Operation All Models
The R404A 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.
Part Number STH026 2/12 199