Scotsman NME1854R Service Manual page 20

NME1854R & FME2404R
OPERATION: Refrigeration
There are two separate refrigeration systems in this machine.
The refrigeration system under low condenser air
temperatures is much the same as it is under
higher temperatures, with the exception that the
resulting low head pressures cause the head
pressure control to close off the liquid line between
the condenser and the receiver.
This forces more refrigerant into the condenser
(with a small amount of discharge gas going into
the receiver to maintain pressure until the head
pressure is built back up to the rated gauge
pressure of 180 psig). At that pressure the valve
opens up the liquid line from the condenser to the
receiver.
Head Pressure Control Valve - Low
Temperature
From
Compressor
From Condenser
During the pump down cycle (usually initiated by
the circuit board de-energizing the liquid line valve)
the discharge gases flow through their normal path
to the remote condenser, through the head
pressure control, and into the receiver.
Low Condensing Temperature
Valve
Disk
To
Receiver
PUMP DOWN
January 2000
Page 20
Head Pressure Control Valve - Normal
Temperature
From
Compressor
From Condenser
At this point the refrigerant flow is stopped by the
closed liquid line valve. This action forces the
refrigerant into the receiver and keeps it out of the
compressor. The pump down continues until the
low pressure control turns the compressor off.
Valve
Disk
To
Receiver