Hussmann Proto-Aire, Installation Manual, Page: 93 / 124

Protocol™ Installation and
Service Manual
Hot Gas
Control Board energizes the Aux Relay Coil, which de-energizes the Main Liquid Line
Pressure Differential Solenoid. The valve reduces liquid supply line pressure. The Control
Board also energizes the Defrost Relay coil, which open Hot Gas Solenoid valves and
closes the Suction Solenoid valves. Each case terminates defrost through individual defrost
termination thermostats, and goes into drip cycle until branch is timed off by the Control
Board.
Note: Only 20% of the cases may be defrosted at once because of the requirement to keep
a refrigeration load on the compressors to provide gas for defrost.
Electric Defrost
Control Board locks out certain compressors to cover heater Amp draw. Control Board
energizes two Defrost Board Relay Coils for each Defrost Circuit:
1) The first closes the POWERLINK™
2) The second opens the Electric Heaters are energized. Branch Liquid Line
Valve closes. Compressors not locked out maintain case refrigeration for
units not in defrost. Defrost is temperature terminated.
3) See POWERLINK™ operation diagram on Page 97. See Page 105 for
further information on defrost operation.
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