3-Pipe Gas Defrost; Electric Defrost - Hussmann Proto-Aire Installation Manual

3-Pipe Gas Defrost

Application
3-pipe gas defrost is designed to operate with different defrost schedules and durations.
Only one lineup or no more than 20 percent of the load should go into defrost at one time.
As a lineup goes into defrost, the other cases will be fed liquid from the Protocol™ and
from the lineup in defrost. For longer lineups, or cases with large evaporator coils,
partitions may be required to split these lineups into smaller sections. All the valves, gas
solenoid and suction solenoid, are located in the cases and are controlled by the Protocol™.
The main liquid differential valve is located in the Protocol™ on vertical units, and field
installed on horizontal units. Isolation ball valves for each lineup branch are recommended
for ease of servicing.
Defrost Operation
1. When a system goes into defrost, the liquid differential valve, located in the
Protocol™ on vertical units and outside the Protocol™ for horizontal units, is de-
energized allowing the valve to modulate at the desired setting of 15-20 lbs. (Note:
higher settings may be required if the Protocol™ unit is located above the
evaporators).
2. The control board will simultaneously switch the appropriate output relay for the
defrosting circuit which will de-energize the suction solenoid and energize the hot
gas solenoid allowing discharge gas to flow through the coil and return through the
liquid line.
3. The defrost cycle can be either temperature terminated using a thermostat or time
terminated. No drip cycle has been built in.

Electric Defrost

Application
Electric defrost is the same with Protocol™ as with any other system. The only exception
is that POWERLINK™ type circuit breaker is used for the defrost loads. This breaker will
provide overload protection, and also be the contactor that switches the defrost heaters on
and off. Liquid solenoids should be used to control temperature and defrost for each
circuit. This prevents a possible pump down problem. The defrost solenoid and
POWERLINK™ breakers are controlled by independent output relays on the defrost board,
but will be assigned (programmed) so the same defrost circuit so that they will be energized
simultaneously. The amp draw for each circuit must be entered into the controller so that a
defrost shedding of compressors can occur reducing the overall amp draw of the unit.
Isolation ball valves for each case lineup are recommended for ease of servicing.
Defrost Operation
Protocol™ Installation and
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Service Manual