Heat And Defrost Mode - Carrier SUPRA 850Mt Operation And Service Manual

2.11.2

Heat and defrost mode

In heat mode, two technologies can be used: Hot gas heating through the hot gas solenoid valves (HGV1, HGV2
& HGV3) or heating by electric heaters in the evaporator.Heating by hot gas is allowed only if no evaporator is in
cooling mode.
If two or three evaporators arein heat mode, one evaporator only will be in hot gas heating. The other evaporators
will be given inductive heating using the electric heaters in the evaporator.
Both hot gas and electric heat are used for defrost.
a. Hot Gas Heating
When refrigerant vapor is compressed to a high pressure and temperature in a reciprocating compressor, the
mechanical energy necessary to operate the compressor is transferred to the gas as it is being compressed. This
energy is referred to as the "heat of compression" and can be used as the source of heat during the heating cycle.
When in the heat mode, with no evaporators calling for cooling, thehot gas solenoid valves HGSV1, HGSV2, and
HGSV3 could be energized. The main heat valve (MHV) will close, diverting the refrigerant to HGSV1, HGSV2, and
HGSV3. The normally closed liquid solenoid valves LSV1, LSV2, LSV3 will energize and open. The normally closed
receiver pressure valve (RPV), situated in the hot gas line to the receiver will open. This allows the receiver to be
pressurized and liquid refrigerant to flow through the drier and sight glass and pass through any liquid line solenoid
valves which would be energized. The refrigerant passes through the expansion valve into the evaporator. At the
same time high temperature, high pressure gas enters theevaporatorvia thesolenoid valves HGSV (1,2 and 3) to give
the required heating. The extra liquid purged from the receiver ensures maximum heating capacity in low ambient
conditions. The evaporator fan passes the air over the hot refrigerant pipes and distributes heated air into the cargo
space.
The hot gas travels through the suction line check valve into the accumulator where it is drawn back through the
compressor pressure regulating valve (CPR) to begin the process again.
Whentemperature is achieved in allevaporators, they will go into null mode. Thecompartment withthe highest set
point will then take the lead and revert back to cool/heat cycles.
b. Principle Of Induction Heating (Electric Heat)
A control box recuperates the self indicated current from the electric motor when the unit is driven by the diesel
engine. This current energizes electrical heaters mounted inside the evaporator.
On standby operation the heaters are directly energized by the main electrical supply.
The system includes a control box, connected with electrical cables to the refrigeration unit and to the heaters
inside the evaporator .
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