2-21
62-11863
2.9
REFRIGERANT CIRCUIT DURING COOLING
Overview
When cooling, the unit operates as a vapor compression refrigeration system. The main components of the system
are the reciprocating compressor, air-cooled condenser, thermostatic expansion valve, direct expansion evapora-
tor, and liquid line solenoid valve. The compressor raises the temperature and pressure of the refrigerant and it
passes through a normally open Main Heat Valve (MHV), through a check valve into the condenser (See
for detailed information).
Condenser
The condenser fan circulates surrounding air over the outside of the condenser tubes. Heat transfer is then estab-
lished from the refrigerant gas (inside the tubes) to the condenser air (flowing over the tubes). The condenser
tubes have fins designed to improve the transfer of heat. This removal of heat causes the refrigerant to liquefy. Liq-
uid refrigerant flows from the condenser and through a check valve to the receiver.
Receiver Charge
The receiver stores the additional charge necessary for low ambient operation, and for heating and defrost modes.
The refrigerant leaves the receiver and flows through a manual receiver shutoff valve (king valve).
Refrigerant Flow
The refrigerant then flows through the subcooler. The subcooler occupies a portion of the main condensing coil
surface and gives off further heat to the passing air.
The refrigerant then flows through a filter-drier where an absorbent keeps the refrigerant clean and dry.
The refrigerant then flows through the accumulator / heat exchanger and then to the liquid solenoid valves (LSV).
These solenoids are electrically energized when in cooling mode and allow the liquid refrigerant to flow through the
externally equalized thermostatic expansion valve (TXV), which reduces the pressure of the liquid and meters the
flow of liquid refrigerant to the evaporator to obtain maximum use of the evaporator heat transfer surface.
Heat Transfer
The evaporator tubes have aluminum fins to increase heat transfer; heat is removed from the air circulated through
the evaporator. This cold air is circulated throughout the refrigerated compartment to maintain the cargo at the
desired temperature.
The transfer of heat from the air to the low-temperature, low-pressure saturated mixture causes the refrigerant to
vaporize. This low-temperature, low-pressure vapor passes into the accumulator tank. The compressor draws the
vapor out of the accumulator through a pick-up tube, which is equipped with a metering orifice. This orifice prevents
the accumulation of oil in the accumulator tank. The metering orifice is calibrated to control the rate of oil flowing
back to the compressor.
Vapor
The vapor refrigerant then enters the compressor pressure regulating valve (CPR), which regulates refrigerant
pressure entering the compressor, where the cycle starts over.
2.10 REFRIGERANT CIRCUIT DURING HEAT AND DEFROST
In heat mode, two technologies can be used; hot gas heating through the hot gas solenoid valves (1HGV and
2HGV) or heating by electric heaters in the evaporator. (See
Heating by hot gas is allowed only if neither evaporator is in cooling mode. If both evaporators are in heat mode,
only one will be in hot gas heating. The other evaporator will be given inductive heating using the electric heaters in
the evaporator. Both hot gas and electric heat are used for defrost.
2.10.1
Hot Gas Heating
Overview
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.
Summary of Contents for SUPRA 950Mt
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