Section III. THEORY OF OPERATION
This section describes the theory of operation of the refrigerant vapor cycle system .
1-6. Theory (fig. 1-11)
During air conditioning operation, the condenser fan draws ambient air through the condenser and discharges it
through the condenser fan discharge door. The evaporator fan draws air from the compartment to be air conditioned into
the evaporator section where it is mixed with fresh air (if desired) and conditioned; then the air is passed back into the
compartment through the evaporator conditioned air outlet port.
Low pressure, low temperature refrigerant vapor (refrigeran-12) enters the compressor through the compressor
inlet leading from the evaporator outlet. The refrigerant vapor is compressed to a high pressure and temperature.
From the compressor, the refrigerant vapor enters the condenser, where it is condensed into a high-pressure
liquid by giving up heat to the condenser airstream. The electrically driven condenser fan draws cooling air through the
condenser and exhausts it to atmosphere. The liquid refrigerant then collects in the receiver, upstream of the condenser.
The receiver stores surplus liquid refrigerant and compensates for inequalities in flow rates.
After leaving the receiver, the liquid refrigerant flows through the subcooler. Subcooling is necessary because of
the line pressure drop in the line leading to the thermo-expansion valve. The subcooler lowers the liquefied refrigerant
temperature sufficiently (approximately 8F) to preclude flashback (vaporization) of the refrigerant on the way to the
From the subcooler, the liquid refrigerant passes through the filter-drier. The liquid refrigerant upon entering the
filter-drier is directed through the drying agent and the filterscreen.
From the filter-drier, the liquid refrigerant flows to the solenoid valve. The remote thermostat, which senses
compartment temperature, controls the operation of the solenoid valve. The valve closes to stop the flow of refrigerant to
the expansion valve when compartment cooling is not required. When the solenoid valve is closed, pressure in line from
the evaporator to the compressor drops because the compressor is still operating. When this pressure drops to 35 to 37
psig, the hot gas bypass valve opens permitting compressor discharge vapor to flow into the compressor inlet line. When
the temperature in the compressor inlet line becomes excessive due to hot bypassed vapor, the liquid quench valve
opens to permit high-pressure liquid refrigerant to vaporize and flow into the line; thereby de-superheating the bypassed
vapor and reducing the line temperature to a safe superheat level entering the compressor.
The liquid refrigerant passes from the solenoid valve through the refrigerant liquid sight indicator, and enters the
inlet port of the thermostatic expansion valve where it is metered by the action of the valve pin. The valve pin is actuated
by a diaphragm whose position is determined by the evaporator load and the superheat level sensed by the valve thermal
bulb. To compensate for the effect of pressure drop across the evaporator, an external pressure equalizing line is
connected between the evaporator outlet and the chamber below the valve diaphragm. Thus, the true evaporator outlet
pressure is exerted beneath the valve diaphragm. The operating pressures on the valve diaphragm are now free from the
effect of the pressure drop through the evaporator, and the expansion valve will respond to the superheat of the refrigerant
vapor leaving the evaporator.
The thermostatic expansion valve is factory adjusted to maintain a nominal 5F superheat setting. The valve
function is to maintain the nominal superheat setting in the evaporator, as dictated by the thermal sensing bulb. This is
accomplished by modulating the flow rate of liquid refrigerant to the evaporator.
The actual cooling effect in this vapor cycle system occurs in the evaporator where the liquid refrigerant is
evaporated under re