Heating and Air Conditioning System Description and
Operation
Engine Coolant
Engine coolant is the key element of the heating system. The
thermostat controls engine operating coolant temperature. The
thermostat also creates a restriction for the cooling system that
promotes a positive coolant flow and helps prevent cavitation.
Coolant enters the heater core through the inlet heater hose, in a
pressurized state.
The heater core is located inside the HVAC module. The heat of
the coolant flowing through the heater core is absorbed by the
ambient air drawn through the HVAC module. Heated air is
distributed to the passenger compartment, through the HVAC module,
for passenger comfort.
The amount of heat delivered to the passenger compartment is
controlled by opening or closing the HVAC module air temperature
door. The coolant exits the heater core through the return heater
hose and recirculated back through the engine cooling system.
A/C Cycle
Refrigerant is the key element in an air conditioning system.
R-134a is presently the only EPA approved refrigerant for
automotive use. R-134a is an very low temperature gas that can
transfer the undesirable heat and moisture from the passenger
compartment to the outside air.
The A/C compressor is belt driven and operates when the magnetic
clutch is engaged. The compressor builds pressure on the vapor
refrigerant. Compressing the refrigerant also adds heat to the
refrigerant. The refrigerant is discharged from the compressor,
through the discharge hose, and forced to flow to the condenser and
then through the balance of the A/C system. The A/C system is
mechanically protected with the use of a high pressure relief
valve. If the high pressure switch were to fail or if the
refrigerant system becomes restricted and refrigerant pressure
continued to rise, the high pressure relief will pop open and
release refrigerant from the system.
Compressed refrigerant enters the condenser in a high
temperature, high pressure vapor state. As the refrigerant flows
through the condenser, the heat of the refrigerant is transferred
to the ambient air passing through the condenser. Cooling the
refrigerant causes the refrigerant to condense and change from a
vapor to a liquid state.
The condenser is located in front of the radiator for maximum
heat transfer. The condenser is made of aluminum and aluminum
cooling fins, which allows rapid heat transfer for the refrigerant.
The semi-cooled liquid refrigerant exits the condenser and flows
through the liquid line, to the thermal expansion valve.
The thermal expansion valve is located in the liquid line
between the condenser and the evaporator. The thermal expansion
valve is the dividing point for the high and the low pressure sides
of the A/C system. As the refrigerant passes through the thermal
expansion valve, the pressure of the refrigerant is lowered. Due to
the pressure differential of the liquid refrigerant, the
refrigerant will begin to vaporize at the thermal expansion valve.
The thermal expansion valve also meters the amount of liquid
refrigerant that can flow into the evaporator.
Refrigerant exiting the thermal expansion valve flows into the
evaporator core in a low pressure, liquid state. Ambient air is
drawn through the HVAC module and passes through the evaporator
core. Warm and moist air will cause the liquid refrigerant boil
inside of the evaporator core. The boiling refrigerant absorbs heat
from the ambient air and draws moisture onto the evaporator. The
refrigerant exits the evaporator back through the thermal expansion
valve and into the suction line and back to the compressor, in a
vapor state completing the A/C cycle of heat removal. At the
compressor, the refrigerant is compressed again and the cycle of
heat removal is repeated.
The conditioned air is distributed through the HVAC module for
passenger comfort. The heat and moisture removed from the passenger
compartment will also change form, or condense, and is discharged
from the HVAC module as water under the vehicle.
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