Heating and Air Conditioning System Description and Operation
Engine Coolant
Engine coolant is the key element of the heating system. The engine thermostat controls the normal engine operating coolant
temperature. Coolant pumped out of the engine enters the heater core through the inlet heater hose. The air flowing through the
HVAC module absorbs the heat of the coolant flowing through the heater core. The coolant then exits the heater core through the
heater outlet hose and returns back to the engine block.
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 a very low temperature gas that can transfer the undesirable heat from the passenger compartment to the outside
air.
A Delphi model CSP-17 compressor is used on this model year vehicle. The A/C compressor is belt driven and operates when the
magnetic clutch is engaged. The compressor builds pressure in the A/C system. 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 A/C 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 tubing 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 TXV.
The TXV is located at the evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the A/C system.
As the refrigerant passes through the TXV, the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the
refrigerant will begin to boil at the TXV. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the TXV 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 to boil inside the evaporator
core.
The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the
evaporator through the suction line and back to the compressor, in a vapor state. This completes 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 moisture removed from the passenger
compartment will also change form, or condense, and is discharged from the HVAC module as water.
© 2011 General Motors. All rights reserved.
HEATING VENTILATION AND AIR CONDITIONING
11
Hvac
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