Porsche P-80 2005, Руководство пользователя

Страница: 18 / 108

Page 1.12
Climate Control Systems Diagnosis & Repair
Air Conditioning Basics
Evaporator
The evaporator is a heat exchanger located in air distribu-
tion housing. It has tubes and fins similar to a radiator or
condenser and it has a large surface area for efficient heat
transfer. The evaporator permits cold refrigerant to
absorb heat from warm air in the passenger
compartment. Outside air or recirculated air is forced
through the air distribution housing and evaporator fins by
blowers.
The evaporator is on the A/C system low pressure side.
The thermostatic expansion valve (TEV) is a restriction
controlling how much refrigerant enters the evaporator.
The refrigerant changes state in the evaporator in this
sequence:
• Liquid high pressure refrigerant sprays into the evapo-
rator, controlled by the TEV orifice.
• The pressure of the refrigerant drops as it passes
through this orifice. At this low pressure, the refrigerant
temperature is above its boiling point. The refrigerant
boils rapidly and changes state.
• As the refrigerant becomes a gas, it absorbs both
sensible heat and large amounts of latent heat from the
cabin air passing through the evaporator fins.
• The refrigerant warms and the cabin air passing over
the evaporator fins cools.
• The refrigerant leaves the evaporator as a cool low
pressure gas, pulled by the suction (intake) side of the
compressor. This low side pressure will be approx. 2
bar (30 psi). Check workshop manuals for exact system
pressure specifications.
In addition to removing heat, the evaporator also dehumidi-
fies the interior air. Cool air cannot hold as much moisture
as warm air. As the air is cooled, moisture condenses and
collects on the evaporator fins. Dehumidifying the air adds
to passenger comfort and also helps to demist the wind-
shield in damp weather. This is why A/C is enabled when
the defrost function is selected.
Evaporator fin temperatures cannot be allowed to drop
below the freezing point of water, 0° C (32° F), or the
condensation can freeze and ice buildup can block the
evaporator fins. Dust and matter carried in with the
airstream tends to collect on the wet evaporator fins, so
there is a cleaning effect. The condensed water that
collects on the evaporator fins is allowed to drain.
Notes: