Porsche P-80 2005, Manual

Page: 19 / 108

Air Conditioning Basics
Climate Control Systems Diagnosis & Repair
Page 1.13
Thermostatic Expansion Valve (TEV)
The Thermostatic Expansion Valve (TEV) is located at the
evaporator inlet. It separates the high and low pressure
sides of the system and creates the restriction that the
compressor uses to build high side pressure. High
pressure liquid refrigerant flows to the expansion valve
inlet from the receiver/dryer.
Porsche uses the H-Type block expansion valve. This valve
works the same way as the old style right-angle valve with
external sensing tubes, but is internally compensated
within the valve and has no external lines to the
evaporator.
The TEV has a variable orifice which controls the amount
of refrigerant admitted into the evaporator. High pressure
liquid refrigerant passes through the TEV orifice at the
evaporator inlet. The liquid refrigerant sprays into the
evaporator where it is exposed to low pressure and rapidly
evaporates. The TEV senses both temperature and
pressure in the evaporator. Both evaporator inlet and
outlet lines pass through the TEV. Refrigerant temperature
is sensed at the evaporator outlet. Increases in evaporator
outlet gas temperature due to increased heat load cause
the diaphragm in the gas filled thermostatic valve element
to move downward, opening the ball valve a greater
amount against spring tension and admitting more refrig-
erant.
The expansion valve also senses evaporator inlet
pressure. Pressure acts on the underside of the
diaphragm in the gas filled thermostatic valve element.
Any decrease in evaporator pressure tends to open the
ball valve and increase the amount of refrigerant admitted.
This would be the case if the evaporator were "starved"
and required more refrigerant. When additional refrigerant
flows into the evaporator, more refrigerant is available to
absorb heat. This reduces the temperature at the
evaporator outlet, causing a pressure drop in the thermo-
static valve element and increases evaporator pressure.
Both conditions act on the diaphragm to reduce the ball
valve opening. The cross-section opening of the valve is
reduced as the spring pushes the ball valve toward its
seat.
Notes: