Porsche P-80 2005, Manual

Page: 11 / 108

Air Conditioning Basics
Climate Control Systems Diagnosis & Repair
Page 1.5
causing it to change state. The temperature of the
material remains the same during the change of state. We
can raise the temperature a pot of water to the boiling
point (100° C/212° F) by adding Sensible Heat energy, but
to make the water boil and change state from liquid to
gas, it takes much more heat energy in the form of Latent
Heat. Similarly, when a gas condenses to a liquid, large
amounts of Latent Heat energy are released (see illustra-
tion, below).
The unit of heat energy is the BTU (British Thermal Unit).
One BTU = the amount of heat energy required to raise
the temperature of one pound of water one degree F. At
sea level pressure, water boils at 212° F. In order to raise
the temperature of one pound of water from 211° F to
212° F, only one BTU heat energy is needed. But to
change that same pound of water at 212° F to one pound
of steam (gas) at 212° F requires 970 BTUs, a much
greater quantity of heat. When the water condenses back
to a liquid, it gives off the same 970 BTUs of heat.
In the A/C system, refrigerant evaporates and condenses
over and over as it is pumped through the system. R-134a
refrigerant absorbs or gives off large amounts of latent
heat energy (about 85 BTU/lb) as it changes state. This is
the key to the efficiency of A/C systems.
3. The pressure of a liquid or gas varies with its
temperature and the temperature of a liquid or
gas varies with its pressure.
The pressure of the refrigerant in the closed A/C circuit
will vary with changes in temperature, and the temperature
of the refrigerant varies with changes in its pressure. With
A/C off and at an ambient temperature of 21° C (70° F),
the pressure in a fully charged system will be
approximately 4.8 bar (70 psi).
As the temperature rises, the pressure will also rise. The
table below shows the temperature/pressure relationship
for R-134a at temperatures between -30° C (-22° F) and
70° C (158° F).
At -30° C (-22° F) refrigerant pressure drops to 0 bar (0
psi). At this temperature, liquid refrigerant in an open
container will not boil and will remain a liquid.
Practical Application
• The job of the A/C compressor is to compress
refrigerant gas, raising its pressure and temperature
and concentrating the heat. The refrigerant flows to the
condenser where it is much hotter than the ambient air
blowing through and gives up latent heat to the air.
• In the evaporator, refrigerant is exposed to low
pressure. This lowers its temperature and the cold
refrigerant absorbs latent heat from the passenger
compartment.
Note:
The temperatures of the A/C lines when the A/C is ON.
The high pressure lines will be warm or hot, the low
pressure lines will be cool or cold.
R-134a Refrigerant Pressure vs. Temperature
R-134a Refrigerant pressure vs. temperature
Temperature ° F
-22
0.0
-4
4.4
14
14.5
32
27.5
50
45.0
86
97.2
104
132.0
122
177.0
140
229.2
158
293.0
68
68.2
Pressure (psi)
Latent Heat
Heat energy
changes liquid
to vapor (gas)
Temperature of
liquid does not
change
Change of
state occurs