Porsche P-80 2005, Manual

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Page 1.6
Climate Control Systems Diagnosis & Repair
Air Conditioning Basics
4. The boiling point of a liquid varies with its
pressure.
The boiling point of a liquid changes with pressure. For
example, engine coolant temperatures can easily climb
above the boiling point of the coolant/water mixture at sea
level pressure, about 108° C (226° F). By increasing the
pressure to 1 bar (15 psi) the boiling point is raised to
128° C (263° F).
Practical Application
• When the A/C system is operating, the refrigerant
pressure in the evaporator is low which lowers its
boiling point. This allows the liquid refrigerant to boil
and to change state to a gas, absorbing latent heat.
• In the condenser, the pressure of the refrigerant gas is
high, raising the boiling point and allowing it to
condense to liquid as heat is removed.
R-134a refrigerant is as a gas at atmospheric pressures
and ambient temperatures. In the closed refrigerant
circuit, some refrigerant is in liquid form because of the
higher pressure. As with water, the boiling point of refrig-
erant increases as its pressure increases.
Reducing pressure to below-atmospheric (partial vacuum)
lowers the boiling point of a liquid. The table shows the
boiling point of water at several low pressures. The value
29.92 in. Hg is atmospheric pressure at sea level. At this
pressure, the boiling point of water is 100° C (212° F). At
higher altitudes, such as in Denver, the pressure is lower,
and therefore the boiling point is lower. This is why baking
or cooking times are longer at high altitudes.
On the table, note the low pressure at which water will boil
at room temperature (shown in bold). This low pressure is
attained during refrigerant system evacuation. At this low
pressure, any water present in the A/C system will boil off.
Notes: