2. The refrigerant is routed to the expansion valve
through high-pressure lines and hoses.
3. The expansion valve reduces refrigerant
pressure to the evaporator by allowing a controlled
amount of liquid refrigerant to enter it.
4. A stream of air is passed over the coils in the
evaporator as refrigerant enters.
5. As the low-pressure refrigerant moves through
the coils in the evaporator, it absorbs heat from the
airstream, which produces a cooling effect.
6. As the refrigerant nears the end of the coils in
the evaporator, greater amounts of heat are absorbed.
This causes the low-pressure liquid refrigerant to boil
and change to a gas as it exits the evaporator.
7. As the refrigerant enters the compressor, the
pumping action increases refrigerant pressure, which
also causes a rise in temperature.
8. The high-pressure, high-temperature gas enters
the condenser, where heat is removed by an outside
ambient airstream moving over the coils. This causes the
gas to condense and return to a liquid form again.
9. The high-pressure liquid refrigerant now enters
the receiver again to begin another cycle. This
continuous cycle, along with the dehumidifying and
filtering effect, produces a comfortable atmosphere on
Figure 13-1 shows the refrigeration cycle. You
should trace the order of the cycle to understand it fully.
COMPONENTS OF THE
Each air-conditioning system must have a
receiver/drier, an expansion valve or metering device,
an evaporator, a compressor, and a condenser. Without
these components, an air-conditioning system will not
function. Additionally, the system must have some
means of control. The following information briefly
covers each air-conditioning component and the
The receiver (fig. 13-3), otherwise known as a
filter-drier or accumulator-drier, is a cylindrical-shaped
metal tank. The tank is hollow with an inlet to the top of
the hollow cylinder. The outlet port has a tube attached
to it that extends to the bottom of the receiver. This tube
assures that only liquid refrigerant will exit the receiver,
Figure 13-4.Possible sight glass conditions.
because any gas entering will tend to float above the
. FilterThe filter is mounted inside the receiver
on the end of the outlet pipe. This filter removes any
impurities from the refrigerant by straining it.
. DesiccantA special desiccant or drying agent,
also, is located inside the receiver. This agent removes
any moisture from the system.
. Relief ValveSome systems use a relief valve
mounted near the top of the receiver. This valve is
designed to open when system pressure exceeds
approximately 450 to 500 psi. As the relief valve opens,
it vents refrigerant into the atmosphere. As soon as
excess pressure is released, the valve closes again so the
system will not be evacuated completely.
. Sight GlassA sight glass is a small, round,
glass-covered hole, sometimes mounted on the outlet
side of the receiver near the top. This observation hole
is a visual aid you use in determining the condition and
amount of refrigerant in the system. If bubbles or foam
is observed in the sight glass while the system is
operating (above 70°F [21°C]) (fig. 13-4), it may
indicate that the system is low on refrigerant. Some
systems have a moisture-sensitive element built into the
sight glass. If excessive moisture is present, the element
turns pink. If the system moisture content is within
limits, the element remains blue. In many later
automotive air-conditioning systems, the sight glass has
been eliminated. In such applications, you must depend
on the system pressures.
THE EXPANSION SYSTEM
The refrigerant expansion system is designed to
regulate the amount of refrigerant entering the
evaporator and to reduce its pressure.