A small liquid line wrapped around a larger
FLOW OF LIQUID REFRIGERANT
A liquid line and suction line side by side and
GAS AND LIQUID
The expansion valve is a refrigerant metering
Figure 11-9.--Expansion valve principle.
device that is located between the high- and
low-pressure sides of a refrigeration system to control
sight glasses to show liquid level during operation. A
the amount of liquid refrigerant that flows into the
receiver is normally about one-third full during
evaporator. There are other types of metering devices
used in refrigeration systems, such as capillary tubes,
but mobile air-conditioners use expansion valves.
The basic function of the expansion valve (fig.
The heat exchanger provides an increase in system
11-9) is to change high-pressure liquid into
efficiency by transferring heat from the hot liquid
low-pressure liquid as it enters the evaporator. The
(flowing from the receiver to the evaporator) to the
expansion valve, also referred to as a metering device,
cooler suction vapor (flowing from the evaporator to
uses a combination of springs, bellows, a needle valve,
the compressor). Because of this heat transfer, the
and a thermostatic bulb (temperature-sensitive bulb) to
liquid refrigerant enters the evaporator at a lower
control the refrigerant entering the evaporator. The
temperature, allowing it to absorb more heat per pound
liquid refrigerant flow is regulated as it is released into
of refrigerant. This heat transfer also ensures that the
the evaporator to ensure maximum operating efficiency
vapor flowing to the compressor from the evaporator
without overloading the compressor. The action of the
contains no liquid. Liquid entering the suction side will
valve is similar to a spray nozzle. It sprays a mist of
damage the compressor.
refrigerant into the evaporator. This is the beginning of
the low-pressure system.
Heat exchangers can be constructed in several
ways. Those you will encounter may be any of the
The type of expansion valve used on mobile
air-conditioners is called a thermostatic expansion
valve (fig. 11-10). The valve meters the liquid
A small liquid line enclosed within a larger
Figure 11-10.--Thermostatic expansion valve.