pressure in the evaporator then activates the
low-pressure switch, which stops the compressor. As
the temperature rises, the increase in bulb pressure
closes the switch contacts, and the refrigerant is
supplied to the expansion valve.
LIQUID LINE.The refrigerant accumulated in
the bottom of the receiver shell is conveyed to the
cooling coils through the main refrigerant liquid line.
A stop valve and thermometer are usually installed in
this line next to the receiver. Where the sight-flow
indicator, dehydrator, or filter-drier is close to the
receiver, the built-in shutoff valves may be used
instead of a separate shutoff valve.
L I Q U I D L I N E F I L T E R - D R I E R O R
DEHYDRATOR. A liquid line filter-drier (fig.
6-28) prevents or removes moisture, dirt, and other
foreign materials from the liquid line that would harm
the system components and reduce efficiency. This
tank like accessory offers some resistance to flow. and,
for this reason, some manufacturers install it in a
bypass line. A filter-drier consists of a tubular shell
with strainers on the inlet and outlet connections to
prevent escape of drying material into the system.
Some filter-driers are equipped with a sight-glass
indicator, as shown in figure 6-28. A dehydrator is
similar to a filter-drier, except that it mainly removes
Figure 6-28.A liquid line filter-drier with sight-glass
SIGHT-FLOW INDICATOR.The sight-flow
indicator, also known as a sight glass (fig. 6-29), is a
special fitting provided with a gasketed glass, single or
double port, and furnished with or without seal caps for
protection when not in use. The double-port unit
permits the use of a flashlight background. The
refrigerant may be viewed passing through the pipe to
determine the presence and amount of vapor bubbles in
the liquid that would indicate low refrigerant or
unfavorable operating conditions. Some filter-driers
are equipped with built-in sight-flow indicators, as
shown in figure 6-29.
SUCTION LINE.Suction pressure regulators
are sometimes placed between the outlet of the
evaporator and the compressor to prevent the
evaporator pressure from being drawn down below a
predetermined level despite load fluctuations. These
regulators are usually installed in systems that require
a higher evaporator temperature than usual.
PRESSURE CONTROL SWITCHES.
Pressure control switches (fig. 6-30), often called
low-pressure cutouts, are essentially a single-pole,
single-throw electrical switch and are mainly used to
control starting and stopping of the compressor. The
suction pressure acts on the bellows of the power
element of the switch and produces movement of a
lever mechanism operating electrical contacts. A rise
in pressure closes the switch contacts and thereby
completes the circuit of the motor controller, which, in
turn, starts the compressor automatically. As the
operation of the compressor gradually decreases the
suction pressure, the movement of the switch linkage
Figure 6-29.Sight-flow indicators with different types of