controls prevent damage to the equipment and injury to
SOLENOID VALVES.--Solenoid valves, similar
to the one shown in figure 11-16, are used in mobile
air-conditioners to control the flow of refrigerant at
various points in the system. These valves are of two
types--normally opened or normally closed. Regard-
The operating controls of an air-conditioning
less of type or use, construction and function of the
system may be simple or complex, depending upon the
valves are essentially the same.
design and use of the system. Some of the operating
In the normally closed type, when the circuit to the
controls are manually controlled by the operator.
solenoid coil is completed, the coil energizes and pulls
Others are automatic and operate in response to
the valve off its seat, opening the valve passage. When
changes in temperature or pressure.
the circuit to the solenoid coil opens, the coil
THERMOSTAT.--A thermostat is a device that
de-energizes and spring pressure pushes the valve into
automatically regulates temperature. Thermostats used
its seat, closing the valve passage.
on mobile air-conditioners, similar to the one shown in
Operation of the normally opened type is the
figure 11-15, consist of a capillary tube filled with a
reverse of the operation of the normally closed type.
volatile liquid, a bellows, and a set of electrical
When the solenoid coil energizes, the valve closes;
when the solenoid coil de-energizes, the valve opens.
The thermostat contacts are connected to a solenoid
CIRCUIT CONTROLS.--Controls for the
valve circuit. The capillary tube bulb is placed in the
various circuits of an air-conditioning unit may vary
discharge airflow immediately downstream of the
from a simple toggle switch to remote-controlled
evaporator. As the temperature of the discharge air
contact relays. These devices may be used for actuating
rises, the liquid in the capillary tube expands and exerts
fans, lights, motors, or for checking various circuits
a pressure on the bellows. This causes the contacts to
while troubleshooting a defective system. Switches
close, energizing the solenoid. The solenoid opens the
used for maintenance work are called service switches.
valve so liquid refrigerant can flow to the expansion
When the desired temperature of the discharge air
is reached, a reverse of the above action occurs. The
Safety devices are installed in air-conditioning
liquid in the capillary tube contracts, the contacts open,
systems to prevent bodily injury to operating personnel
the solenoid valve closes, and the flow of refrigerant to
and damage to equipment and spaces receiving the
the expansion valve is stopped.