1. Commpressor breakers.
4. High-pressure controls.
2. Compressor starters.
5. Oil failure controls.
3. Fan cycle controls.
6. Solid-state staging thermostat.
Figure 7-29.Packaged air-cooled chiller controls.
cooling unit. The temperature-sensitive element may
be a bimetallic strip or a confined, vaporized liquid.
The thermostats used with refrigerative air
conditioners are similar to those used with heating
equipment, except their action is reversed. The
operating circuit is closed when the room temperature
rises to the thermostat control point and remains closed
until the cooling unit decreases the temperature
enough. Also, cooling thermostats are not equipped
with heat-anticipating coils.
Wall type of thermostats most common for heating
and air conditioning in the home and on some
commercial units use a bimetallic strip and a set of
contacts, as shown in figure 7-30. This type of
thermostat operates on the principle that when two
dissimilar metals, such as brass and steel, are bonded
together, one tends to expand faster than the other does
when heat is applied. This causes the strip to bend and
close the controls.
Figure 7-30.Bimetallic thermostat.
As a Utilitiesman, you may be required to make an
adjustment that sets the temperature difference between the
cut-in and cutout temperatures. For example, if the system is
set to cut in at 76°F and cut out at 84°F, then the differential
is 8°F. This condition prevents the unit from cycling
continually as it would if there were no differential.
A room "humidistat" may be defined as a
humidity-sensitive device controlling the equipment
that maintains a predetermined humidity of the space
where it is installed. The contact of the humidistat is
opened and closed by the expansion or contraction of
natural blonde hairs from human beings, which is one
of the major elements of this control. It has been found
that these types of hairs are most sensitive to the
moisture content of the air surrounding them.
Pressure-flow controllers are discussed in chapter 6. The
purpose of these controllers in air conditioning is to act as
safety switches for the system, so if either the head pressure is
too high or suction pressure too low, the system will be
secured regardless of the position of the operating switches.
The refrigerant-flow controllers used with air
conditioners are also similar to the ones discussed in
chapter 6. These controllers are either of the capillary
type or externally equalized expansion valve type and