ENGINEMAN 1 & C
TWO-POSITION CONTROL
This control may be used on three types of
systems:
1. Systems employing a simple thermostatic-
ally controlled single-pole switch to control flow
of refrigerant to the cooling coil.
2. Systems using reheaters, employing a ther-
mostatic element actuating two interlocked
switches.
3. Systems using reheaters in the same man-
ner as in item 2 with control of humidity added,
where specified.
The type 1 system above is the most commonly
used and requires little explanation. A thermostat,
consisting of a temperature sending element,
actuating a single-pole, single-throw switch, opens
and closes a magnetic valve to start and stop the
flow of refrigerantchilled water or commercial
refrigerant. This type of control is similar to ther-
mostatic control for the refrigeration plant.
Although the type 1 system requires single-pole
thermostats, the 2PD used in type 2 and type 3
systems can be used. The cooling switch would
then be connected in the normal manner with the
heating switch inoperative.
The use of the type 2 system has greatly in-
creased, due to the present effort to make living
and working spaces more comfortable and to the
rapid development of various types of weapons
systems. Such spaces often use a common cool-
ing coil serving several different spaces. Assume
that three spaces are being cooled by a common
coil. Since the load changes seldom occur
simultaneously, electric or steam reheaters are
installed in the cooling air ducts and the cooling
thermostats of the various spaces are connected
in parallel so that any one may open the cooling
coil valve.
Suppose space B (figure 6-11) has a load
change and spaces A and C do not. These
spaces would become too cold for comfort with
the coil operating to take care of space "B". In
order to prevent this condition, the thermostat
would close the heating switch and energize the
reheaters for spaces A and C.
The type 3 system is identical to the type 2
system, except that a humidistat is wired in
parallel with the thermostatic heating switch. This
type system is used mostly in weapons and elec-
tronic spaces. The humidistat is set for the relative
humidity condition desired. In most installations,
it is only necessary to prevent the humidity from
exceeding 55%. Where the humidistat is installed,
an increase in temperature beyond the thermostat
setting will close the thermostat cooling switch and
an increase in relative humidity beyond the
Figure 6-11.Typical air conditioning system.
6-16
