Systems may accumulate dirt and scale as a result of improper procedures used during repair or installation of the system. If such dirt is excessive and a tank-type cleaner is available, connect the cleaner to the compressor suction strainer. Where such a cleaner is not available, insert a hard wool felt filter, about 5/16 inch thick, in the suction strainer screen. The plant should then be operated with an operator in attendance, for at least 36 hours or until cleaned, depending upon the size and the condition of the plant.

AIR CONDITIONING CONTROL

Most of the information presented to this point applies to the refrigeration side of a system, whether it is used for a refrigeration plant or for air conditioning. The compressor controls for both type systems are nearly identical, however, the devices used to control space temperatures differ. A two-position control, sometimes called the on/off control, is used for the automatic control of most shipboard air conditioning systems.

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 thermostatic element actuating two interlocked switches.

3. Systems using reheaters in the same manner 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 thermostatic 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 increased, 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 cooling 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 electronic 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.

humidistat setting will close the heating switch, energizing the reheaters.