Most of the information presented so far 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 types of systems are nearly identical; however, the devices used to control space temperatures differ, The two-position dual control, called 2PD, is used for the automatic control of most shipboard air-conditioning systems.

TWO-POSITION DUAL CONTROL (2PD)

This control is used on three types of systems:

Type 1. Systems employing a simple thermo-statically controlled single-pole switch to control flow of refrigerant to the cooling coil

Type 2. Systems using reheaters, employing a thermostatic element actuating two interlocked switches

Type 3. Systems using reheaters in the same manner as those in type 2, with control of humidity added where specified

The type 1 system, because of its simplicity, requires little explanation. The thermostat consists of a temperature-sensing element actuating a single-pole, single-throw switch. It opens and closes a magnetic valve to start and stop the flow of refrigerant-chilled water or commercial refrigerant. This type of control is similar to the thermostatic control for the refrigeration plant. The type 1 system requires single-pole thermostats, but type 2 and type 3 systems can use two-position dual controls (2PD). The cooling switch would then be connected in the normal manner with the heating switch inoperative.

The type 2 system is most commonly used to make living and working spaces more habitable and for various types of weapons systems that require cooling. These systems often use a common cooling coil serving several different spaces. Since load changes seldom occur simultaneously, electric or steam reheaters are installed in the cooling air ducts. The cooling thermostats of the various spaces are connected in parallel so that any one of the thermostats may open the cooling coil valve.

Suppose three spaces are being cooled by a common coil. Space B in figure 5-6 has a load change and spaces A and C do not. With the coil operating to take care of space B, these spaces would become too cold for comfort. 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 of system is used mostly in weapons and electronic spaces. The humidistat is set for the relative humidity desired. In most installations, it is only necessary to prevent the humidity from exceeding 55 percent. Where the humidistat is installed, an increase in temperature beyond the thermostat setting will close the thermostat cooling switch. An increase in relative humidity beyond the humidistat setting will close the heating switch and energize the reheaters.

MAINTENANCE

Proper attention to the planned maintenance system often exposes developing troubles in time to take corrective action. Since most breakdowns occur at the most inopportune times, periodic checks and maintenance will help to avoid malfunctions. The 2PD control system can easily be checked out in a reasonably short time. The checkout should be made at least every 3 months or more often if necessary. Inspections and checks should be made at the beginning of, and midway through, the cooling season and heating season.

You should inspect the sensing elements and remove any dust accumulations. Remove dust and dirt from thermostatic sensing elements with a soft brush. Use air to gently blow off any dust on the sensing elements in humidistats. The air will not damage the element but will remove any problem-causing dust. Magnetic valves should be checked for operation. Be sure that they open and close completely. Set points of the thermostats and the humidistats should be checked with a calibrated thermometer and a reliable humidity indicator.

When servicing the two-position control system, look for three possible sources of trouble:

l The sensing element and its associated mechanism

Figure 5-6.A typical air-conditioning system.

l The magnetic valves that control the flow of refrigerant

l The wiring system that connects the sensing elements to the solenoids of the magnetic valves and the controller of the electric heaters