similar to the thermostatic control for the refrigerationplant. The type 1 system requires single-polethermostats, but type 2 and type 3 systems can usetwo-position dual controls (2PD). The cooling switchwould then be connected in the normal manner with theheating switch inoperative.The type 2 system is most commonly used to makeliving and working spaces more habitable and forvarious types of weapons systems that require cooling.These systems often use a common cooling coil servingseveral different spaces. Since load changes seldomoccur simultaneously, electric or steam reheaters areinstalled in the cooling air ducts. The coolingthermostats of the various spaces are connected inparallel so that any one of the thermostats may open thecooling coil valve.Suppose three spaces are being cooled by a commoncoil. Space B in figure 5-6 has a load change and spacesA and C do not. With the coil operating to take care ofspace B, these spaces would become too cold forcomfort. To prevent this condition, the thermostat wouldclose the heating switch and energize the reheaters forspaces A and C.The type 3 system is identical to the type 2 system,except that a humidistat is wired in parallel with thethermostatic heating switch. This type of system is usedmostly in weapons and electronic spaces. Thehumidistat is set for the relative humidity desired. Inmost installations, it is only necessary to prevent thehumidity from exceeding 55 percent. Where thehumidistat is installed, an increase in temperaturebeyond the thermostat setting will close the thermostatcooling switch. An increase in relative humidity beyondthe humidistat setting will close the heating switch andenergize the reheaters.MAINTENANCEProper attention to the planned maintenance systemoften exposes developing troubles in time to takecorrective action. Since most breakdowns occur at themost inopportune times, periodic checks andmaintenance will help to avoid malfunctions.The 2PD control system can easily be checked outin a reasonably short time. The checkout should be madeat least every 3 months or more often if necessary.Inspections and checks should be made at the beginningof, and midway through, the cooling season and heatingseason.You should inspect the sensing elements andremove any dust accumulations. Remove dust and dirtfrom thermostatic sensing elements with a soft brush.Use air to gently blow off any dust on the sensingelements in humidistats. The air will not damage theelement 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 humidistatsshould be checked with a calibrated thermometer and areliable humidity indicator.When servicing the two-position control system,look for three possible sources of trouble:l The sensing element and its associatedmechanismFigure 5-6.—A typical air-conditioning system.5-12
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