ENGINEMAN 1 & CTWO-POSITION CONTROLThis control may be used on three types ofsystems:1. Systems employing a simple thermostatic-ally controlled single-pole switch to control flowof refrigerant to the cooling coil.2. Systems using reheaters, employing a ther-mostatic element actuating two interlockedswitches.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 commonlyused and requires little explanation. A thermostat,consisting of a temperature sending element,actuating a single-pole, single-throw switch, opensand closes a magnetic valve to start and stop theflow of refrigerant—chilled water or commercialrefrigerant. This type of control is similar to ther-mostatic control for the refrigeration plant.Although the type 1 system requires single-polethermostats, the 2PD used in type 2 and type 3systems can be used. The cooling switch wouldthen be connected in the normal manner with theheating switch inoperative.The use of the type 2 system has greatly in-creased, due to the present effort to make livingand working spaces more comfortable and to therapid development of various types of weaponssystems. Such spaces often use a common cool-ing coil serving several different spaces. Assumethat three spaces are being cooled by a commoncoil. Since the load changes seldom occursimultaneously, electric or steam reheaters areinstalled in the cooling air ducts and the coolingthermostats of the various spaces are connectedin parallel so that any one may open the coolingcoil valve.Suppose space “B” (figure 6-11) has a loadchange and spaces “A” and “C” do not. Thesespaces would become too cold for comfort withthe coil operating to take care of space "B". Inorder to prevent this condition, the thermostatwould close the heating switch and energize thereheaters for spaces A and C.The type 3 system is identical to the type 2system, except that a humidistat is wired inparallel with the thermostatic heating switch. Thistype system is used mostly in weapons and elec-tronic spaces. The humidistat is set for the relativehumidity condition desired. In most installations,it is only necessary to prevent the humidity fromexceeding 55%. Where the humidistat is installed,an increase in temperature beyond the thermostatsetting will close the thermostat cooling switch andan increase in relative humidity beyond theFigure 6-11.—Typical air conditioning system.6-16
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