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184 CHAPTER 21 1953 Guide Steam Heating Systems 485 range of the average vapor system.' Fig. 9 illustrates a typical low pressure system with condensate pump.; Two-Pipe Vapor Systems Two-pipe vapor systems operate at pressures varying from 20 in. vacuum or more (depending upon the tightness with which the system is assembled) to 15 psig without the use of a vacuum pump. A typical two-pipe up-feed vapor system is shown in Fig. 10, and a typical two-pipe down-feed system is illustrated in Fig. 11. The method of dripping drop risers in a down-feed system is illustrated in Fig. 12. Radiators discharge their condensate and air through thermostatic traps to the dry-return main. Air is eliminated, when the system is under pressure, at the ends of the supply and return mains just before they drop to the wet return. The vent valves are of the float and thermostatic type which opens when cool air contracts the thermo static element, and closes when steam expands the element. The float element of the valve closes the valve when, due to pressure differences, water rises to the point of overflow in the main. The vent valves are also provided with a small check disc which closes to,prevent the inflow of air to the system when the pressure drops below atmospheric pressure. . This enables the system to operate under vacuum conditions at lower steam temperatures for a period of four to eight hours, depending on the tightness of the system. Vapor systems may also be provided with an automatic return trap or alternating receiver which automatically returns condensate to the boiler when the boiler is steaming under pressure conditions which would prevent the return of condensate by gravity. The typical connections for an auto matic return trap are illustrated in Fig. i3. 10.Fig. Typical Up-Feed Two-Pipe System'with Automatic Return Trap* ' * Proper piping connections are essential with special appliances for pressure equalizing and air elimination- . tions at Bottom op Down- ..... Feed Steam Drop Automatic Return Trap