Document OzMBKqoo09dmrRZ1wJ6KOZDJv

490 CHAPTER 21 1955 Guide Steam Heating Systems I ' 491 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 rather than float and thermostatic type since there should be no steam in the return main so long as the thermostatic traps on the radia tors or convectors are operating properly. 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. 13. Feed Steam Drop