Document JvmzR7enpG695DjJGYQDO4K2B

1935of andAmerican Society Heating Ventilating Engineers Guide, 13.Fig. Typical Atmospheric System with Automatic Return Trap* Proper piping connections are essential with special appliances for pressure equalizing and air elimination 512 31Chapter --Steam Heating Systems VACUUM SYSTEM In the vacuum system, a vacuum is maintained in the return line nractically at all times but no vacuum is carried on the steam side, and the usual accessories include graduated valves on the radiator supply and thermostatic traps on the radiator return. The air is expelled from the system by a vacuum pump and all drips must pass through thermostatic traps before connecting to the return side of the system. These systems are often fed from high pressure steam mains through pressure-reducing valves but they may be fed direct from a low-pressure -steam-heating- boiler-as-shown-in-Fig.-14,- in- which a typical- up-feed vacuum system is illustrated. The supply main slopes down in the direction of flow; the runouts pitch down toward the riser if the riser is dripped (Fig. 3) or up toward the riser if the riser is not dripped (Fig. 2); both conditions are indicated in Fig. 14. The matter of dripping the risers depends largely on the height of the riser and the judgment of the designer. Ordinarily risers less than three stories high are not dripped and those more than four stories high are dripped, but there is no set rule for this. When risers are dripped the runouts from the steam main may be taken from the bottom if desired and each runout then serves as a drip for the main. The risers are carried up to the highest radiator connection and are connected to the radiator through runouts sloping back toward the riser. The radiators usually have graduated valves on the supply end, although this is not absolutely necessary. Angle-globe valves and gate valves may be used where graduated manual control is not desirable. The return valves must be of the thermostatic type which will pass air and water but which will close against the passage of steam. The return risers are carried down to the basement and are connected into a common return line, care being taken that no air pockets exist in the runouts or in the horizontal return main which slopes downward toward the vacuum pump to which it is connected. The air and water are taken by the vacuum pump, which discharges the air from the system and pumps the water back to the boiler, or Other receiver, which may be a feed-water tank or a hot well. It is essential on these systems that no connection from the supply side to the return side be made at any point except through a trap. While the best practice demands a return flowing to the vacuum pump in an interrupted downward slope, in some cases limitations make it necessary to drop the return below the level of the vacuum pump inlet before the pump can be reached. In such event one of the advantages of the vacuum system is that the return can be raised by the suction of the vacuum pump to a considerable height, depending on the amount of vacuum maintained, by means of a lift fitting inserted in the return. When the lift is considerable, several lift fittings are used in steps (Fig. 15), more successful operation being obtained by this method than when the lift is made in one step. If the lift occurs close to the vacuum pump, a special arrangement is used as shown in Fig. 16. Down-Feed Vacuum System The piping arrangement for the down-feed vacuum system is similar on the supply side to the down-feed vapor system in that it has similar 513