Document oM9yaEmkDpyO5m6rZR1vy9X4E

American Society of HEATiNG- and' Ventilating Engineers Guide, 1934 X, air temperature will be but slightly above the room temperature. To establish and maintain a constant heat flow, provision must be: made for removing the air in the room, after it has cooled to the desired room tem perature, by a system of vent flues or ducts. As the air flow is maintained by natural draft and this gravity head is very slight, it is necessary to make all ducts as short as possible, especially the runs from the heating units to- the base of-the vertical warm air flues. 418 \ Chapter 31 STEAM HEATING SYSTEMS Gravity and Mechanical Return, Gravity' One-Pipe Air-Vent System, Gravity Two-Pipe Air-Vent System, One-Pipe Vapor System, Two-Pipe Vapor System, Atmospheric System, Vacuum System, Sub-Atmospheric System, Orifice System, Zone Control, Condensation Return Pumps, Vacuum Pumps, Traps STEAM heating systems may be classified according to the piping arrangement, the accessories used, the method of returning the con densate to the boiler, the method of expelling air from the system, or the type of control employed. The essential features of the common types are described in this chapter." Information concerning the design and layout of steam heating systems is given in Chapter 32. GRAVITY AND MECHANICAL RETURN In gravity systems the condensate is returned to the boiler by gravity due to the static head of water in the return mains. The elevation of the boiler water line must consequently be sufficiently below the lowest heating units and steam main and dry return mains to permit the return of condensate by gravity. The water line difference* must be sufficient to overcome the maximum pressure drop in the system and, when radiator and drip traps are used as in two-pipe vapor systems, the operating pressure of the boiler. This applies only to closed circuit systems, where the condensation is returned to the boiler. If the condensation is wasted, no water line difference is required. In mechanical systems the condensate flows to a receiver and is then forced into the boiler against the boiler pressure. The lowest parts of the supply side of the system must be kept sufficiently above the water line of the receiver to insure adequate drainage of water from the system, but the relative elevation of the boiler water line is unimportant in such Cases except that the head on the pump or trap discharge becomes greater as the height of the boiler water line above the trap or pump increases. There are three general types of mechanical returns in common use, namely, (1) the mechanical return trap, (2) the condensation return pump, and (3) the vacuum return pump. Further information on pumps and traps will be presented later in this chapter. GRAVITY ONE-PIPE AIR-VENT SYSTEM In the gravity one-pipe air-vent system each radiator has but a single connection through which steam must enter- and condensation must *Tbe water line difference is the distance between the water line of the boiler and the low point of the water in the dry return main. 419