Document B5MdOD97qyeaRp1xw1OrY1LRm

492 CHAPTER 21 1955 Guide Each heating unit in a vapor system, as in all two-pipe systems, is pro vided with a graduated or modulating valve which permits the control of heat in the radiator by varying the opening of the valve. Two-Pipe Vacuum Systems Vacuum systems operate under conditions of both low pressure and vacuum, but employ the use of a vacuum pump to insure maintenance of "sub-atmospheric pressures in the return piping for all operating conditions. The system may operate transiently with sub-atmospheric pressure in the supply piping during the time the rate of steam generation is equivalent to.or less than the total connected load. A typical two-pipe upTeed vacuum system is illustrated in Fig. 14, and a down-feed arrangement in Fig. 15. The return risers are connected in the basement into a common return main which slopes downward toward the vacuum pump. The vacuum Steam Heating Systems 493 float control for the pump at the low point of the return main,-located adjacent to the vacuum pump. ' When the vertical lift is considerable, several lift fittings should be used in steps as shown in Fig. 16. This permits a given lift to be secured with a somewhat lower vacuum than where the vertical distance is served by a single lift. Where several lifts are present in a given system at different locations, the lifting cannot occur until the entire system is filled with steam. A lift connection for location close to the pump, where the size may be above the commercial stock sizes, is shown in Fig. 17. It is desirable that means be provided for manually draining the low point of the lift fittings to eliminate danger of freezing. TWO-PIPE SUB-ATMOSPHERIC SYSTEMS Sub-atmospheric systems are similar to vacuum systems but, in con trast, provide control of building temperature by variation of the heat pump withdraws the air and water from the system, separates the air from the water and expels it to atmosphere, and pumps the water back to the boiler or other receiver, which may be a feed-water heater or hot well. It is essential that no connection be made from the supply side to the return side at any point except through a trap. The desirable practice demands a return flowing to the vacuum pump by an uninterrupted downward slope. In some instances local conditions make it necessary to drop toe return below the level of the vacuum pump inlet before the pump caD be reached. In such an event one of the advantages of the vacuum sys tem is the ability to raise the condensate to a considerable height, by the suction of the vacuum pump, by means of a lift connection or fitting in serted in the return. The height the condensate can be raised depends on the amount of vacuum maintained. It is preferable to limit lift con nections to a single lift at the vacuum pump. A still more preferaDi arrangement is the use of an accumulator tank, or receiver tank, with irom t_n_e__r_a_d_ia__to_r_s_. me radiator neat emission is controlled by varying the pressure, temperature and specific volume of steam in circu lation. These systems differ from the ordinary vacuum system in that hey maintain a controllable partial vacuum on both the supply and return sides of the system, instead of only on the return side. In the vacuum system, steam pressure above that of the atmosphere exists in the ^Pply mains and radiators practically at all times. In the sub-atmosPheric system, atmospheric pressure or higher exists in the steam supply P'Pmg and radiators only during severe weather. Under average winter temperature the steam is under partial vacuum which in mild weather ay reach as high as 25 in. Hg, after which further reduction in heat Ptput is obtained by restricting the quantity of steam. The rate of steam supply is controlled by a valve in the steam main or y thermostatically controlling the rate of steam production in the boiler.