Document EdmrK5EVYw8Go81d5ooED1gzV

i HEATING VENTILATING AIR CONDITIONING GUIDE 1940 CHAPTER 15. STEAM HEATINC SYSTEMS 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. 14. It- is desirable that means be provided for manually draining the low point of the lift fittings to eliminate from the return piping all water in danger of freezing-in case the system is shut down for a considerable length of time. 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 runouts, radiator valves, drips on the bottom of the steam drops, andenlargement of the drops for the lower radiator connections. The return varying the pressure, temperature and volume of steam in circulation. These systems differ from the ordinary vacuum system in that they main tain 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 supply-mains and radiators practically at all times. In the sub-atmospheric system, atmospheric pressure or higher exists in the steam supply piping and radiators only during severe weather. Under average winter temperature the steam is under partial vacuum which in mild weather may reach as high as 25 in. Hg., after which further reduction in heat output is obtained by restricting the quantity of steam. The rate of steam supply is controlled by a valve in the steam main or by thermostatically controlling the rate of steam production in the boiler. side of the system is exactly the same as the up-feed system except that the steam- riser- drips at the bottom are' connected into the 'return line through thermostatic traps. It is preferable to take the runouts for the risers from the bottom or at a 45-deg angle down from the steam main so that they may serve as steam main drips. When this is done.it is practical to run the steam main level if a runout is located at every-change in pipe'size, or if eccentric fittings are used (Fig. 15). A slight pitch in the -steam main, however, should be used when possible. An overhead vacuum down-feed system is shown diagrammatically in Fig. 16. SUB-ATMOSPHERIC SYSTEMS / i Sub-atmospheric systems are similar to vacuum systems but, in con>trast - provide control of building temperature by variation of the heat output1 from the radiators. The radiator heat, emission is controlled by &76 VERTICAL UFT TO BE ONE SIZE. .- smaller than the UFT HTT1NQ VACUUM 6ETUQN fs VACUUM RETURN /Uft fitting Fig. 13. Method of Making Lifts on Vacuum Systems, when Distance is Over 5 ft Fig. 14. Detail of Main Return Lift at Vacuum Pump .1 eccentric reducing (fCOUPUNQ.. Fig. 15. Method of Changing Size of Steam Main when Runouts are Taken from Top The control-valve may be of the automatic modulating or floating type governed thermostatically from selected "control points in the building or it may be a special pressure reducing valve which will maintain the; desired sub-atmospheric pressures by continuous flow into the heating main. All radiator supply Valves have incorporated adjustable'orifices or are equipped with regulating orifice plates. The sizes of orifices used are larger, than for orifice systems because for equal radiator sizes the volume flowing is larger. These'orifipes are . omitted on some systems, depending upon the type of control. Radiator traps and drips are designed to operate at any pressure from 15 lb gage to 26 in. of Hg. A vacuum pump capable of, operating at high vacuum is preferable to promote accuracy in the distribution of steam throughout the system, particularly in mild weather. This vacuum is partially self induced, by the condensation of the steam in the system under conditions of restricted supply for reduction of the radiator heat emission.' 277