Document KRbKMX6avoR8B24v0RXwLJwmK

1-J HEATING : VENTILATING -AIR CONDITIONING GUIDE 1943 ' 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 steami drops, and enlargement of the drops for the lower radiator connections. The return side of the system is exactly the-same as the up-feed system except thatthe 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 inpipe size, or'if eccentric fittings are used (Fig.' 15). A slight pitch in the CHARTER 14. STEAM HEATING SYSTEMS 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. 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. Alt radiator1 supply valves have incorporated adjustable orifices of are equipped with regulating orifice plates. The sizes of orifices used are larger than for orifice systems because for equal radiator sizes the .steam main, however,- should be used when possible. An overhead . vacuum down-feed system is shown diagrammatically. in Fig. 16. - SUB-ATMOSPHERIC SYSTEMS r.- Sub-atmospheric systems are similar to-vacuum systems but, in contrast; .'provide-control of. building'temperature by variation bf the; heatoutput-from the radiators. . The. radiator heat emission is controlled by. varying the- pressure,-. temperature -and- volume- of steam in circulation. These;systems differ from the ordinary vacuum system inthat'they main tain a controllable partial vacuum on both 'the:supply and retum 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-mainsand radiators practically. at. all times..; In the'sub-atmospheric system, atmospheric' pressure:or higher;'exists in - the steam supply piping; and 278 n Fig. 13. Method of Making.Lifts -on Vacuum Systems when Distance is Over 5 ft . nt--1* Fig.. 14. Detail of Main Return Lift at Vacuum Pump . ECCENTWC deducing [COUWJNG. Fig. 15. Method of Changing Size of Steam Main when Runouts , are Taken, from Top .. volume flowing is larger. These orifices are omitted oh some systems, depending upon the. type of :control. ; Radiator traps and drips are designed to operate at any pressure from 15.1b 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. .. The returns must grade downwardconstantly and uninterruptedly from the radiator return outlets to the inlet of the receiver of the vacuum pump. One radical difference between this and the ordinary vacuum .system is that no lifts should be made in the, return line,-except at the vacuum pump. The receivers are placed at a lower level than.-the pump and equipped with float control so the pump may operate-as a return pump under night" conditions. The system may be operated in the same manner as the ordinary vacuum system-when desired.... 279 . . y'