Document wqJ9pKn4bY0BmVp6xdy1X0YBD

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 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, 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 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 CHAPTER 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. 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 steam main, however, should be used when possible. An overhead vacuum down-feed system is shown diagrammatically in Fig. 16. 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 output 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 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 276 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 bxentwc beojomq 'COUPLING. Fig. 15. Method of Changing Size of Steam Main when Runouts are Taken from Top volume flowing is larger. These orifices 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. The returns must grade downward constantly 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. 277