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s -r-`7ey *> Heating Ventilating Air Conditioning Guide 1939 pump withdraws the air and water from the system, separates the a' from the water and expells it to atmosphere and pumps the water back / 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 best practice demands a return flowing to the vacuum pump by an uninterrupted down ward slope. In some instances local conditions make it necessary to dron the return below the level of the vacuum pump inlet, before the pump can be reached. In such an event one of the advantages of the vacuum system 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 Chapter IS. Steam Heating Systems Down-Feed Vacuum System - , nioing arrangement for the down-feed vacuum system is similar V i cunnly side to the down-feed vapor system in that it has similar ?n radiator valves, drips on the bottom of the steam drops, and ^l^ement of the drops for the lower radiator connections. The return f the system is exactly the same as the up-feed system except that l team riser drips at the bottom are connected into the return line l wh thermostatic traps. It is preferable to take the runouts for the re from the bottom or at a 45-deg angle down from the steam main Jfv6) so that they may serve as steam main drips. When this is done 't practical to run the steam main level if a runout is located at every Fig. 16. Method of Making Lifts on Vacuum Systems when Distance is Over 5 ft Fig. 17. Detail of Main Return Lift at Vacuum Pump ECCENTRIC REDUCING (COUPLING. Fig. 18. Method of Changing Size of Steam Main when Runouts are Taken from Top inserted in the return. The height the condensate can be raised depends on the steam pressure and the amount of vacuum maintained. It is preferable to limit lift connections ter a single lift at the vacuum pump. A still more preferable arrangement is the use of an accumulator tank, or receiver tank with a 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 (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 elimi nate from the return piping all water in danger of freezing in case the system is shut down for a considerable length of time. \ 294 Fig. 19. Typical Down-Feed Vacuum System change in pipe size, or if eccentric fittings are used (Fig. 18). _ A slight pitch in the steam main, however, should be used when possible. . An overhead vacuum down-feed system is shown diagrammatically in Fig. 19. 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, 295