Document 6LMyZdb7xNwNrj3QmVdnZGb6

264 Chapter 14 1945 Guide in 10 It. Where this pitch cannot be "obtaih'ed'runouts over 8 ft'in length should'be one size larger than called for in the table. - 3. In general it is not desirable to have a supply main smaller than 2 in. 4. When necessary, supply main, supply risers, or branches to supply risers should be dripped separately into a wet return, or may be connected into the dry return through a thermostatic drip trap. VACUUM SYSTEMS -- In the vacuum system, a vacuum is maintained in the return line practically at all times. The pump is usually controlled by a vacuum regulator which operates the pump to maintain the vacuum within limits and operates in response to a pressure difference between the atmosphere Steam Heating Systems and Piping. 265 ~ nections to' a single lift~at the vacuunrpump7~ 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 as shown in Fig. 12. ' This permits a given lift to be secured with a somewhat lower vacuum Ilian 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. 13. It is and the return to control the vacuum in the return main. The source of steam supply may be a low pressure boiler as shown in Fig. 11, or a high pressure line through a pressure reducing valve. The piping and other details are the same as for the vapor systems. The return risers are connected in the basement into a common return main which slopes downward toward the vacuum pump. .The vacuum pump withdraws the air and water from the system, separates the air from, the water and expells 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 down ward slope. In some instances local conditions make it necessary to drop 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 inserted in the return. The height the condensate can be raised depends on the amount of vacuum maintained. It is preferable to limit lift con- Fig. 12. Method of Making Lifts on Vacuum Systems when Distance is Over 5 ft . Fig. 13. Detail of Main Return Lift at Vacuum Pump ECCENTRIC REOUCIWS Rl --15------ I(CCODUPUNO. fi1---- Fig. 14. Method of Changing Size of Steam Main when Runouts are Taken from Top desirable that means be provided for manually draining the low point of the lift fittings to eliminate danger of freezing. , 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 m pipe size, or if eccentric fittings are used (Fig. 14). A slight pitch in the steam main, however, should be used when possible. An overhead vacuum down-feed system is shown diagrammatically in Fig. 15. Vacuum, atmospheric, sub-atmospheric and orifice systems are usually employed in large installations and have total drops varying from % to