Document MG8KJXOgM8JKx5DGm66wOxDpL

426 CHAPTER 23 1946 Guide fire, and (3) because with large variations in pressure the value of gradu ated valves on radiators is destroyed. For small vapor systems where the equivalent length, of run does not exceed 200 ft, it is recommended that the niain and any runouts to risers that may be dripped should be sized from Column D, Table 4, while riser runouts not dripped and radiator runouts should employ Column I. The up-feed steam risers should be taken from Column H. On the returns, the risers should be sized from Tables 5 and 6, Column U, (lower portion) and the mains from Column U (upper portion). It should again be noted that the pressure drop in the steam side of the system is kept the same as on the return side except where the flow in the riser is concerned. On a down-feed system the, main vertical riser should be .sized from . Column H, but the down-feed risers can be taken from Column D al though it so happens that the values in Columns'"!? and H for small systems correspond. This will not hold true in larger systems. For vapor systems over 200 ft of equivalent length, the drop should notexceed'^ psi to M psi, impossible. Thus, for a 400 ft equivalent run the , drop per 100 ft should be not over Hpsi divided by 4, or M2 psi. ; In this case the steam mains would be sized from Column B, the radiator and undripped riser runouts from Column I; the risers from Column B, . because Column H gives a drop in excess of psi. On a down-feed system, Column B would have to be used, for both the main riser and'the smaller risers feeding the radiators in order not to increase the drop over % psi. The return risers would be sized from the lower portion of Column 0 and the dry return main, from the upper portion, of the same column, while any wet returns would be sized from Column N. The same pressure ., drop is applied on both the steam and the return sides of the system. ' Steam Heating Systems and Piping 427 Notes on Vapor Systems 1. Pitch of mains should not be less than M >n- >n 10 ft. 2. Pitch of horizontal runouts to risers and radiators should not be less than M in. in 10 ft. Where this pitch cannot be obtained 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 runouts 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 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 nections to 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 as shown in Fig. 12. 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. 13. It is 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 connection's. The return