Document 4v9Lbb7jXza6bn2xVoGNJk2Qe

424 ,.................... .............: ............................... CHAPTER 23, . ' - ...............................- 1948 Guide 1 ............ ..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 Ruction 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.ampunt pf vacuum maintained, It is preferable to limit lift com" 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. Steam Heating Systems and Piping; \ -. ;;. _j /; ' 425 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 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 controled 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 floatingtype 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. In some systems radiator supply valves include adjustable orifices When the vertical lift is considerable, several lift fittings should be used in steps as shown in 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 eliminate danger pf freezing. TWO-PIPE 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 specific volume of steam in circu lation. These systems differ from the ordinary vacuum system in that they maintain a controllable partial vacuum on both the supply and return sides of the system, instead of only on the . return side.In the - Fig. 16. Method of Making Lifts on Vacuum Systems when Distance . ` is Over 5 ft or are equipped with regulating orifice, plates. The sizes of orifices; used are larger than for other types of..Orifice systems:because, for equal radiator, sizes the volume flowing is larger. . Orificesjare omitted bn some systems.. Radiator traps and drips, are designed to, ..operate at any pressure frpm 15 lb gage to 26 in, Hg,-. A vacuum pump capable of operating at high vacuum is preferable to promote, accuracy in: the distribution of. steam throughout the.system, particularly,inrmild.weather. . This.vacuum i? partially, self-induced by the condensation of. jthie. steam ;in ,thSystem under conditions of restricted, supply, for,reduction pf the radiator, heat emission. . .... ' .... l - The returns must grade downward constantly and uninterruptedly from the radiator return outlets to.the inletqf.the receiverof 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 that 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. Steam for heating domestic hot water should be taken from the boiler header back of the control valve so that pressures sufficiently high, for heating the water may be maintained on-the heater.. The sub-atinospheric method of heating cambe-used for:the heating coils of ventilating