Document a1evpG6BmrDkGNkzpKR3Gog1a

American Society of Heating and Ventilating Engineers Guide, 1934 Automatic Return Traps In the general heating plant, where thermostatic traps are installed on the heating units, it becomes necessary to provide a means for returning the water of condensation to the boiler, if a condensation or vacuum pump is not used. When the return main can be kept sufficiently high above the boiler water line for all operating conditions, the water of condensation will flow back by gravity, and no mechanical device is required. But actually this does not work out in practice. It follows, therefore, that a direct return trap is needed for the handling of the condensation even though it may not be called into action except under some operating condition where the pressure differential exceeds the static head provided. The installation of a direct return trap assures safety for such systems, and the operation of the plant under varying conditions. Automatic return traps, sometimes called alternating receivers, may be of the counterbalanced, tilting type, or spring actuated. These consist of a small receiver with an internal float, and when the condensate will not flow into the boiler under pressure, it will feed into the receiver of the trap, and in so doing, raise or tilt the float or mechanism which actuates a steam valve automatically. This admits steam to the receiver, at boiler pressure, and the equalizing of the pressures which follows allows the water to flow into the boiler. Fig. 21 shows a direct return tilting trap and receiver properly connected for automatically feeding a boiler from a system of returns delivering the condensate to the receiver. 440 Chapter 32 STEAM SYSTEM PIPING Flow of Steam in Pipes, Pipe Sizes, Initial Pressure, Pressure Drop, Maximum Velocity, Reaming, Equivalent Length of Run9 Tables far Pipe Sizing, Sizing One-Pipe Gravity Air Vent Systems, TwoPipe Gravity Air. Vent Systems, Two-Pipe Vapor Systems, Atmos pheric Systems, Vacuum Systems, Sub-Atmospheric Systems, Orifice Systems, Pressure-Reducing Valves, Expansion in Steam and Return Lines, Piping Connections and Details, Boiler Con nections, Hartford Return Connection THE design of a steam heating system may be divided into four parts, namely, (1) the details of the heating units, (2) the arrangement of the general piping scheme, (3) the details of connections, and (4) the sizing of the lines. Items 1 and 2 are covered in Chapters 30 and 31, respectively, while this chapter considers the two latter items. The functions of piping are to supply the heating units with steam and to remove the condensation. In some systems both the air and con densation are removed from the heating units by the return piping. To accomplish this effectively, the distribution of the steam should be efficient and equitable, without noise, and the returns should be as short as possible. When air is handled its escape should be facilitated to the utmost since an air-bound system will not heat properly. Condensation takes place in a steam- system not only in the heating units, but through out the piping system as well, and the returns also condense any steam or vapor that may be contained. At the same time part of the condensation may flash back into steam when the vacuum or pressure in the return is considerably below the steam pressure. It is essential that steam piping systems not only distribute steam at full load but also at partial loads, as the average winter demand is less than half of the demand in most severe outside temperatures. Further more, in heating up rapidly the load on the steam main may exceed the maximum operating load even in extreme weather, due to the necessity of raising the temperature of the metal in the system to the steam tem perature. This may require more heat than would be emitted from the system itself after it once is thoroughly heated.. STEAM FLOW IN PIPES The rate of flow of dry steam or steam with a small amount of water flowing in the same direction is in accordance with the general laws of gas flow and is a function of the length and diameter of the pipe, the density of the steam and the pressure drop through the pipe. This relationship has been established by Babcock in the following formula: . 441