Document 2jzLvDLO3aDwwgOo6GakOLR66

American Society of Heating and Ventilating Engineers Guide, 1936 3 Iti the ordinary vacuum system1 of steam heating, where does1 the vacuum usually exist? On the .return side of the system only, between the radiator trap and the vacuum pump. If the radiator supply valve is closed off, the vacuum may extend, back through the radiator as far as the supply valve; if an adequate supply of steam is furnished to the system, some vacuum may be developed in the steam main, but neither of these can be termed notmai Operation: 4 What is the distinction between the open and the closed vapor systems? The open vapor system has the return line always Open to the atmosphere, while the closed vapor system has an automatic device, on the air vent .so that air once expelled froiti the system through the vent cannot re-enter via this route. - 5 On a vacuum system, what device must be placed on all drips before they enter the vacuum return line? A thermostatic drip trap or occasionally, where large volumes of condensation are to be handled, a float trap. 6 How does the sub-atmospheric system differ in operation from the ordinary vacuum system? The ordinary vacuum system has pressure in the steam line, and a vacuum produced by the vacuum pump in the return line, usually varying between 5 and 10 in. of water. The sub-atmospheric system may have either a vacuum or pressure on the. steam and return lines, but a constant difference in pressure is maintained between the iines regardless of what pressure or vacuum may be carried. The vacuum, which is generally produced by condensation in the system under conditions of throttled steam supply, may run much higher than in the ordinary vacuum systems. 7 What is generally understood by zoning in building steam heating systems? Zoning is a term applied to the placing of certain sections of a building on a single temperature control instead of having either individual room control or a single tempera ture control governing the whole building.. Zones may be horizontal, such as a single story, a basement, or an attic, or. vertical such as the north side, or the west side. 8 Why does the water line in the far end of a wet return in a gravity steam system rise higher than the water line in:the boiler? The friction of the steam flowing through the steam main from the boiler to the far end of the system causes a drop in steam pressure at the point where the wet return is connected; consequently, the steam pressure on,top of. the water in the wet return is less than the steam pressure on top of the water in the boiler, so the water in the end of the wet return rises until a balanced condition is set up. '\ 9 # On gravity one-pipe systems as indicated in Fig. 1 and Fig. 3, why is the drip on the steam runout'connected to Wet return? Because if it were Connected to dry return, the pressure drops to two different points would not necessarily be the same,and the system would short circuit. 10 t What is the function of the automatic return trap? To insure the return of condensate to the boiler when the operating condition is such that the boiler pressure exceeds the static head on the returns. 11 What advantage is there to an air valve with a check to prevent the re entrance of expelled air? A system equipped with such valves builds up a vacuum and holds the heat longer. With proper controls on the boiler, lower radiator temperatures can be' maintained in mild weather, giving better plant efficiency. 556 Chapter 32 PIPING FOR STEAM HEATING SYSTEMS Flow of Steam in Pipes, Pipe Sizes, Tablesfor Pipe Sizing, Sizing One-Pipe Gravity Air Vent Systems, Two-Pipe Gravity Air Vent Systems, Two-Pipe Vapor Systems, Atmospheric Systems, Vacuum Systems, Sub-Atmospheric Systems, Orifice Systems, High Pressure Steam, Expansion in Steam and Return lines. Piping Connections and Details, Boiler Connections, Hartford Return Connection THE design of a steam heating system should be considered under four headings, namely, (1) the details of the heating units, (2) the arrange ment 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 heait than would be emitted from the system itself after it once is thoroughly heated. STEAM FLOW 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: