Document a1NJK3VbywkmroBVx7RvZEE3B

American Society of Heating arid Ventilating Engineers Guide, 1937 condensation. Fig. 21 shows a method which may be used for disposing of the greater part of the vapor of re-evaporation." An expansion chamber often is installed between the high- and low-pressure traps. 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 guarantees the operation of the plant under varying'conditions. Automatic return traps, sometimes called alternating receivers, may be of the counter-balanced, 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. 22 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. PROBLEMS IN PRACTICE 1 What is meant by water line difference in a gravity steam heating system? The water line difference is the distance between the level of the water in the dry or wet return and the boiler water line. This difference is equivalent to the pressure required to overcome the maximum drop in the system and the operating pressure of the boiler. 2 How many types of common mechanical returns are there and what are they? Three: (1) the mechanical return trap, (2) the condensation return pump, and (3) the vacuum pump. 3 In the ordinary vacuum system of steam heating, where does 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 inadequate supply of steam is furnished to the system, some vacuum may be developed in the steam main, but neither of these can be termed normal 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 from the system through ;the vent cannot re-enter via this route. Chapter 31--Steam Heating Systems. 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. g 0 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 mercury. 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 lines regard less of what pressure or vacuum may be carried. The vacuum, which is generally produced jointly by condensation and the exhausting action of the pump, in the system under conditions of throttled steam supply, will run much higher than in the ordinary vacuum system, and as high as 25 in. of mercury in the radiators. 7 t 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 t 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 and the pressure reduction resulting from the condensing action of the radi ators 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 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. 12 With a one-pipe steam heating plant designed for a total pressure drop of H ib with a check valve on the return, how high must the lowest part of the steam main be above the boiler water line? Water line difference X 28) Flow head required Friction head of check valve Factor of safety 7 in. 3 in. 4 in. 6 in. Total required 20 in. 13 What are the essentials of a two-pipe closed vapor system? Packless graduated valves on radiators; thermostatic return traps on return and drips; an automatic return trap to prevent water from backing out of the boiler. 569