Document mOMv6B8GN6rVOGd1Z6OyXeqQ

Heating Ventilating Air Conditioning Guide 1939 valve. Water entering the trap fills the bucket which sinks and through compounj leverage opens the valve, and the trap discharges. It is impossible to install a wat<2 gage glass on an inverted bucket trap, but if visual inspection is necessary, a gage g|j? can be placed on the line leading to the trap. No air relief cocks can be used 6ut this j! unnecessary, as the elimination of air is automatically taken care of by air passing through the vent in the top of the inverted bucket regardless of temperature. 8 Thermostatic Traps. Thermostatic traps are of two types, those in whjch the discharge valve is operated by the relative expansion of metals, and those in which the action of a volatile liquid is utilized for this purpose. Thermostatic traps of large capacity for draining blast coils or very large radiators are called blast traps. Float and thermostatic traps have both a thermostatic element to release air and a float element to release the water. Impulse traps operate with a moving valve actuated by a control cylinder. When the trap is handling condensate, the pressure required to lift the valve is greater than the reduced pressure in the control cylinder and consequently the valve opens allowing a free discharge of condensate. As the remaining condensate approaches steam tempera ture, flashing results, flow through the valve orifice is choked and the pressure builds up in the control chamber closing the valve. 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. Tilting Traps. With this type of trap, water enters a bowl and rises until its weight overbalances that of a counter-weight, and the bowl sinks to the bottom. As the bowl sinks, a valve is opened thus admitting live steam pressure on the surface of the water and the trap then discharges. After the water is discharged, the counter-weight sinks and raises the bowl, which in turn closes the valve and the cycle begins again. Tilting traps are necessarily intermittent in operation. They are not ordinarily equipped with glass water gages, as the action of the trap shows when it is filling or emptying. The air relief of tilting traps is taken care of by the valves of the trap. 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. 304 Chapter IS. Steam Heating Systems How many types OI cumiuuu uicuuauiuii they* fl) the mechanical return trap, (2) the condensation return pump, and (3) tThherevea*cuum pump,,. In the ordinary vacuum system of steam heating, where does the vacuum usually exist. . re(Urn side of the system only, between the radiator trap and the vacuum pump, tfrhe radiator supply valve is closed off, the vacuum may extend back through the H'ator as far as the supply valve; if an inadequate supply of steam is furnished to 1?ai~ tem, some vacuum may be developed in the steam main, but neither of these can betermed '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 dosed vapor system has an automatic device on the air vent so that air once expelled fiom the system through the vent cannot re-enter via this route. g ^ Qn 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, or combination float and thermostatic 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 mercury. The sub-atmospheric system may have either a vacuum or pressure on the steam and return lines according to the weather conditions, but a constant difference in pressure is maintained between the lines regardless of what 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 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 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? . X *LSy.re ^e-return Qf condensate to the boiler when the operating condition is such that the bouer pressure .exceeds the static head on the returns. 11 What advantage is there to an air valve with a check to prevent the reentrance of expelled air?