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HEATING VENTILATING AIR CONDITIONING GUIDE 1940 and their piston speed in feet per minute should not exceed 20 times the square root of the number of inches in their stroke. They are usually supplied with an air separating tank, open to'atmosphere, placed-on the discharge side of the pump and at an elevation sufficiently high to allow gravity flow of the condensate to the boiler. If the boiler pressure is too high for such gravity feed then an additional steam pump for feeding the boiler is desirable. The extra pump is sometimes avoided by using a closed separating tank with a float controlled vent. In both arrangements, the air taken from the system must be discharged against the, full dis charge pressure of the vacuum pump.' In the case of high or medium pressure boilers, it is better, to use the atmospheric separator and the second pump. In figuring the required displacement for such pumps; a value of from 6 to 10 times the volumetric flow of condensation is used for average vacuums and systems. TRAPS Traps are generally classified as to function as (a) separating traps, (6) return, lifting or vacuum traps, and (c) air traps. Separating traps may be either float operated, thermostatically operated or float and thermostatically operated. Return traps for low pressure service are referred to later as alternating receivers in this chapter. Return traps may also operate to receive condensate under a vacuum and return it to atmosphere or a higher pressure.. Air traps are generally float operated. Separating traps are used to release water of condensation but to retain steam. The thermostatic, and float and thermostatic types release, both condensate and air but retain steam. Separating traps are used for draining condensate from radiators, indirect air heaters, steam piping systems, kitchen equipment, laundry equipment, hospital equipment, drying equipment and many other kinds of apparatus. Air traps release air but retain water. Devices known as air ventsare, in principle, traps which allow the passage of air but prevent the passage of either water or steam. - ,' ^Return traps are used for returning condensate either-by gravity, by steam pressure, of by both, to a boiler or other point of disposal, and for lifting condensate from a lower to a higher elevation, or for handling condensate from a lower to a higher pressure. t. ; '.. ; The fundamental principle upon which the operation of practically all traps depends is that the pressure within the trap at the time of discharge shall be equal to, or slightly in excess of, the pressure against which the trap must discharge; including the friction head, velocity head and static head on the discharge side of. the trap. If the static head is in favor of the trap discharge it is a minus quantity and may be deducted from the other factors of the discharge head. Traps may also be classified according to the principle of operating device which supplies the power to cause them to-function as (1) float, (2) bucket, (3) thermostatic, (4). float and thermostatic, (5) impulse, or (6) tilting traps. 284 CHAPTER IS. STEAM HEATING SYSTEMS float Traps. A discharge valve is operated by the rise and fall of a float due to the change of water level in the trap. When the trap is empty the float is in its lowest position, a'nd the discharge valve is closed. A gage glass indicates the height of water in the chamber. . ... - -, Unless float traps are well made and proportioned there is danger of considerable steam leakage through.the discharge valve due to unequal expansion of the valve and seat and the sticking of moving parts. The discharge from a float trap is usually con tinuous since the height of the float, and consequently the area of the outlet, is propor tional to the amount of,water present. Bucket Traps. Bucket traps are of two types, the upright and inverted, and although they are both of the open float construction, their operating principle is entirely different. In the upright bucket trap, the water of condensation enters the trap and fills the space between the bucket and the walls of the trap. This causes the bucket to float and forces the valve against its seat, the valve and its stem usually being fastened to the bucket. When the water rises above the edges of the bucket it flows into it and causes it to sink, thereby withdrawing the valve from its seat. This permits the steam pressure acting on the surface of the water in the bucket to force the water to a discharge opening. When the bucket is emptied it rises and closes the valve and another cycle begins. The discharge from this type of trap is intermittent. In the inverted bucket trap, steam floats the inverted submerged bucket and closes the valve. Water entering the trap fills the bucket which sinks and through compound leverage opens the valve, and the trap discharges. It is impossible to install a water gage glass on an inverted bucket trap, but if visual inspection is necessary, a gage glass can be placed on the line leading to the trap. No air relief cocks can be used, but this is 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. Thermostatic Traps. Thermostatic traps are of two types, those in which 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 285