Document 6RVkk7yX095QjDJ1BRk47nkxo

Heating Ventilating Air Conditioning Guide 1939 % Chapter IS. Steam Heating Systems obtained which limit the vacuum the pump can produce even though the traps are in normal operating condition and the system is reasonably tight. It is for this reason the condensate from equipment using steam at ^ high pressures should not be connected directly to a vacuum return line $ but should drain to a receiver through a high pressure trap. The receiver should have an equalizing connection to a low pressure steam main and I drain through a low pressure trap to the vacuum return main as indicated I in Fig. 20. ' Vacuum Pump Controls In the ordinary vacuum system the pump is controlled by a regulator I fwhich cuts in when the vacuum drops to the lowest point desired and cuts out when the vacuum has been increased to the desired high point to * economize on current consumption. The cut-in point is usually about 3 in. and the cut-out point approximately 8 in. This, is done largely to -I eliminate frequent starting and stopping of the vacuum pump which would otherwise occur without serving any particular purpose in the ordinary vacuum return line system. In addition to this vacuum control, a float control is included which automatically starts the pump whenever sufficient condensation accumulates in the receiver, regardless of the vacuum in the system. A selector switch is usually provided to allow operation at night as a condensation pump only and to permit continuous operation if desired. In sub-atmospheric systems the vacuum pump control maintains a pressure difference between the supply and the return piping which is held within relatively close limits. Such limits pernfit securing higher vacua !on the supply piping as the upper limit of the pump's vacuum is ap proached. A pilot switch is also provided to enable operation as a con densation pump or to give continuous operation. In those cases where the condensation returns at a level lower than the pump, an accumulator tank is provided with a float control, to start and stop the pump whenever sufficient condensate accumulates. Piston Displacement Vacuum Pumps Piston displacement return vacuum heating pumps may be either power or steam driven. They should be provided with mechanical lubricators 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 discharge 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. 302 TRAPS Trans are generally classified as to function as (o) separating traps, (h) return, lifting or vacuum traps, and (c) air traps. Separating traps av be either float operated, thermostatically operated or float and thermostatically operated. Return traps for low pressure service have heen referred to previously 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 quick vents and vent valves are, 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, or 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. 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. 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, and 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 contmuous 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 me 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 th h s."r .ce f tbe water in the bucket to force the water to a discharge opening. When e bucket is emptied it rises and closes the valve and another cycle begins. The discharge irom this type of trap is intermittent. In the inverted bucket trap, steam floats the inverted submerged bucket and closes the 303