Document Z4q4yKmOvQyJ4ZJQNXKO6o8MO

American Society of Heating and Ventilating Engineers Guide, 1936 In the vacuum system, steam pressure above that of the atmosphere exists in the supply mains and radiators practically at all times. In the sub-atmospheric system, steam pressure exists in the steam main and radiators only during the most severe weather, while under average winter temperatures the steam is under a partial vacuum which in mild weather may reach as high as 25 in. after which further reduction in heat output is obtained by partially filling the radiation with steam. This vacuum is partially Self-induced by the condensation of the steam in the system due to the supply of steam being furnished through the control which admits it, and it being proportioned to balance the existing heat loss. In the sub-atmospheric system, a control valve is inserted on the steam main of an ordinary, vacuum' system, near the boiler or the boiler is automatically controlled, a high-vacuum pump is substituted for the ordinary type and is supplied with- a- pressure+lifference control, and traps are placed on the radiators and drips which will operate satisfactorily. at any pressure from 5 lb gage to 26 in. of vacuum. .. The control valve is either a special pressure-reducing valve which .may be controlled manually, or a control: valve or combustion equipment which .may be operated' thermostatically, from points selected in the building. The vacuum pump-regulator is simply a diaphragm so ar ranged that, when the vacuum in the return line is insufficient to hold the desired difference in pressure between the steam and return sides of the system, the vacuum pump is automatically started and the vacuum increased to the necessary amount. The actual pressure difference main- 546 "1 Chapter 31--Steam Heating Systems tained between the two sides of the system is only enough to secure adequate circulation and is often about 2 in. of mercury. This fixed pressure difference between the supply and return sides of the system results in practically constant circulation under all pressure conditions. In order to distribute the steam equally when the system is being warmed up and also to reduce the amount of steam delivered to the radiators on mild days, orifice plates are used in the graduated radiator control valves. The heat emitted from the radiators in mild weather and under conditions of high vacuum is not only reduced in proportion to the difference in the steam temperature between that for 2 lb gage and for 25 in. of vacuum but it is reduced still further by a reduction in the amount of steam which can pass through the orifice when the steam is expanded due to the vacuum. This renders possible the control of heat emission from the radiators to a point not indicated entirely by the difference in steam temperatures, but far beyond it. Sub-atmospheric operation has advantages even where individual thermostatic radiator control is installed. By operating the system with --- . steam'temperatures in parallel with the outside temperature require. mentis, a large part of the load is removed from the temperature control system, it makes fewer operations and the radiator follows an even tem perature without fluctuating from extreme hot to extreme cold. The high-vacuum pumps on this system are equipped with receivers having float control so that the pump can be placed on a receiver-returnpump basis at night if desired so no high vacuum will be carried. One radical difference between this system and the ordinary vacuum system is that no lifts can be made in the return line, except at the vacuum pump. The returns must grade downward constantly and uninterruptedly from the radiator return outlet to the inlet on the high-vacuum pump receiver. No attempt should be made to heat service water on this system unless the steam line for water heating is taken off the boiler header back of the heating system control valve, and then only when 2 lb or more will be carried on the boiler at all times. ORIFICE SYSTEM Orifice systems of steam heating may have piping arrangements identical with vacuum systems but some of these systems omit.both the radiator thermostatic traps and the vacuum pump in cases where the returns are wasted to a sewer or delivered to some type of receiver in which no back pressure exists. The principle on which they operate is embodied in the well-known fact that an orifice will deliver varying velocities when the ratio of the absolute pressures on the two sides of the orifice exceeds 58 per cent. If the absolute pressure on the outlet side is less than 58 per cent of the absolute pressure on the inlet side no further increase in velocity will be obtained. As a result, if an orifice is so designed in size as to exactly fill a radiator with steam at 2-lb gage on one side and 34-1b gage on the other, the abso lute pressure relation is 14.7 + 0.25 14.7 + 2.0 90 per cent 547 /