Document y2ej2ONmGVXYk9MrzrLEegjD

American Society of Heating and Ventilating Engineers Guide, 1934 runouts, radiator valves, drips on the bottom of the steam drops, and enlargement of the drops for the lower radiator connections. The return side of the system is exactly the same as the up-feed system except that the steam riser drips at the bottom are connected into the return line through thermostatic traps. It is preferable to take the runouts for the risers from the bottom or at a 45 deg angle down from the steam main (Fig. 6) so that they may serve as steam main drips. When this is done it is practical to run the steam main level if a runout is located at every change in pipe size, or if eccentric fittings are used (Fig. 17). A slight pitch in the steam main, however, should be used when possible. An overhead vacuum down-feed system is shown diagrammatically in Fig. 18. Chapter 31--Steam Heating Systems 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. This vacuum is largely self-induced by the; condensation of the steam in the system when an inadequate supply of steam is being furnished through the control valve which admits jt. In the sub-atmospheric system, a control valve is inserted on the steam main of an ordinary vacuum system near the boiler, a high-vacuum pump is substituted for the ordinary type and is supplied with a pressuredifference 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. Fig. 15. Method of Making Lifts on Vacuum Systems when Distance is Over 5 ft Fig. 16. Detail of Main Return Lift at Vacuum Pump __ an--1*- ra i teuc.centoc deducing coueuml Fig. 17. Method of Changing Size .of Steam Main when Runouts are Taken from Top SUB-ATMOSPHERIC SYSTEMS The sub-atmospheric systems are similar to the vacuum system except that a pump capable of operating up to 25 in. of vacuum is used, and a control is placed on the pump so that the vacuum or absolute pressure carried in the return can be maintained a certain amount below that existing in the steam line to cause a constant circulation. The.traps are designed to operate in high vacuum. It is apparent that this system differs from the ordinary vacuum system by having a vacuum on both sides of the system, instead of only on the return side, in order to secure controlof the heat emission from the radiators and thus to-control the temperature in the building. The system can be operated in the same manner as the ordinary vacuum system when desired. 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 430 Fig. 18. Typical Down-Feed Vacuum System The control valve is a special pressure reducing valve which may be controlled manually or 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 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 431