Document 6BbnQxQeBe6b6J10XBq3KXkD6

Chapter 14_________________ ' 1945 Guide' 34 lb. Systems where the maximum equivalent length does not exceed 200 ft preferably employ the smaller pressure drop while systems over 200 ft equivalent length of run more frequently are designed for the higher . drop, owing to the relatively greater saving in pipe sizes. For example; a system with 1200 ft longest equivalent length of run.would employ a drop per 100 ft of 34 lb divided by 12, of '/^ lb. In this case the steam main, would be sized from Column C, Table 4, and the risers also from Column C (Column H could be used as far as critical velocity is concerned but the drop would exceed the limit of '/j4 lb). Riser runouts! if dripped, would use Column C but if undripped would use Column I; radiator runouts; Column /; return risers, lower part of Column S, Table 5; return runouts to radiators, one pipe size larger than the radiator trap connections. Fig. 15. Typical Down-Feed Vacuum System Notes on Vacuum- Systems 1. It is not generally considered good practice to exceed Ys lb drop per 100 ft of equivalent run nor to exceed 1 lb total pressure drop in any system. 2. Pitch of mains should not be less than Y in. in 10 ft. 3. Pitch of horizontal runouts to risers and radiators should not be less than Yi in. in 10 ft. .Where this pitch cannot be obtained runouts over 8 ft in length should be one size larger than called for in the table. 4. In general it is not considered desirable to have a supply main smaller than 2 in. 5. When necessary, the supply main, supply riser, or branch to a supply riser should be dripped separately through a trap into the vacuum return. A connection should not be made between the steam and return sides of a vacuum system without interposing a trap to prevent the steam from entering the return line. 6. Lifts should be avoided if possible, but when they cannot be eliminated they should be made in the manner described in this chapter. 7. No lifts can be used in orifice and atmospheric systems. In sub-atmospheric systems the lift must be at the vacuum pump. . Steam Heating Systems and Piping________________________________________ 267 SUB-ATMOSPHERIC SYSTEMS Sub-atmospheric systems are similar to vacuum system's but, in con1 trast, provide control of building temperature by variation of the heat output from the radiators. The radiator heat emission is controlled by ' varying the pressure, temperature and-volume of steam in circulation. These systems differ from the ordinary vacuum system in that they main tain a cbntrollable partial vacuum on both the supply and return sides.of the system, instead of only on the return side. 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, atmospheric pressure or higher exists in the steam supply piping and radiators only during severe weather. Under average winter temperature the steam is under partial vacuum which in mild weather may reach as high as 25 in. Hg, after which further reduction in heat output is obtained by restricting the quantity of steam. The rate of steam supply is controlled by a valve in the steam main or by thermostatically controlling the rate of steam production in the boiler. The control valve may be of the automatic modulating or floating type governed thermostatically from selected control points in the building, or it may be a special pressure reducing valve which will maintain the desired sub-atmospheric pressures by continuous flow into the heating main. All radiator supply valves have incorporated adjustable orifices or are equipped with regulating orifice plates. The sizes of orifices used are larger than for orifice systems because for equal radiator sizes the volume flowing is larger. These orifices are omitted on some systems, depending upon the type of control. Radiator traps and drips are designed to operate at any pressure from 15 lb gage to 26 in. Hg. A vacuum pump capable of operating at high vacuum is preferable to promote accuracy in the distribution of steam throughout the system, particularly in mild weather. This vacuum is partially self-induced by the condensation of the steam in the system under conditions of restricted supply for reduction of the radiator heat emission. The returns must grade downward constantly and.uninterruptedly from the radiator return outlets to the inlet of the receiver of the vacuum pump. One radical difference between this and the ordinary vacuum system is that no lifts should be made in the return line, except at the vacuum pump. The receivers are placed at a lower level than the pump and equipped with float control so the pump may operate as a return pump under night conditions. The system may be operated in the same manner as the ordinary vacuum system when desired. Steam for heating domestic hot water should be taken from the boiler header back of the control valve so that pressures sufficiently high for heating the water may be maintained on the heater. The sub-atmos pheric method of heating can be used for the heating coils of ventilating and air conditioning systems. The flexible control of heat output secured by this method materially reduces the required size of by-pass around the heaters. Some applications of sub-atmospheric systems are proprietary. ORIFICE SYSTEMS Orifice systems of steam heating may have piping arrangements identi cal with vacuum systems. Some of these omit the radiator thermostatic traps but use thermostatic or combination float and thermostatic traps on all drip points. A return condensation pump with receiver vented to atmosphere, a return line vacuum pump, or a return trap, is generally