Document 916JxEqGX5KZvYaODJ1x5wmOD

478 CHAPTER 20 1952 Guide The control valve may be of the automatic modxdaling 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. In some systems radiator supply valves include adjustable orifices, or are equipped with regulating, orifice plates. The sizes of orifices used are larger than for other types of orifice systems because, for equal radiator sizes, the volume flowing is larger. Orifices are omitted on some systems. Radiator traps and drips are designed to operate at any pressure from 15 psig to 26 in. Hg. A vacuum pump capable of operating at high vacuum is pref erable to promote accuracy in the distribution of steam throughout the system, particularly in mild weather. This vacuum is partially selfinduced by the condensation of the steam in the system under conditions of restricted supply used 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. Fig. 16. Method of Maxing Lifts on Vacuum Systems when Distance is Over 5 ft ' Fig. 17. Detail of Main Return Lift at 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 that 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. TWO-PIPE ORIFICE SYSTEMS Orifice steam heating systems may have piping arrangements identical with vacuum systems. Some of these omit the radiator thermostatic traps, but use thermostatic or combination float and thermostatic traps oh all drip points. A return condensate pump with receiver vented to Steam Heating Systems 479 atmosphere, a return line vacuum pump, or a return trap, is generally used to return the condensate to the boiler or place of similar disposition, such as a feed-water heater or hot well. The heat emission from the radiators is controlled by varying the pressure differential maintained. The principle on which these systems operate is based on the fact that the steam flow through an orifice will vary with square root of pressure drop when the ratio of the absolute pressures on the two sides of the orifice exceeds 58 percent. If the absolute pressure on the outlet side is less than 58 percent of the absolute pressure on the inlet side, no further increase, in flow will be obtained as a result of the increased pressure difference. If an orifice is so designed in size as to exactly fill a radiator with 2 psig on one side and $ psig on the other, -the absolute pressure relation is 14.7 + 0.25 14.7 + 2.0 0.90 or 90 percent. Should the steam pressure be dropped to J psig on the supply pipe, the pressure on each side of the orifice would be balanced and no steam flow would take place. From this it will be apparent that if an; orifice of a given diameter will fill a given radiator with steam when there is a given pressure on the main, reducing this steam main pressure will permit filling various desired portions of the radiator down to the point where the main pressure equals the back pressure in the radiator, provided the supply pipe pressures may be controfled sufficiently dose. If orifices are designed on a similar basis for a given system and proportioned to the heating capacity of the radiators they serve, all radiators will heat proportionately to the steam pressure. The range of pressure variation is limited by the per-, missible noise level of the steam flowing under, the pressure difference required for maximum heat output. The control of the steam supply is obtained by a valve placed in the steam main, which maintains a deter mined pressure, and by varying the vacuum in the return lines. The valves are frequently set manually from a remote location, guided by tem perature indicating stations in the building; or thermostatically controlled from a thermostat on the roof, which' automatically measures the dif ferential of outside and inside temperatures. Since the range through which the pressures may be varied is usually from 0 to 4 psig, the control should be capable of maintaining close regulation to maintain the desired space temperatures, particularly in mild weather. A recommended orifice schedule is shown in Table 1. Some systems use orifices not only in radiator inlets, but also at different points in the steam supply piping for the purpose of balancing the system to a greater extent. In this manner the difference between the initial and terminal pressure in the steam main may be compensated to a great extent. For example, if the initial pressure is 3 psig and the pressure at the end of the main is 2 psig, an orifice could be used in each branch for the purpose of obtaining a more uniform pressure throughout the system. Such a provision may be particularly useful in this system for branches close to the boiler where the drop in the main has not yet been produced. Some orifice systems are proprietary. SIZING PIPING FOR STEAM HEATING SYSTEMS The functions of the piping system are the distribution of the steam, the return of the condensate and, in systems where no local air vents are provided, the removal of the air. The distribution of the steam should be rapid, uniform and without noise, and the release of air should be facili-