Document DDRyko5zn4VekBN6ge1Lzo2oo

HEATING VENTILATINC AIR CONDITIONING GUIDE 1940 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-atmqspheric 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. 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 used to return the condensation to the boiler or place of similar disposition, 278 CHAPTER 15. STEAM HEATING SYSTEMS such as a feed-water heater or hot well. The heat emission from the radiators is controlled by varying the pressure maintained in the steam supply piping. The principle on which these systems operate is based on the fact that the steam flow through an orifice will vary when the ratio of the absolute pressures on the two sides of the orifice exceeds 58 per cent. If the abso lute pressure on the outlet side is less than 58 per cent 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 lb gage on one side and 34 lb gage on the other, the absolute pressure relation is: 14 7 4- 0 2*i 14.7 + 2 0 = 0 90 or 90 per cent' Should the steam pressure be dropped to 34 lb 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 portion 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 controlled sufficiently close. 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; or by a boiler pressure control. The valves are frequently manually set from a remote location, guided by temperature indicating stations in the building; or thermostatically controlled from a thermostat on the roof, which automatically measures the differential of outside and inside temperatures. Since the range through which the pressures may be varied is usually from 0 to 4.0 lb gage,, the control should be capable of maintaining close regulation to maintain the desired space temperatures, particularly in mild weather. 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 was 3 lb gage and the pressure at the end of the main was 2 lb, 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. Orifice systems are proprietary. ZONE CONTROL Often certain portions of a building may require more heat than others even if occupied for the same daily periods and the same maintained 279