Document XREXk6MGLOLxR3Y7dRQ2EgE1B

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 : 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. 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, 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 272 CHAPTER 14. STEAM HEATING SYSTEMS size as to exactiy fill a radiator with 2 lb gage on one side and on the other, the absolute pressure relation is: 14.7 + 0.25 ,,M",,,, 14 7 + 2 0 = '9 r 90 per Cent' lb gage Should the steam pressure be dropped to J4 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 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 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 arid inside temperatures. Since the range through which the pressures riiay 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 fpr 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. CONDENSATION RETURN PUMPS Condensation return pumps are used for gravity systems when the local conditions do not permit.the condensation to return to the boilei; under the existing static head. The return of the condensate permits the. , water to repeatedly go through the.cycle of vaporization, with subsequent condensation and return to die boiler. During such repeated cycles any incrustants or other substances in solution are precipitated and the water de-activated to a considerable extent so that corrosion of a serious nature is seldom ever encountered where the: conderisate is-repeatedly used. Serious corrosion is more frequently found in systems where the conden sation, is not repeatedly used-but is wasted and-fresh make-up water is continually being introduced. , The most generally accepted condensation pump unit for low pressure 273