Document J3JZkEG01RVKn9gVMm8YB0o9r

268 Chapter 14 1945 Guide usedtoretum thecondensationtcrthe boiler orplace 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 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 lb gage on the other, the absolute pressure relation is: 14 7 4- 0 14 7 + 2 0 = -90 0r 90 *** cent- Should the steam pressure be dropped to 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 by varying the vacuum in the return lines, or by varying the pressure in the supply lines and the vacuum in the returns. The valves are frequently manually set 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 4.0 lb gage, 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 7. 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 mam 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. Some orifice systems are proprietary. HIGH PRESSURE STEAM SYSTEMS Many of the recent installations of heating systems for large industrial type buildings have been designed for the use of high pressure steam, that is, without the use of pressure reducing valves. Such systems usually involve the use of unit heaters or large built-up fan units with blast . Steam Heating Systems and Piping 269 "Keating"coilsrPressureson these systems vary froih-30 to 150 psi. Tem peratures are controlled by a modulating or throttling type thermostatic valve controlled by the air temperature in the fan outlet. Tables 8 to 11 may be used for the, sizing of steam and return piping for systems of 30 and 150 psi pressure at various pressure drops. These tables are based on Babcock's formula, and have been used as the basis of design for a number of years. Capacities at other pressures may be computed by means of Table 1. The steam lines can be sized for greater pressure drops than the return piping. For a system using steam at 30 psi pressure, the total pressure drop can be 5 to 10 lb, and for 150-psi systems 25 to 50 lb. It has been observed that the maximum pressure in the returns of a Table 7. Orifice Capacities Capacity Expressed in Square Feet Equivalent Direct Radiation This table is based on data from actual tests* Orifice Diameter &4THS OF an Inch 7 8 9 10 . 11 12 13 14 15 16 17 18 19 20 21 6 In. Hg Differential 5 In. Hg Differential 4 In. Hg Differential 2 In. Hg 1 In. He Differential Differential 18-23 23-29 29-36 36-44 44-52 52-62 62-72 72-83 83-94 94-106 100-119 119-133 133-148 148-163 163-179 16-21 21-27 27-33 33--40 40-48 48-57 57-66 66-76 76-86 86-97 97-109 109-122 122-135 135-149 149-164' 15-19 19-25 25-30 30-37 37-44 44--51 51-59 59-67 67-76 76-86 86-97 97-108 108-120 126-133 133-145 10-13 13-17 17-21 21-26 26-31 31-37 37-43 43-49 49-56 56-64 64-72 72-80 80-88 88-98 . '98-107 8-11 . 11-14 14-i7 17-20 20-24 24-28 28-32 32-37 37-42 42-47 47-52 52-58 58-64 64-71 Note.--The radiator orifice plates recommended in this table are made of brass stampings 0.023 in* thick, cup-shaped to be inserted in radiator valve unions, Flow of steam through Orifices into Radiators, S. S. Sanford and C. B. Sprenger (A.S.H.V.E. Trans actions, Vol. 37, 1931, p. 371). 30-psi system is about 5 lb, and that of. a 150-psi system is about 20 lb. The pressure in the return mains is of course caused by the discharge of traps or by leaky traps and by flashing due to the lower pressure in the return line. The total pressure drop in the returns can be approximately 2 lb for the 30-psi returns,, and about 10 lb for the 150-psi returns. The usual practice in the sizing of high pressure returns has been to size on the basis of 3^ lb per 100 ft of pipe for 30-psi systems, and 1 lb per 100 ft for 150-psi systems. This is an average figure which corresponds generally to several of the previously published tables for the design of high pres sure'return piping. The returns generally discharge to a vented receiver. It is, of course, necessary to provide for the elimination of air from high pressure systems, the same as in low pressure systems. If possible, it is desirable to design the system so as to. condense some of the steam escaping through the vents by passing it through a heat exchanger. -When high pressure steam is being supplied and lower steam pressures are required for use in heating, domestic hot water, utility services, etc.,