Document g2yX5aLe2eNp8pq9QzqwLnkBJ

516 CHAPTER 20 1957 Guide 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 waives 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 tated as much as possible, as an air bound system will not heat readily nor properly. In designing the piping arrangement, it is desirable to maintain equivalent resistances in the supply and return piping to and from a radiator. Arranging the piping so the total distance from the boiler to the radiator is the same as the return piping distance from the heating unit back to the boiler, tends to obtain such a result, The condensate which occurs in steam piping as well as in radiators must be drained to prevent impeding the ready flow of the steam and air. The effect of back pressure in the returns and excessive re-vaporization, such as occurs where condensate is released from pressures considerably higher than the vacuum or pressure in the return, must be avoided. It is important that steam piping systems distribute steam not only at full design load, but during excess and partial loads. Usually the average winter steam demand is less than half of the demand at the design outside temperature. Moreover, in rapidly wanning up a system even in moder ate weather, the load on the steam main and returns may exceed the maximum operating load for severe weather, due to the necessity of raising the temperature of the metal in the system to the steam temperature, and the building to the design indoor temperature. Investigations of the return of condensate have revealed that as high as 143 percent of the design condensation rate may exist under conditions of actual operationExpressed in gallons per minute per 1000 sq ft equivalent direct radiation the theoretical condensing rate`of the system at the design indoor tezn- Steam Heating Systems 517 perature is 0.5. During the warming up period .this value will approach 0.72 gpm. The piping design of a heating system is greatly influenced by its operat ing characteristics. Heating systems do not operate under constant condi tions, as conditions change, continually, due to variation in load. As the system is being filled with steam, the pressures existing in various locations may be different from those which exist for appreciable periods at. other locations, although at equilibrium conditions the pressures are Table X. Orifice Capacities tor Low Pressure Steam Systems This table is based on data from actual tests* Orifice Diameter 64ths of an Inch 6 in. He' Differential 5 in. He' Differential 4 IN. He. 2 in. He 1 IN. He Differential Differential. Differential Capacity Expressed In Square Feet E D R 7 8 9 10 11 12 13 14 15 15 17 18 19 20 21 18-23 23-29 29-36 36-44 44-52 52-62 62-72 72-83 83-94 94-106 106-119 119-133 133-148 148-163 163-179 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 97-108 108-120 120-133 133-145 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 14-17 17-20 20-24 24-28 . 28-32 32-37 37-42 42-47 47-52 52-58 58-64 1 64-71 Capacity Expressed in Pounds per Hour 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 4.5-5.8 5.8-7.3 7.3-9.0 9.0-11.0 11.0-13.0 13.0-it>.> 15.5-18.0 18.0-20.8 20.8--23.5 23.5-26.5 26.)"/y.8 29.8-33.3 33.3-37.0 37.0-40.8 40.8-44.8 4.0-5.3 5.3-6.8 6.S-8.3 8.3-10.0 10.0-12.0 12.0^14.3 14.3-16.5 16.5-19.0 19.0-21.5 21.5-24.3 24.8-27.3 27.3-30.5 30.5-33.8 33.8-37.8 37.3-41.0- 3.S-4.8 4.8-6.3 6.3-7.S 7.5-9.3 9.3-11.0 11.0-12.8 12.8-14.8 14.8-16.8 16.8-19.0 19.0-21.5 21.5-24.3 24.3-27.0 27.0-30.0 30.0-33.3 33.3 36.3 2.5-3.3 3.3-4.3 4.3-5.3 5.3-6.5 6.5-7.8 7.8-9.3 9.3-10.8 10.8-12.3 12.3-14.0 14.0-16.0 16.0-18.0 18.0-20.0 20 0-22.0 22.0-24.5 24.5-26.8 2.0-2 8 2.8-3 5 3.5-43 43-5.0 5.ft-flO fi.o-7 n 7.0-8 0 8.0-03 93-ift 5 10.5-11.8 11.8-13.0 13.0-143 14.5-1 5 0 16.0-17.8 cu>6hap--edmtoebraediniasteorrteodrifinicerapdlaiatetosrrevcaolvmemuenniodnesd. in_________-- ---____> au>ui,,y.iug,,ti__u_.u_a_o_in_. uuck t ri * low of Steam Through Orifices into Radiators, by S. 8. Sanford and C. B. Sprenger (A.8.H.V.E. TransVol. 37, 1931, p. 371). -innately the same. In designing piping it is of especial importance 1 arrange the system to preclude trouble caused by such pressure dif- efences. The systems which readily release the air, permit uniform PJ^sures to be attained in much shorter, time intervals than those which sluggish. Results are given in Fig. 21 from investigations1 to deternune "e rate of condensate and air return from a two-pipe gravity heating