Document 6wrnbbkDY7rZ68aXdvpzxxzM9

428 CHAPTER 23 1948 Guide boiler to the radiation 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 pf back.pressure in the/retums and excessive, reyaporization, 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 pipirig'systethsJdistnbute steam not only at full design load but during excess and partial loads. Usually the average wint;er steam demand is less than half of the demand at the design outside temperature. Moreover, in rapidly warming up a system even in moder ate weather, the load on the steam mainland returns may exceed the maximum operating load for severe weather due to the necessity of raising . . the .temperature ofiihe metal in the system to the steam temperature and the building to'the design indoor temperature. Investigations-of the Fig.- 18. ' . ELAPSED TIME. MINUTES: Relation Between Elasped Time; Steam Pressure, .Condensate and . ' . Air Elimination Rates return of condensate have; revealed that as high as 143 per cent of the design condensation rate may exist under conditions of actual operation. . The piping design, of a heating system is greatly influenced by its operating characteristics! Heating systems do not operate under constant conditions as'they are continually changing 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 approximately the same. In designing piping it is of especial importance to arrange the system to preclude trouble caused by such pressure dif ferences. The systems which readily release the air permit uniform pressures, to be attained in much shorter time intervals than those which are sluggish. Results are given in Fig. 18 from investigations1 to deter mine the rate of condensate and air return from a two-pipe gravity heating system. Variations in the steam pressure during the warming-up period when,.the rate of air elimination and condensation is high are clearly indicated ;in these, curves. It is evident that the condensate flow during the initial warming-up .period reaches a peak which, is greater than the constant condensing rate eventually, reached when the pressure becomes uniform. Moreover, the peak condensing rate is obtained;when the system steam pressure is Steam Heating Systems and Piping 429, lower than that existing.during a period of. constant condensing rate. It will also be noted that the' peak rate of air elimination does not coincide with the higher condensing rate. Steam Flow The rate of flow of dry steam or steam with a small amount of water flowing in the same direction is in accordance with the general laws of gas flow and is a function of the length and diameter of the pipe, the density of the steam, and the pressure drop through the pipe. This relationship. has been established by Babcock in the formula given at the top of Table 2. In Columns 1, 2, 3, and 4 of this table, the numerical values of the factors for different pressure losses, pipe diameters, steam densities and lengths of pipe have been;worked out in convenient form so that the steam flowing in any: pipe may be calculated by multiplying together the proper factors in each column as shown in the example at the bottom of the table. Pipe Sizes . The determination of pipe sizes for a given load- in steam heating depends on the following principal factors: . , . ... ,1. The initial pressure and the total pressure drop which may be allowed between the source of supply and the end of the return system. . 2., The maximum velocity of steam allowable for quiet and dependable,operation of the system'; taking into consideration the direction of condensate flow. ':. 3. , The equivalent length of the run from .the boiler or source of steam supply to the farthest heating unit. . . 4. The direction of flow of the,,condensate, whether against or -with, the steam. -. Initial Pressure and Pressure Drop " : Theoretically, there are several factors to be considered, such as initial pressure and-pressure required at the end of the line,' but it is most import ant that: (1) the total pressure drop does not exceed the initial gage pressure, of the system and in actual practice it should never exceed one-, half of the initial gage pressure ; (2) the pressure drop is not so great as to cause excessive velocities; (3) there is a constant initial pressure, except oh systems specially designed for varying initial pressures, such as the sub-atmospheric, which normally operate under controlled partial vacua,' and orifice and vapor systems which at times operate under such; partial vacua as may be obtained due to the condition of the fire; and (4) the rise in water due to pressure drop does not exceed the difference in level, for gravity return systems, between the lowest point on the steam main,1 the heating units, or the dry return, and the boiler water line. All systems should be designed for a low initial pressure and a reason ably small pressure drop for two reasons: first, the present tendency in steam heating unmistakably points toward a constant lowering of pres sures even to those below atmospheric; second, a system designed in this manner will operate under higher pressures without difficulty. When a system designed for a relatively high initial pressure and a relatively high pressure drop is operated at a lower pressure, it is likely to be noisy and have poor circulation. The total pressure drop should never exceed one-half of the initial gage pressure when condensate is flowing in the same direction as the steam. Where the condensate must flow counter to the steam, the governing factor is the velocity permissible without interfering with the condensate flow. _ A.S.H.V.E. Research Laboratory experiments limit this to the capacities given in Table, 3 for horizontal pipes at varying grades.