Document zon1annV4qZ1nGdqk7QzJ4R16

CHAPTER 14 Steam Sweating. Sgilemi and piping Classification, Piping lor Steam Heating Systems, Steam Flow, Pipe Sizes,'Indirect Heating Units, Types of Heating Systems, High Pressure Steam Systems, Boiler Connections, Condensa tion Return Pumps, Vacuum Pumps, Traps, Control Valves, Connections to Heating Units STEAM heating systems may be classified as gravity or mechanical according to the method of returning the condensation to the boiler. They may also be classified according to any one of, or combination of the following features: the pipe arrangement, the accessories used, the method of expelling or removing the air from the system, the type of control used, and the pressure or vacuum conditions obtained in operation. GRAVITY AND MECHANICAL RETURN ' In gravity systems.the condensate is returned by gravity due to the static head of water in the return pipes or mains. The elevation of the boiler water line must be sufficiently below the lowest heating unit, steam pipe or dry return pipe to permit the return by gravity. The water line difference forming the static head must be sufficient to overcome the maximum pressure drop in the system, including the pressure drop due to the condensing effect of the radiation. When radiator and drip traps are used, as in two-pipe vapor systems, the static pressure must also, exceed the operating pressure of the boiler. The pressure drop caused by condensing rate of the radiation is especially important during those portions of the operating periods where changing pressure conditions prevail, as for example,- when the system is being initially filled with steam. In systems where the condensate is wasted to the sewer, no water line difference is required. However, the waste of condensate may introduce conditions which warrant the use of an appropriate mechanical return system. Whenever the conditions of a heating system are such that the returns cannot gravitate to the boiler, they must be returned by some mechanical means. In mechanical systems the condensate flows to a receiver by gravity and is then forced into the boiler against its pressure. In all instances the preferable practice is to provide for gravity flow even where a vacuum pump is used. The lowest parts of the supply side of the system must be kept sufficiently above the water line of the receiver to insure adequate drainage of water from the system. There are three general types of mechanical return devices in common use, namely, (1) the mechanical return trap, (2) the condensation return pump, and (3) the vacuum return line pump. 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 Steam Heating Systems and Piping 249 - maintain-equivalent resistances in the- supply and_return piping_to and from a radiator. Arranging the piping so die total distance from the 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 condensation 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 revaporization, such as occurs where condensation 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 warming up a system even in moder ate weather, the load on the steam main and returns may exceed the Fig. 1. Relation Between Elapsed Time, Steam Pressure, Condensate and Air Elimination Rates maximum operating load for severe weather due to the necessity of raiising 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 condensation 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. 1 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 . ^*rH*,y** Research Report No. 954--Condensate and Air Return in Steam Heating Systems. Dy ** C. Houghten and J. L. Blackshaw (A.S.H.V.E. Transactions. Vol. 39, 1933, p. 199).