Document oGggnK8dnpYJd9G40a0YxoDr

HEATING VENTILATING AIR CONDITIONING CUIDE1944 effect of a cloth cover extending downward 6 in. from the top of the radiator was to make the performance unsatisfactory arid inadequate. Some commercial enclosures and shields for use on direct radiators are equipped with water pans for the purpose of adding riioisture to the air in the room. Tests17 show that an average evaporative rate of about 0.235 lb per square foot of water surface per hour may be obtained from such pans, when the radiator is steam hot and the relative humidity in the room is between 25 and 40 per cent. This source of supply of moisture alone is not adequate to maintain a relative humidity above 25 per cent on a zero day. `^University of Illinois, Engineering Experiment Station Bulletin No. 230, p. 20. CHAPTER 14 Steam Sweating. Sy-ilemi Gravity and Mechanical Return, Gravity One-Pipe Air-Vent, Gravity Two-Pipe Air-Vent, Air Line Heating, One-Pipe Vapor, Two-Pipe Vapor, Atmospheric, Condensation Return, Vacu um, Sub-Atmospheric, Orifice, Zone Control, Condensation Return Pumps, Vacuum Heating Pumps, Traps STEAM heating systems may be classified according to the pipe arrangement, the accessories used, the method of returning the con densate to the boiler, the method of expelling air from the system, or the type of control employed. Information concerning the design and layout of steam heating systems will be found in Chapter 15. GRAVITY AND MECHANICAL RETURN Systems are classified as gravity or mechanical according to the method of returning the condensate from the system to the boiler. 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 con densing 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 differ ence is required as is the case with closed systems. However, the waste of condensate may introduce conditions which warrant the use of an appropriate mechanical system. Whenever the conditions of a heating system are such that the returns from the radiation 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. 261