Document LKdrjR9qYm5eqnw3n0bvz4qr3

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 4. The comfort level (approximately 2 ft-6 in. above floor) is below the breathing line level (approximately 5 ft-0 in. above floor), and temperatures taken at the breathing line may not be indicative of the actual heating effect of a radiator in the room. The comfort-indicating temperature should be-taken below the breathing line level.' . 5. High column radiators placed at the sides of window openings do not produce as comfortable heating effects as long, low, direct radiators placed beneath window openings. HEATINC UP THE RADIATOR AND CONVECTOR The maximum condensation occurs in a heating unit when the steam is first turned on. Tests15 on an old-style column type cast-iron radiator indicated that in the first 10 min the condensation rate reached a peak of 0.95 lb per square foot of radiator per hour and 10 to 15 min later lowered to a. rate of 0.24 lb. In practice the rate of steam supply to the heating unit while heating up is frequentlyretarded by"controlled elimination of air through air valves or traps. Automatic control valves may also retard the supply of steam. Vacuum types of air venting valves may be used to reduce the length of the venting periods. ENCLOSED RADIATORS .The general effect of an enclosure placed about a direct radiator is to restrict the air flow, diminish the radiation and, when properly designed, improve the heating effect. Investigations15 indicate that in the design of the enclosure three things.should be,considered:. 1. There should be better distribution of the heat below.the breathing line level to produce greater heating comfort and lowered ceiling temperatures.1: 2. The lessened steam consumption may not materially change the radiator heating performance. 3. The enclosed radiator may inadequately heat the space. , A comparison between a bare or exposed radiator (A) and the same radiator with a well-designed enclosure (B), with a poorly-designed enclosure (C), and with a cloth-cover (Z)) will illustrate the relative heating effects. In Fig. 4 the curve (B) reveals that the enclosed radiator used less steam than the exposed radiator, but gave a satisfactory heating - performance. A well-designed shield placed over a radiator gives about the same, heating effect. Curve -(C) shows, the unsatisfactory effects produced by improperly-designed enclosures. Curve (D) shows that the effept of. a cloth cover extending downward 6 in, from the top of the radiator was to make the performance unsatisfactory and-inadequate. Some commercial enclosures and shields for use on direct radiators are equipped with water pans for the purpose of adding moisture 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 issteam 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. "A.S.H.V.E. Research Report No. 1087--The Cooling and Heating Rates of a Room with Different- Radiators and Convectors, by A. P. Kratz, M. K. Fahnestock and E. L. Broderick (A.S.H.V.E. Transactions, Vol; 43. 1937. p. 389). : - "University of Illinois, Engineering Experiment Station Bulletins Nos. 192 and 223, and Investigation ot Hearing Rooms with Direct Steam Radiators Equipped with Enclosures and Shields, by A. C. Willard. A. P. Kratz. M. K.'Fahnestock and S. Konzo (A.S.H.V.E. Transactions, Vol. 35.1929, p. 77). . vUniversity of Illinois. Engineering Experiment Station Bulletin'No. 230, p. 20. 268 Chapter 14 STEAM HEATINC SYSTEMS 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. 1 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, stea.m 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 thei 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.