Document jmExbBR6VRq201kw0YwR7L4mZ

528 CHAPTER 25 1949 Guide line level (approximately 5 ft-0 in. above floor), and temperatures taken at the breath ing 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 windows. HEATING DP THE RADIATOR AND CONVECTOR i The maximum condensation occurs in 'a heating unit when the steam is first turned on. Tests16 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 dropped to a rate of 0.24 lb. In practice the rate of steam supply to the heating unit, while heating up, is frequently retarded by controlled elimination of air through air valves or traps. Automatic control valves may also retard Fig. 4. Steam Consumption op Exposed and Concealed Radiators 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. Investigations10 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. , 2. The lessened steam consumption may not materially change the radiator heat ing performance. 3. The enclosed radiator may inadequately heat the space. i'.3 v .' '* A comparison between a bare or exposed radiator (A) and the same radi ator with a well-designed enclosure'(B), with a poorly-designed enclosure (C), and with a cloth cover (D) 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 Radiators, Convectors, Coils 529 effect. Curve (C) shows the unsatisfactory effects produced by improperlydesigned enclosures. Curve (D) shows that the effect 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 a radiator is steam heated 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. COILS Coils described in this chapter are used for heating or cooling an air1 * 3 stream under forced convection. Surface coil equipment may be made up of a number of banks assembled in the field, or the entire assembly may be factory constructed. The applications of each type of coil are limited to the field within which it is rated. Other limitations are imposed by code regulations, by proper choice of materials for the fluids used and the condi tion of the air handled, or by lan economic analysis of the possible alternates on each installation. For heating service, coils are used as tempering coils, preheaters, reheaters or booster heaters. -The function of the coils is air heating only, but the apparatus assembly may include means for humidification and air cleaning. Steam or hot water are the usual heating media, although others are used; -in special cases, such as reheating by means of discharge gas from a refriger ating system. Coils are used for air cooling with or without accompanying dehumidi fication. Examples of cooling applications without dehumidification are precooling coils using well water or other relatively high temperature water to reduce the load on the refrigerating machinery, or water cooled coils to remove sensible heat in connection with chemical moisture-absorption apparatus. By proper coil selection it is possible to handle both sensible cooling and dehumidification together as explained later. The assemblyusually includes air cleaning means to protect the coil from accumulation of dirt and to keep dust and foreign matter out of the conditioned space. Although cooling and dehumidification are the usual functions, there are' cases of cooling coils purposely wetted to aid in air cleaning and odorabsorption. The usual cooling media used in surface coils are cold water or Group I . (ASA Classification) refrigerants, but others are used in special cases. Brines are seldom required for the range of applications covered by thischapter, although there are cases where low entering air temperatures with large latent heat loads require a refrigerant temperature so low that use of water becomes impracticable. Sometimes, also, brine from an industrial system already installed is the only convenient source of refrigeration. For combined cooling and dehumidifying, surface coils present an alter-' nate to spray dehumidifiers. ' For many applications it is possible, by proper selection of apparatus, choice of air velocities, refrigerant tempera-' tures, etc., to perform the same duty with either. In a few cases both' sprays and coils are used. The coils may then be installed within the spray chamber, either in series with the sprays or below them. In making the selection between spray and surface dehumidifiers, certain advantages of