Document 5L4MN2ZeaJqypy5E6VNjm8mge

S74 CHAPTER 31 1948- Guide Table 3. Highest Safe Surface Temperatures for Heating Panel Type of Panel Plastered Ceiling (Pipes Imbedded)..-- Plastered Walls (Pipes Imbedded)____ Floor, Any Method___________________ Floor, Border and Aisles._____________ Iron, Hot Water Medium3.......... ............ Iron'Steam Vapor. Electrically Heated Panels. Surface Temperature F Dec 115 120 85 120 160 180 200 Low surface temperature radiation is recommended regardless of the heating medium employed. Practical limits (or surface temperatures of heating panels are given in.Table 3. In this example water will be selected as the medium. 8. Determine the size, length, and location of the pipe coils in the panels. When hot-water pipes are imbedded in concrete slabs or attached to plastered sur faces, their rate of heat emission varies with many factors. If the pipes are imbedded in dense concrete slabs, it may be assumed that the rate of heat emission of 54-in. pipe, spaced 6 in. on centers; J-in. pipe, spaced 9 in. on centers; and 1-iri. pipe spaced 12 in. on centers: per foot of length of pipe and per degree difference between the temperature of the water in the pipe and that of the air in the space to be heated, is 0.8, 1.0, and 1.2 Btuh, respectively. If the distance between the pipes is increased, the rate of heat emission, per foot of pipe, is also increased; if the distance is doubled, the rate of heat emission is increased about 15 per cent. If the pipes are attached to plastered ceilings, the rate of heat emission is slightly less, probably about 10 per cent less, than when the pipes are imbedded in concrete slabs. The data given regarding heat emission of panels are intended as general guides for the designer. Additional experience and research are needed to develop definite and complete data. However, after a heating panel has been designed and installed, any small error can easily be corrected by modifying the temperature of the water circulating through the coils. When the heating pipes are attached to a plastered ceiling, a portion of the heat emitted by the pipes is delivered to the space below the ceiling and a portion to the space above the ceiling. The relative quantities depend on the degree of insulation applied above the heating coils. When the heating pipes are imbedded in a concrete floor slab a portion of the heat emitted by the pipes will flow upward into the space to be heated, and the remainder will flow downward into the ground. When the heating pipes are placed below the concrete floor slab instead of being imbedded in the slab, a larger portion of the heat will flow into the ground, and a smaller portion into the space to be heated. In the following example it is assumed that the insulation above the pipe coils is such that 90 per cent of the heat emitted by the pipe coils willflow into the room and 10 per cent into the space above. Since the room is to receive 35,117 Btuh, and since the room is assumed to receive only 90 per cent of the heat emitted by the coils attached to the plastered ceiling, the coils must emit 35.117/0.9 or 39,000 Btuh. If %-in. pipe and a mean water temperature of 140 F are selected, the heat emitted, per foot of pipe, will be 0.9 (140 -- 68) or 65 Btuh. The quantity of pipe required will therefore be 39,000/65 = 600 lineal feet. The pipe coils can be arranged in any convenient manner, but should be arranged so that the temperature of the water in the pipe will vary only slightly; otherwise, the temperature distribution over the ceiling will not be uniform. Generally, it is best to arrange the pipes so as to form two-pipe reversed-return systems, as suggested by the two sketches in Fig. 10. By using 33 runs of %-in. pipe, welded to two 1H in. mains, sufficient pipe surface is secured; the %-in. pipes will then be spaced about 8J4 in. on centers, which is satisfactory. While coils can be designed with pipe and fitting resistances which will insure proper distribution to each coil it is advantageous to provide adjustable flow control valves or resistances for final regulation of the water temperature or flow to the various coils. It Panel Heatinu and Radiant Heating 575 is desirable, to divide large heating systems into sections and to install, valves so that individual sections can be disconnected without interfering with the operation of the system as a whole. Part II--Radiant Heating The term radiant heating is applied in this chapter to a system in which only the heat radiated from the panel is effective as in outdoor and semi- outdoor conditions. . The outstanding example of radiant heating is the transfer of heat from the sun to the earth. The sun radiates large quantitiesof energy of which a very small portion is intercepted by the earth. A part of the intercepted radiation is transformed into heat when it strikes the earth's Fig. 10. Arrangement of Pipe Coil for Even Distribution of Panel Temperature surface. In this manner heat is received by the earth from the sun by radiation. In industry,, radiant heating is employed in manufacturing processes, particularly in drying, baking, and dehydrating operations; in agricul ture, it is employed to improve living and growing conditions for young plants and young animals.' The heating engineer employs radiant heat primarily in the heating of open-air schools and open-air hospitals. When a surface radiates heat,' and every surface does unless its temperature is absolute zero, every point of the surface radiates heat in all directions. The total quantity of heat radiated by a point or by an elementary area is it times the .quantity of heat radiated at right angles to the surface. Thus, if in an elementary cube the upper face is the heating panel, the lower face would receive only about 32 per cent of the radiated energy and the four sides would receive each about 17 per cent.