Document 06MV59L69N5ebjZ0RondLewzn

748 CHAPTER 35 1950 Guide Air.Heating and Cooling Coils , 749 cooling , and dehumidification, the logarithmic./difference does not. apply strictly, and such problems should be handled as described in a later section on Performance of Dehumidifying Coils. With volatile refrigerants there is often an appreciable pressure drop and corresponding change in evaporat PERFORMANCE OF HEATING AND DRY COOLING COILS, r The performance of heating and dry cooling coils depends in general upon: ' ' - j ., ing temperature through the refrigerant circuit. The problem is further complicated by the fact that the refrigerant is evaporating in part of the circuit, and superheating in the remainder. In spite of this, heat transfer and ratings for coils using volatile refrigerants are usually based on a refrigerant temperature corresponding to the average pressure in the coil. 1. The overall coefficient of heat transfer from the fluid within the coil to the air it heats or cools. . 2. The mean temperature difference between the 'fluid within the coil and the air flowing over the coil.. ' ,' ' 3i The physicaldimensions of the coil. The design and surface arrangement of the coil include such items as materials, type, thickness, height and spacing of the fins, and the ratio of this surface to that of the tube, the use of the staggered or in-line tube Thus, for any one definite operating condition, the heating or cooling capacity of a given coil is expressed by the,following basic formula: arrangement, and provisions to increase the air turbulence such as the use ' " ; ' q,;= U X (At,,) X A X N - (1) :of corrugated as against flat fins. Staggered tubes increase the total heat where transfer, as against the in-linearrangement,and corrugated fins may be more effective than flat. This design and surface arrangement has a large effect qt -- total heat transferred by the coil, Btu per (hour) (square foot.of coil on the air film heat transfer, resistance. face area). ! The velocity of the air usually considered is the coil face velocity. This U = overall coefficient.of heat,transfer, Btu per (hour), (square foot, of exter- - - bears a varied relation to the actual velocity over the surface, depending upon nal coil surface) (Fahrenheit degree temperature difference between the the individual coil design. As long as a fixed design of coil is under con fluid within the coil and the air flowing over the coil). , sideration face velocities may be used, but they may be unsatisfactory in Aim = mean temperature difference, Fahrenheit degrees, between the fluid comparing different designs, as it is the actual surface velocity that is signifi ,, within the coil and the air passing over it. (This is commonly taken as cant. The air volume is often based on standard air at 70 F and a baro metric pressure of 29.92 in. Hg. The use of air volume in coil roiling information may be misleading. The significant value is mass velocity in pounds per (minute) (square foot of face area) and not cubic feet'per minute, . the logarithmic mean temperature difference). . .'. \;j ,.. A:= external surface area of the given coil,/square feet per (square.foot-of - coil face area) (row of coil depth). N,M number of rows of cod depth. " .... because for a fixed volume the corresponding weight may vary widely, depending upon the temperature and barometric pressure. ' At the same mass air velocity, varying performance can be obtained depending upon the turbulence'of the air flow, into the coil, and upon the uniformity of distribution of air over the coil face. The latter is very im portant in obtaining reliable test ratings and in realizing rated performance :in practical installations. The resistance through the coils will assist in distributing the air properly, but where .the inlet duct connections are brought in at sharp angles to the coil face,'the effect is frequently bad and there may even be reverse air currents through the coils. This reduces Overall Coefficient of Heat Transfer ' < - * Of all factors affecting the performance of heating or dry cooling coils, . the overall coefficient of heat transfer is the most difficult to determine, as it is influenced by several factors which depend upon coil design and con ditions of operation. V- .Considering any coil, whether .of bare;pipe or of finned type, the overall heat transfer coefficient for a given size and`design of coil can always;.be considered as a combined effect of three individual heat transfer coef ficients, namely: the capacity, but can be avoided by proper layout or by the use of directing baffle. . T.' Heat transfer depends also upon the velocity of the medium in the tubes and upon its character, whether.flowing water; condensing steam or evapo rating volatile refrigerant. Heat transfer rates expressed as Btu per (square foot of internal surface) (degree" logarithmic mean effective temperature difference between the fluid and tube wall) are, for example: about 150 to 300 for evaporating dichlorodifluoromethane, about 350 to 1200 for water at 2 and 6 fps, and about 1200 for condensing steam. The influence of the medium in the tubes on the overall heat transfer rate is therefore apparent. Because of these variables, reliable rating and performance information for any design of coil must be based on actual tests on that coil under the 1. The film coefficient of heat transferbetween air and the external surface of the coil, usually given in Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference). . . 2. The coefficient of heat transfer through the coil material--tube wall,'fins, ribs, etc. * / i 3. The film coefficient of heat transfer between the internal surface of the coil-and the fluid flowing within the coil, usually given in'Btu per (hour), (square foot internal surface) (Fahrenheit degree mean temperature difference). ; -; . These, three individual coefficients acting in series ;result in an overall coefficient of.heat transfer in accordance-With the basic laws given in Chap ters 5 and 9. For/a bare pipe coil the overall coefficient of heat transfer, whether, for.heating or for, cooling (without dehumidification), can be ex pressed by a simplifiedbasiclormula as follows: expected conditions of operation. A comparison between the performance of two designs, unless based on such tests on each, may lead to entirely erroneous conclusions. Details on coil calculation and performance follow. . ; ... " ' ' , . -f/. <= -------- .. . - . 1 ; .-= : A, .,... f. * fo ....... y<