Document v66O1QRaMrmypynvLqadZ540Y

474 Chapter 25 1945 Guide HEAT TRANSFER AND AIR FLOW RESISTANCE The transfer of heat between the heating or cooling medium and the air stream is influenced by several variables: 1. The temperature difference. 2. The'design and surface arrangement of the coil. 3. The velocity and character of the air stream. 4. The velocity and character of the medium in the tubes. The driving force is usually taken as the logarithmic mean'temperature difference for heating or cooling without dehumidification. For combined cooling and dehumidification, a special measure of.the propelling force is used as described later. Logarithmic differences are generally employed in practice although there are special flow relationships used, such as cross-flow, where they do not strictly apply. With volatile refrigerants there is often an appreciable pressure drop and corresponding change in evaporating 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 transfers and ratings for coils using volatile refrigerants are usually based in practice on a refrigerant temperature corresponding to the average pressure in the coil. The design and surface arrangement of the coil includes 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 arrangement,.and provisions to increase the air turbulence such as the use of corrugated as against flat fins. Staggered tubes increase the total heat transfer as against the in-line arrangement and corrugated fins are more effective than flat. Of especial importance is the bond between fin and tube. - The velocity of the air usually considered is the coil face velocity. This bears a varied relation to the actual velocity over the surface, de. pending upon the individual coil design. As long as a fixed design of coil is under consideration face velocities may be used, but they may be unsatisfactory in comparing different designs, as it is the actual surface velocity that is significant. The air volume is often based on standard air at 70 F and a barometric pressure of 29.92 in. Hg. The use of air volume.in coil rating information may be misleading. The significant value is mass velocity in pounds per minute and not cubic feet per minute, because for a fixed volume the corresponding weight may vary widely, depending upon the temperature and barometric pressure under con sideration. 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 properly distributing the air, 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 the capacity, but can be avoided by proper layout or by the use of directing baffles. The heat transfer depends also upon the velocity of the medium in the tubes and upon its character, whether flowing water, condensing steam or evaporating volatile refrigerant. Heat transfer rates, expressed as Btu Heat Transfer Surface Coils 475 per square foot of internal surface per 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,1 reliable rating and performance information for any design of coil must be based on actual tests on that coil , under the expected conditions of operation. A comparison between the perfor mance of two designs, unless based On such tests on each, may lead to entirely erroneous conclusions. PERFORMANCE OF HEATING AND COOLING COILS Heating and cooling coils are heat exchangers and as such their per formance depends in general upon: ; 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. 3. The physical dimensions of the coil. Thus, for any one definite operating condition, the heating or cooling, capacity of a given coil is expressed by the following basic formula: q = UX MfD X A (1) where q = total heat transferred by the coil, Btu per (hour) (square foot of coil face area). U -- overall coefficient of heat transfer, Btu per (hour) (square foot of external coil surface) (degree Fahrenheit temperature difference between the fluid within the coil and the air flowing over the coil). MTD = mean temperature difference, degrees Fahrenheit between the fluid within the coil and the air passing over it. (This is commonly taken as the logarithmic mean temperature difference.) A = external surface area of the given coil, square feet per square foot of coil face area. . The performances of heating and cooling coils are influenced by the same factors in all but one very important exception, that is, when cooling coils operate wet or act as dehumidifying coils. For this, reason, in the later discussion, heating and dry cooling coils are treated as one. group and dehumidifying coils as another. OVER-ALL COEFFICIENT OF HEAT TRANSFER Of all factors affecting the performance of heating or 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. 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: 1. The. film coefficient of heat transfer between air and the external surface of the coil, usually given in Btu per (hour) (square foot external surface) (degree Fahrenheit mean temperature difference). - 2. The coefficient of heat transfer through the coil material--tube wall, fins, ribs, etc.