Document b5a2xVv5e1OwbKaK8KY0aab70

320 CHAPTB? 23 1959 Guide Air Heating and Cooling Coils 321 ;l ; such items as materials, type, thickness, height, and spacing U overall coefficient of heat transfer, Btu per (hour) of the fin* and the ratio of this surface to that of the tube (square foot of external coil surface) (Fahrenheit the use of the staggered or in-line tube arrangement, and degree temperature difference between the fluid provisions to increase the air turbulence such as the use of within the coil and the air flowing over the coil). corrugated instead of flat fins. Staggered tubes increase aim m mean temperature difference; Fahrenheit degrees, be the total heat transfer, as against the in-line arrangement, tween the fluid within the coil and the air passing L *=* of tube wall inches. U B ratio between external and internal surface of the bare tube usually varying from 1.03 to 1.15 for the tube used in typical heating or cooling coils. This ratio R is inserted in the formula in order to place internal fluid coefficient of heat transfer on the basis of external point temperature, the less will be the difference between the leaving dry-bulb temperature and the leaving dew-point tem perature. The first portion of a cooling coil (in the direction of air flow) may function in the same manner as a dry cooling coil. Where the moisture removal starts, the cooling surfaces also and corrugated fins may be more effective than flat. This over it. (This is commonly taken as the logarithmic surface. design and surface arrangement has a large effect on the airfilm heat-transfer resistance. The velocity of the air usually considered is the coil face velocity. This bears a varied relation to the actual velocity over the surface, depending upon the individual coil design. As long as a fixed design of cod is under consideration face velocities may be used, but they may be unsatisfactory in comparing different designs, as it is the actual surface ve mean temperature difference.) A ~ external surface area of the given coil, square feet per (square foot of coil face area) (row of coil depth). N " number of rowB of coil depth. Overall Coefficient of Heat Transfer While the overall coefficients of heat transfer for plain pipe coils have been defined by numerous tests within close limits, Frequently, when jape or tube walls are thin and of ma terial having high conductivity (as is the case in construction of typical hating and cooling coils) the term L/k in Equa tion 2 becomes negligible and is generally disregarded. (The effect of the term L/k in typical bare-pipe heating or cooling coils seldom exceeds 1 to 2 percent of the overall coefficient.) Thus, in its simplest form, for bare pipe: locity that is significant. The air volume is usually based on the verification of a plain coil design by a series of tests is an standard air at 70 F and a barometric pressure of 29.92 in. accepted commercial practice. The overall coefficients of heat mercury. At the same mam air velocity, varying performance can . transfer for finned coils should always be obtained from tests. The data from a series of tests may be used for purposes of /i + /. be obtained depending upon the turbulence of the air flow extending coil data beyond test range but such data should For finnpH coils the formula* for the overall coefficient of into the coil, and upon the uniformity of distribution of later be verified by tat. heat transfer can be conveniently written: air over the coil face. The latter is very important in ob taining reliable test ratings, and in realizing rated perform Considering any coil, whether of bare pipe or of finned type, .the overall heat-transfer coefficient for a given size and Rg. 12 .... Performance of Dehumidifying Coil ance in actual installations. The air resistance through the design of coil can always be considered as a combined effect continue the removal of sensible heat, thereby carrying the coils will assist in distributing the air properly, but where of three individual heat-transfer coefficients, namely: f* nf* load due to both. As saturation is approached in the cooling 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 a portion of the coils. 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) (Fahrenheit degree tem in which the term tj, called the fin efficiency, is introduced to allow for the resistance to hat flow encountered in the coil, each degree of sensible cooling is approximately matched by a corresponding degree of dew-point decrease. However,. while the sensible heat removal from the dry air remains ap This reduces the capacity, but can be avoided by proper perature difference). fins. proximately constant per degree change, the amount of la layout or by the use of vanes or baffles. 2. The conductivity of the coil material-tube wall, fins, - The term R, in this case, is the ratio of total external sur tent heat removal per degree of dew-point change varies con Heat-transfer information on plain pipe coils has been developed and verified through many tests. In the case of finned coils, the beat transfer from the cooling or. heating medium to the'air stream is dependent on so many factors that reliable rating and performance information for any ribs, etc., usually given in Btu per (hour) (square foot of sur face) (Fahrenheit degree per inch). 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) (Fahreneit degree mean temperature difference). face to internal surface. For typical designs of finned coils for heating or cooling, this ratio varies from 10-to 30. Term R is again introduced to place the internal surface coefficient of heat transfer on a basis of external surface. The performances of all heating and dry cooling coils are siderably because moisture content varies widely at different temperatures. For example, the following tabulation compares tire amount of moisture removal involved in a reduction of one degree of dew point from 60 to 59 F with the removal from 50 to j design of coil must be based upon actual teste of the spe cific coil. Mathematical comparisons of different designs of coils on a square foot of surface and face area basis may be misleading. The'selection of finned coils should be made These three individual coefficients acting in series result in ah overall coefficient of heat transfer in accordance with the basic laws given in Chapters 5 and 9. For a bore-pipe coil the influenced by these same factors. But, when cooling coils operate wet or act as dehumidifying coils, the performance cannot be predicted on the basis of overall coefficients. 49 F: Daw Point W, X Daw Point W. X 10*U/(fl>) from curves or tables of coil.performance prepared from a series of adequate and reliable tests. There are cases in which the engineer must extend available data or design for a single unique installation. For such purposes the following coil calculations will be useful. overall coefficient of heat transfer, whether for heating or for cooling (without dehumidification), can be expressed by a simplified basic formula as follows: U (2) PERFORMANCE OF DEHUMIDIFYING COILS When the dew point of the air leaving a cooling coil is lower than the dew point of the air entering the coil, some moisture removal has been accomplished. A coil that nor 60 59 Difference 11.080 10.690 0.390 50 49 Difference 7.658 7.374 0.284 PERFORMANCE OF HEATING AND mally accomplishes (or is designed to accomplish) moisture Tbe abova vahn an firva in TaMa 9, Chapter $. DRY COOLING COILS where removal in addition to sensible-heat cooling is termed a dehumidifying coil. When cooling coils act as dehumidifying coils, the perform ance can be predicted accurately only from tests at a suffi The performance of heating and dry cooling coils 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 heat ing or cooling capacity of a given coil is expressed by. the following basic formula: q, -U X (At.) X A X N (1) where qi -- total heat transfer of the coil, Btu per (hour) (square foot of coil face area). U = overall coefficient of heat transfer, Btu per (hour) (square foot external surface) (Fahrenheit degree moan temperature difference between afr and fluid within the coil). /, -- film coefficient of heat transfer between the internal surface of the coil and the fluid flowing within the coil, Btu per (hour) (square foot internal surface) (Fahren heit degree mean temperature difference between that surface and the average fluid temperature). /, -- film coefficient of heat transfer between air and the external- surface of the coil, Btu per (hour) (square foot external surface) (Fahrenheit degree mean tem perature difference between the mass of air and the external surface). k = conductivity of material from which the bare pipe is constructed, Btu per (hour) (square foot) (Fahrenheit degree per inch thickness). In most air-conditioning processes, the air may be con sidered as a mixture of water vapor and the dry components. Both dry components and the water vapor enter an airconditioning coil at the same dry-bulb temperature; both the dry components and the water vapor lose sensible heat during the contact with the first portion of the coding coil in the same manner as in a dry cooling coil. As the dry-bulb temperature of the mixture approaches the dew point of the water-vapor components, moisture removal starts. The psychrometric path of air through a cooling coil is generally assumed to follow a path similar to that shown in fig. 12. When the dry-bulb temperature of the air mixture in the coil falls below the entering dew-point temperature, moisture removal proceeds. The indications are that the lower the leaving dry-bulb temperature below the entering dew- * Rafa'noal development tad mtiag erf extended air mrJrnf surface, by H. B. PownaU {Btfrigtrotatf Enfinttrtnf, October 193$, p. til). cient number of points to establish the definite performance characteristics of the coil under varying conditions of loading and of entering air. Dehumidifying coils employing volatile refrigerants are generally rated in conjunction with specific refrigerant distributing and flow control equipment. The com bination of the coil with its refrigerant control equipment (such as distributor and expansion valve, and capillary tube or float valve) must be tested at both the higher and lower capacities of its rated range. The lower capacities impose a test on the distributor to provide equal distribution and on the control to modulate without hunting at the lower capaci ties. The higher capacities result in a greater pressure drop through the coif system and a test of the maximum feeding capacity of the flow control device at various head pressures. Most coil manufacturers have their own .methods of pro ducing performance rating tables from a suitable number of coil-performance tests. A method of testing and rating coils, It