Document zodBypgmxZ9DG3B5peGx0oJgB

330 CHAPTER 23 1960 Guide 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 instead of flat fins. Staggered tubes increase the total heat transfer, as against the in-line arrangement, and corrugated fins may be more effective than flat. This 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 coil is under consideration face velocities may be used, but they may be unsatisfactory in comparing different designs, as it is the actual surface ve locity that is significant. The air volume is usually based on standard air at 70 F and a barometric pressure of 29.92 in. mefeury. 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 important in ob taining reliable test ratings, and in realizing rated perform ance in actual installations. The air 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 a portion of the coils. This reduces the capacity, but can be avoided by proper layout or by the use of vanes or baffles. Heat-transfer information on plain pipe coils has been developed and verified through many tests. In the case of finned coils, the heat 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 design of coil must be based upon actual tests 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 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. PERFORMANCE OF HEATING AND DRY COOLING COILS The performance of heating and dry cooling coils depends in genera] 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: (it - 17 X (Mm) XA X N (1) where q, TM 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 of external coil surface) (Fahrenheit degree temperature difference between the fluid within the coil and the air flowing over the coil). = mean temperature difference, Fahrenheit degrees, be tween the fluid within the coil and the air passing over it. (This is commonly taken as the logarithmic mTi 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 rows 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, the verification of a plain coil design by a series of tests is an accepted commercial practice. The overall coefficients of heat transfer for finned coils should always be obtained from tests. The data from a series of tests may be used for purposes of extending coil data beyond test range but such data should later be verified by test. 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 coefficients, 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) (Fahrenheit degree mean tem perature difference). 2. The conductivity of the coil material--tube wall, fins, 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 riven in Btu per (hour) (square foot internal surface) (Fahren heit degree mean temperature difference). These three individual coefficients acting in series result in an overall coefficient of heat transfer in accordance with the baric laws given in Chapters 5 and 9. For a bore-pipe coil the overall coefficient of heat transfer, whether for heating or for cooling (without dehumidification), can be expressed by a simplified baric formula as follows: where U - overall coefficient of heat transfer, Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference between air and fluid within the coil). fi TM 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 externa) surface). k *= conductivity of material from which the bare pipe is constructed, Btu per (hour) (square foot) (Fahrenheit degree per inch thickness). Air Heating and Cooling Coils 1, thickness of tube wall inchesR = ratio between external and internal surface of the bare tube, usually varying from 1.03 to 1.15 for the tube used in typical beating 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 surface. Frequently, when pipe or tube walls are thin and of ma terial having high conductivity (as is the case in construction of typical heating 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: 331 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 For finned coils the formula* for the overall coefficient of heat transfer can be conveniently written: in which the term tj, ealled the fin efficiency, is introduced to allow for the resistance to heat flow encountered in the fins. The term R, in this case, is the ratio of total external sur 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 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. 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 mally accomplishes (or is designed to accomplish) moisture removal in addition to sensible-heat cooling is termed a dehumidifying coil. In most air-conditioning proce&es, 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 cooling 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 psyehrometric 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- * Rational development end retin* extended air oooiin* surface, by H. B. PowbsII (Hennaesatufa Ejrw*e*n*a, October 1035, p. 211). Fig. 12 .... Performance of Dehumidifying Coil continue the removal of sensible heat, thereby carrying the load due to both. As saturation is approached in the cooling 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 proximately constant per degree change, the amount of la tent heat removal per degree of dew-point change varies con siderably because moisture content varies widely at different temperatures. For example, the following tabulation compares the 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 49 F: Dew Pourf 60 59 Difference w, x iott/Ub)| Dew Point 11.080 10.690 0.390 1 50 1 49 | Difference W. X 10* ib/((b) 7.658 7.374 0.284 The above values are given is Table 2. Chapter 3. When cooling coils act as dehumidifying coils, the perform ance can be predicted accurately only from tests at a suffi 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 coil 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,