Document 99YogD4n8J1qVqaog1L81QYx5

152 v CHAPTER f 1949 Guide 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 over-all 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) (Fahrenheit degree mean temperature difference). 2. The coefficient of heat transfer through the coil.material--tube wall, fins, riba etc. ' 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 over-all coefficient of heat transfer in accordance with the basic laws given in Chap ters 5 and 6. For a bare pipe coil the over-all coefficient of heat transfer, whether for heating or for cooling (without dehumidification), can be ex pressed by a simplified basic formula as follows: where - " " ?4. i hf k ~na (2) U = over-all coefficient of heat transfer, Btu per {hour) (square foot external sur face) (Fahrenheit degree mean temperature difference between air and fluid within, the coil). hx = 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 sur face) (Fahrenheit degree mean temperature difference between that surface - and the average fluid temperature). h* = film coefficient of heat transfer between air and the external surface of the coil, Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature 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). L = thickness of tube wall, inches. \ R = 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 oh the basis of external surface. . Frequently, when pipe or tube walls are thin and of material having high conduc tivity (as is the case in construction of.typical heating and cooling coils) the term L/k in Equation 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 Der cent of the over-all coefficient). Thus, in its simplest form, for bare pipe: ) pgfo'rmance of Air Heating and-Cooling Coils 153 , For finned coils the formula1 for the over-all coefficient of heat transfer can be conveniently written: U J. (4) ht in which the term 17, called the Jin efficiency, is introduced to allow for the resistance to heat flow encountered in the fins. The term B, in this case, is the ratio of total external surface 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 oh a basis of external surface. In the discussions which follow, coefficients hr and vK will be considered sepa rately, and also various ways of combining them will be outlined. The performances of all heating and dry cooling coils are influenced by these same factors. But, when cooling coils operate, wet or act as de- humidifying coils, the performance cannot be predicted on the basis of over-all coefficients and an analysis must be made on the basis of individual film coefficients as will be explained. ' PERFORMANCE OF DEHXIMEDIFYING COILS When a cooling coil operates with a surface temperature which is below the dew-point of the air entering the coil, moisture i? condensed and the air leaves the coil with a humidity ratio lower than it had when it entered _ the coil. To understand the performance of surface coils under such con ditions, assume that air enters a cooling coil at conditions corresponding to point 1 in Fig. 1. As long as the surface temperature of the coil is above the dew-point, the air is cooled without dehumidification, and its conditioh leaving the coil will be somewhere on line 1-A. Its exact position on this line depends on the air velocity and the external film coefficient as well as upon the surface temperature. When the surface temperature just equals the dew-point, the air leaves with conditions represented by point A. If the surface temperature is below the dew-point, condensation takes place, and the air has a final condition somewhere along the line A-2-3 which is a