Document 3QqmemnynVMwNMgJ5xJj06kpO

142 CHAPTER 7 1948-Guide In the discussions.which follow, coefficients ht arid ^ha will be considered^ separately, 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 dehumidifying coils, the performance cannot be predicted ori the basis of over-all coefficients and ah'analysis must be made on the basis of individual. film coefficients as will be explained. PERFORMANCE OF DEHUMIDIFYING COILS ' When a cooling coil operates with a surface temperature which is below the dew-point of the air entering the coil, moisture is condensed arid the air leaves the coil with a humidity ratio lower than it had when Fig. 1. Performance of Dehumtoifying Con. it entered the coil. To understand the performance of surface coils under such conditions, 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 dehumidifi cation, and its condition leaving the coil will be somewhere on line 1-A. Its exact position on this line depends ori 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 some where along the line A-2-3 which is a line at a constant horizontal distance frorinthe saturation curve. It should be understood that the line l-A-2-3 is not intended to represent the path of the condition of the air as it passes through the coil from row to row. It is simply the path traced by the exit air conditions as the surface temperature is gradually reduced with other conditions remaining constant2. In the process of dehumidification, since heat is being transferred to the coil surface by two different mechanisms, (convection and conden sation), it is evident that an over-all coefficient of heat transfer cannot be determined by the sarrie method used for heating and for dry cooling coils. However, if it is assumed that the sensible heat transfer of a dehumidifying: coil is unaffected by the presence of moisture on its1 surface, Equation 5 may be obtained to express this part of the heat Performance of Air Heating and Cooling Coils 143 transfer in terms of the external film coefficient and. the surface tem perature. . e,, = fta X A X N X (MTDq) (5) where ' qB = sensible heat transferred, Btu per (hour) (square foot of coil face area). h = dry-bulb temperature,of air entering coil, Fahrenheit degrees. tt = dry-bulb temperature of air leaving coil, Fahrenheit degrees: ta = average temperature of coil external surface, Fahrenheit degrees. MTDa = logarithmic mean temperature difference between air and coil surface = h- h If Equation 5 is combined with another equation expressing sensible heat transfer in terms of mass velocity and temperature difference, the variables may be arranged in the following form (which is useful for the solution of dehumidification problems and for the determination of ha from test data): ha A N ((. - fa) log. (x -- h = 0.243 G (t, - tt ) ft -- ts or, hqA N 0.243 G (l -- fa log.' t, - h where 6.243 = specific heat of humid air, Btu per (pound) (Fahrenheit degree). G = air mass velocity, pounds per (hour) (square foot of coil face area). (6) An examination of Fig. 1 will reveal that when 4 is at the dew-point of the entering air: tl -- ts _ t\ --(dpi la -- h la -- (dpi and when 4 is below the dew-point: h -- Is _ ti -- (dpi It -- la la -- (dpi Therefore, Equation 6 may be written in its most useful form as: hqA N 0.243G It -- (dpi loge tt -- (dpt (i - la loge It -- ta (7) where ' ta =". minimum dry-bulb possible without dehumidification, Fahrenheit degrees, (dpi = dew-point of air entering coil, Fahrenheit degrees. (dpi = dew-point of air leaving coil, Fahrenheit degrees. This equation may be used to establish a line as A-2-3 for. a given coil if ha is known for the coil, or it may be used to determine ha from test data for the purpose of rating coils. The use of this equation for.coil selection is; illustrated in Example 1 at the end of the chapter. Equation 7 is also important as a means of determining the external film coefficient. External Film Coefficient While formulas have been developed expressing the film coefficient ha for air passing parallel to a plane surface, they cannot be used directly