Document mmv2Ex6zZdezXenXbaB9J1r3B

75 CHAPTER 35 1950 Guide . 9.78 * 57.3-102 8;78 (5) The total coil loadmay. be calculated fromthe'erithalpy difference across the coil and the air quantity using the weight iof dry air instead of the weight of the mixture. 9, - <?. (hi - k,) , , , v = 1700 (49.24 - 23.77) = 43,200 Btu per (hr) (sq.ft of face area) where ' : >"' 1 j G, = weight of dry air per (hour) (square foot of coil face area). hi = enthalpy of air vapor mixture entering coil, Btu per pound of dry air. hi = enthalpy of air vapor mixture leaving coil, Btu per pound of dry air. Therefore, t; = (52.3-22.1) = 30.2.' - '. . " . w \ i. ^ Xhus coil 6 rows deep, operating at a refrigerant temperature; of. 30.2 F and a face; velocity of 400 fpm, is required; and it will carry a total load of 43,200 Btu per (hour) (square foot of face area). ..The air. conditions leaving.the coilare too low for the con-; ditions of the problem and therefore it is necessary to by-pass air at the entering condition to obtain the desired result of 80,5 F dry-bulb and 73 F wet-bulb. Although the preceding solution is satisfactory, it may be more desirable in some cases to use a higher refrigerant temperature and employ reheat to obtain the desired load ratio. Such a solution is shown in Fig. 14. In thus case, the coil load ratio line intersects the saturation curve and, therefore, a coil of any depth may be selected. If a coil depth of 6 rows is maintained, the exit air conditions for the coil are indi cated at point B Fig. 14 as 72.3 F dry-bulb and 70.8 F wet-bulb and the surface temper ature will be: V? Air-Heating and Cooling Coils ; 75? 9.78X72.3 r-102 t. fa' 69.0 8.78 . The coil load will be: 9t = 1700(49.24 -- 34.66) = 24,800 Btirper (hour} (sqtfare foot of face area) and the refrigerant temperature will be. found from Equation 9: 24,800 (1. - Q = 12.7 325 15X.6X -- lo Therefore,, t =. 69,0 -- 12.7 = 56.3.. Thus, for the case where reheat is used, a coil 6 rows deep operating at a refrigerant temperature of 56.3 F is required. ' The total coil load will be 24;800 Btu per (hour) (square foot of face area) but the actual effective load will be less by the'amount of reheat.required... Therefore, for a given load, a larger coil and more refrigerating capacity'are required when reheat is used. - - - - - - ! : LETTER SYMBOLS USED IN CHAPTER 35, ij = fin efficiency. - - . A = external area of coil, square feet per (square foot of coil face area) (row . , of coil depth). .. - - . . d h fupi M- 1dpi' - .... . : . ' D = internal diameter of tube; inches. G = air mass velocity, pounds per (hour) (square foot of coil face area). , <? = dry air mass velocity; pounds dry air per (hour) (square foot of coil face area). . ' p - ' - ' fi = filin coeflicient of,heat transfer; between fluid and internal coil surface, ; Bti per'(hour) (square.fopt^internal.surface),(Fahrenheit degree mean temperature between fluid and surface)/ ' . . . . ' ; ... .., ; /o -- film coefficient of heat transfer between air and external coil surface, Btu per (hour) (square foot external surface) (Fahrenheit degree mean temperature difference between air and coil). hi = enthalpy of air-vapor mixture entering coil, Btu per pound of dry air. ht = enthalpy of air-vapor mixture leaving coil, Btu per pound of dry air. k -- conductivity of pipe or tube material, Btu (square foot) (hour) (Fahren heit degree per inch thickness). L = thickness of tube wall, inches. N = number of rows of coil depth. n = a constant, exponent of G in Equation 8, obtained by plotting, on loga rithmic coordinates, G against values of /,,. The value of n is the Blope of the line. q, = sensible heat transferred, Btu per (hour) (square foot of coil face area). 9t = total heat transferred by coil, Btu per (hour) (square foot of face area). B = ratio between external and internal surface of tube. t -- average water temperature, Fahrenheit degrees. = dry-bulb temperature of air entering coil, Fahrenheit degrees, ft = dry-bulb temperature of air leaving coil, Fahrenheit degrees. I. -- minimum dry-bulb temperature possible without dehumidification, Fahrenheit degrees. fdpi = dew-point of air entering coil, Fahrenheit degrees.