Document 2q0q4G1NVd1K6b62kBpvv6ak7

886 CHAPTER 34 1958 Guide circulating water as a result of evaporation; that the water suspended in the tower is surrounded by a film of air which is saturated with water vapor and at the temperature of the water surrounded; and that the basic theory of cooling' tower operation proposed by Lewis13 and developed by Merkel14 is applicable. Thus theory refers to the fact that the numerical value of the coefficient of sensible heat transfer, when divided by the numerical value of the coefficient of diffusion, equals the specific heat (at constant pressure) of air. The reader should observe that this relationship refers to the numerical values of three distinct constants, the units for each being different. The above relationship makes it possible to simplify the heat transfer equation by combining the two driving forces into one potential represented as the difference between the enthalpy of the air film (at water temperature) surrounding the water, and the enthalpy of the main air stream. conditions 1 WATER L FLOW LB PER HR ACTIVE TOWER VOLUME ir V # < AIR G FLOW LB PER HR CONDITIONS 2 Fig. 10. Operations in a Typical Water Cooling Tower Tower Performance Factor The operations taking place in a typical water-cooling tower are shown in Fig. 10. If the reduction in water flow rate, due to evaporation within the volume, is neglected, and the usual concepts of heat flow and mass heat transfer are applied, the equations typifying cooling tower operation are: K~GaV~ = Jr2l h'd-h K (3) and ~ J2KaV _ f1 da L h' - h. (4) where a -- overall average wetted area (surface of water drops plus wetted tower sur face), square feet per cubic foot of active tower volume. G = weight rate of flow of air, pounds of dry air per hour. h = enthalpy, Btu per pound of dry air. Spray Apparatus 887 ha = enthalpy of air-vapor mixture, Btu per pound of dry air. h" = enthalpy, of saturated air-vapor mixture at water temperature, Btu per pound of dry air. K = overall energy unit conductance, Btu per (hour) (square foot overall average wetted area) (Btu enthalpy difference per pound of dry air). h = water rate, pounds per hour. " B = temperature of water in tower, Fahrenheit. di = temperature of inlet water, Fahrenheit. Bi = temperature of outlet water, Fahrenheit. 1 V = active tower volume, cubic feet. Ether term -~^r~ or may be called the Tower Performance Factor or Number of Tower Units (NTU). these equations indicate that the rate of heat transfer from the water to the air depends primarily upon the enthalpy of the air, the latter being dependent only on the wet-bulb temperature of the air. This explains the common observation that cooling tower performance is independent of inlet dry-bulb air temperature, and that adiabatic conditions exist. The integration of Equations 3 and 4 must be performed by mechanical r graphical means, because direct mathematical integration would be cural* only within narrow temperature limits. The temperature en- PY diagram in Fig. 11 represents the conditions for either of the above entb i0nS' ,wa*er ls cooled from the temperature Bi to 02, and the ent a t^le air film surrounding it follows the saturation line h". Air It it,TM tower at a wet-bulb temperature of t'2, and an enthalpy of hi. of h Q.te<* an outlet wet-bulb temperature of t[, with an enthalpy the a' ti?06 keat rejected by the water equals the heat absorbed by water1'the k0at a^sor^e(^ Per pound of air is a function of the pounds of nno^*Per Pund of air going through the tower, and the slope of the air operating line is the L/G ratio.