Document jBYDOvmpKGbEOBmZDYm1eZ45k

^^C_VENTILAT,NC AIR CONDITIONING rnmc ,944 In, h, -- hi . 30.7 - 18.4 = 24 In. 30.7 18.4 NTU = --1 = 1.5 The Number of Transfer Units required as determined by use of the logarithmic mean driving potential equals 1.5 which compares favorably with the correct magnitude, 1.72. For small size counterflow spray towers intended, for a low cooling range only considerable data*2,13 are available on the performance of the Fig. 7. Graphical Integration to Determine Number of Transfer Units Required for Desired Operating Conditions of Example 1 spray, system in terms of the logarithmic mean enthalpy potential. These data show the importance of proper nozzle arrangement and spray distribution within the tower. The greatest cooling effect is shown to be obtained in a region close to the nozzles; and the beneficial effects of packing: added in the lower section of the tower follow as increasing the cooling/ange. A variation of the design method according to Equation 6 is further possible14 through the introduction of Equation 2 to yield STaF L (8) The advantage of this form is in the fact that the left side of the equation may be dealt with as a thermodynamic function only, while the "A.S.H.V.E. Research Report No. 11S9--Performance Characteristics of a Forced Draft, Counterflow . Spray Cooling Tower, by H. H. Niederman, E. D. Howe, J. P. Longwell, R. A. Seban and L. M. K. Boelter (A.S.H.V.E. Transactions, Vol. 47,1941, p. 413), "Spray Nozzle Performance in a Cooling Tower, by L. M. K. Boelter and S. Hori (Beating, Piping and Air Conditioning, May, 1943, p. 259), MPerformance and Selection of Mechanical-Draft Cooling ToyrerB, by Joseph Lichtenstein (AS.M.E. Transactions, October, 1943. Voi 65, No. 7. p. 779). ' 542 CHAPTER 27. SPRAY EQUIPMENT right side depends upon the tower construction and operation. A series of performance curves may be determined from a graphic evaluation of the integral, which will serve either as a convenient means of analyzing experimental data to determine the conductance Ka, or as a rapid method of performance prediction, once the tower characteristic, --Kdjf--V , is known. These performance curves may be expressed in terms of the following variables; approach to wet bulb, cooling range, LJG, and wet-bulb temperature of the entering air; and as such they will serve to accurately cover the tower performance in consistent terms. The application of the foregoing design method to atmospheric towers is difficult because rate coefficients and flow conditions are not yet well defined for such equipment. If these are known, application of Equation 7 to sections of the tower small enough to justify use of the logarithmic mean potential will yield the tower volume required for each section. The sections must be taken perpendicular to the path of water flow. A correction to adjust the logarithmic mean potential, evaluated as for counter-flow, to the reduced effectiveness of cross-flow, has been derived for heat transfer and may be applied to this case15. Atmospheric towers operate with natural draft, produced in a vertical direction by the stack action of the tower structure, at zero velocity of the approach wind. Approach wind of sufficient magnitude (the magnitude depending on the baffle arrangement which is designed to reduce drift) will cause cross-flow augmenting the natural draft. An adequate design requires the consideration of both .flow conditions. Expression for Cooling Tower Performance ' The performance of a cooling tower is described in terms of its effective ness as an energy exchanger. The effectiveness is defined as the ratio of the energy actually exchanged to the energy available for exchange. Effectiveness expressions: * Case 1. The slope of the operating line on the t -- h diagram exceeds the slope of the saturation line in the region of w^ter temperatures considered. h - ht (9) Case 8. The slope of the saturation line exceeds that of the operating line. hi -- h, _ ti -- h ~ (ti - twb) `` " l-b (10) This equation represents the approach to wet-bulb. Usual tower operating conditions conform to Case. 1. Because of the curvature of the saturation line, operating conditions may present them selves to which neither Case 1 nor 2 applies. Since a simple expression for the intermediate case is not.available, the expression of Case 1 may be utilized for the small number of operating conditions falling into tiie intermediate classification. "Heat Transmission, by W. H. McAdams (McGraw-Hill Co., 1933, p. 157). 543