Document xdv81rjw80BrRY30G4072yg9y

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 by comparison of area At, At, and A, of Fig. 7. Each represents the Number of Transfer Units required to achieve a water temperature reduction of about 10 F. A i = 0.471 NTU (110 to 100 F) At = 0.595 NTU (100 to 90 F) At = 0.657 NTU ( 90 to 80 F) The use of the logarithmic mean driving potential is illustrated by applying Equation 7 to example 1: NTU-TMV^Ht-Ht G A/lIm (ft" - fti) - (hn2 - ht) ft, - fti In, *! - ht _ 30-7 - 18.4 30.7 In, 18.4 24 Fig. 7. Graphical Integration to Determine Number of Transfer Units Required for Desired Operating Conditions of Example 1 NTU 61.1 - 25.1 24 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. 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 counterrflow, to the reduced effectiveness of cross-flow, has been derived for heat transfer and may be applied to this case10. "Heat Transmission, by W. H. McAdams (McGraw-Hill Co.. New York City. 1933, p. 157). 558 CHAPTER 27. SPRAY EQUIPMENT 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 water temperatures considered. b_ 1 -- fa h\ - (8) Case 2. The slope of the saturation line exceeds that of the operating line. e ti t\ -- ^wb (9) 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 the intermediate classification. . Make-Up Water Since the atmospheric water cooling equipment performs its functions chiefly by evaporating a portion of the water in order to cool the re- Table 5. Comparison of Various Types of Atmospheric Water Cooling Equipment Figures indicate order-of desirability Cooling Sphat Pond Pond Cost....... Area.. Height________,,.................,,.......... ......... _..... Weight per square foot......... 1.................. . Independence of wind velocity..................... Drift nuisance.................................. ................. Make-up water required............................... Pumping hfiad Maintenance......... ............................................ Suitability for congested districts............... Water quantity required for definite result z 5 1 X 6 1 1 1 2 X 6 2 4 2 X 3 6 6 2 1 5 5 Sprat Tower i 3 3 1 4 5 5 4 3 4 4 Deck Tower Mechanical Indoor Draft Tower 345 21X 4-5 4-5 X 342 5 1-2 1-2 4 2-3 2-3 4 2-3 2-3 536 456 312 1-2 1-2 3 *Not comparable. 559