Tower characteristics

The coordinates refer directly to the temperature and enthalpy of any point on the water operating hne but refer directly only to the enthalpy of a point on the air operating line. The corresponding wet-bulb temperature of any point on CD is found by projecting the point horizontally to the saturation curve, then vertically to the temperature coordinate. The integral [Eq. (12-8)] is represented by the area ABCD in the diagram. This value is known as the tower characteristic, vaiying with the L/G ratio. 

In order to predict tower performance it is necessary to know the required tower characteristics for fixed ambient and water conditions. 

The tower characteristic KaV/L can be determined by integration. Normally used is the Chebyshev method for numerically ev uating the integral, whereby... 

FIG. 12-13 Nomograph of cooling-tower characteristics. [Wood and Betts, Engineer, 189(4912), 337 (1950).]... 

The cross-flow-tower manufacturer may effec tively reduce the tower characteristic at very low approaches by increasing the air quantity to give a lower L/G ratio. The increase in air flow is not necessarily achieved by increasing the air velocity but primarily by lengthening the tower to increase the air-flow cross-sec tional area. It appears then that the cross-flow fill can be made progressivelv longer in the direction perpendicular to the air flow and shorter in the direction of the air flow until it almost loses its inherent potential-difference disadvantage. However, as this is done, fan power consumption increases. 

The number of transfer units or tower characteristic is based on overall heat and mass transfer ... 

Figure 9-111. Typiccil effect of hot water temperature on tower characteristic, KaV/L at constant L, Ga wet buib temperature and packed height. Note L and G shown in chart are hourly rates. Reproduced by permission of the American Institute of Chemical Engineers, Kelly, N. W., and Swenson, L. K., Chemical Engineering Progress, V. 52, No. 7 (1956) p. 263 all rights reserved.

Because the heat load, L, Ga and temperatures are known for an operating tower, its performance as represented by the number of transfer units, or tower characteristics can be determined. Solve Equation 9-129 for Ka V/L, or use the modified Merkel diagram, Figure 9-127. This is the number of transfer units operating in the tower. For relative comparison of Ka values see Figure 9-128. 

Cooling in the crossflow mode requires an incremental trial and error technique, best suited to computer analysis. The tower characteristic KaV/L can then be plotted against varying L/G ratios, and this gives a measure of the ability of the packing to effect the transfer (Figure 34.18). 

Cooling tower characteristics can only be deduced from actual field tests. Tower characteristics are generally presented in the form of an empirical correlation. This correlation defines the relationship between the available... 

Using Simpson s rule or some other appropriate numerical integration technique, determine the area under the curve obtained from step (3). From the area, the required value for the tower characteristic (KaV/L) can be determined. 

Estimate the effect on the tower characteristic KaV/L on the deviation of the water inlet temperature from the 120°F for which the data of Table 6.2 were prepared. 

From this and the gas mass velocity, it is determined that 3 towers are needed, each one with a diameter of 18.5 ft. The tower height is determined by the height of a transfer unit which is 20.7 ft. 05). Therefore, the height of the packed tower is 41.4 ft. Other detailed tower characteristics are given in Table III. 

NTU is the number of transfer units or the tower characteristics based on the overall simultaneous heat and mass transfer, as defined by Eq. 9.21. For a more complete development of the NTU concept, see Chapter 13. 

This relates the tower characteristic to the number of packing decks in the tower and the L /Ga ratio. Values of A and n are given in Table 9-50. 

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