Tower sizing

Suppose that the actual commercial tower size has an area of only 910 fr. Within reasonable hmits, the shortage of actual area can be compensated for by an increase in air velocity through the tower. However, this requires boosting fan horsepower to achieve 110 percent of standard tower performance. From Fig. 12-15, the fan horsepower is found to be 0.057 hp/fr of actual tower area, or 0.057 x 910 = 51.9 hp. 

Data for determining the size of natural-draft towers have been presented by Chilton [Proc. Inst. Elec. Eng., 99,440 (1952)] and Rish and Steel (ASCE Swuposium on Thermal Power Plants, October 19.58). Chilton showed that the duty coefficient Df of a tower is approximately constant over its normal range of operation and is related to tower size by an efficiency factor or performance coefficient as follows ... 

From Figure 9, a material balance across the scrubber section shown is as follows providing a basis for a graphical method of tower sizing ... 

Solvent reeovery systems would also neeessitate the speeifieation of eondenser duties, distillation tower sizes, holding tanks, piping, and valves. It is important to note that the engineering design of an adsorption system should be based on pilot data for the partieular system. Information ean usually be obtained direetly from the adsorbent manufaeturer. The overall size of the unit is determined primarily by eeonomie eonsiderations, balaneing the operating eosts against the eapital eosts. 

Example 9-7 Koch-Sulzer Packing Tower Sizing (used by permission, Bulletin KS-1, Koch Engineering Co. Inc.)... 

This type of tower uses fans at the base to force air through the tower fill or packing (Figure 9-102). Due to the relatively low oudet air velocity, there is a tendency for discharged hot air to recirculate into the fan intake and reduce tower performance. The fan handles only atmospheric air thereby reducing its corrosion problem when compared to the fan on an induced draft tower. The tower size for the forced as well as the induced draft unit is considerably less than for an atmospheric or natural draft unit due to the higher heat transfer rates. 

The effects of wet bulb, approach and range on mecbanical draft cooling tower size is indicated in Figure 9-118. 

Economical tower sizes usually require fan horsepower between 0.05 and 0.08 hp/ft of ground plan area [19], and motors larger than 75 hp are not often used due to... 

Maze. R. W. Hydrocarbon Process., 46(2) (1967) 123. Practical tips on cooling tower sizing. 

Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures ... 

Tower size Prill tube height, ft Rectangular cross section, ft Cooling air rate, Ib/h inlet temperature temperature rise.°F Melt Type Urea 130 11 by 21.4 360,000 ambient 15 Ammonium Nitrate... 

As noted, two principles of heat transfer are involved evaporation and convection. The rate of heat transfer by both convection and evaporation increases with an increase in air-to-water interfacial surface, relative velocity, contact time and temperature differential. Packing and fill in a tower serve to increase the interfacial surface area the tower chimney or fans create the relative air-to-water velocity and contact time is a function of tower size. These three factors all may be influenced by the tower design. 

The determination of the heat to be dissipated by a cooling tower is an essential factor that not only affects the tower size, but also its effectiveness. If the heat load. determination is not accurate, either too high or too low, a larger or smaller size tower than is needed for a particular job could be selected. 

Figure 5.9 Effect of flow variance on cooling tower size factor.

Figure 5.10 Effect of range variance on tower size factor.

The approach has a significant effect on the tower size, as shown in Figure 5.11. For a given heat load, gpm and wet-bulb temperature, the cooling tower size increases as the approach decreases, and the closer the cold water temperature approaches the wet-bulb temperature, the greater the increase in the cooling tower size. 

Figure 5.11 Variation in tower size factor with approach.

In this plot, the size corresponding to the selected condition for a typical tower design is called the 100% design. The curve shows percentage variation of the tower size if any two of the conditions are kept constant and the others varied. 

The total volume of air required has a direct bearing on the required area of drift eliminators. Thus, it is indirectly related to the tower size itself. 

Practical Tips on Cooling-Tower Sizing Maze, R. W. 

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