Cooling towers driving force

Mechanical draft cooling towers either force or induce the air which serves as the heat-transfer medium through the tower. For their driving force, natural draft cooling towers depend upon the density difference between the air leaving the tower and the air entering the tower. 

If we decrease the air rate (i.e., increase L G), then in effect the driving force is decreased and a greater degree of difficulty is reflected in the form of a larger value for Ntu(. This is illustrated by the enthalpy-temperature diagram of Figure 6.1. The plot reflects a counterflow cooling tower at constant conditions but variable L G ratios. 

If, under the most severe conditions, ice does accumulate on the louvers to a detrimental degree, operation of the fans in reverse will force warm air out through the louvers, melting the accumulated ice. Reverse operation of fans is used only to eliminate ice, not prevent it. Unfortunately, most fan drive units are not designed for continuous reverse operation. (See Chapter 9 for a discussion of ice prevention systems for cooling towers.)... 

It is important that drive shafts be properly balanced. Imbalance not only causes tower vibration but also induces higher loads and excessive wear on the mechanical equipment coupled to the shaft. With drive shafts approaching speeds of 1800 rpm in most cooling tower applications, it is necessary that the shafts be dynamically balanced to reduce vibrational forces to a minimum. 

The effect of condensation upon transfer rates with application to flue-gas washing plants and cooling towers are discussed. Theoretical models were developed for determining the rate of heat and mass transfer under conditions where fog formation prevails. Derived relationships are functions of the vapor and liquid equilibria and local heat and mass transfer of driving forces. They were used for a numerical study of the amount of fog formation as a function of the operational variables of a flue-gas washing plant in which the inlet gas temperature is typically... 

The most generally accepted theory of the cooling-tower heat-transfer process is that developed by Merkel (op. cit.). This analysis is based upon enthalpy potential difference as the driving force. 

Finally, Table 3.2.1 contains two economic relations or rules-of-thumb. Equation 3.2.20 states that the approach temperature differences for the water, which is the difference between the exit water teir jerature and the wet-bulb temperature of the inlet air, is 5.0 "C (9 °F). The wet-bulb temperature of the surrounding air is the lowest water temperature achievable by evaporation. Usually, the approach temperature difference is between 4.0 and 8.0 C. The smaller the approach temperature difference, the larger the cooling tower, and hence the more it will cost. This increased tower cost must be balanced against the economic benefits of colder water. These are a reduction in the water flow rate for process cooling and in the size of heat exchangers for the plant because of an increase in the log-mean-temperature driving force. The other mle-of-thumb. Equation 3.2.21, states that the optimum mass ratio of the water-to-air flow rates is usually between 0.75 to 1.5 for mechanical-draft towers [14]. 

Figure 9.16, the enthalpy-temperature diagram, shows the relationship between the water and air as they exist in a counterflow cooling tower. The vertical difference at any given water temperature between the water operating line and the air operating line is the enthalpy driving force. 

Figure 9.16. Driving force diagram for cooling tower.

It frequently happens that for commercial cooling-tower packings, only overall coefficients of the form Kya, and not the individual phase coefficients, are available. In that case, an overall driving force representing the enthalpy difference for the bulk phases but expressed in terms of H must be used, such as the vertical distance SU in Figure 8.7. Then,... 

The water cannot be cooled below the wet bulb temperature. The driving force for the evaporation of the water is approximately the vapor pressure of the water less the vapor pressure it would have at the wet bulb temperature. The water can be cooled only to the wet bulb temperature, and in practice it is cooled to about 3 K or more above this. Only a small amount of water is lost by evaporation in cooling water. Since the latent heat of vaporization of water is about 23(K) kJAcg, a typical change of about 8 K in water temperature corresponds to an evaporation loss of about 1.5%. Hence, the total flow of water is usually assumed to be constant in calculations of tower size. 

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