Water cooling, optimum flow rate

The design conditions set the values of q and and the heat capacity of water may ordinarily be approximated as 1 Btu/(lbX°F). Therefore, Eq. (52) shows that the flow rate of the cooling water is fixed if the temperature of the water leaving the condenser (t2) is fixed. Under these conditions, the optimum flow rate of cooling water can be found directly from the optimum value of t2-The annual cost for cooling water is wHyCw. From Eq. (52),... 

The optimum value of t2 can be found from Eq. (56) by a trial-and-error solution, and Eq. (52) can then be used to determine the optimum flow rate of cooling water. The trial-and-error solution can be eliminated by use of Fig. 11-6, which is a plot of Eq. (56).f... 

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]. 

Optimum flow rate of cooling water in condenser, 368-371... 

The lube oil must be maintained within a specific operating band to ensure optimum equipment performance. This is accomplished by controlling the flow rate of the cooling water with a temperature control loop. 

An important thermodynamic parameter in cooling tower calculations is the ratio of the thermal capacity of the water stream to that of the sir stream. This parameter is referred to as, the tower capacity factor. It is shown that when air or water efficiency, are plotted against the capacity factor test points for a given tower are found to lie on a single smooth curve. The correlation is obtained, irrespective of whether the equipment is used as a water cooler or air cooler, and irrespective of the temperature levels, temperature ranges and barometric pressures. The paper also shows that when a specified amount of heat has to be rejected into a specified air stream, optimum performance giving the lowest average water temperature is obtained when the water flow rate is chosen so that its thermal capacity is equal to the potential thermal capacity of the air stream. 13 refs, cited. 

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