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February 2012 Go to Page 1 2 3 4
Cooling tower fill selection
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Figure 1. Process water and air flows to a cooling tower. Process conditions are taken from an example in Reference 1.
Figure 1. Process water and air flows to a cooling tower. Process conditions are taken from an example in Reference 1.

Efficient heat transfer in cooling towers is vital for maintaining optimum performance. Cooling tower process optimization is influenced by many factors, with one of the most important being selection of the proper fill. Two interrelated critical factors in fill choice are selection of fill that provides the highest heat transfer, but minimizes fouling. High-efficiency fill can be great for a while, but if it suffers from severe fouling the efficiency benefits will quickly disappear. This article examines cutting-edge fill designs, and how the quality of the water influences fill selection. For example, water that has high fouling potential might dictate the use of open fill, or perhaps even splash fill.

In other cases, a more compact fill might be utilized to maximize heat transfer.  

Cooling tower efficiency is, of course, important for existing cooling towers, but especially so for the new towers that are being constructed or are anticipated. In fact, Kiewit Power Engineers recently designed and constructed the equipment (two hyperbolic cooling towers) for

References

  1. Potter, M.C., Ph.D., and C.W. Somerton, Ph.D., Thermodynamics for Engineers, Schaum’s Outline Series, McGraw-Hill, 1993.
  2. Personal conversation with Ray Post of ChemTreat.
  3. Wallis, J. and R. Aull, “Improving Cooling Tower Performance with Thermal Fills”; Process Cooling & Equipment, March/April 2000.
  4. Buecker, B., Aull, R., P.E., and R. Post, P.E., “Chemical Treatment and Fill Selection Methods to Minimize Scaling/Fouling in Cooling Towers”; the 72nd Annual Meeting of the International Water Conference, November 13-17, 2011, Orlando, Fla.
the Brayton Point Power Plant to convert the “open-cycle” system to a “closed-cycle” system.

Some cooling tower heat transfer fundamentals
The figure below illustrates process conditions that could easily exist in a cooling system. We will calculate the mass flow rate of air needed to cool 150,000 gpm of tower inlet water to the desired temperature. We also will calculate the water lost by evaporation.

The first step is to determine the energy balance around the tower, where the blowdown is a negligible flow.[1]

(ma1*ha1) + (w3*hw3) = (ma2*ha2) + (mw4*hw4), where

ma = mass flow rate of dry air
ha = enthalpy of dry air streams
mw = mass flow rate of water streams
hw = enthalpy of water streams

Utilizing algebra, the fact that ma1 = ma2, and that a mass balance on the water flow is m4 = m3 – (W2 –W1)*ma, where W = humidity ratio; the energy balance equation can be rewritten in the following form.

ma = (m3*(h4 – h3))/((h1 – h2) + (W2 – W1)*h4)

The value for 3 has already been defined as 150,000 gpm. From a psychrometric chart and the steam tables, we find the following:

    h1 = 24.6 Btu/lbm
    h2 = 52.5 Btu/lbm
    h3 = 72.0 Btu/lbm
    h4 = 45.1 Btu/lbm
    W1 = 0.0075 lbs moisture per lb of dry air
    W2 = 0.0286 lbs moisture per lb of dry air

So, with an inlet cooling water flow rate of 150,000 gpm (1.251 million lb/min), the calculated air flow rate (a) is 1.248 million lb/min, which by chance in this case is very close to the cooling water mass flow rate. Obviously, the air flow requirement would change significantly depending on air temperature, inlet water temperature and flow rate, among other factors, and that is why cooling towers typically have multiple cells, often including fans that have adjustable speed control. The volumetric air flow rate can be found using the psychrometric chart, where inlet air at 68°F and 50 percent RH has a tabulated specific volume of 13.46 ft3/lb. Multiplying the mass flow rate by this conversion factor gives a volumetric air flow rate of nearly 17 million ft3/min. Cooling towers must move a great deal of air to perform properly.

The amount of water lost to evaporation can be simply calculated by a mass balance of water only. 


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