Air cooling is an option, especially for the process industry, installed in locations where water availability is constrained and it may have significant economic advantage over conventional water cooling.
Employing air cooling implies using air cooler exchangers instead of the usual shell and tube heat exchanger. Air coolers can be either induced or forced draft, the latter being one of the most common arrangements. It consists of several heat transfer sections of finned tubes, where hot fluid passes, mounted on a frame. A fan, located either above or below the tube section induces or forces air through it.
The heat transfer coefficient of an air cooler is relatively smaller than the average water cooled exchanger. To obtain the same heat load with lower heat transfer coefficients large transfer surfaces are demanded and since the pressure drop through the finned tube sections has to be small, large areas for installation are required. Performance of air cooling is also greatly dependent on ambient temperature. At operating temperatures higher than design conditions, cooling efficiency drops, while for very cold climates, air temperature passing the exchanger has to be controlled, to prevent the temperature of the fluid being cooled, from falling below freezing or pouring point. Air flow control can be obtained by changing fan pitch. For high temperature ambient conditions, humidification can help, while for low temperature, recirculation of air can be used, depending on design.
Considering all these points, air coolers capital cost can be twice the cost of water coolers for the same service. But this option may have a much lower environmental impact, considering overall capital and operational costs, by the avoidance of water use and disposal. It may be an interesting and feasible alternative for once-through water cooling systems. As a matter of fact, cooling air may not be considered a utility itself, but an option to reduce or avoid usage of water.
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