Experimental Analysis of the Effects of Particulate Fouling on Heat Exchanger Heat Transfer and Air-Side Pressure Drop for a Hybrid Dry Cooler

Abstract

It is well known that significant fouling by particulate matter can have a deleterious effect on the performance of enhanced surface heat exchangers, and the same is true for hybrid heat exchangers. Hybrid heat exchangers are heat exchangers that are typically run in dry mode to reject heat. When the ambient conditions require more heat rejection than can be provided by sensible heat transfer, a water pump is turned on and water flows over the fins, and the evaporation of water provides a further cooling effect. Fouling in dry-mode operation is physically similar to that of air-cooled heat exchangers, but in evaporative mode the flow of the water over the coil eliminates the impact of fouling. A hybrid dry cooler heat exchanger of 60 cm × 60 cm frontal area has been installed in a well-instrumented wind tunnel to measure the heat exchanger's performance. Hot water flows through the coil to provide the load, and air flows over the coil to provide cooling. During evaporative mode operation another stream of water flows over the outside face of the coil, adhering mainly to the louvered fins. The louvered fins are specially designed for optimized water flow during wetting mode. The fins are made of aluminum, the tubes are copper, and protection against corrosion is realized by a special E-coating. This coil has been tested clean and fouled with ASHRAE standard dust, for both dry and wet operation. Results are presented for the air-side pressure drop and overall heat transfer conductance of the coil under all conditions for which 50% increases in air-side pressure drop are found under heavy fouling. The influence of fouling on heat transfer is small. Also, using the wetting water to wash the fouling off the coil is investigated and is found to be of some limited utility.