Enhancement of heat transfer by an air/water spray in a transformer oil cooler

Water has been used for many years by the power industry to cool overheated transformers. Transformer oil coolers are compact finned-tube heat exchangers. However, the quantity of water applied to the oil coolers has not been optimized. This has motivated the present research to investigate the effects of water spray on a transformer cooler. The development of an experimental test facility consisting of a full-scale oil transformer cooler is described. An experimental program was carried out to evaluate the performance of a cooler under air-cooled conditions and deluged-water conditions. Significant performance enhancement was found by spraying relatively small amounts of water onto the heat exchanger. Tests were performed for various air velocities, oil flow rates, and water spray now rates. From the air-cooled data, individual airside heat transfer coefficients were "backed-out" for each row of tubes. A model was then developed to predict the local air/water-cooled performance that incorporates the heat and-mass-transfer analogy to determine the water evaporation rates. For this "wet" model, the water distribution is a required input. The inlet (top) oil temperatures thus predicted are in good agreement with the experimental values.