Anti-freezing prediction model for different flow patterns of air-cooled heat exchanger

Abstract

Water heat load may get lower than the huge cooling capacity of air for air-cooled heat exchanger in winter, resulting in the freezing risk of finned tube bundles, so the water temperature distribution inside the tube under various working conditions stands the premise issue for the security and energy efficiency of dry cooling system. Based on the energy balance and heat transfer analyses, the anti-freezing prediction model of air-cooled heat exchanger is developed and experimentally validated at different low ambient temperatures with the maximum deviation of 16%. Taking a six-row plate finned tube heat exchanger as the research object, the water temperature distribution in any tube and the critical anti-freezing parameters are obtained with different air/water flow patterns at various ambient temperatures, air face velocities, inlet water temperatures, and water flow rates. The results show that the finned tube bundle with slotted fins is superior to the flat fins in heat transfer performance, but faces more serious freezing risks in cold winter. In the baseline case, the average outlet water temperature of the slotted finned tube bundle decreases by 3.9 °C compared to the flat finned tube bundle. For the air/water cross-counter flow pattern, the critical freezing position is fixed and appears at the water exit of upwind row. While for the air/water cross-cocurrent flow, the minimum water temperature varies with operating conditions, but it always happens at the row 1 or row 4. The anti-freezing of air-cooled heat exchanger can be realized by reasonably matching the air/water heat capacity as well as transforming the air/water flow patterns. The proposed anti-freezing prediction model is of potential benefit to the real-time temperature distribution forecast, freezing analyses, and control strategies for air-cooled heat exchanger in winter.