Investigation on the control mechanism of spray pre-cooling the inlet air of natural draft dry cooling tower

Abstract

• A 3-D model of spray pre-cooled NDDCT was developed. • The control mechanism of nozzle spray was investigated from three aspects. • The highest modified cooling efficiency of 73.2% occurs in spray scheme 18. • The heat rejection rate of the NDDCT increases from 123.67 MW to 176.08 MW. • The regional regulation principle of spray water flowrate is proposed. Spray pre-cooling the inlet air can improve the thermal performance of natural draft dry cooling towers (NDDCTs). To achieve the purpose of good performance improvement with minor water consumption, it is necessary to investigate the control mechanism of spray pre-cooling system for NDDCT performance improvement. This paper developed a 3-D model of spray pre-cooled NDDCT using Fluent 16.2. The NDDCT was 120 m high and 1/12 NDDCT was simulated due to the symmetry. The control mechanism of nozzle spray was investigated from three aspects, i.e., the spray angles, arrangement of nozzles, and spray water flowrates. Simulation finds that: (1) To achieve the goal of maximal cooling performance improvement with benchmark spray water flowrate, the recommended scheme is 18, which consists of 1 upward nozzle at height of 3 m, 3 co-flow nozzles at height of 5 m, 3 nozzles with the spray angle of 45° at height of 5 m, 3 upward nozzles at height of 8 m, 3 nozzles with the spray angle of 45° at height of 11 m and 3 nozzles with the spray angle of 45° at height of 13 m. The heat rejection rate of the whole cooling tower increases from 123.67 MW to 176.08 MW because the natural convection is enhanced; (2) The highest modified cooling efficiency at the heat exchanger surface is 65.0 % when the spray water flowrate reduced to 75 % of benchmark spray water flowrate while it is 47.1 % with 50 % of benchmark spray water flowrate; (3) To achieve more uniform cooling, it is suggested that the nozzles should be arranged at the tower inner region in low-height zone while the nozzles should be arranged at the tower outer region in high-height zone. Besides, large spray water flowrate at high-height zone is recommended to achieve maximal cooling effect with low water consumption.