This paper presents a practical case study using the computational fluid dynamics (CFD) technique in retrofitting the large cabinet fans and airflow channels in malt processing air-conditioning systems in order to get uniform airflow field, to increase fan capacity and reduce energy consumption. Site measurements were used to validate the models before they were used. CFD simulation results were used to evaluate and optimise the design and construction of splitter vanes in the airflow channels and scrolls of the fans. Simulation tests showed that the fan capacity (maximum total air flow rate delivered at full fan speed) could be increased by 8.3-20% using different shapes of scroll, and 30-34% by adding splitter vanes further in the airflow channels. After retrofitting the fans and airflow channels, site measurements showed that the fan capacity increased 21-24 and 28-29%, respectively in the two buildings validated. The approach of using the CFD technique in the retrofitting, the simulation results, and the site measurements before and after fan retrofitting are presented and compared in this paper.