High efficiency cleanroom air-conditioning: A decoupler-bypass air handling unit with variable outdoor airflow and its optimization for full-range weather and load conditions

To ensure strict indoor environment control, cleanrooms commonly employ constant temperature and humidity air-conditioning systems, which often cause energy waste due to unnecessary overcooling and subsequent reheating. The decoupler-bypass air handling unit system addresses this issue by decoupling the airflow treated by the cooling coil from the total supply airflow required for cleanliness. However, the lack of systematic design guidelines, control strategies, and practical validation has hindered its wider adoption in cleanrooms. This paper proposes an enhanced decoupler-bypass system with variable outdoor airflow and a systematic design and control optimization framework to achieve both robust environmental control and energy efficiency. Through coordinated optimization of outdoor airflow ratio and primary return airflow ratio, the proposed system utilizes free cooling and minimizes reheating, effectively eliminating thermodynamic energy waste and reaching a near-optimal operating state. A one-month field test in a pharmaceutical factory in Hong Kong verifies the practical operational performance and energy-saving capability of the proposed system. Using experimentally validated models, the energy-saving potential of the proposed system is systematically evaluated under full-range weather and load conditions, in comparison with a conventional system and a dedicated outdoor air system. Results show that the proposed system reduces the annual energy consumption by 59.9% compared with the conventional system, and by 25.7% compared with the dedicated outdoor air system.