Optimum duct design for variable air volume systems

Scope and method of study. Current duct design methods for variable air volume (VAV) systems are based on the use of peak constant airflow. However, VAV systems operate much of the time at an off-peak load condition and the impact of varying airflow rates to the sizing of duct systems has not been considered. This study introduces an optimum duct design procedure for VAV systems to investigate the importance of the varying airflows to the system design. Hourly airflow requirements, part-load fan characteristics, and duct static pressure control are incorporated into the problem formulation. Constraints such as discrete duct sizes and velocity limitation are incorporated into the duct design procedure. First, the domain of a VAV optimization problem is analyzed to define the problem characteristics and to suggest an optimization procedure. Second, the VAV optimization procedure is applied to several VAV duct systems with different parameter values. The results are analyzed to compare duct design methods and the effect of several factors that influence optimal design are investigated.; Findings and conclusions. The VAV optimization procedure including the Nelder and Mead downhill simplex method with a penalty approach was successfully applied to find the global optimum discrete solution. The VAV optimization procedure yields significant life cycle cost savings compared with the equal friction and static regain methods. However, compared with the T-method, the life cycle cost savings was not as great. This is due to duct size constraints, static pressure requirement at the end of the longest duct line, and relatively flat life cycle cost domain. This indicates that the T-method can be used favorably even in VAV system optimization. The T-method has great potential to save costs over the non-optimization-based methods, without the input data and computation time requirements of the VAV optimization method. Therefore, it is recommended that the T-method be utilized for duct design in VAV systems.