scope:

INTRODUCTION

Many constant air volume (CAV) systems were installed in the 1950s and 1960s. Unfortunately, many of these systems are still in use due to the high cost of converting them to VAV systems. Constant air volume systems, like VAV systems, are often oversized and incur a significant energy penalty due to their incapability of modulating airflow. Zone loads are often much lower than the design load due to partial occupancy and maintain cooler air temperatures than design weather conditions. Since a constant supply air temperature is required for humidity control, more energy may be used during moderate weather conditions. CAV systems consume more fan power, more heating energy, and more cooling energy. The conventional method of reducing energy consumption is to install variable frequency drives (VFDs) on the supply and return air fans and convert the terminal box. Retrofitting the existing terminal box requires significant investment and interrupts the building's normal occupancy pattern.

Liu and Claridge (1999) presented a method to convert dual-duct constant volume systems without having to retrofit the terminal boxes to a VAV system. A damper is installed in the main hot duct. During summer, the hot air is shut off. This method reduces both fan and thermal energy. For single-duct constant volume systems, Liu et al. (2002) suggested installing VFDs on both supply and return air fans to reduce airflow at night and on weekends. This practice can be extended to normal operating hours with use of a proper control sequence for the CAV system.

The objective of this study is to develop a method for VFD control through a case implementation. This paper presents facility information, procedures for supply fan speed control, and the results of the case study application.