Two energy efficiency measures for constant air volume exhaust systems : using dampers and variable air frequency drives

The conventional constant air volume exhaust fan system is actually a variable air volume system. The fan airflow increases as the fume hood airflow decreases. Under partial fume hood exhaust airflow, the fan power is higher than the design fan power. Two energy efficiency measures are developed in this study to reduce the fan power of the conventional constant air volume exhaust system. In the first measure, a modulation damper is added in the main exhaust air duct and
a static pressure sensor at the inlet or outlet of the exhaust air fan. The fume hood static pressure is controlled by the modulation damper, while the fan inlet or outlet static pressure is controlled by the makeup air damper. This measure can potentially reduce exhaust fan power by up to 15%. The second measure consists of adding a variable frequency drive (VFD) to the fan motor and a static pressure sensor to the outlet of the fan. The fume hood static pressure is controlled by the makeup
air damper. The fan outlet static pressure is controlled by the VFD. This measure can potentially reduce fan power by up to 60%. The annual exhaust fan energy savings depends on the
exhaust usage patterns and the exhaust system duct layout.