The unit airflow is typically modulated in some way to match the server fan though a constant volume system
can still be employed.
has been awarded a contract to install new variable frequency drives (VFD's) to replace the constant volume system
at Chicago's main post office.
Unlike the constant volume system, there is only one requirement that must be satisfied to make this a successful pass.
For the constant volume systems, the recommendation for optimum air flow and minimum energy use was to use the minimum amount of required CRAC units (that minimum amount should include the redundant units).
One option for the air-distribution system is to convert dual-duct, multi-zone, or constant volume systems
to a variable volume system.
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.