Air conditioning and ventilation to xxx is supplied by variable air volume (VAV) fan system xxx. Variable inlet vanes (VIVs) on the supply and return fans modulate to maintain the required duct pressure and air flow. The VIVs throttle air flow by creating flow resistance. Under these conditions, which occur during partial system load, the fans operate inefficiently.
We recommend replacing the variable inlet vanes and fan motor with variable frequency motor drives (VFDs) and inverter-duty motors on both the supply and return fans. The duct pressure and air flow will then be maintained by automatic adjustment of the fan motor speeds. The fan motors will draw less power because the fans will have less flow resistance to overcome.
<The new VFDs are electronic components, and although they are designed for continuous operation, they will occassionally fail. Since this fan system serves a critical area that must have cooling available at all times, we recommend designing the new system with manual bypasses. The bypasses will allow either fan to operate at full speed by diverting power directly to the fan motor if the VFD fails. To prevent the fans from over-pressurizing and damaging the ductwork, the VIVs must be left in place to limit duct pressure when the new system is in bypass mode. Under normal operation, the VIVs would remain fully open.>