Quote:
Originally Posted by mrd
Air speed at ducts affects temperature stratification and air distribution noise levels. Typically, your ductwork is sized to provide a certain exit velocity for a specific flow rate. Changing the fan speed throws off duct design, unless your ductwork contains active components to maintain exit velocities (very unlikely.)
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Although, I might add, as the delta T of the tempering air to distributed air is reduced, meeting stratification expectations requires lower exit velocities. This equates to lower flow rates, which improves efficiency due to lower friction losses, as you mentioned earlier.
As such, the fan speed could be reduced with lower delta T to even further improve efficiency. Certainly there are algorithms to optimize all of these factors together to maximize efficiency while meeting the design requirements. It just requires all the components to work together, flexibly, and be designed properly.