Where does an air damper cause less resistance in a supply duct?

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An air damper in a supply duct causes less resistance to flow when positioned closer to the outlet rather than the fan, as the pressure drop (Δ P) is lowest where air density is highest. For a given flow rate, the irreversible pressure drop across any restriction is minimized downstream. To determine mass flow for a known volume and pressure drop, the appropriate pressure and relative humidity (RH) to use is typically the measurement taken after the restriction. This ensures accurate calculations based on the conditions affecting the flow. Understanding these principles is crucial for optimizing airflow in HVAC systems.
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Where in an air supply duct (after the fan) an air damper (or any other resistance to flow) will have less Δ P : closer to the fan, or closer to the outlet ?
 
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For any given flow rate and restriction in a flow stream, the irreversible drop in pressure across that restriction will be lowest where density is highest.
 
Thank you very much !
(all is clear now).
 
One little qwestion more please. For the known volume (for the known dP) to find the mass flow, what pressure (and RH) do we take : one of before the restriction, after, or a mean ?
 

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