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  1. Can airflow through an HVAC duct be considered inviscid?

  2. Does viscid flow ever occur for HVAC?

If the answer to (1) is yes, how do we know airflow in an HVAC pipe is inviscid? In general, what's a good method for determining whether a flow is viscid or inviscid?

Intuitively, I associate a viscous flow with something like honey. Airflow is definitely not like honey. But I'm wondering if there's a more quantitative or theory based approach to verify that airflow would be inviscid.

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    $\begingroup$ I would be looking at laminar or turbulent for airflow in a duct, and would be surprised if it was not turbulent... $\endgroup$
    – Solar Mike
    Commented Apr 3, 2022 at 20:05
  • $\begingroup$ air has viscosity ... it varies with temperature $\endgroup$
    – jsotola
    Commented Apr 3, 2022 at 22:58
  • $\begingroup$ this is not a term that is applied to HVAC systems. What are you trying to do or learn here? $\endgroup$
    – Tiger Guy
    Commented Apr 4, 2022 at 14:39

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Airflow in a duct is viscid. One of the important factors in designing ductwork for airconditioning systems is the cross-section and surface friction of the duct material.

Both of these two factors correlate directly with the pressure drop per lineal length of the duct and directly affect the required power of the system fan.

eg, cylindrical ducts are preferred over rectangular ducts because for the same cross-section area a cylinder has a smaller surface, thus less friction. Or galvanized surface has less friction with the airflow than a plastic surface.

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