Flue Gas Velocity in Duct

Volume flow would not account for the density; mass flow would account for it.

The eqetions you have referred is generally used for liquid.

v = Sq. rt of ((2gh)/ density) is a general equetion for any liquid, but when used for water density = 1.

There's some confusion here.

Firstly, what do you mean by h? The formula applies when h is a head difference across some element, and usually v is lower than Sq. rt of (2gh) as the coefficient of discharge is < 1. E.g. for an orifice plate under most conditions CD = 0.62.

If h is in m fluid, v is in m/s. If you want the mass velocity kg/(m²*s) you need to multiply v by the density, not divide.

But you're dealing with gas flow, and you wouldn't (normally) have pressure drop in m fluid. It would be in pressure units, Pa (= N/m²). In that case the formula is

v (m/s) = CD*√(2*ΔP/ρ) where ρ = density kg/m³ about 1.2 for air.

or mass velocity v_m (kg/(m²*s)) = CD*√(2*ΔP*ρ)

Cheers........Codey