A_c = a x b

O = 2 x (a + b)

d_h = 4 x A_c/O = 2 x a x b/ (a + b)

d_e = 2 × b × (π^2-n × (1 + a/b)¹⁺ⁿ/(a/b)3)^1/(n-5)

where n = 1/(1,05 × log (Re) -0,45)
where Re = Reynolds number for air at 20ºC

Rectangular air duct system

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