dp = 7.57 q1.85 L 104 / (d5 p)
where
q = air volume flow at atmospheric conditions(m3/min)
dp = pressure drop (kg/cm2)
d = inside diameter of pipe (mm)
p = initial abs. pressure (kg/cm2)
L = length of pipe (m)
1 kg/cm2 = 98068 Pa = 0.97 atmosphere = 0.98 bar =736 mm Hg = 10 m H2O = 10000 mm H2O= 2050 psf = 14.2 psi = 29 in Hg = 394 in H2O = 32.8 ft H2O