Re = DhV / ( 720 v ) For standard air and temperature between 40 and 100°F, Re can be calculated by Eq. 3 Re = 8.50 Dhv Eq. 4 Dh= 4 A / P Where: ε=material absolute roughness factor, ft Dh= hydraulic diameter, in A = duct area, in2 P = preimeter of cross section, i...