Velocity in Duct
Velocity in duct can be expressed as
V = Q / A = 144 x Q / a x b
Where
• V = air velocity in ft per minute (FPM)
• Q = air flow through duct in cubic ft per minute (CFM)
• A = cross-section of duct in sq-ft
For rectangular ducts
• a = Width of duct side (inches)
• b = Height of other duct side (inches)
Equivalent Round Duct Size for a Rectangular Duct
Equivalent round duct size for a rectangular duct can be expressed as
Deq = 1.3 x (a x b)0.625 / (a +b)0.25
Where
• Deq = equivalent diameter
• a = one dimension of rectangular duct (inches )
• b = adjacent side of rectangular duct (inches)
Equations for Flat Oval Ductwork
P= ((3.14XDS) + 2 x (DL – DS)) / 12
Deq = (1.55 x A0.625)/p0.625
A=((((DL –DS) x DS) + (3.14 x DS2)/4))/144
Where
• DL = Major Axis Dimens ion (Inches)
• DS = Minor Axis Dimension (Inc hes)
• A = Cross-Sectional Area (Sq-ft)
• P = Perimeter or Surface Area (Sq-ft per linear feet)
• Deq = Equivalent Round Duct Diameter
Duct Air Pressure Equations
TP = SP + VP
Where
• TP = Total Pressure
• SP = Static Pressure, friction losses
• VP = Velocity Pressure, dynamic losses
Velocity Pressure
VP = (V / 4005)2
Where
• VP = Velocity pressure
• V = Air velocity in FPM
