Ductwork Equations

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 area 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 Dimension (Inches)
   DS = Minor Axis Dimension (Inches)
   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

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