DUCTWORK EQUATIONS

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 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