Plant Sizing Duct Sizing  Page 1 2 3 4 5 6 7 8
Pressure Loss in Fittings
Zeta (z) factors
are to be used with the Velocity Pressure
to find fittings resistances.
Zeta (z) factors
are pressure loss factors.
These
allow for the resistance of fittings in ductwork systems which can be quite
significant compared to straight runs of duct.
Pressure
Loss (or resistance) (Pa) = zeta factor (z) x
Velocity Pressure (Pa)
and Velocity Pressure (Pa) V.P. = 0.5 x r x v^{2}
Where: V.P. = Velocity pressure (Pa)
r = Density of air
(1.2 kg/m^{3})
v = Actual air velocity (m/s)
Remember, for rectangular
ductwork, the velocity used to find Velocity Pressure should be the actual velocity
related to a rectangular duct with a Cross Sectional Area.
Examples
of Zeta Factors
1.
Zeta factor = 0.67
2. Rectangular Duct
Branch  read
table below.
Decide
which duct is to be sized, No2 or No.3.
Straight Thro 1 to 2 
To Branch 1 to 3 

Velocity pressure ratio VP_{2}/VP_{1} 
Zeta factor (z) 
Velocity
pressure ratio VP_{3}/VP_{1} 
Zeta factor (z) 
0.6 
0.44 
0.6 
1.60 
0.8 
0.09 
0.8 
0.78 
1.0 
0.04 
1.0 
0.55 
1.2 
0.02 
1.2 
0.45 
3.
Tapered Reduction
Read Table Below.
Included angle q 
Zeta factor 

Taper both sides 
Taper one side 

30^{o} 
0.02 
0.07 
45^{o} 
0.04 
0.20 
60^{o} 
0.07 
0.40 
Plant Sizing Duct Sizing  Page 1 2 3 4 5 6 7 8