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 v2

Where:            V.P.      =          Velocity pressure (Pa)

r        =          Density of air (1.2 kg/m3)

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. Bend – mitred and radiused on both inside and outside.

 

 

 

 

 


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

VP2/VP1

Zeta factor

(z)

Velocity pressure ratio

VP3/VP1

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

30o

0.02

0.07

45o

0.04

0.20

60o

0.07

0.40

 

 

 

 

 

 

 

 

 

 


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