Pressure Drop

Three criteria are needed to determine the pressure drop across a static mixer.

• Reynolds Number: Re
• Pressure drop in an empty pipe of the same length as the mixer elements.
• A flow coefficient, or friction factor correction factor for the static mixer.

Reynolds number can be calculated; see Reynolds # section.

Pressure Drop in an Empty Pipe

For calculation of the pressure drop in an empty pipe, use the standard Darcy equation and the Darcy friction factor.



The equation for the pressure drop is as follows:

An estimation of the friction factor can be made from the equations below:

Where e is the surface roughness and D is the pipe diameter in consistent units.

If this equation were to be used, the recommended value for e would be 0.0457 mm (0.0018 inch).

The flow coefficient is basically a correction factor to compensate for the increased pressure loss caused by the mixer elements compared with that of an empty pipe.

The coefficient is given as separate equations for flow regimes.

The following shows the relevant equations for both 1.5:1 and 1:1 pitch elements.



Note that the Pitch is shown as the element length compared with the ID of the pipe. A 1" element, for example, would have a 1.5:1 Pitch as standard and an ID of 1.049". The element length should therefore be 1.5735". Also, the equations are based on elements of standard thickness. Changes in either the pitch or the element width from such things as coatings will affect the flow coefficient and therefore the pressure drop.

HENCE,



Static Mixer pressure drop can be calculated by multiplying the standard pipe pressure drop with the flow coefficient.