**Pressure loss** – The pressure loss constitutes the pressure differential generated between two defined points through wall friction and internal friction in static mixers, pipes, moulded parts, fittings, etc. With static mixers, these points are the intake and outlet of the mixer. In engineering, a resistance coefficient ζ is used for elements installed in a pipe (mixing elements, valves, baffles, etc.) which can be taken from tables. The pressure loss generated by the wall friction is determined by the friction factor for pipes λ. In case of laminar flow the friction factor for pipes depends on the Reynolds number. The surface roughness plays a role especially with turbulent flows.

The theory

The equation for pressure losses in passed through flows with the prerequisite of a constant density is:

This is the Bernoulli’s energy equation, whereby the term of the static height is not taken into account because it does not represent a pressure loss.

Design criteria ρ density in kg/m3

u medium flow velocity in m/s

λ friction factor for pipes

l length of the tubing in meters

d diameter of the tubing in meters

ζ resistance coefficient