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Like in the coefficient of velocity, the rotation speed n,
in the diagram above left, has a great influence. A power
function, altogether presenting a positive correlation, can
be seen. The influence is weak in the area of approx.. n =
3,5 1/s and 5,5 1/s. For the inclination angle β there is initially
a linear positive correlation, which increasingly lessens.
Initially the influence of the filling level φ, pictures above
right is negative. This influence disappears from values
of approx. φ = 0,4 onwards. The bulk material parameters
that are presented in the lower line of picture 9 reveal the
following correlations. However it should be noted that only
certain combinations of the 3 parameters that describe the
bulk material, are possible. Therefore the influence of all
three parameters on the coefficient of power λ has to be
considered. Nevertheless, since the influence of the bulk
material should be simplified and described through these
parameters, they are reviewed individually in the following.
In case of the bulk density the influence initially increases,
but after a maximum of approx. ρ = 740 kg/m³ it sinks
again. Altogether the friction coefficient against the screw
pitch and tube has a positive influence, which initially is
very weak and becomes stronger from a value of approx.
µ S = 0,35. The correlation of the inner friction
Issue 04 | 2010
TRANSFER OF TECHNOLOGY
coefficient µ i is similar. The initially very weak negative
influence becomes clearly positive from a value of
µ i = 0,60.
After having analyzed the influences on the coefficient of
power, the last needed step is finding the simplest formula
of correlations, in order to simply and reliably calculate the
coefficient of power from the mentioned parameters.
Summary
Pic. 9:
Influence on the coefficient of power
The project to determine guidelines for dimensioning
and layout of highly inclined screw conveyors examines
the conveyance of bulk material in screw conveyors
that are operated with high speed and filling levels, in at
inclinations between 20° and 60°. The goal is to develop
simple, practical and safe methods to calculate the
achievable volume flow and the needed driving power.
While the predetermination of the kinetic behaviour and
therefore the possible volume flow is possible with analytical
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