Educ. Reso. for Part. Techn. 014Q-Rhodes
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Copyright © 2001 Martin Rhodes, Licensed to ERPT
Pneumatic Transport of Powders, by Martin Rhodes
-- 1: Dilute phase and dense phase transport --
In this section we will look at the distinguishing characteristics of dense
and dilute phase transport and the types of equipment and systems used with each.
The design of dilute phase systems is dealt with in detail and the approach
to design of dense phase systems is summarised.
The pneumatic transport of particulate solids is broadly classified
into two flow regimes: dilute (or lean) phase and dense phase.
Dilute phase transport in its most recognisable form is characterised
by high gas velocities (greater than 20 m/s), low solids concentrations
(less than 1% by volume) and low pressure drops per unit length of transport line
(typically less than 5 mbar/m). Dilute phase pneumatic transport is limited
to short route, continuous transport of solids at rates of less than 10 tonnes/hour
and is the only system capable of operation under negative pressure.
Under these dilute flow conditions the solid particles behave as individuals,
fully suspended in the gas, and fluid-particle forces dominate.
Fig. 1: Dilute phase transport
(taken from the video -- flow is left to right)
Video 1 [5 min to download at 28.8kbaud]:
Dilute Phase Transport: Horizontal Flow above the Saltation Velocity
At the opposite end of the scale is dense phase flow, characterised
by low gas velocities (1-5 m/s), high solids concentrations (greater than 30% by volume)
and high pressure drops per unit length of pipe (typically greater than 20 mbar/m).
In dense phase transport particles are not fully suspended
and there is much interaction between the particles.
Fig. 2: Sequence showing an example of horizontal dense-phase transport
(sequence from video 3 -- flow is right to left)
Video 2 [3 min to download at 28.8kbaud]:
Dense Phase Transport: Slugging in Vertical Flow
Video 3 [4 min to download at 28.8kbaud]:
Dense Phase Transport: Slugging in Horizontal Flow
Video 4 [11 min to download at 28.8kbaud]:
Dense Phase Transport: Dune Movement in Horizontal Flow
Unfortunately, the boundary between dilute phase flow and dense phase flow
is not clear cut, and there are as yet no universally accepted definitions
of dense phase and dilute phase transport.
Konrad (1986) lists four alternative means of distinguishing dense phase flow
from dilute phase flow:
a) on the basis of solids/air mass flowrates
b) on the basis of solids concentration
c) dense phase flow exists where the solids completely fill
the cross section of the pipe at some point.
d) Dense phase flow exists when, for horizontal flow, the gas velocity
is insufficient to support all particles in suspension, and,
for vertical flow, where reverse flow of solids occurs.
In all these cases different authors claim different values
and apply different interpretations. In this chapter
the "choking" velocity will be used to mark the boundary
between dilute phase transport and dense phase transport in vertical pipelines
and the "saltation" velocity will be used to mark the boundary
between dilute phase transport and dense phase transport in horizontal pipelines.
These terms are defined below through relationships involving gas velocity,
solids mass flowrate and pressure drop per unit length of transport line
in both horizontal and vertical transport.