Shorter Communications = 28*12(V,)Ai -cms/sec when Pa is expressed in ems.
(3) Consider now fluidised particles of average diameter 80 p and assume that the minimum bubble size the bed could sustain is 050 cm dia. From (3) above the flow rate to produce these minimum sized bubbles is 2.9 cmYsec. The minimum fluidisation velocity for such particles with atmospheric air is about O-3 cm/set or about 15 cma/sec in an 8 cm dia. tube. Thus, in this example, a 20 per cent increase in gas flow rate is necessary to initiate bubbling and this is the kind of increase found necessary with particles of this size.
There are many crude assumptions in the above calculation but it illustrates that a minimum possible bubble size means that a limited increase in gas flow will not necessarily produce bubbles and particles separation must occur to reduce the drag coefficient[7] or particles would be lifted from the bed. Since observable uniform particle separation does not occur with larger particles, it is concluded that the minimum bubble size increases at a slower rate than the minimum fluidisation velocity does so that Gti becomes a smaller proportion of the minimum fluidisation flow rate as the particle size increases.