Dave Dickey Forum Moderator 324 Posts
Re: How do I go about scaling using a single impeller?16 July 2014 at 8:32am
With geometric similarity, equal blend time should be achieved at equal mixer speed. Since the scale change has retained the geometric ratio of impeller diameter to tank diameter, the only remaining adjustment is the rotational speed of the small-scale mixer. Going from two impellers to one impeller probably means that the best course of action is to increase the speed of the single impeller mixer to get double the power. To double the power means that for turbulent conditions the speed needs to be increased by a factor of 2 raised to the 1/3 power, or 1.26 times. That means that for turbulent conditions the small-scale speeds need to be 700 x 1.26 = 882 rpm or 1192 x 1.26 = 1502 rpm. These calculations assume that turbulent conditions exist which should be true for a water like fluid.
All of these calculations are theoretical. Actual blend times will be determined as much by the flow patterns in the vessels and the point (and/or) rate of injection of the added component. Chemical reactions that are a function of rapid blend times can be strongly influenced by the local turbulence intensity, which is local power per volume dependent. Local power per volume is proportional to total power per volume as long as the flow patterns are similar. Maintaining equal power is about the best that can be done for equivalent blending.
Tests run in such small-scale equipment will always blend very rapidly and the effects of blend time may be difficult to identify.
I am trying to do a scale down of a mixing step. This is my first time doing so. After scaling based on mixing time mixing tank diameter is 39" scale down is 12.5". Impeller diameter is 6.3 scaled down is 2". The mix speed is 700-1192 scaled down is 972 -1585. The issue I now have is that large scale uses two mixers and I cannot do that on smaller scale. How do I go about scaling using a single impeller?
Have an insight or suggestion?
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