Dave Dickey Forum Moderator 295 Posts
Re: When is it appropriate to use a recirculation loop?17 July 2009 at 1:29pm
The simple answer to your question is that a recirculation loop is never a good substitute for a well-designed mixer, for at least two major reasons:
To accomplish the same volumetric pumping of a mixer, the pump would require about 10 times the power of the mixer.
Return flow from the recirculation loop creates an unpredictable and often short-circuit path through the vessel, therefore not providing good bulk uniformity in the vessel.
As a simple guide to the use of a recirculation loop, the loop should be designed for a specific purpose other than bulk mixing.
The loop may withdraw material from a poorly mixed region at the bottom of the tank, as from a stagnant region in the drain valve or from a pile of settled solids.
The loop may provide additional heat transfer through an external heat exchanger.
The loop may pass material through an inline, high-shear disperser to eliminate lumps.
The loop may pass through a static mixer for rapid addition of a reactive chemical.
The loop is essential for effective processing.
Other than a clear indication that effective mixing is not present in the vessel with the existing agitator, I cannot think of a good reason for using a recirculation loop as a substitute for an agitator.
It sounds as if you should attend a course on basic mixing.
How does the mixing from recirculation loop from the bottom of the vessel compare to that provided by the agitator? I am guessing this will depend on various parameters like agitator rpm, agitator impeller types, recirculation flow rate, recirculation discharge height, where is recirculation discharged from, volume to be mixed.
Have an insight or suggestion?
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