# torque required to stir liquid?

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i'm trying to calculate how much motor torque is required to stir a
liquid at a constant speed. liquid viscosity is 1.04. required speed is
30rpm. stirrer tool (impeller) needs to be approx. 200mm diameter. can
anyone help me with this calculation? thanks.
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Liquid viscosity is 1.04 ?[units]?
Rory
hob wrote:
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inadequate information. A stirrer could be a flat plate, or it could be a wire square. No viscosity unit.
Still, the torque is likely to be less than 5 lb-ft At 30 rpm we are talking pi X 0.2 m/rev X 0.5 rev/sec X 5 lb X 9.8 N/kg / 2.2 lb/kg = 7 newton.meters/sec = 7 watts.
Brian W
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thanks. the figure of 1.04 is the original gravity (not the viscosity as i previously said!). the stirrer will be a flat plate of total length 200mm, width 30mm.
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Answer depends on impeller design and baffle configuration. Impeller/stirrer manufacturer should be able to supply these data.
Alternatively, see Perry's Chemical Engineering Handbook (pages 19-6 through 19-8 in the 5th edition). Or see unit operations book by McCabe, Smith, and Harriot. Other ChE books should have this too.
What you will see is a plot of the dimensionless "power number" versus impeller Reynolds number. Calculate Reynolds number, look up power number for your specific impeller design, and then find power. Torque is power/angular velocity in appropriate units. (If power is in ft-lbf/s, and angular velocity is in radians/s, then torque will be in ft-lbf.)
Note that the conventional definition of Reynolds number for the stirrer is somewhat illogical.
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IMHO that is the kind of thing where an old engineer looks for a like product and then does a quick test - only students have to actually figure such things
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I would agree however even young (freshly minted grads) engineers cling to "we can calc it". They just love that false precision.
THey don't have the experience; one good test is worth a thousand expert opinions (or 100 pages of calcs)
:)
Bob
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One good source of information is a company that makes mixers and impellers. Try: