torque required to stir liquid?

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.
Reply to
hob
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Liquid viscosity is 1.04 ?[units]?
Rory
hob wrote:
Reply to
Rory Johnson
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
Reply to
Brian Whatcott
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.
Reply to
hob
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.)
See also:
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Note that the conventional definition of Reynolds number for the stirrer is somewhat illogical.
Reply to
Olin Perry Norton
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
Reply to
Hobdbcgv
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
Reply to
Bob K 207
One good source of information is a company that makes mixers and impellers. Try:
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They (or any company in this business) should be able to supply information on the torque and power required by their products.
Reply to
Olin Perry Norton

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