- posted
17 years ago

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.

- posted
17 years ago

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.

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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- posted
17 years ago

Liquid viscosity is 1.04 ?[units]?

Rory

hob wrote:

Rory

hob wrote:

- posted
17 years ago

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

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

- posted
17 years ago

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.

- posted
17 years ago

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:

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.- posted
17 years ago

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

- posted
17 years ago

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

THey don't have the experience; one good test is worth a thousand expert opinions (or 100 pages of calcs)

:)

Bob

- posted
17 years ago

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.PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.