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Anonymous Poster #1

### How to Calculate Water Velocity

05/06/2012 2:32 PM

hi....

i have 10350 liter water tank (4.156 m x 2.336 m x 1.066m ) with a 16' 5 blades propeller fan at 1 end that runs by a 3 phase 7.5hp motor (1500rpm) how do i calculate the velocity of the water?

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#1

### Re: how to calculate water velocity

05/06/2012 3:05 PM

AFAIK You cannot calculate anything from the information you have provided. I find it very odd that you have a five blade, sixteen foot long propeller fan but your tank is dimensioned in metric units. Also the longest dimension of this tank is less than the length of one of these propeller blades.

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#2

### Re: how to calculate water velocity

05/06/2012 3:20 PM

Giving you the benefit of the doubt on the fan diameter and your units soup, do you really expect us to believe that the fan is turning at 1,500 RPM's?

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#4

### Re: how to calculate water velocity

05/06/2012 3:53 PM

the 1500 is the maximum motor speed without load, the fan speed is around 250 rpm

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#8

### Re: how to calculate water velocity

05/06/2012 4:14 PM

What is the geometry and diameter of the impeller? (You should not call it a fan because fans apply to air movement).

If you have the diameter of the impeller, the best you can do is obtain an approximation of a maximum velocity which will happen at the impeller blade tips.

It would help if you would tell us WHY you are trying to find what the velocity would be.

Also:

• What is the diameter and height of the tank?
• Does the tank have baffles?
• Are you trying to mix something into the water?
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#10

### Re: how to calculate water velocity

05/06/2012 4:34 PM

Maybe he's going for his PhD.

What's the impeller tip speed at 250 RPM over 16'?

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#11

### Re: how to calculate water velocity

05/06/2012 4:38 PM

Maybe it's a water tunnel, as opposed to a wind tunnel.

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#3

### Re: how to calculate water velocity

05/06/2012 3:28 PM

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#5

### Re: how to calculate water velocity

05/06/2012 3:54 PM

After it ejects 10.35 m3 of water

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#13

### Re: how to calculate water velocity

05/06/2012 5:07 PM

Weird that Wikipedia would misspell Old Faithful with an extra L.

--Ed. C.

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#14

### Re: how to calculate water velocity

05/06/2012 7:38 PM

Native American spelling?

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#6

### Re: how to calculate water velocity

05/06/2012 4:04 PM

The water velocity is zero; a fan moves air.

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#7

### Re: how to calculate water velocity

05/06/2012 4:07 PM

Give the guy a break. Water is composed of two gasses. Maybe this is fuzzy physics.

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#12

### Re: how to calculate water velocity

05/06/2012 4:58 PM

I think you have the correct number but for the wrong reason. As long as the water remains in the tank, the velocity of the water will be zero. The fan and tank dimensions are just red herrings. There is nothing to calculate.

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#9

### Re: how to calculate water velocity

05/06/2012 4:30 PM

If it is actually a propeller (or impeller), we need to know its theoretical pitch and efficiency in order to calculate the water velocity.

So-called "square" propellers have a pitch equal to their diameter, and their efficiencies could be around 0.65. This information can enable at least a rough estimate of water velocity. I suspect that the 16' was actually intended to be 16", but I don't know.

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#15

### Re: How to Calculate Water Velocity

05/06/2012 8:36 PM

Lay a few yardsticks end to end, drop die at prop, use stopwatch, time die movement 1 second, take picture at 1 second, measure for velocity ft per sec....

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#19

### Re: How to Calculate Water Velocity

05/06/2012 10:12 PM

or you could just buy one of these....

http://www.globalw.com/products/flowprobe.html

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#16

### Re: How to Calculate Water Velocity

05/06/2012 9:21 PM

the fan diameter is 16 inch, and the 1500 is d motor full speed without load not the fan speed, the fan speed is 250prm

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#18

### Re: How to Calculate Water Velocity

05/06/2012 9:40 PM

So, are you in Purchasing, or Accounting?

Relative of the owner?

You still haven't defined the problem, in any relevant terms.

I have to take a nap.

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#17

### Re: How to Calculate Water Velocity

05/06/2012 9:38 PM

v = pnη

(Exercise for reader to match up equation terms w/ verbal terms in earlier post.)

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#20

### Re: How to Calculate Water Velocity

05/08/2012 7:45 AM

what i understand what you meant in your statement is that the tank is rectangular and on one side there's a fan inside the tank coupled to the driver motor outside having an rpm of 1,500. And your asking what could be the velocity of the fan.

well, if my description is correct, the fan is just for agitation not for pumping water. And for the fan rpm, it is very impossible to directly couple the fan to the motor since the result would damage the motor because it needs more torque (unless for air). the fan could be indirectly driven by a belt and pulley, chain and sprocket or a gearbox (reducing gear). so for the velocity of the water after it hits the fan blade, then there are things you have to consider. The angle of the blades, the speed of the blade, etc.

but if the water is flowing to a pipe, then you can calculate V = Q/A, where Q is the volume flow rate and A for cross sectional area A=(pi x pipe dia (squared))/4. to get Q just fill any container and use stop watch to time how long it fills the container and calculate the volume of the container then you have the flow rate. then you can get the velocity of the water passing through the pipe.