Calculating Litres Per Minute to achieve a Reynolds Number of 4000 for flushing

[stuckoutthere]

Good Evening,

I am trying to work out the litres per minute required to flush a length of 10mm O/D tubing with 1.5mm Wall Thickness. I know I need to achieve a reynolds number of 4000 to flush it.

I calculated it as below but I think it may be wrong:

1. Find out fluid kinematic viscosity at 20 degrees Celsius in centistokes for the flushing fluid you will be using.

2. Convert from centistokes to m2/s by multiplying by x10^-6

3. Find tube internal diameter in inches or metric

4. Divide bore diameter in inches by 39.370 to give you metres, If in mm then convert to metres by dividing by 1000

5. Find tube cross sectional area by multiplying pi by the bore diameter squared divided by 4

6. Find the flow rate in m^3/s by multiplying the tube cross sectional area by 4000(Reynolds number required for turbulent flow)by the fluid kinematic viscosity in m^2/s then divide it by the bore diameter.

7. Find the flow rate in L/Min by multiplying the flow rate in m^3/s by 1000 then by 60

8. This will give you required flow rate in l/min to achieve a Reynolds number of 4000 at 20 degrees Celsius(fluid temperature)

See Calculations Below: (Example)
1. Fluid Kinematic Viscosity at 20 degrees is 4.4cSt

2. 4.4 x10^-6 = 0.0000044m^2/s

3. Tube internal diameter is 0.125”

4. 0.125/39.370 = 0.003175m

5. 3.14x((0.003173^2)/4)=7.91329x10^-6m^2

6. 7.91329x10^-6((4000x0.0000044)/0.003175)=4.38658x10^-5

7. (4.38658x10^-5*1000)x60 = 2.6L/Min@20Degrees Celsius.

Is this correct or am I completely wrong. If this is wrong can someone point me in the correct direction.

Thanks

 

[IRstuff]

That matches the Wikipedia entry for the calculation:

http://en.wikipedia.org/wiki/Reynolds_number#Flow_in_pipe

and the value is the same as what I get when I type everything into Mathcad:

2.6333 L/min

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[katmar]

I agree with your and IRstuff's calculated flowrate, but you should also be aware that this gives a velocity of around 5.5 m/s which will give you a friction pressure drop of about 2 bar per metre of tube length in such small bore tubing.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[ione]

I got a different result. Mine is 0.0352 m3/hr (0.587 l/min) corresponding to a velocity of 1.23 m/s, which in turn gives a pressure drop of approx 0.13 bar per metre of tube length.

 

[katmar]

ione, the flow of 2.633 litre/min is 4.4 times as big as yours. Is it a coincidence that this is the same as the viscosity value given? I have attached my results.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[ione]

Hello Harvey,

It's definitely my fault as for kinematic viscosity I've entered the value corresponding to water instead of the one reported in the OP.

 

[stuckoutthere]

Thanks for all the replies. At least i know i have the calculation correct and i can now go on and calculate for the various flushing fluids and tube sizes i use.

I hadnt taken into account pressure drop as the runs i am involved in are at the most 4-5m in length.

Does anyone know off any software that would calculate flushing rates for various fluids and pressure drop because at the moment i am doing manual calculations and inputting it all on an excel spreadsheet for quick reference.

 

[MikeHalloran]

At least learn how to make Excel do the math for you.

Mike Halloran
Pembroke Pines, FL, USA

 

[stuckoutthere]

I have mike but i am not very trusting and wanted to check the initial calculation was correct.

 

[badservo]

stuckoutthere,
"10mm O/D tubing with 1.5mm Wall Thickness" does not equal "internal diameter is 0.125”"

 

[stuckoutthere]

Yes i know that was a typo mistake from a previous calculation i was doing for tube in inches and what i wrote the example from.