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Old 13th January, 2002, 10:22 PM
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Bends

Any one know a good resource for fluid dynamics info, I am trying to calculate the best routs for tubing around the case bends and the like, is it better to have a long sweeping curve or a short 90 degree bend, and where is the sweet spot for 10mm inside diameter tubing in terms of radius.
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Old 13th January, 2002, 11:22 PM
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long sweeping bends will cause less backpressure than short 90 degree ones, as for the optimum radius for 10mm hose, I have no idea :P
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Old 14th January, 2002, 12:17 AM
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thanks that’s what I thought, always nice to get a second opinion
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Old 15th January, 2002, 10:07 AM
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Yup... things get pretty crazy when ya got The Bends... pretty painful too..
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Old 15th January, 2002, 03:03 PM
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I'm a source of fluid dynamics info

Probably the easiest method is to use the method of equivalent lengths:

Pressure drop in pipes due to friction is given by:

Pf =8 f (L / di ) .(pu^2)/2

Where Pf = pressure drop in N/m^2
f = friction factor
L = total pipe length in m
di = pipe inside diameter in m
p = fluid density in kg/m^3
u = fluid velocity in m/s

The bits you'll need:

Friction factors: found from a nasty looking graph. A typical value is 0.0035

p for water is approx 1000kg/m^3

Method of equivalent diameters: For each of the following fittings, multiply the number of equivalent pipe diameters by the inside diameter of the pipe and add this to the length of your tube. Use the total sum in the above equation:

45deg std elbow = 15
45deg long radius elbow = 10
90deg std radius elbow = 30 - 40
90deg long radius elbow = 23
90deg square elbow = 75
Tee - entry from leg = 60
Tee - entry into leg = 90
Union and coupling = 2
Sharp reduction (tank outlet) = 25
Sharp expansion (tank inlet) = 50
Gate valve (fully open) = 7.5
Globe valve (fully open) = 300
Plug valve (fully open) = 18

(Numbers after the = are the number of equivalent pipe diameters). This method assumes that the pressure drop due to each of the above fittings is equal to that caused by an equivalent length of the straight tubing.

Whatever value you get for the pressure drop is the pressure head your pump needs to provide to make the system flow.

Shout if you need more info.

(Source: Coulson and Richardson's Chemical Engineering volume 6, ed 3, R K Sinnot, 1999, Butterworth Heinemann)
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Old 15th January, 2002, 04:52 PM
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sweeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeet

nice one will apply and see what happens,

thanks so much
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