2 replies to this topic

[siretb]

If a tube is used to distribute puffs of air, (as in a pulse jet filter), with small holes (6-10mm), with the constaint that the flow is critical at the orifices (choking occurs at every hole),
what constraints would I have in terms of velocity inside the tube?
pressure inside the tube would be around 3 bars, outside 1 bar.

More specifically, I would like to know if anyone would recommend a miminum ratio of the tube internal cross section area to the the total hole area, or if I can allow velocities at the tube inlet almost as high as the gas velocity at hole?

[hollerg]

although sonic flow will eliminate the flowrate dependency on the outside pressure, the degree of maldistribution will depend on the flow profile in the pipe and the pressure drop profile in the pipe.

D_orifice=(D_pipe/n^.5)*(1/(N*(Co^2)*alpha)^0.25) will give a distributio of +/- 5% where:

A) N=10 for discharge distribution and 15 for withdrawal distribution

B) alpha = 1.1 for fully turbulent flow
alpha = 2.5 for distortion of the flow profile (e.g. -- elbow just in front of the device or a turn into a lateral

C) Re_orifice ==> Do*Vo*rho/mu > 5000

D) Pipe wall thickness less than 75 % of the hole diameter

E)friction losses in the pipe are low ===> 4f*L_pipe/D_pipe <=3

F) n is the number of orifices in the lateral

G) Co is .675 for liquid into liquid, gas into gas, gas into liquid. Liquid into gas varies

[siretb]

Thank you.
This confirms my preliminary calculations. This leads to slightly smaller holes (or large pipe diameter) than what I calculated,
I'll used a 2.5" schedule 40 pipe, with n=50 holes and use 6mm holes.

The criteria of wall thickness to hole diameter is slightly exceeded.

The criteria you've given is for established, steady state flow. I have puffs, pulsing air (valve with large Kv upstream) for time durations of less than a half a second. It may change things a bit. Thank you again.