I need to design an air plenum 1

[gutobom]

I need to finish the design of an air plenum, which is actually a distributor of air. This plenum is 4.200 mm long and 146 mm wide, and would have the final shape of a long and narrow oblique parallelepiped. Along the length, on one of the walls only, I have a series of 400 orifices with 9 mm diameter, stamped as Venturis, which will allow the air to be blown over a paper tissue which needs to be dried. Hot air needs to go into the plenum at 130 C. Air velocity on the orifices is rated at 25 m/s. Considering that I have to feed 0.636 m3/hour of air through the Venturis, what is the recommended section of the plenum on the infeed side? Should I work with which air speed at the entrance in order to guarantee the uniform pressure in the plenum to provide uniform air speed through the Venturis along the length? What would be the ideal shape of the parrallelepiped?

 

[poetix99]

You have 0.636 m3/sec, not /hr.

I'm not certain what it means that the nozzles are "rated at 25 m/s"; that is a relatively low velocity. The practical way to hold that velocity is by controlling the incoming flow rate, rather than the plenum pressure.

A 150 mm supply line would have an average velocity of 36 m/s to supply your nozzles; that should be OK.

Note that the total area of your nozzles is 0.0254 m2, so the plenum cross-sectional area should be larger than that area. If you are not severely limited by space for your plenum, make the constant cross-sectional area (at least) twice the total nozzle area (> 0.05 m2).

 

[gutobom]

- Yes, it is 0.636 m3/sec, by saying the nozzles are rated at 25 m/s is the air speed we would like to have there. I have some limits of space, I think the maximum I will get is about 65% more cross-sectional area than the total nozzle area. If I am supplying air through one end of the plenum, should it have an oblique parallelepiped shape? Which would be the best angle? In case I have several plenums side to side, is it better to have the ports to the plenums made in circular shape instead of the rectangular shape of the plenum and then make an adapter?

 

[prex]

Assuming the pressure drop across the orifices will be close to &[ignore]Delta[/ignore];p_o=0.5&[ignore]rho[/ignore];V_o², as the pressure change (recovery) along the distributor is mainly due to the recovery of the velocity head &[ignore]Delta[/ignore];p_d=0.5&[ignore]rho[/ignore];V_di², to keep constant flow across the orifices you need &[ignore]Delta[/ignore];p_di<<&[ignore]Delta[/ignore];p_o.
Of course how much less depends on the degree of uniformity you require: if you wanted a uniformity to &[ignore]plusmn[/ignore];10% then I guess you should target V_di&[ignore]asymp[/ignore];0.1V_o.
As you can't do that, you should make the distributor of varying height: as the width is constant, the height should vary linearly down to almost zero at the end opposite to entrance (triangular shape), to keep the velocity along the distributor as constant as possible. prex

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