Equalizing air flow into long slitted tube? 1

[shane21]

Assume a 10" diameter 10' long tube or pipe laying horizontally with a 2" wide slit all along its top side running it's full length.

One end is capped and the other has a fan drawing air into the pipe through this 2" slit all along its length.

Without changing the slit dimensions, is there anything about the tube diameter itself, if that could be varied along its length, that would then help have air being drawn in all along the slit be more equalized and not mostly just coming in through the slit nearest the end with the fan?

IOW's, if you wanted it to draw in near to the same volume of air at one end of slit compared to other end, is there anything about changing pipe dimensions make any difference?

Thanks for any thoughts.

 

[Compositepro]

Make the tube ten feet in diameter and you will get closer to your goal.

The only factors that determine how much air flows through any portion of the slit is the slit width (or geometry), which you say must stay constant, and the pressure difference across the slit at any point. So, to stay constant anywhere along the slit length, there can be no pressure gradient along your tube, which means little or no flow along the tube.

To do what you are asking, the slit must have a relatively high resistance to air flow compared to all other parts of your system.

 

[shane21]

Compositepro, makes sense, about what I figured, wanted to make sure I wasn't missing anything, thanks.

 

[berkshire]

The other thing you can do is to taper the tube from the diameter of the slit at the far end i.e. 2" to 17.5" dia at the near end. this assures constant volume and pressure regain, as the air traverses down the tube. You would then have to install a reducer to get back to the 10" dia . Away from the slit. Or if you want to keep the 10" dia pipe, Put a 5/8" wide slit.
B.E.

You are judged not by what you know, but by what you can do.

 

[EnergyProfessional]

if you stay at below 1000 ft/ min it will self-balance somewhat due to static regain if you keep same diameter over entire length.

At 10'length it really should be easy, at least better than a balancer would do. And even if each foot has slightly different flowrate - it probably won't matter.

 

[Compositepro]

Tapering the tube helps IF you are blowing from the tube out of the slot. The taper allows for a gradual conversion of air velocity (momentum) entering the tube into pressure, which will make the flow from the slot more uniform. Tapering works against you if you are sucking air into the tube through the slot, because it increases the resistance to flow at the far end where you need to get more flow through the slot.

 

[berkshire]

I am afraid Composite pro is correct, You could put a perf screen in the slit , Or put a plenum chamber with a mesh screen behind the slot .
drawing from multiple points also helps.
B.E.

You are judged not by what you know, but by what you can do.

 

[willard3]

You can also install a multi-blade damper and balance it for equal velocity in the opening.

 

[shane21]

Restricting airflow entering tube nearer the fan end, if more air from farther end desired,
might could be done upstream of the slot if it has to stay same width all along its length.

 

[LittleInch]

Or insert a 6 inch pipe inside and gradually drill more and larger holes as you go down the tube?? Then connect the 6 inch pipe to the fan.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RossABQ]

Can you draw out of the middle of the duct, instead of the end?

 

[DrRTU]

You did not specify the CFM. Based upon a max flow at 1200fpm you will be at 650CFM. A 2” slot 10’ would be 1.67ft2 = 390 fpm at equal face velocity. Use volume dampers from a 2” slot diffuser installed in a stubby plenum with a perforated inlet face. I would do 5 2’ units. Balance with the face dampers. If your flow goes above 800cfm it will make whistling noises.

 

[DrRTU]

If this is a high velocity collection system, determine you capture / conveying velocity, build the 10” plenum as a taper based upon constant velocity. We would roll (2) 5’ tapers and but weld and cap the small end. Build small blast gates to balance the flow. Roll the blast gates in short lengths to the taper and mount to the outside of the duct. Bolt gates to surface. No tek screws.

 

[shane21]

Discovered opportunity to create dozen separate channels leading down to the 10" diameter tube all along it's 10' length,
and will play with restricting the size of those channel openings, more on the fan end than the closed end to achieve more
uniform draw into the tube all along its full length.

Basically accomplishing same as having 2" slit be less wide at fan end drawing air in, and ever wider the further away.

 

[John2025]

Are you guys talking about something like this? If so, is there any way to calculate sizes based on air flow, velocity, etc. either as a slot or maybe a series of holes on the inner pipe?

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