Question re: Aperture sizes for Resistance to Airflow

[Fadingtoblack]

Hi,

I'm a final year Biomed. Eng. student currently working with a team on a project investigating Threshold Responses to Inspiratory loads, and their relation to sleep apnea.

The key to this study is a box which provides the loads.

It is a rectangular box containing 6 screens or 'stages', each designed to apply a specific load, such as 0.5cm.H2O, 1cm.H2O etc as a flow of 60L/min is drawn from one end of the box. These are 'switched on' and off using balloon valves along the top of the box.

These loads will combine in series to form a total load of 6cm.H2O when air is being drawn through all 6 screens.

Now the problem is calculating the correct area of the screen (a sheet of laser drilled stainless steel) to expose at each stage to get the correct pressure drops.


I calculated them from a sample I had of the material, but found when I tested them finally in reality, they were giving incorrect pressure drops.


All I can think of is that by enlarging the area of the screen exposed, the air now has more area to flow through, meaning it is passing slower through the screen, giving me lower pressure drops than it should.

If I'm right then all future screens I've calculated to be 'right', will end up being too low once put into practice, so I'm hoping someone can help get me to grips with the correct calculations and factors involved.



I hope that's the least bit understandable! Please let me know if you need more information, and I'm extremely grateful in advance for any help anyone can give me!

 

[AlephZero]

I suspect trying to calculate accurately will be a waste of time. If you are making small holes, the exact hole shape, thickness of the screens, roughness round the edges of the holes, etc, could all be significant.

You could measure the resistance from your existing screens, plot the results and use the experimental data to redesign them with the correct number of holes.

Or, make screens which have too many holes and then block off part of the screen to set the resistance accurately (by experiment).

Once you have one accurate set of screens, I would think laser drilling should give consistent results if you want to copy them accurately.

 

[Fadingtoblack]

Firstly thanks for the reply!


I see what you mean in terms of accuracy of calculations in real life!

Let me just clarify some aspects of the setup in case that will affect your answer.


-The screens are stainless steel, 1mm thick, with holes 0.1mm diameter (from memory) drilled across the face by a laser. (so it's like a mesh, with a certain 'open area', just on a much smaller scale, as mesh isn't much good as a resistance element to air flow!)

What we've done for the setup, is have a piece of plastic, with a recess on one side to place a square of steel.
Depending on the required drop in pressure, we cut a square through the plastic therefore exposing a certain area of the steel to airflow.
In practice, like you said, the measurements were a little off, due to the guys only being able to cut mm to about 2 decimal places, but I think I can pass this off as neglible as it won't result in a much greater or lower pressure drop (in mm.H2O)

These two elements make up a 'screen' or stage, which we then slide into the main box.



Now as you've suggested I have been working from my experimental data, but after this first box has turned out as a bit of a bust I'm just worried I'm not taking everything into account.


-The new required sizes I have calculated from our results on the current box will result in a new box roughly double the size - this is where I'm getting worried as I'm just not sure if having a greater area of steel exposed to that same 60L/sec flow will result in a lower velocity through it -> resulting in a lower pressure drop than expected.

Again I hope I'm making sense, I feel it's likely I'm just being stupid though, if it isn't obvious already! :P
(I'd just like to be sure before I make them spring to cover this new box getting made!)

 

[AlephZero]

That's pretty much like what I imagined you were describing.

Cutting through all the details, the bottom line is you need to have the correct number of 0.1mm holes exposed in each screen. I was thinking you could blank off the mesh with something low-tech like masking tape to set up the system, then make your plastic blanking sheets to match the prototype. Something like 3M "magic tape" (from office suppliers) should be easy to work with. Start with an exposed area that is too small, then cut the tape with a scalpel and peel it off to increase the area, till you get what you want.

 

[Fadingtoblack]

Sorry for the late reply!


I think you're right, we're just going to have to trial and error it until we get what we need.

So I guess the next question is - what's the easiest way to do it?
We've been doing it with a vacuum->flowmeter->(our current box)->Angle manometer
but to check these new sizes we'd have to do it from our large sheet of steel still remaining.

Obviously we need a chamber each side, holes for airflow, and a small one to the manometer, I was thinking maybe an icecream container on each side? I'm just trying to think how to make these measurements without cutting the steel first (as we have a limited supply)


And last question, just to put my mind at rest, as the steel is only 1mm thick, exposing a greater surface area (or more holes as you say) to the flow would only make a neglible difference to the measured pressure drop right?


Thanks again for the help :)

 

[FredGarvin]

In our nozzle flow rig, which is essentially the same thing you are doing, we use metallic tape to block off holes when we need to. It works like a charm.

In regards to the screen itself, can't you open up the hole size in stead of making the sheet larger? Or are your numbers so far more that you couldn't fit it on the current sheet?

One thing you may look into is wire mesh cloth. We use many different size grids to make distortion screens and the like for some of our testing. They can come in a lot of different wire size and % open areas. Here is the company that we deal with a lot and their listing of screens:
http://www.phoenixwirecloth.com/spacech.htm