Air Nozzles and Impinging Pressure

I think this would be an example of try it and see...

I can't think of any formulae that you would be able to calculate the force given the fact that a jet will vary in flow rate over a wide range, depending on pressure, and nozzle dimensions etc...

Stick to a circular jet as that shape is a little more predictable, generally the longer the internal jet is and the smoother the bore the more laminar will be the air outlet and so the jet might not need to be so big a diameter to blow off your components?

Your component shape and size is also information you will need.

John.

I do agree that this is an example of try it and see as iam tired of searching a material relevant to this and also as you rightly mentioned that it depends on so many factors.

I wish to know if there are any low cost pressure pads (sensor arrays) available in the market which could detect the peak pressure pattern of the nozzle blow for me to test the different orifice sizes to ascertain the best sizes and its cross section.

I got some material on the water jet cutting application nozzles, which is available at

http://engr.smu.edu/rcam/research/waterjet/Par4.html

This sounds very similar to grain threshing where a blower is used to separate grain from chaff. In this case, I think velocity on the solid mixture is the critical element. Too much velocity will carry or blow everything out of the mixture. Too little velocity will not blow off enough of the "chaff". Threshing equipment will have adjustable slots so the velocity can be varied for different crops, field conditions, and speed. The slots are rectangular shape to try to maintain a uniform velocity across the entire material stream going through the separator.

What is the relative difference in density, volume, or specific gravity of the items you are trying to separate? Is this a batch process or continuous material stream conveyed past the orifice? How much "chaff" or dross is being separated from the desired product? Why such a short pulse of air flow? I would suggest a continuous blower at a lower pressure instead of a short pulse of compressed air for a variety of reasons.

Thank you for the details and related application example.

The equipment iam working on is a cotton contamintation sorter used at the blow room stage of the textile spinning mill.

The cotton is pneumatically conveyed through ducts at negative pressure (slightly lower than atmosphere) through the equipment where the contaminant such as jute, plastic sheets, cloth bits, plastic strings are detected by the intelligent optical system and the same conveyed to solenoid valves through electronic communication. This is a continuous process taking place through out the year (24 x 7 x 365)

You have rightly said that the air pulse should be optimal to throw the contaminant from the main line to the secondary chamber.

There will be number of nozzles and only those nozzles getting the signal of the contaminant will be activated.

The cotton tuft (Flock) size will vary at each of the mill based on the location where the sorter is placed, hence iam trying to find out a nozzle design which would give the highest impinging pressure to securely remove the contaminant from the cotton with minimum good cotton loss

You need to do this practically by installing a 1/4" needle valve (not Ball Valve) and manually adjusting the air flow velocity till you get the right result.

What is the variation of the diameter of the pipe or duct the flock is passing through?

What is the variation of cotton flock?

What is the velocity of the flock in the system?

As someone else previously noted, some degree of adjustment will probably be required to adjust for production rate variations, whether in blow off timing, air pressure, or nozzle diameter.

I agree with the needle valve and if you want to get fancy use a two or three way system like used in air brushes to control velocity, volume, pressure or diffusion.

Hooking that up to solenoids or actuators will be a bit of a challenge with the double action trigger. But once setup you could run it from a pc controller that automatically adjusts to your requirements

2 cents from

Brad

Thanks for the details.

Iam more concerned on the nozzle design rather than the valve design, since we use solenoid actuated large 3 way direction control valve and it is controlled for the time of opening through electronics. The Cv of the valve is 1.5. The compressed air supply to this valves is from an air reservoir @ 6 bar(g) pressure. There are number of such valves based on the duct width (Normally 24 numbers)

I wish to understand the impact force of the flow from the nozzles. Given the flow rate through the nozzle as fixed what will be the role of nozzle design.

I wish to test this in a laboratory which can measure the pressure or force of the impinging.

Then a pressure pad will give you incorrect data.

Your target is a string/line and behaves much different. Try measuring the tension forces on the different lines in similar angles and situations.

You may even want to mock up a line going threw at real time speeds and clean it while monitoring the speed deflection and tension.

As for design a nozzle, that can be as much art as science. At least for rocket nozzles I'm familiar with.

Hope this is of some help

Brad

I was under the impression that pressure pads would give correct data, since it is the method used by the air blow off nozzle manufacturers create the specification sheet like blow force @ pressure @ distance and the jet diameters.

But your suggestion seems to be meaningful for my case.

Let me give it a try.

Thanks a lot for the guidance.

I'm curious about why you want to know the pressure applied by any given nozzle. How will you use that information?

You seem to be dealing with objects that are relatively light - low density and small volume - objects that would likely be picked up by a light to moderate wind if lying on open ground. (Of course, I could be way off base here, but the principle of what I'll say next may still apply).

If a light wind is sufficient to displace your foreign particles, then the ambient pressure around the particle is not the measure that makes a difference - it's the velocity (speed and direction) of the wind that determines whether the particle will be picked up.

Clearly, your system is intended to do more than simply move the offending particle - it must move it far enough, fast enough, and in a specific direction.

Electroman's point about ensuring laminar flow seems to be a more important issue than the pressure you deliver at a given distance from the nozzle: the more laminar the flow, the longer your stream of air will maintain its velocity and width (and density).

Electroman also points out that a circular jet is more predictable. I couldn't say for sure, but I bet they produce a more stable (i.e. the laminar flow lasts longer) jet than other shapes. The only reason I can think of for using anything other than a round nozzle is if the jet air velocity and the transverse transport air velocity are similar: I might then use an elliptical nozzle - with the long axis in the direction of the transverse transport air.

HTH

Chris

Please refer to the figure above where the cotton is flowing in the main line at a velocity ranging from 8 - 20 m/sec depending on the production rate required since this factor will determine the minimum conveying velocity for the cotton to flow without any choking.

The duct size is not varying it is 1200mm x 80mm rectangular shape made of GI sheet.

There are two suction blowers, one connected to the main line and the other connected to the evacuation line.

The rectangle is the nozzle attached with solenoid valves. The direction of the main line and the direction in which the nozzle has to hit the contaminant is perpendicular and hence when struck by the high pressure air blast from the nozzle it travels to the evacuation chamber and the suction blower connected to it collects them in a netted bag.

There are two positions in a textile mill where our machine will be placed preferably after a opener which opens the cotton in to flocks. In the beginning stages the flock sizes are big and hence high momentum is necessary to carrry the contaminant away from the main line.

In the end stage, the cotton flocks is well open and hence little momentum is necessary to push the contaminant to the evacuation line.

It is pretty difficult to estimate the openness of the cotton flock after a opener as it is dependent on too many parameters. Also the material transport speed which i have already mentioned varies based on the production rate.

Taking in to account all the above, iam unable to come to a conclusion on the flow rate of compressed air required and the nozzle design suitable for both the stages.

So i thought a nozzle design which could deliver the highest impining pressure would meet my requirement at both stages.

Our office is closed until 24th October (Vacation), hence i may not have access to this forum until then.

I will try my best to be in touch with all of you personally.

Though i regret keeping away for time being. Please bear with me.

Hello every one,

Back to the forum

Is there any further inputs in this regard to help me?

Raghu