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Air Consumption of a Machine

05/26/2016 3:48 PM

I have a machine with a total of 400 holes in 8 - 15' runs of 1/4" SS. tubing at 15psi. The holes are 1mm each (which gives a total opening size of 12.4sq." How many scfm will the supplying compressor need to provide?

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#1

Re: air consumption of a machine

05/26/2016 4:05 PM

The compressor would need to produce more than that, the holding tank size and compressor capacity will be determined by cycle time intervals desired....

http://www.tlv.com/global/TI/calculator/air-flow-rate-through-orifice.html

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#2

Re: air consumption of a machine

05/26/2016 4:09 PM

SCFM won't fit through any quantity of any size mm holes (round or square, matters not).

Units must match, or the air will get stuck.

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#3

Re: air consumption of a machine

05/26/2016 4:31 PM

Well, let's see you want to know how many SCFM* (standard cubic feet per minute) will flow through a 1 mm orifice @ 15 PSI differential and multiply that by 400.

In theoretically near perfect conditions you would need about .56 SCFM flow per hole so given you have 400 holes that works out to around ~225 SCFM minimum or roughly 13 compressor HP!

*(Not sure where some others are having a hard time working with SCFM in an air flow application but, okay?)

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#4

Re: air consumption of a machine

05/26/2016 4:43 PM

Each opening is 1mm diameter, so:

A = πr2 = 3.14159*0.52 = 0.78540 mm2

Total area is 400A = 314.16 mm2

and 314.16 mm2/(25.4 mm/in)2 = 0.52338 in2 or 3.6346x10-3 ft2, so you miscalculated your total area.

To get SCFM, you need another dimension, namely the average velocity in ft/min.

What is the average velocity of air you want coming out of the holes? That, plus the operating temperature can be used to approximate what flow rate of air you will need.

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#11
In reply to #4

Re: air consumption of a machine

05/27/2016 12:11 AM

"...so you miscalculated your total area."You were a lot closer, but my calculators show 314.16 mm2/(25.4 mm/in)2 to be 0.488 in2.

As Lyn pointed out, with 50 holes in a 1/4" pipe, there is going to be a LOT of pressure drop! In fact, I'm guessing that there will be virtually no flow through the last hole. If so, the average hole will only have about half the 15psi. Using the calculator in the link from SE (Thanks SE!), the flow at that pressure will be about 0.01 SCFM per hole, so the total flow would be around 40 SCFM.

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#12
In reply to #11

Re: air consumption of a machine

05/27/2016 1:29 AM

Whoa there! We DON"T KNOW what he wants for flow rate. The pressure drop is proportional to that. I don't disagree with almost everything you have said, but I think there are a lot of unfounded assumptions being made here. Unless we get some feedback from the OP, we are "jumping the gun". More of my take on this at http://cr4.globalspec.com/comment/1153519/Re-air-consumption-of-a-machine

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#25
In reply to #12

Re: air consumption of a machine

05/27/2016 9:58 AM

Mainly, jumping the air gun. Some little home compressor will not do this job. He needs a fan blower. Really, it is not clear why 15 psig is important in a textile context.

It does not take 15 psig to move a piece of thread.

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#26
In reply to #25

Re: air consumption of a machine

05/27/2016 10:08 AM

This is a new piece of equipment that was engineered overseas and has been running with success there so we're slow to change settings (ie. pressure). This is where it has been set by the OEM. Currently air is sourced from a nearly unlimited plant supply but we're looking to switch to a smaller local compressor. They filter then regulate the air supply to the tubes and each 1/4" tube is fed from each end so at the center it is surely a lower pressure. The gist of what I'm seeing here is a consumption of 200 to 250ish and depending on the compressors duty rating I'll need a 10-15 HP machine.

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#28
In reply to #26

Re: air consumption of a machine

05/27/2016 10:41 AM

Smaller compressors are nearly always less efficient than large ones. If you already have a large plant air supply, and apparently already have supply lines in place, why do you want to reduce the efficiency?

If you are having problems with variation in pressure, just add an additional storage tank and an additional pressure regulator, so that tank runs at a lower pressure than the plant lines minimum.

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#29
In reply to #28

Re: air consumption of a machine

05/27/2016 10:51 AM

This next part falls under the heading of "do as you are told". We have some question re: desiccant migration downstream in our plant air supply. These small holes blow out directly onto a spinning shaft which is in contact with our product. Now there are small micron filter sets before each pair of tubes and they are clean ...... However, when the boss says get a new air supply I have to do it. The questions of moisture and oil in the new air supply seem to fall on deaf ears ....... so. Ingersol Rand here I come. Around here we call this "kicking the crutch out from under them".

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#35
In reply to #28

Re: air consumption of a machine

05/27/2016 4:25 PM

i would store the air at the higher pressure and regulate after the receiver tank.

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#30
In reply to #26

Re: air consumption of a machine

05/27/2016 11:05 AM

OK, if the tubes are fed from both ends the tube area (total both ends) is ~ same as the total orifice area (per tube). Assuming the headers at each end are a decent size, the entry to each end of the tube behaves much like an orifice, so as a rough guess the pressure inside tube finishes up at ~ 7.5psig. Guesstimated orifice flow per tube 12scfm, total 96scfm. Velocity just inside the tube about 500fps so there will be high losses and a lot lower orifice flow near the centre.

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#27
In reply to #12

Re: air consumption of a machine

05/27/2016 10:30 AM

As I read the original post, the OP does not know what he wants for a flow rate, either.

He has since clarified that the 1/4" tubes are fed from both ends, which helps considerably, although the 1/2" feed line, now effectively feeding 16 1/4" tubes with 25 holes each, is too small at that pressure.

Since someone who knows the OP has suggested a $200 Sears compressor, I gather it is not for an industrial plant of any size.

Since the installation is already working, replacing the tubes with larger ones is probably not an option, so I'd approach this from a totally different angle. As long as the compressor has a reasonable sized tank, I'd forget the 15 psi value, set the compressor to produce a more normal 75-125 psi, and put a pressure regulator between the tank and the machine. Then you would simply adjust the pressure until it does the job. Since (s)he is working with textiles there should be an oil filter in that line as well.

Unfortunately, the Sears website I looked at gives no indication of the SCFM ratings of any of their compressors, so this does not answer the OP's real question, which is 'how much does (s)he need to spend on a compressor', but I'm pretty sure the $200 one would not last long.

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#5

Re: air consumption of a machine

05/26/2016 4:52 PM

So, if I interpret your inquiry correctly... you have 8 ea. 15' long 1/4" diameter tubes. Each tube has 50 holes that are 1mm in diameter... assuming evenly distributed along the 15' length. Further assuming your supply is inputting from only one end and the other end of the 15' length is capped.

Fist, ID your desired CFM/PSI through the 1mm orifice that is furthest from your supply.

Calculate the PSI/CFM you would need on the other side of the orifice to provide you that output.

Then, ID pressure drop through 15' of quarter inch tubing.

Add additional loss due to the preceding 49 orifices that are also contributing to pressure drop.

Multiply by 8

Or, make a sketch and call a compressor sales representative.

Unless this is homework... then go back to your lecture notes and figure it out for yourself.

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#6

Re: air consumption of a machine

05/26/2016 4:59 PM

JavaHead may have the more precise way of getting at an answer, where the pressure drop along the tube is determined by integrating the discrete integral for flow per hole per applied pressure, where you may assume that the 15 psig is provided at the inlet to the tubes with the holes.

The simple approach: TCMtech suggested learning the flow for one 1 mm hole, and then simply multiply. That approach will get you somewhere close, but then go ahead and oversize your compressor by a factor of three, so you don't burn it up the first hour.

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#8
In reply to #6

Re: air consumption of a machine

05/26/2016 7:20 PM

"That approach will get you somewhere close, but then go ahead and oversize your compressor by a factor of three, so you don't burn it up the first hour."

Why by three? My commercial units are rated for continuous duty at their rated peak flow and pressure ratings.

Personally, if it was me and given the less than 20 PSI I would use a simple positive displacement rotary lobe or screw type compressor. Either that or a multistage regenerative blower system.

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#22
In reply to #8

Re: air consumption of a machine

05/27/2016 9:47 AM

I suppose those could handle that job, probably a lot cheaper also.

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#10
In reply to #6

Re: air consumption of a machine

05/26/2016 10:57 PM

Maybe. But, as usual, we're left with a very unclear picture of what is going on. The temptation for those of our ilk () to try to fill in the gaps and try to piece together a partially-formed scenario like this is almost impossible to ignore.

I wager 3000 quatloos that the OP does not return to (t)his thread!

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#17
In reply to #10

Re: air consumption of a machine

05/27/2016 7:56 AM

Had to leave work early and take care of family business..... So I'm reading all the posts and trying to balance how much my boss will spend vs. how long this will need to last. I'll post again on the main thread a few clarifications and kind of "average" the good guesses. Liked the second reply from "Dorman". Air getting stuck??!! THATS how I know I'm dealing with REAL process engineers.... Sorry to cost you the Quatloos.

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#34
In reply to #17

Re: air consumption of a machine

05/27/2016 3:54 PM

That's OK. Please don't take my remark personally, but it is seldom when an Original Poster returns to answer the inevitable questions to clarify this or that part of the problem.

Thank you for returning to do just that! There are a lot of engineers here more qualified to help you with this, so I am going to "bow out", though I will be following this thread with interest.

Welcome to CR4

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#7

Re: air consumption of a machine

05/26/2016 5:13 PM

I get 0.510675 SCFM per orifice....@ 80°F
or 204.27 SCFM total....

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#9

Re: air consumption of a machine

05/26/2016 9:29 PM

Have you given any thought at all to how much different the flow will be from the first hole closest to the inlet as opposed to the last hole 15' feet away?

The imbalance will be astronomical in a 1/4" tube.

Good luck.

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#32
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Re: air consumption of a machine

05/27/2016 1:11 PM
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#36
In reply to #32

Re: air consumption of a machine

05/27/2016 7:03 PM

You just gave me a great idea!

I have a number of micro drill bit that are in the .004" - .010" range I doubt I will ever put to any use so maybe I could set up a micro drill press and drill holes in the sides of pennies to give people something to think about whenever they find them.

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#38
In reply to #36

Re: air consumption of a machine

05/27/2016 7:33 PM

Fortunately, pennies made since 1982 are copper-plated zinc, which should be relatively easy to drill. Put a real copper/brass/bronze penny in, and your chances of drill breakage go up significantly.

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#39
In reply to #36

Re: air consumption of a machine

05/31/2016 8:20 AM

Don't give the ancient aliens freaks ideas! They will come up these drill holes in modern pennies being related to Puma Punku, Bolivia, formerly on the shores of Lake Titicaca.

source: (Wikipedia)

My question: How many Llama stomachs used as bladders (or even whale stomachs) would be needed to float a 100 ton block quarried from the shores of Lake Titicaca, 10 miles away from Puma Punku, and on farther to Tiwanacu? Not to mention how to fish these huge blocks of stone from the lake once transported. Not to mention how to move over land without wheels, and not to mention the cutting and drilling to shape the intricately geometric patterns in these blocks.

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#13

Re: air consumption of a machine

05/27/2016 2:47 AM

Perhaps the poster should mention what he expects the air to do once out of such a thin pipe.is it to move dust or to aerate something, perhaps like sparging in a fermenter? Or even levitate something? Knowing that may trigger more valid responses.

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#14

Re: air consumption of a machine

05/27/2016 4:13 AM

Just guessing:-

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#21
In reply to #14

Re: air consumption of a machine

05/27/2016 9:41 AM

haha good guess. it's a textile application though. Not really that much difference though.

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#15

Re: air consumption of a machine

05/27/2016 5:19 AM

Given that you are only operating at 15psi. I looked at using a side channel blower rather than a compressor but 1038mbar is too high. If you can work with 720mbar (approx 11psi.) then capital cost is 70-80% of compressor, running costs are about 65-75%, maintenance about 30-40% and equipment life double.

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#16

Re: air consumption of a machine

05/27/2016 7:35 AM

Firstly, I agree with others about total orifice area, I make it 314mm2 = 0.487in2. Orifice area per run (50 orifices) 39.3mm2.

I make flow per orifice 0.414scfm, using discharge coefficient 0.62. Total 165.5scfm for all 400. With those pressures orifice velocity is sonic.

But your tube size ¼ inch is a non-starter. I assume that is OD, and taking ID = 5mm, tube area is ~ ½ total orifice area per tube. That gives huge pressure loss along the tube. To get good distribution of air among the orifices, pressure drop along the tube should be low compared with orifice pressure drop. Trying 20mm tube, I get pipe loss 0.35psi, 15mm tube gives 1.55psi. Allowing for reducing flow along the tube 15mm might be OK, depends how good you want the distribution, but I would go for 20mm.

Roots blower should be OK, though 15psi pressure rise is on the limit. Assuming typical efficiency 60%, shaft power 9.8kW, probably 11kW motor.

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#18

Re: air consumption of a machine

05/27/2016 8:04 AM

Awesome responses! Thank all of you. A couple of clarifications. I have no idea of velocity. It has to blow a little thread out of the way. This installation is currently in production and maybe what I need to do is put a rotometer inline and just check it that way. Each line (1/4" tube) is back fed from each end to help the pressure loss you have mentioned. Each of the 8 tubes is individually fed from a 1/2" pipe. We basically need to change air sources to a new compressor and many people think a little $200 Sears compressor will do it because it's just tiny little holes ..... right? I was looking for some science that doesn't come from the seat of someone's pants. I sure found it here. Thanks!

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#19
In reply to #18

Re: air consumption of a machine

05/27/2016 8:46 AM

If you specify the orifice upstream pressure (and assuming downstream is to the atmosphere) the velocity is what it is, I make it about 250 m/s (actual).

Sears compressors no use for this application, pressure much higher than you need and power much lower, so flowrate well down.

When sizing the header that supplies the tubes, to get good distribution you need to apply same principle as for sizing the tubes.

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#24
In reply to #18

Re: air consumption of a machine

05/27/2016 9:56 AM

IF you want that Sears compressor to do this job, the holes will have to be much much smaller, and once the American Engineers are done with those holes, British ones will be happy to weld them shut and provide even smaller holes, then the Texan Engineers will just blow into the pipes and you can bore the holes out even larger.

If you harnessed some hot air in Foggy Bottoms, you would not need such small holes, and no Sears compressor at all, just politician breath (and farts) can be pretty stinky after the bean soup.

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#20

Re: air consumption of a machine

05/27/2016 9:31 AM

15 psi thru a 1/4 orifice would have 26.9 scfm.

The holes in the tube are too large and numerous to create a back pressure.

So multiply the 26.9 times 8.

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#23
In reply to #20

Re: air consumption of a machine

05/27/2016 9:51 AM

That would be with no back pressure. Actual flow will be lower.

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#33
In reply to #23

Re: air consumption of a machine

05/27/2016 3:53 PM

What would be the cause of the back pressure?

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#37
In reply to #33

Re: air consumption of a machine

05/27/2016 7:14 PM

No air will flow through the Ø 1mm holes unless there is a difference in pressure from the inlet side of each hole to the outlet side; therefore the pressure inside the 1/4" tube must be above the (atmospheric) outside pressure.

No air will flow into the 1/4" tube unless there is a difference in pressure from the inlet side of the 1/4" tubing opening/orifice (inside the 1/2" tube) to the outlet side of the opening/orifice (inside the 1/4" tube); therefore the pressure inside the 1/4" tube must be below the stated 15 psi source pressure.

It should be obvious that the pressure inside the 1/4" tube will gradually drop from the ends (inputs) to the center, since some air is lost through each hole. I haven't done the math to find out what that center pressure might be, but my intuition tells me it will be only a little above atmospheric, and there will be very little flow through the centermost holes. Thus the pressure in the 1/4" tube will be greatest at the input end. That pressure is pushing back towards the entrance to the 1/4" tube.

When in steady-state operation, the pressure inside the 1/4" tube will automatically adjust until there is equilibrium between the quantity of air flowing in through the 1/4" orifice and the quantity of air flowing out through the Ø 1mm holes.

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#31

Re: air consumption of a machine

05/27/2016 12:28 PM

Talk these guys.

They are X Paxton engineers who made a better blower and this is their specialty.

Sonic air system - For Drying & Blow Off

They have a great warranty program.

I used their products for years and quit using Paxton's.

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#40

Re: Air Consumption of a Machine

06/01/2016 2:34 PM

I was going to paste a picture in this thread, but I think the network I am on is messing up the upload feature.

Since my system will not allow me to paste anything here, follow the link below. You can download the relevant file from there. It is taken from the current version of the Womack Fluid Power Data book, page 47.

https://www.dropbox.com/s/o9ubdksasnjyn2y/FPData_OrificeFlow.pdf?dl=0

You can use the numbers provided to get you in the ball park, or you can extrapolate from these numbers to get the particular orifice size or flow that you need.

Have a great day!

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