Taking a Break From the Basics - Air Flow Through An Orifice

We're taking a small break from the Compressed Air Basics Series, but we'll be back to it next week.  I noticed something - a certain page on our website that draws a lot of traffic and I wanted to discuss it.

When looking at a list of our most visited web pages, this one is always the most popular or in the top three:  http://www.aircompressorworks.com/airflowthroughorifices.html.

It is just a chart of how much air can flow through a hole at different pressures and different hole sizes.  It seems simple, but this chart it very important to many compressed air applications.  As a customer, this chart can mean a lot to you.

LEAKS

The first reason is leaks.  Leaks in a compressed air system can cost you a lot of lost money in electrical bills, maintenance, and wear and tear on the compressor.  I've seen customers who needed a 20 hp air compressor just to keep up with leaks. 

Looking back at that chart, if you just have a tiny 3/32" hole - that's less than 1/10th of an inch - it will leak out about 17 CFM if the pressure is 125 psi.  That's 5 hp worth of compressor right there just to keep up with a tiny 3/32" hole.  At the average Florida electrical costs and a 60 hour work week, that's about $1200 per year.  If you have a large air system it's common to have several of these leaks.  That tiny hole doesn't seem so tiny once you do the math.

That chart gives you a great idea of how much air is coming out of your leak.  You might notice that for the same size hole, the lower the pressure is then the less CFM the leak is.  It's a great example of how turning down your system pressure can save you money.  Not only do you get the energy savings of running the compressor at a lower pressure, but because the air lost from leaks is reduced, then the compressor runs less.  It's a positive feedback loop:  Lower the pressure and your leaks are smaller;  the leaks are smaller so you need less CFM; you need less CFM so you can lower the system pressure a little, which then reduces the leaks even more, ect. 

BLASTING

Another way that chart comes in handy is with abrasive blasting.  Abrasive blasting is sandblasting,
bead blasting, soda blasting, and some other media blasting applications.

We have customers that use a lot of air for sandblasting, and yes, a lot of that air is justified - they really do need it.  However, there are many other customers who waste tons of air and media by using too much pressure or nozzles that are too big.

The blasting is just a leak, and the size of the leak is the bore size of the blasting nozzle.  A very common thing to do is just use unregulated air from the compressor and blast away - whatever's coming out of the compressor, that's the psi they're using.


Well, look again at that chart.  With an 1/8" bore and 125 psi, you need about 31 CFM.  However, if you regulate the pressure down to 90 psi, then you only need about 23 CFM.  The price difference in a compressor that can do 23 CFM at 90 psi and one that can do 31 CFM at 125 psi is between $1500 and $2000.  Also, the electrical cost to run the bigger one is more.  The majority of sandblasting applications need 90 psi or less, and some can go as low as 40 psi.  It all depends on what you're blasting and the media that you're blasting it with.  If you could blast at 40 psi, then you'd only need about 12 CFM.

Another way to reduce the CFM needed is to reduce the bore size.  Let's say you had an 1/8" bore, but you could use a 3/32" bore and still do the job just as fast or nearly as fast.  At 90 psi, you'd need only about 13 CFM.  So we started out at 125 psi and an 1/8" bore needing 31 CFM.  However, reducing the pressure down to 90 psi and using a smaller tip, we now only need about 13 CFM.  That's a $2000-$3000 difference in the initial price of the compressor, and you save money in electricity and maintenance. 

Also, as the nozzles get used, they get worn down.  What happens is that the hole gets bigger and bigger.  If you're compressor is running more than it used to or can't keep up anymore, then it's time to get new nozzles.

If you can reduce the pressure and use a smaller bore, not only does it use less compressed air, but it also uses less media.  So there is even more savings to be had.

So how do you figure out what you need?  If you have a set-up already, regulate down the air to the lowest psi and still get the job done.  What media you're using, the material you're blasting, and what you're trying to remove will determine the pressure you need.  Once that's done, try a smaller tip size and see if you can do the job in the same amount of time or really close to it.

 CLEANING

In a lot of plants, the personnel used compressed air for cleaning.  They blow dust off machines, they knock the debris off radiators, and they even use the air to clean themselves.  Sometimes they just use the air from an open hose hooked into the air system.  Looking at the chart again, you can see that blowing air from a 3/8" hose at 125 psi is about 284 CFM!  That's a ton of air. 

This is why the air needs to be regulated, and blowguns need to be used.  Regulating down to about 30 psi and using a blowgun will knock that 234 CFM down to under 20 CFM.  Some applications require larger blow guns and a little more pressure.  However, even with a huge blowgun used at 90 psi, it's still less than half of just an open hose using unregulated air.

OTHER APPLICATIONS

The power of that chart is that it tells you the CFM needed for any application where compressed air blows through a hole and onto something else.  There are thousands of applications out there that this applies to.  Using it that chart you can see how manipulating the size of the hole or the pressure can affect your compressed air usage. 

We didn't make that chart - it was made by some engineer ages ago.  The name of the person who came up with it has been lost in antiquity, to the best of our knowledge.  You can see the equation that's used at the bottom of the page.  We put it on our website, because it comes in handy so many times for us.  We're glad so many people are finding it as handy as we do.