Pressure – Absolute, Gauge, and Units of Both

Compressed air is a common utility used throughout industrial facilities and it has to be measured like any other utility in order to know just how much a facility is using. When dealing with compressed air a common unit of measurement that readily comes up is psi, pound-force per square inch. This unit of measure is one of the most basic units used to measure pressure in the compressed air industry. There are other means to measure this though, so let’s discover the difference.

Again, the pressure is a force distributed over an area, the Earth’s atmosphere has pressure, if it didn’t we would all balloon up like the Violet from Willy Wonka, just without eating some prototype gum causing internal pressure. PSIA is a unit of measure that is relative to a full vacuum. It is pounds per square inch absolute (PSIA). The absolute pressure is calculated as the sum of the gauge pressure plus the atmospheric pressure. If you were to travel into space, the atmospheric pressure would be absolute zero which is actually a vacuum. There is nothing pushing from the outside in so the inside pushes out, hence the ballooning.

The atmospheric pressure on earth is based on sea level. This is 14.7 pounds per square inch absolute pressure. This pressure will change along with the weather and the altitude at which the measurement is taken.

So how do we get to the pressure that is displayed on a pressure gauge?  When shown open to room air, my pressure gauge reads zero psi. Well, that is zero psi gauge, this already has the atmosphere showing. It is not showing the Absolute pressure, it is showing the pressure relative to atmospheric conditions. This is going back to the fact that gauge pressure is the summation of absolute pressure and atmospheric conditions, for sea level on earth that is 14.7 psia. So how do we increase this and get the gauge to read higher levels?

We compress the air the gauge is measuring, whether it is using a screw compressor, dual-stage piston compressor, single-cylinder, or any other type of compressor, it is compressing the ambient, atmospheric air. Some materials do not like being compressed. Air, however, reacts well to being compressed and turns into a form of stored energy that gets used throughout industrial facilities.  By compressing the air, we effectively take the air from atmospheric conditions and squeeze it down into a storage tank or piping where it is stored until it is used. Because the air is being compressed you can fit larger volumes (cubic feet or cubic meters) into a smaller area. This is the stored energy, that air that is compressed always wants to expand back out to ambient conditions. Perhaps this video below will help, it shows the GREAT Julius Sumner Miller explaining atmospheric pressure, lack of it, and when you add to it.

Lastly, no matter where you are, there is a scientific unit that can express atmospheric pressure, compressed air pressure, or even lack of pressure which are vacuum levels. To convert between these scientific units, some math calculations are needed. While the video below is no Julius Sumner Miller, it does a great job walking through many of the units we deal with daily here at EXAIR.

 

If you want to discuss pressures, atmospheric pressure, how fast the air expands from your engineered nozzle to atmospheric, why all the moisture in the air compresses with it, and how to keep it out of your process, contact an application engineer and we will be glad to walk through the applications and explanations with you.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

1 – Willy Wonka & the Chocolate Factory – Violet Blows Up Like a Blueberry Scene (7/10) | Movieclips, Movieclips, retrieved from https://youtu.be/8Yqw_f26SvM

2 – Lesson 10 – Atmospheric Pressure – Properties of Gases – Demonstrations in Physics,  Julius Sumner Miller, Retrieved from https://www.youtube.com/watch?v=P3qcAZrNC18

3 – Pressure Units and Pressure Unit Conversion Explained, Chem Academy, retrieve from https://www.youtube.com/watch?v=2rNs0VMiHNw

 

O-Rings, Seals, Gaskets, Maintenace, Filtration – They All Matter

I’ve mentioned it before and I’ll say it again. You can’t teach experience. This was told to me by a mentor at a previous job and of course, younger me thought, “Yeah, yeah I know all I need to know.”  Well, younger me was an idiot and learned many things through experience. Sometimes I am still a slow learner and eventually, I remember those experiences and make decisions based on them. So what does this have to do with o-rings, seals, and gaskets?

I’m in the midst of a light construction project in my house and have reached a stage where some tools that I do not have would come in handy and make the job faster. Younger me would have justified purchasing a new one, experienced me understands a budget and reached out to my network of friends and a good friend said they had the tool I needed. This was a compressed air powered framing nail gun. Straight through nailing, no-problem, toe-nailing, no-problem, this thing won’t break a sweat and your arms will be stronger by the time you are done using it while your thumbs are screaming thank you for not smashing me a hundred times.

The Framing Nail Gun in question

This loan did come with two conditions, one was, he didn’t have any nails to give with it. This was not a problem as I wouldn’t expect a friend to give me free fasteners with a tool loan. The second is the one that concerned me, he said, it does leak a little air but it should still shoot just fine. After working in the compressed air industry for over a decade I have experienced this many times. At that point I knew if you could hear it, chances were it was a bad leak. Upon further inspection, there was a cylinder gasket and rubber spring that were in pieces.

Old Spring Bumper and Main Cylinder Gasket
Gasket pieces and dirty air can result in catastrophic failures.

Nothing that a trip to a local business couldn’t take care of.  A few new parts and discussion with their knowledgeable staff and I had the information needed to rebuild this nail gun to functioning status.

New vs. Old

Oddly enough, my experience and expertise with how the EXAIR products like the No-Drip Air Atomizing Liquid Spray Nozzles operate and how to rebuild them, provided a good foundation about how this tool worked. This repair ended up being very similar to the rebuild on a No-Drip Spray Nozzle.

This story is two-fold, filtration could have prevented a lot of the damage to this gun. This gun uses a good amount of air volume at an expedient pace so keeping it clean and clear of debris helps extend the lifetime of internal parts.  See my video on what happens without filtration below.

The second part is that maintaining and understanding processes to clean/rebuild are crucial to sustainable function of a machine. The cleaning process for this gun was fairly straightforward and using the correct lubricant for reassembly was another critical role. This culminated in a framing nail gun that can now be used to further my project and will more than likely live another decade before needing a rebuild again. That is if filtration and proper lubrication are followed.

Had I not obtained experiences throughout my career that helped me to understand how this tool functioned, the worth of a reliable network of vendors, and the necessity to complete tasks that take me out of my comfort zone I wouldn’t be in the place I am today. Because I have the experience and the network to ask for help it enables me to keep machines running that could have cost valuable production hours had this been a production environment.

EXAIR stocks rebuild kits, gaskets, shims, and parts for all of our product lines which may require a repair. For products which need to be cleaned in order to return back to new performance, we have the instructions or can do it for you here. From time to time they may need a repair or refurb in order to keep functioning at peak performance. If you want to build your trusted network or learn more about how to rebuild or clean EXAIR products, contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF

The Makeup of Earth’s Air

Most people know that oxygen, makes up about 20% of the earth’s atmosphere at sea level, and that almost all the rest is nitrogen. But did you know there’s an impressive list of other gases in the air we breathe

whats in air
Reference: CRC Handbook of Chemistry and Physics, edited by David R. Lide, 1997.

We can consider, for practical purposes, that air is made up of five gases: nitrogen, oxygen, argon, carbon dioxide, and water vapor. But because water vapor is a variable, this table omits it, water vapor generally makes up 1-3% of atmospheric air, by volume, and can be as high as 5%.  Which means that, even on a ‘dry’ day, it pushes argon out of third place!

There are numerous reasons why the volumetric concentrations of these gases are important.  If oxygen level drops in the air we’re breathing, human activity is impaired.  Exhaustion without physical exertion will occur at 12-15%.  Your lips turn blue at 10%.  Exposure to oxygen levels of 8% or below are fatal within minutes.

But here at EXAIR we care about how compressed air can be used efficiently to better your process! 

Any of our products are capable of discharging a fluid, but they’re specifically designed for use with compressed air – in basic grade school science terms, they convert the potential energy of air under compression into kinetic energy in such a way as to entrain a large amount of air from the surrounding environment.  This is important to consider for a couple of reasons:

  • Anything that’s in your compressed air supply is going to get on the part you’re blowing off with that Super Air Nozzle, the material you’re conveying with that Line Vac, or the electronics you’re cooling with that Cabinet Cooler System.  That includes water…which can condense from the water vapor at several points along the way from your compressor’s intake, through its filtration and drying systems, to the discharge from the product itself.
  • Sometimes, a user is interested in blowing a purge gas (commonly nitrogen or argon) –  but unless it’s in a isolated environment (like a closed chamber) purged with the same gas, most of the developed flow will simply be room air.

Another consideration of air make up involves EXAIR Gen4 Static Eliminators.  They work on the Corona discharge principle: a high voltage is applied to a sharp point, and any gas in the vicinity of that point is subject to ionization – loss or gain of electrons in their molecules’ outer valences, resulting in a charged particle.  The charge is positive if they lose an electron, and negative if they gain one.  Of the two gases that make up almost all of our air, oxygen has the lowest ionization energy in its outer valence, making it the easier to ionize than nitrogen.  You can certainly supply a Gen4 Static Eliminator with pure nitrogen if you wish, but the static dissipation rate may be lesser.

If you want to learn more about the compressed air or any of our point of use compressed air products, you can contact an Application Engineer.  We will be happy to help you.

Jordan Shouse
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_JS

Air photo courtesy of Barney Moss Creative Commons License

Rotary Scroll Compressors

Over the years, my EXAIR colleagues and I have blogged about different types of air compressor types including single and double acting reciprocating, rotary screw and sliding vane air compressors. You can click on the links above to check those out. Today, I will review the basics of the rotary scroll-type compressor.

The rotary scroll type compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Positive displacement type compressors

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber. The rotary scroll uses two inter-meshing scrolls, that are spiral in shape. One of the scrolls is fixed, and does not move (red).  The other scroll (black) has an “orbit” type of motion, relative to the fixed scroll. Air would be drawn in from the left, and as it flows clockwise through the scroll, the area is reduced until the air is discharged at a high pressure at the center.

How it Works

There is no metal to metal sliding contact, so lubrication is not needed.  A drawback to an oil free operation is that oil lubrication tends to reduce the heat of compression and without it, the efficiency of scroll compressors is less than that of lubricated types.

The advantages of the rotary scroll type compressor include:

  • Comes as a complete package
  • Comparatively efficient operation
  • Can be lubricant-free
  • Quiet operation
  • Air cooled

The main disadvantage:

  • A limited range of capacities is available, with low output flows

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Jordan Shouse
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_JS

 

 

Rotary Scroll GIF:  used from  Public Domain
Images Courtesy of  Compressor1 Creative Commons.