Air: What is it?

Air Balloons

What is Air? Air is an invisible gas that supports life on earth. Dry air is made from a mixture of 78% Nitrogen, 21% Oxygen, and 1% of remaining gases like carbon dioxide and other inert gases.  Ambient air contains an average of 1% water vapor, and it has a density of 0.0749 Lbs./cubic foot (1.22 Kg/cubic meter) at standard conditions.  Air that surrounds us does not have a smell, color, or taste, but it is considered a fluid as it follows the rules of fluid dynamics. But unlike liquids, gases like air are compressible.  Once we discovered the potential of compressing the surrounding air, we were able to advance many technologies.

Bellows

Guess when the earliest air compressor was used?  Believe it or not, it was when we started to breathe air.  Our diaphragms are like compressors.  It pulls and pushes the air in and out of our lungs.  We can generate up to 1.2 PSI (80 mbar) of air pressure.  During the iron age, hotter fires were required for smelting.  Around 1500 B.C., a new type of air compressor was created, called a bellows.  You probably seen them hanging by the fireplaces.  It is a hand-held device with a flexible bag that you squeeze together to compress the air.  The high stream of air was able to get higher temperature fires to melt metals.

Then we started to move into the industrial era.  Air compressors were used in mining industries to move air into deep caverns and shafts.  Then as the manufacturing technologies advanced, the requirements for higher air pressures were needed.  The stored energy created by compressing the air allowed us to develop better pneumatic systems for manufacturing, automation, and construction.  I do not know what the future holds in compressed air systems, but I am excited to find out.

Since air is a gas, it will follow the basic rules of the ideal gas law;

PV = nRT  (Equation 1)

P – Pressure

V – Volume

n – Amount of gas in moles

R – Universal Gas Constant

T – Temperature

If we express the equation in an isothermal process (same temperature), we can see how the volume and pressure are related.  The equation for two different states of a gas can be written as follows:

P1 * V1 = P2 * V2  (Equation 2)

P1 – Pressure at initial state 1

V1 – Volume at initial state 1

P2 – Pressure at changed state 2

V2 – Volume at changed state 2

If we solve for P2, we have:

P2 = (P1 * V1)/V2  (Equation 3)

In looking at Equation 3, if the volume, V2, gets smaller, the pressure, P2, gets higher.  This is the idea behind how air compressors work.  They decrease the volume inside a chamber to increase the pressure of the air.  Most industrial compressors will compress the air to about 125 PSI (8.5 bar).  A PSI is a pound of force over a square inch.  For metric pressure, a bar is a kg of force over a square centimeter.  So, at 125 PSI, there will be 125 pounds of force over a 1” X 1” square.  This amount of potential energy is very useful to do work for pneumatic equipment.  To simplify the system, the air gets compressed, stored as energy, released as work and is ready to be used again in the cycle.

Air Compressor

Compressed air is a clean utility that is used in many different applications.  It is much safer than electrical or hydraulic systems.  Since air is all around us, it is an abundant commodity for air compressors to use.  But because of the compressibility factor of air, much energy is required to create enough pressure in a typical system.  It takes roughly 1 horsepower (746 watts) of power to compress 4 cubic feet of air (113L) to 125 PSI (8.5 bar) every minute.  With almost every manufacturing plant in the world utilizing compressed air in one form or another, the amount of energy used to compress air is extraordinary.  So, utilizing compressed air as efficiently as possible is mandatory.  Air is free, but making compressed air is expensive

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Products, you can contact an Application Engineer at EXAIR.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Picture: Hot Air Rises by Paul VanDerWerf. Creative Commons Attribution 2.0 Generic.

Picture: Bellows by Joanna Bourne. Creative Commons Attribution 2.0 Generic.

Picture: Air Compressor by Chris Bartle. Creative Commons Attribution 2.0 Generic.

EXAIR Blogs This Week Are Almost As Cool As Shark Week

Yes, ALMOST. This week, the EXAIR Blog has featured some excellent explanations of the science behind the operation of compressed air products. On Tuesday, John Ball posted the best explanation of SCFM vs ACFM that I’ve come across, and I’ve been explaining this to callers for almost four years now. I’m using his blog to perfect my “elevator pitch” on this topic. It will still likely require a building with more than ten floors, but I think that’s OK.

Also on “Two Blog Tuesday,” (this week only; I’m not trying to start anything) Dave Woerner’s gem of a blog detailed the terminology associated with pressure measurement, and why we use “psig” (g = gauged) – in a nutshell, the compressed air inside the pipe doesn’t care what the pressure outside the pipe is. And, since he mentioned it, I might add that most of agree that we care even less about how a certain NFL team’s footballs were (or were not) properly inflated.

Brian Farno’s “One Blog Wednesday” entry was a quite useful (if not alphabetical…OK; now I AM trying to start something) list of some common terms and expressions we use on a regular basis while discussing the operation and performance of EXAIR compressed air products. If you liked his photo demonstration of the Coanda effect with the foam ball & Super Air Amplifier, I encourage you to experience the Coanda effect for yourself, if you have access to a leaf blower and a volleyball:

I mention these earlier blogs to get to the point of MY blog today…a bit of theory-to-practice, if you will. Once you’ve gotten a decent understanding of these principles (or have the above links bookmarked for quick reference,) we can apply it to what’s needed for the proper operation of a compressed air product itself.

With a working knowledge of air flow (SCFM) and compressed air supply pressure (psig,) we can more easily understand why certain pipe sizes are specified for use with particular products. For instance, the longer the Super Air Knife and/or the longer the run of piping to it, the larger the pipe that’s needed to supply it:

This table comes directly from the Installation & Operation Instructions for the Super Air Knife.
This table comes directly from the Installation & Operation Instructions for the Super Air Knife.

The reasons for this are two-fold: First, the pipe…longer runs of pipe will experience more line loss (a continuous reduction in pressure, due to friction with the pipe wall…and itself) – so, larger diameter pipe is needed for longer lengths. For another practical demonstration, consider how much faster you can drink a beverage through a normal drinking straw than you can through a coffee stirrer. Not as dramatic as the leaf blower & volleyball (you really want to try it now, don’t you?) but you get my point.

Second, the Air Knife…the longer the Air Knife, the more air it’s going to use. And, if it’s longer than 18”, you’ll want to feed it with air at both ends…line loss will occur in the plenum as well.

In closing, I want to leave with another video, shot right here at EXAIR, showing the actual reductions in pressure due to line loss through different lengths, and diameters, of compressed air supply line to a Super Air Knife.

If you ever have any questions about compressed air use, or how EXAIR products can help you use your compressed air more efficiently, safely, and quietly, please give us a call.

Russ Bowman
Application Engineer
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Why 5 PSIG Matters

Last week I pointed out the important locations for measuring your compressed air system pressure throughout your compressed air system.   One of the critical points to measure system pressure was before and after each filter.  This leads into another question that I receive every once in a while, “How do I tell when the filter needs to be changed?”  The answer to this is easy, when you see more than a 5 PSIG pressure drop across the filter.  This means that the element within the filter has become clogged with sediment or debris and is restricting the volume available to your downstream products.

Filter
EXAIR 5 micron Auto Drain Filter Separator

 

This can lead to decreased performance, downtime, and even the possibility of passing contaminants through the filter to downstream point of use components.  In order to maintain an optimal performance when using EXAIR filter separators and oil removal filters, monitoring the compressed air pressure before and after the unit is ideal.

Replacement filter elements are readily available from stock, as well as complete rebuild kits for the filter units. Changing the filters out can be done fairly easily and we even offer a video of how to do it.

The life expectancy of a filter element on the compressed air is directly related to the quality of air and the frequency of use, meaning it can vary greatly.  If you tie a new filter onto the end of a compressed air drop that has not been used in years, you may get a surprise by the filter clogging rather quickly.   However, if you maintain your compressor and your piping system properly then the filters should last a long time. Generally we recommend checking your filters every 6 months.

If you have questions about where and why to filter your compressed air contact us.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF