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.


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
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.

Taking Professional Advice

I have an old house, turn of the last century, plaster and lath walls, remnants of knob and tube wiring, blocked over fireplaces in every room – old! Wiring in houses this old can be brittle which can make installing new ceiling fans tricky. First the ceiling is ten feet so you may be high on the ladder, and every time you need something from below you have to get a helper, carry a tool belt or climb down the ladder. Now if you are an experienced electrician, you have a tool belt filled with exactly what you need and a little extra. Cooks call this Mise en Place. My old man called it having your stuff together.

I don’t do it very well, so I have to climb down the ladder often. I tried to get a helper, but my nine month old son had trouble differentiating between a flat head screw driver and chew toy and wire nuts are clearly a choking hazard. Speaking of wire nuts. They seem to be such an innocuous widget. A wire nut is a wire nut. I have never thought much about them. Well it turns out that after the brittle wire has broken inside the wall. And then you drop the fifteenth wire nut, because it wouldn’t grab your old brittle wire. You spend some time thinking about the wire nuts as you are writing the check to the electrician. He might recommend an expensive brand of wire nuts that he uses that work great in these old houses. He gave me about twenty extra for my next attempt at electrical work, so I have a start on my electrical Mise en Place, but the next problem will probably involve plumbing…

Ceiling fan
Not my house, but similar.

Wire nuts remind me of air nozzles to some degree. They are such simple products but provide tremendous protection, and utility. There is also an incredible amount of brands, styles, and sizes. It is easy to think that the nozzle that comes installed on the thumb gun works great for home use, why should you spend time or money in investing in an upgrade. For the professional electrician, the expensive wire nuts made his day easier, more productive and his final installation safe. If you need to use compressed air to clean, dry or cool your parts, investing in an intelligent compressed air product will make the application quieter, more efficient and more effective.

Nozzle Lineup
EXAIR Engineered Solutions

EXAIR Super Air Nozzles are engineered solutions that meet OSHA requirements for dead end pressure – this makes them safe. The air nozzles utilize the Coanda effect to amplify compressed air flow up to 25 times – this makes them more effective. The small orifices build up pressure inside the supply the line in order to produce higher velocities – this makes the engineered air nozzles more efficient. Stop using cheap inefficient nozzles, think about your tools, and use what the professionals use.

Dave Woerner
Application Engineer


Ceiling Fan Photo Courtesy of Kevin GalensCreative Commons License