## 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

## A (Sample) Lexicon For Compressed Air

Every industry and different technical subject matter comes with it’s own lexicon of terms or vocabulary words.  More often than not, when speaking to an Application Engineer here at EXAIR you are going to hear words within our lexicon. The list I have compiled below is merely a sampling to help translate some terms that we forget not everyone knows.  Some of these are merely acronyms that get thrown around a good amount.

SCFM – Standard Cubic Feet per Minute – This is the unit we use to represent the volumetric flow rate of compressed gas that has already been corrected to standardized conditions of pressure and temperature.

PSIG – Pounds per square inch gauge – This is the unit which we use to represent the operating inlet pressure of the device.  When requesting this, we generally are looking for a pressure gauge to be installed directly on the inlet to the device with no other form of restrictions between the two.  For the most part, catalog consumption values are given in SCFM at 80 psig.  The main exception to that rule are the Vortex Tube based products.

Compressed Air – This is a utility that most industrial manufacturing facilities have available to them.   It is regular, atmospheric air which has been compressed by an air compressor to a higher pressure than atmospheric.  Generally speaking, compressed air systems will be at a range of 85-120 psig.

OSHA – Occupational Safety and Health Administration – This is the main federal agency that enforces two of the major conformance standards that EXAIR products meet or exceed.

29 CFR- 1910.95 (a) – Maximum allowable noise level exposure.  The great majority of EXAIR products meet or exceed this safety standard, our largest Super Air Nozzles
1910.242 (b) – This is the standard which states compressed air blow off devices cannot exceed 30 psig of dead end pressure.  This means, if the exit point of the air can be blocked the operating pressure must be below 30 psig.  The reason for this standard is to prevent air embolism which can be fatal.  All EXAIR products meet or exceed this standard by having multiple orifice discharge.

Coanda Effect – This is the effect that numerous EXAIR products utilize to amplify and entrain ambient air.   The Coanda effect is when a fluid jet (stream of compressed air) tends to be attracted to a nearby surface.  This principle was found by a Romanian aerodynamics pioneer, Henri Coandᾰ.  The picture below shows a Super Air Amplifier blowing a foam ball into the air and suspending it due to the Coanda effect on the surface of the ball.

Rigid Pipe or Hard Pipe – This is the term we will often use when discussing the compressed air line that can be used to support and supply certain EXAIR products.  Generally we are referring to a Schedule 40 steel pipe, Type L copper line, stainless steel tube, or any form of pressure rated hard pipe that can be used for supplying compressed air.

Plenum – the state or a space in which a gas, usually air, is contained at pressure greater than atmospheric pressure. Many of our products feature a plenum chamber.

Again, this list is only a sample of the terminology you will hear us use when discussing compressed air applications.  If there are any other air/compressed air/fluid dynamic terms you may be unsure of, please contact us.

Brian Farno
Application Engineer Manager
BrianFarno@EXAIR.com
@EXAIR_BF

## Positive Crankcase Ventilation

This week I am going to share with you a recent application I did for a customer with a stationary gas compressor. Transporting natural gas through pipe lines across the nation requires substations along route to overcome pressure loss. Some of the gas is used to power up engines that turn large compressors to push the gas further down the line.

With all combustion engines some of the combustion gasses along with carbon soot gets past the piston rings and into the crankcase. Common practice is to vent this back into the intake manifold using the vacuum in the manifold to draw it in.

In this case, the customer wanted to filter the “blow by” before returning it into engine. The pressure drop across the filter impeded the flow too much to be effective.

They installed an EXAIR Line Vac  model HT6063 High Temperature line Vac to generate a vacuum to pull the gasses through the filter. Problem solved. It was a simple solution to a major problem.

Do you have an overwhelming problem? Give one of our application engineers a call and the will do their best to find you a simple solution.

Joe Panfalone
Application Engineer
Phone (513) 671-3322
Fax (513) 671-3363
Web: http://www.exair.com