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

 

How to Calculate the Cost of Leaks

Leaks are a hidden nuisance in a compressed air system that can cause thousands of dollars in electricity per year. These leaks on average can account for up to 30% of the operation cost of a compressed air system. A leak will usually occur at connection joints, unions, valves, and fittings. This not only is a huge waste of energy but it can also cause a system to lose pressure along with lowering the life span of the compressor since it will have to run more often to make up for the loss of air from the leak.

There are two common ways to calculate how much compressed air a system is losing due to leaks. The first way is to turn off all of the point of use compressed air devices; once this has been complete turn on the air compressor and record the average time that it takes the compressor to cycle on and off. With the average cycle time you can calculate out the total percentage of leakage using the following formula.

The second method is to calculate out the percentage lost using a pressure gauge downstream from a receiver tank. This method requires one to know the total volume in the system to accurately estimate the leakage from the system. Once the compressor turns on wait until the system reaches the normal operating pressure for the process and record how long it takes to drop to a lower operating pressure of your choosing. Once this has been completed you can use the following formula to calculate out the total percentage of leakage.

The total percentage of the compressor that is lost should be under 10% if the system is properly maintained.

Once the total percentage of leakage has been calculated you can start to look at the cost of a single leak assuming that the leak is equivalent to a 1/16” diameter hole. This means that at 80 psig the leak is going to expel 3.8 SCFM. The average industrial air compressor can produce 4 SCFM using 1 horsepower of energy. Adding in the average energy cost of $0.25 per 1000 SCF generated one can calculate out the price per hour the leak is costing using the following calculation.

If you base the cost per year for a typical 8000 hr. of operating time per year you are looking at $480 per year for one 1/16” hole leak. As you can see the more leaks in the system the more costly it gets. If you know how much SCFM your system is consuming in leaks then that value can be plugged into the equitation instead of the assumed 3.8 SCFM.

If you’d like to discuss how EXAIR products can help identify and locate costly leaks in your compressed air system, please contact one of our application engineers at 800-903-9247.

Cody Biehle
Application Engineer
EXAIR Corporation
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Video Blog: How To Calculate Air Consumption At A Pressure Other Than Published Values

The below video shows how to calculate the air consumption when operating at any pressure.

If you want to discuss efficient compressed air use or any of EXAIR’s engineered compressed air products, give us a call or email.  We would enjoy hearing from you!

Steve Harrison
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How to Calculate SCFM (Volume) When Operating at Any Pressure

If you need to operate at a different pressure because you require less or more force or simply operate at a different line pressure, this formula will allow you to determine the volume of air being consumed by any device.

Volume Formula

Using the EXAIR 1100 Super Air Nozzle as our example:

1100

Lets first consider the volume of the 1100 Super Air Nozzle at a higher than published pressure.  As shown in the formula and calculations it is simply the ratio of gauge pressure + atmospheric divided by the published pressure + atmospheric and then multiply the dividend by the published volume.  So as we do the math we solve for 17.69 SCFM @ 105 PSIG from a device that was  shown consume 14 SCFM @ 80 PSIG.

higher

Now lets consider the volume at a lower than published pressure.  As shown it is simply the ratio of gauge pressure + atmospheric divided by the published pressure + atmospheric and then multiply the dividend by the published volume.  So as we do the math we solve for 11.04 SCFM @ 60 PSIG from a device that was shown to consume 14 SCFM @ 80 PSIG.

lower

When you are looking for expert advice on safe, quiet and efficient point of use compressed air products give us a call.  Experience the EXAIR difference first hand and receive the great customer service, products and attention you deserve!  We would enjoy hearing from you.

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