Absolute Pressure Ratio

This model 1101 Super Air Nozzle requires 14 SCFM @ 80 PSIG. How much air will it consume at 60 PSIG?

Compressed air driven devices are always given a specification for the compressed air flow at a certain pressure.  For example, an EXAIR model 1101 Super Air Nozzle has a specified flow of 14 SCFM at 80 PSIG.  This means that when this nozzle is operated at 80 PSIG, it will require 14 SCFM of compressed air flow.  But what if the force from the nozzle is too high when operated at 80 PSIG and a lower operating pressure is needed?

Thankfully, we can calculate the compressed air flow at a different pressure using the absolute pressure ratio.  The absolute pressure ratio says that for any given change in absolute operating pressure, there will be a proportional change in the air consumption of a device.  So, what is an absolute pressure?

Put simply, an absolute pressure is the value which you would measure on pressure gauge plus the atmospheric pressure (PSIA, or Pounds per Square Inch Atmospheric).  So, our 80 PSIG operating pressure mentioned above is an absolute pressure of 94.5 PSI (80PSIG + 14.5 PSIA).  Similarly, if we wanted to determine the compressed air flow at an operating pressure of 60 PSIG, our absolute pressure would be 74.5 PSI (60 PSIG + 14.5 PSIA).

The absolute pressure ratio is a ratio of the new absolute operating pressure (new PSIG + PSIA) compared to the known absolute operating pressure (known PSIG + PSIA).  For example, when comparing an operating pressure of 60 PSIG to an operating pressure of 80 PSIG, we will end up with the following ratio:

This means that our absolute pressure ratio in this case is 0.7884.  To determine the compressed air flow for the model 1101 Super Air Nozzle at 60 PSIG, we will take this ratio value and multiply it by the known flow value at 80 PSIG.  This will yield the following:

Utilizing this formula allows us to truly compare a compressed air powered device at different operating pressures.  If we did not use the absolute pressures when comparing compressed air devices at differing pressures, our values would be erroneously low, which could yield to improper compressed air system planning and performance.  And, using the absolute pressure ratio allows anyone to make a true comparison of compressed air device performance.  If specifications are given at different pressures, performance data can be misleading.  But, by using the absolute pressure ratio we can make a more exact evaluation of device operation.

If you have a question about your compressed air device and/or how a change in pressure will impact compressed air flow, contact our Application Engineers.  We’ll be happy to help.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Can Counting Carbs Help in Your Compressed Air System?

Breakfast Cereal
Breakfast Cereal

Have you ever counted the amount of carbs that you eat?  People typically do this to lose weight, to become healthier, or for medical reasons like diabetes.  Personally, I like to eat cereal in the morning.  I will pull a box of cereal down from the cupboard and look at the Total Carbs field.  One morning, I looked at a box of gluten-free rice flakes and compared it to a peanut butter nugget cereal.  I noticed that the carbs were very similar.  The rice cereal had 23 grams of total carbs while the peanut butter nuggets had only 22 grams of total carbs.  Then I looked at the serving size.  The rice cereal had a serving size of 1 cup while the nuggets only had a serving size of ¾ cups.  So, in comparison, for one cup of nugget cereal, the total amount of carbs was 27.5 grams.  Initially, I thought that they were similar, but the peanut butter nugget was actually 20% higher in carbs.  This same “misdirection” occurs in your compressed air system.

Here is what I mean. Some manufacturers like to use a lower pressure to rate their products.  This lower pressure makes it seem like their products will use less compressed air in your system.  But, like with the serving sizes, it can be deceiving.  It is not a lie that they are telling, but it is a bit of misconception.  To do an actual comparisons, we have to compare the flow rates at the same pressure (like comparing the carbohydrates at the same serving size).  For example, MfgA likes to rate their nozzles at a pressure of 72.5 PSIG.  EXAIR rates their nozzles at 80 PSIG as this is the most common pressure for point-of-use equipment.  You can see where I am going with this.

To compare nozzles of the same size, MfgA nozzle has a flow rate of 34 SCFM at 72.5 PSIG, and EXAIR model 1104 Super Air Nozzle has a rating of 35 SCFM at 80 psig. From an initial observation, it looks like MfgA has a lower flow rating.  To do the correct comparison, we have to adjust the flow rate to the same pressure.  This is done by multiplying the flow of MfgA nozzle by the ratio of absolute pressures.  (Absolute pressure is gage pressure plus 14.7 PSI).  The ratio of absolute pressures is:  (80PSIG + 14.7) / (72.5PSIG + 14.7) = 1.09.  Therefore; the flow rate at 80 PSIG for MfgA nozzle is now 34 SCFM * 1.09 = 37 SCFM.  Now we can compare the flow rates for each compressed air nozzle.  Like adjusting the serving size to 1 cup of cereal, the MfgA will use 9% more compressed air in your system than the EXAIR model 1104 Super Air Nozzle.  This may not seem like much, but over time it will add up.  And, there is no need to waste additional compressed air.

Family of Nozzles
Family of Nozzles

The EXAIR Super Air Nozzles are designed to entrain more ambient air than compressed air needed. This will save you on your pneumatic system, which in turn will save you money.  The other design features gives the EXAIR Super Air Nozzle more force, less noise, and still meet the OSHA compliance.

If you want to run a healthier compressed air system, it is important to evaluate the amount of compressed air that you are using. To do this correctly, you always want to compare the information at the same pressure.  By using the EXAIR Super Air Nozzles in your compressed air system, you will only have to worry about your own weight, not your pneumatic system.

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

 

Picture: Breakfast Cereal by Mike Mozart Creative Commons Attribution 2.0 Generic License

Deflated Footballs? What’s the Big Deal, We Talk Air Pressure Everyday

This week we prepare for the professional football championship game, that phrase is trademarked within the Woerner household. For a few years, we have had my friends from college over for guacamole, chicken wings, French fries, and beverages. This year our small family is now three, so we are in for a quiet evening at home. My son will most likely be asleep at kick off, but my wife and I might stay awake for the end of the first quarter. Even with the small amount of people that we will watch the game, I will still make a small spread for our family, because tradition. Tradition says, it’s Super Bowl Week – we buy avocados early in the week so they have time to ripen.

In the build up to the big game, it seems like we always get a very silly story that the media grabs a hold of and just will not let go.  I want to join them. Have you heard about the fact that the footballs that the one of teams used on offense might not have been inflated to the correct pressure. I don’t know that the fotballs were under inflated on purpose, but I also think that LaDainian Tomlinson might have been on to something, when he said “The Patriots live by the saying if you ain’t cheating, you ain’t trying.”

That was a long introduction into my blog today about pressure. The NFL Rule Book states,

“The ball shall be made up of an inflated (12 1/2 to 13 1/2 pounds) urethane bladder enclosed in a pebble grained, leather case (natural tan color) without corrugations of any kind. It shall have the form of a prolate spheroid and the size and weight shall be: long axis, 11 to 11 1/4 inches; long circumference, 28 to 28 1/2 inches; short circumference, 21 to 21 1/4 inches; weight, 14 to 15 ounces.”

From an engineering perspective this is ambiguous at best. If I read this with no knowledge of football, I would have no idea how to test whether the ball is inflated. The rule states that the ball should be an inflated urethane bladder. Then in the parenthetical phrase it lists 12 1/2 to 13 1/2 pounds. Last time I checked pounds is a measure of weight. If I received this specifications, I would put the ball on a scale to weigh it. Using some common sense a quarterback isn’t going to be able to throw a 12 pounds ball, like a bullet, 10 yards. Let alone 60 yards for that deep bomb.

If I was writing the rule book, it would read that “the ball shall be inflated to a pressure of 12 1/2 to 13 1/2 pounds per square inch gauge pressure.” With this wording there is a clear standard to be met for football to be worthy for use.

What Is Gauge Pressure?

Gauge pressure is the pressure determined by a gauge or instrument. The term is used to differentiate pressure registered by a gauge from absolute pressure. Absolute pressure is determined by adding gauge pressure to atmospheric (aka barometric) pressure. Barometric pressure can be calculated based on elevation or measured by a barometer.

What is Atmospheric Pressure?

Andrew Gatt
This bottle was sealed at 10,000 ft above sea level then moved to the beach. At the beach the bottle spontaneously crushed by the increased atmospheric pressure

 

Atmospheric pressure is the force per area that the air around us compresses our world. Above is a photo with a simple illustration of atmospheric pressure. At roughly 10,000 feet above sea level, the bottle is sealed trapping the atmospheric pressure inside the bottle. As the bottle drops in elevation, the pressure outside the bottle rises compressing bottle and the air inside.

When do I use Gauge Pressure?

Gauge pressure is used in a majority of industrial applications. For instance, EXAIR’s air nozzle performance is based on 80 Pounds per Square Inch Gauge (PSIG). No matter what elevation the air nozzles are used the flow rate and the force of the nozzle will be the same as long as the gauge at the inlet to the nozzle reads 80 PSIG.

When do I use Atmospheric Pressure?

I seldom use atmospheric pressure by itself. I often use atmospheric pressure in conjunction with gauge pressure. Meteorologists reference atmospheric pressure when referring to low pressure or high pressure weather systems.

When do I use Absolute Pressure?

In one word: calculations. Absolute pressure is equal to gauge pressure plus atmospheric pressure. In a majority of formulas or calculations, absolute pressure is used. Specifically, whenever you are using pressure to multiply, divide, or raise to a power, absolute pressure is used. There may be exceptions, but I would need to be very familiar with the formula, before I would only use gauge pressure to multiply. For instance, if you need to calculate the air usage at of an air nozzle at a different pressure (as seen in this earlier blog), you would use the absolute pressure. The flow through a nozzle is governed by Bernoulli’s principle.

Dave Woerner
Application Engineer
@EXAIR_DW
DaveWoerner@EXAIR.com

 

Photo Courtesy of Andrew Gatt. Creative Commons License