How to Calculate the Effects of Back Pressure on a Vortex Tube

Karate Chop

As any like-minded parent would do, I woke up this morning with the intention of scaring my son before breakfast.  As he came down the stairs, I tucked myself into an unlit corner of the adjacent room.  Just at the right moment I walked toward him really fast, not saying a word.  Got him!  He almost karate chopped me, but managed to see it was me before swinging.

So, that was the start of my day.  Now I’m helping people all over the world karate chop their compressed air use and integrate EXAIR products into their applications.  For example, I worked with our distributor in the U.K., Good Hand U.K. to determine the effective cooling capacity of an EXAIR Vortex Tube at higher than normal operating pressure, and with a back pressure above 5 PSIG.

In such a case, the cooling capacity of the Vortex Tube can be calculated as follows:

1.  Calculate the absolute pressure ratio with the back pressure

2.  Determine the effective pressure coming through the cold end with the non-typical back pressure

3.  Correlate the new, calculated effective pressure, to the Vortex Tube Performance Chart to determine the temperature drop (hold this value aside for use in later equation)

4.  Calculate the new air consumption based on the calculated effective pressure

5.  Multiply the new air consumption by the cold fraction value

6.  Enter these figures into the equation below to determine the new cooling capacity

BTU/hr. = K ΔTc (CFMc)

Where:  K = 1.0746

ΔTc = (100 – (Inlet compressed air temperature – Temperature drop created by Vortex Tube)

CFMc = Actual cold airflow from Vortex Tube under operating conditions

Using this information, we can calculate the effective cooling capacity of a Vortex Tube for any application.

For example, if we were to use a 3225 Vortex Tube in an application that desired a panel temperature of 100°F, with an operating pressure of 125 PSIG, compressed air temperature of 70°F, and a back pressure of 10 PSI, we can determine the cooling capacity as follows:

Calculate the absolute pressure ratio with the back pressure

(125PSIG + 14.7PSIA) / (10PSIG (backpressure) + 14.7 PSIA) = 5.66

Determine the effective pressure coming through the cold end with the non-typical back pressure

(X + 14.7) / 14.7 = 5.66

X + 14.7 = 83.2

X = 83.2 – 14.7

X = 68.5

This is the new, effective operating pressure of the Vortex Tube

Correlate the new, calculated effective pressure, to the Vortex Tube Performance Chart to determine the temperature drop (hold this value aside for use in later equation)

Considering a Cold Fraction value of 70%, we will achieve approximately 62°F in temperature drop

Calculate the new air consumption based on the calculated effective pressure

X / 25 SCFM = (68.5 PSIG + 14.7 PSIA) / (100 PSIG + 14.7 PSIA)

X / 25 SCFM = 83.2 / 114.7

X / 25 SCFM = 0.73

X = 18.25 SCFM

Multiply the new air consumption by the cold fraction value to determine volume of cold airflow

18.25 SCFM * 0.7 (70% Cold Fraction) = Actual volume of cold airflow from Vortex Tube

12.8 SCFM of actual cold air flow

Enter these figures into the equation below to determine the new cooling capacity

BTU/hr. = K ΔTc (CFMc)

BTU/hr. = 1.0746 * (100 – (70 – 62)) * 12.8

BTU/hr. = 1,265

So, in this case, the effective cooling capacity of the 3225 is decreased over 400 BTU/hr. simply due to back pressure.  For this reason, EXAIR Application Engineers always recommend to keep back pressure on a Vortex Tube below 5 PSI.  This ensures the best cooling and most efficient use of the compressed air.  This example also highlights the importance of compressed air pressure and compressed air temperature when using a Vortex Tube.

Lee Evans
Application Engineer

Compressed Air and Halloween Candy

Tomorrow night is Halloween. I live in a suburban subdivision with a high ratio of young families, which makes for a target-rich environment for Trick or Treaters…my boys included. We’ll allow them to enjoy a sensible portion of their haul tomorrow night, and the rest will go into the cabinet over the stove for rationing out over the next week or so.

First to disappear will be the brand name chocolates, candy bars, peanut butter cups, etc. The occasional bags of pretzels/salty snacks will find their way into school lunch bags quickly. Novelty lollipops – the kind with candy or gum in the middle – go fast in my house, but only after the chocolate has been completely exhausted. Individual hard candies linger at the bottom until they’re forgotten about, and eventually get thrown away the next time we need one of those pans we keep in that cabinet…usually when we’re preparing Thanksgiving dinner.

One of the trade publications I read regularly is “Compressed Air Best Practices” Magazine. Every month, there are featured articles that highlight how someone just saved a TON of compressed air by applying various methods and fixes to their systems. Understandably, the opportunities for the largest savings are the most popular…let’s call these the “brand name chocolates.” Common examples of this are:

  • Fixing leaks – I know of a company that saved a million SCF per year this way. (Spoiler alert: it was us.)
  • Replacing open ended blow offs with engineered products. (Spoiler alert again: we’re the undisputed industry leader for this.)

Of course, you can’t manage what you can’t measure. If you’re serious about efficiency, you’ve already got flow meters in place. If not, it’s time for a look at what’s available, and how much of a benefit you’ll get from knowing what your usage is at any given time. Continuing with my candy surplus analogy, these could be considered the novelty lollipops. Departing from the analogy, though, this should be done first (OK; it’s not a perfect analogy). An EXAIR Digital Flowmeter will give you instant, accurate indication of your air usage, and you can make a “before/after” comparison, once those leaks are fixed and your blow offs are upgraded.

Lastly, don’t forget about regular maintenance. Our USB Data Logger is a great addition to the Digital Flowmeter – it allows you to track your usage over time. Those leaks you just fixed weren’t there when the system was new. If you start to see your usage creeping up, you’ll want to find out why. Our Ultrasonic Leak Detector is a great tool for periodic checks. Use it to find any new leaks that pop up, and you’ll stop throwing away compressed air like it was candy.

Russ Bowman
Application Engineer
EXAIR Corporation
(513)671-3322 local
(800)923-9247 toll free
(513)671-3363 fax

Improve Your Compressed Air System: Improve Point of Use Applications

While compressor controls and efficiency are an important part of any comprehensive compressed air audit, so too, are your point of use applications. Many times these point of use locations are quickly and inexpensively improved. The first step is to identify which area of your system you would like to improve first. Certainly you will have that “problem area”, the part of the plant you know is using compressed air more than it should. This area of your plant is usually outfitted with open tubes that have the ends crimped down as a homemade nozzle or the operators are using blow-guns with commercial grade nozzles or worse yet, no nozzle at all. It’s the area of the plant that may require hearing protection due to the loud hissing of air or where that pipe with drilled holes was the quickest and cheapest fix for the application (or so you thought).

Document these areas of the plant and address these points of use by measuring the current consumption. Many times, we find, the volume of air provided by open tubes, inefficient nozzles and drilled pipes is much more than is required for the application.  Accurate compressed air measurement will be important to properly calculate the compressed air cost and savings. These points of use can be retrofitted or optimized in a couple of ways. First, you can retrofit open tubes by placing a compression fitting and engineered air nozzle on it. This will both reduce the air consumption and noise levels within the plant. Drilled pipes have holes, or slots, along the length to provide a wide area blow off. These applications can show dramatic improvement by using compressed air knives or air amplifiers which are engineered to reduce air consumption, reduce noise and maintain OSHA Compliance for dead end pressure. The second way to improve these end use applications is to install pressure regulators and lower the end use pressure which will result in lower air use.

Don’t let these end use applications go unchallenged, just because they were this way when you joined the firm does not mean they should not, or cannot be improved upon. If you get the right folks involved and keep them updated about the actions or changes you are making, you will find advocates for the projects. Remember that quantifying the savings is key so don’t start without measuring how much air you are currently using at these problem areas. Flow meters on each leg of your system or at specific high use areas of the plant will prove invaluable to providing data expressed in dollars of savings to those making decisions within your firm. The compressed air supply side personnel will also be helpful in locating or prioritizing where to start saving compressed air. Keep employees and management informed of savings and improvements and the savings ball will have more potential to keep on rolling.


  • Measure – baseline the current conditions of compressed air use with flow meters
  • Upgrade – retrofit inefficient open blow offs, commercial grade nozzles, drilled pipes etc. with engineered  and intelligent compressed air products
  • Control air pressure – lower pressure results in lower air consumption

If you would like any assistance or support to improve your compressed air system, we’re here to help.

Kirk Edwards
Application Engineer

Evolution of the Super Efficient Super Air Nozzle

To survive an ever demanding competitive market and meet sustainability goals, efficient use of compressed air energy is mandatory. EXAIR is there to help with its line of engineered nozzles. The following video demonstrates the progression from an inefficient open pipe, to EXAIR’s first energy efficient Safety Air Nozzle, and to the current super efficient and quiet Super Air Nozzle.

If you would like assistance in selecting the right product for your application call our application engineers at 1-800-903-9247

Joe Panfalone
Application Engineer
Phone (513) 671-3322
Fax (513) 671-3363

Testing A Freeze Out Valve

I recently had a customer contact me about an application where they were testing freeze out valves that go on the exterior of a locomotive.  The valve would be holding back about 45 psi of water in the pipe, and should open when the valve temperature reaches 35°F.  This would be used to keep any of the coolant / water lines from freezing during outdoor storage.

Mechanical Thermostat

The customer was using a chemical spray to freeze the mechanical thermostat and test that the valve opens.  Recently, an operator that was testing the valve used the wrong chemical and was hospitalized due to exposure so they decided it was time to change their testing method.

The customer purchased a Cold Gun Aircoolant System from our website, however, their compressed air temperature was 85-90°F and the Cold Gun is preset for a 50°F temperature drop at 100 PSIG inlet pressure. This means it wouldn’t quite reach the 35°F target reliably. Fortunately, EXAIR has a number of cooling products to cover an wide array of applications and our Adjustable Spot Cooler fit the application well since it can be adjusted to achieve much colder temperatures, up to a 100°F temperature drop from compressed air temperature.

EXAIR performed a test for the customer with the 25 SCFM generator installed and operated at 100 psig inlet pressure.   The valve was kept at a room temperature of 71°F and the compressed air inlet temperature was approximately 72°F.  The result was the valve would open in just under a minute.  This was even faster than the chemical test and it is adjustable to allow for the variance in the ambient temps. Success!

If you have any questions about how to use an Adjustable Spot Cooler in your application, please let me know.

Brian Farno
Application Engineer

US Economy Growing and Energy Usage Lower

The National Resource Defense Council (NRDC) issued a report this October that America had used the same amount of energy (measured in BTU’s) as it had used in 1999.  We have reduced our energy usage per person, while still growing the economy, a feat that I would not have thought possible.

  Economy and Energy Growth

You can read the full report here: NRDC Energy Report. So often environmental news is dire and gloomy, but this news shows the power of energy efficiency.  As noted in the report, politicians and media members focus on where we are going to find new energy resources.  As opposed to opening up new energy reserves, we have reaped larger rewards from spending time and money conserving the energy over the last thirty-five years.  The energy report uses the household refrigerator as an example of an appliance, whose increasing energy efficiency greatly decreased our electrical load per person.  Refrigerators use 1/4 as much energy as the same size refrigerator used in 1975.  This decrease in energy usage is a huge gain for the user who replaces their refrigerator and for the power grid that doesn’t need to build a new power plant to keep up with the increased load. Average Household Refrigerator Energy Use As an EXAIR employee, I can not help but notice that EXAIR opened in 1982, which is one of the first years in the graph above were economic growth was not directly tied to energy usage.  At EXAIR, we realize the impact of conserving compressed air can have on your compressed air system.  By replacing home-made blow offs like open tubes or holes drilled in pipe with Super Air Nozzle or Super Air Knife engineered solutions, you conserve compressed air and save money. This also reduces wear on the your compressor and can extend its life. A model 1100 Super air nozzle uses 14 SCFM when fed with 80 PSIG, which is a 58% reduction from 1/4″ open copper tube, which uses 33 SCFM when fed with 80 PSIG.  Go to our Air Savings Calculator to see how much compressed air and money you can save by replacing those home made blow offs.

Dave Woerner
Application Engineer

Shock Cooling on Road Simulator

Some time ago I had the opportunity to see a setup which cycles shock absorbers prior to their market release.  This particular design mimicked the contraction and expansion of the shock as if it were installed on a vehicle. Earlier this week I had a chance to work through a similar application on a more realistic setup.

Road Simulator

The photo above shows a complete chassis resting on wheel pans that simulate road conditions.  During testing, the shock absorbers increase in temperature because the entire system is stationary and there is no airflow to remove heat.  To make the testing more accurate and to cool the shock absorbers, Super Air Amplifiers were recommended to provide large volumes of air directed over the shocks and other chassis components.

This application is perfect for an Air Amplifier because Air Amplifiers are efficient at providing large volumes of air to a given area.  They can be used for cooling, light blow off, and even fumigation.

Contact an EXAIR Application Engineer for more information.

Lee Evans
Application Engineer

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