How to Calculate and Avoid Compressed Air Pressure Drop in Systems

EXAIR has been manufacturing Intelligent Compressed Air Products since 1983.  They are engineered with the highest of quality, efficiency, safety, and effectiveness in mind.  Since compressed air is the source for operation, the limitations can be defined by its supply.  With EXAIR products and pneumatic equipment, you will need a way to transfer the compressed air from the air compressor.  There are three main ways; pipes, hoses and tubes.  In this blog, I will compare the difference between compressed air hoses and compressed air tubes.

The basic difference between a compressed air hose and a compressed air tube is the way the diameter is defined.    A hose is measured by the inner diameter while a tube is measured by the outer diameter.  As an example, a 3/8” compressed air hose has an inner diameter of 3/8”.  While a 3/8” compressed air tube has an outer diameter that measures 3/8”.  Thus, for the same dimensional reference, the inner diameter for the tube will be smaller than the hose.

Why do I bring this up?  Pressure drop…  Pressure Drop is a waste of energy, and it reduces the ability of your compressed air system to do work.  To reduce waste, we need to reduce pressure drop.  If we look at the equation for pressure drop, DP, we can find the factors that play an important role.  Equation 1 shows a reference equation for pressure drop.

Equation 1:

DP = Sx * f * Q1.85 * L / (ID5 * P)

DP – Pressure Drop

Sx – Scalar value

f – friction factor

Q – Flow at standard conditions

L – Length of pipe

ID – Inside Diameter

P – Absolute Pressure

 

From Equation 1, differential pressure is controlled by the friction of the wall surface, the flow of compressed air, the length of the pipe, the diameter of the pipe, and the inlet pressure.  As you can see, the pressure drop, DP, is inversely affected by the inner diameter to the fifth power.  So, if the inner diameter of the pipe is twice as small, the pressure drop will increase by 25, or 32 times.

Let’s revisit the 3/8” hose and 3/8” tube.  The 3/8” hose has an inner diameter of 0.375”, and the 3/8” tube has an inner diameter of 0.25”.  In keeping the same variables except for the diameter, we can make a pressure drop comparison.  In Equation 2, I will use DPt and DPh for the pressure drop within the tube and hose respectively.

Equation 2:

DPt / DPh = (Dh)5 / (Dt)5

DPt – Pressure drop of tube

DPh – Pressure Drop of hose

Dh – Inner Diameter of hose

Dt – Inner Diameter of tube

Thus, DPt / DPh = (0.375”)5 / (0.25”)5 = 7.6

As you can see, by using a 3/8” tube in the process instead of the 3/8” hose, the pressure drop will be 7.6 times higher.

Diameters: 3/8″ Pipe vs. 3/8″ tube

At EXAIR, we want to make sure that our customers are able to get the most from our products.  To do this, we need to properly size the compressed air lines.  Within our installation sheets for our Super Air Knives, we recommend the infeed pipe sizes for each air knife at different lengths.

There is also an excerpt about replacing schedule 40 pipe with a compressed air hose.  We state; “If compressed air hose is used, always go one size larger than the recommended pipe size due to the smaller I.D. of hose”.  Here is the reason.  The 1/4” NPT Schedule 40 pipe has an inner diameter of 0.364” (9.2mm).  Since the 3/8” compressed air hose has an inner diameter of 0.375” (9.5mm), the diameter will not create any additional pressure drop.  Some industrial facilities like to use compressed air tubing instead of hoses.  This is fine as long as the inner diameters match appropriately with the recommended pipe in the installation sheets.  Then you can reduce any waste from pressure drop and get the most from the EXAIR products.

With the diameter being such a significant role in creating pressure drop, it is very important to understand the type of connections to your pneumatic devices; i.e. hoses, pipes, or tubes.  In most cases, this is the reason for pneumatic products to underperform, as well as wasting energy within your compressed air system.  If you would like to discuss further the ways to save energy and reduce pressure drop, an Application Engineer at EXAIR will be happy to assist you.

 

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

How to Calculate Compressed Air Consumption at a Different Inlet Pressure OR Math Doesn’t Lie and Neither Will Your Results

EXAIR Application Engineers field a wide variety of technical assistance questions. Many are quantifiable, and we just need to do a little math.  For instance:

Q. You publish the compressed air consumption of your products assuming a supply pressure of 80psig. What if my supply pressure is different?

A. Compressed air consumption is going to be directly proportional to ABSOLUTE pressure supply. That means you have to add atmospheric pressure of 14.7psia (a=absolute) to your gauge pressure, measured in psig (g=gauged, and zero on the gauge is atmospheric pressure,) and calculate the ratio. For example:

Our catalog publishes most products' performance and specification data for a compressed air supply pressure of 80psig.
Our catalog publishes most products’ performance and specification data for a compressed air supply pressure of 80psig.

Model 1100 Super Air Nozzle consumes 14 SCFM @80psig. How much will it consume @95psig?

1100 recalc

This is good news…if you need that extra amount of flow and force from a little higher pressure supply, you’re still FAR below the air consumption of an open-ended 1/4″ copper tube (33 SCFM @80psig or 38 SCFM @95psig)* or SCH40 pipe (140 SCFM @80psig or 162 SCFM @95psig.)*

*Using the same formula above.  Check my math if you like.  I’m right, but it’ll be good practice.  Those values come from this chart in our catalog, by the way:

open blow air consumption
You can get your own personal copy of our current catalog here.

Of course, if your application doesn’t need all that flow and force, this formula works the other way too…it, in fact, works in your favor, air consumption-wise.  Consider the savings associated with dialing back your supply pressure.  Let’s say, for instance, you replace a open ended 1/4″ SCH40 pipe with a Model 1100 Super Air Nozzle, regulate the supply down to 55psig, and find that it still does what you need it to:

1100 recalc-1

(Remember, the value you’re solving for is ALWAYS the numerator of the fraction, because…Algebra! )

Now, let’s do just a little more math.  Don’t worry; I’m almost finished.  Plus, this is the part you can show your boss and be the hero.  So, we find out that you’re saving 151.7 SCFM by replacing that open pipe blow off with a Super Air Nozzle, and regulating its supply pressure down from your full line pressure of 95psig to 55psig:

162 SCFM – 10.3 SCFM = 151.7 SCFM saved

You may know your facility’s cost of compressed air generation.  If not, $0.25 per 1,000 Standard Cubic Feet (SCF) is a reasonable estimate:

151.7 SCFM X 60 minutes/hour X 8 hours/day X 5 days/week X 52 weeks/year =

18,932,160 SCF/year X $0.25/1,000 SCF = $4,733.04 annual savings

Now, this is just an example…one in which a $34.00 (Model 1100 Super Air Nozzle’s current 2014 List Price) product pays for itself before the end of the second day (again, feel free to check my math and see how right I am.)  Keep in mind that your mileage, as they say, may vary, but the math…and our products’ performance…will hold true according to whatever your conditions are.

How much can you save by using engineered, Intelligent Compressed Air Products from EXAIR?  Call me, and we’ll start the process of finding out.

Russ Bowman
Application Engineer
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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
LeeEvans@EXAIR.com
@EXAIR_LE

The Case of the Missing Nozzle

I recently discussed Safety Air Guns with a customer who was the head of safety in his facility.  The issue he was having was all of the operators had been taking the regulators off his traditional style air guns to get the cleaning of their parts done.   When he asked them the reasoning, they would say there was no way they could get the debris removed when they are restricted to 30 PSIG.  They were simply not able to get the job done because they didn’t have a good distribution of air or enough force to get the job done.

I asked him what style gun he was using and he told me it was a cheap gun he got from his industrial supplier.  He gave me a picture of the gun below.

As you can see the gun has an extension on it which is simply an open pipe.  Because it is just a section of pipe and can be dead ended, they had to regulate the gun to lower than 30 PSIG inlet pressure to meet OSHA standards or face a fine.

The application was to simply remove metal chips from holes and cavities after a machining process.  They need the extension to reach around the extents of the part and the thinner pipe to get into some of the blind holes.   Our Precision Safety Air Gun was an ideal replacement for this gun.   The extension was nearly the same and because our Engineered Nano Super Air Nozzle is at the tip it can not be dead ended.  This means their operators can run full line pressure and still not be able to dead end the gun.

The other part to the equation is because they were using our Engineered Solution, they were also able to save compressed air.  The gun they were using would consume 19.3 SCFM @ 80 PSIG.  Our Precision Safety Air Gun is going to consume   8.3 SCFM @ 80 PSIG.  This means the customer will save $0.17 per hour of use on the guns.   Multiply that by the 25 guns they have in house means they will be saving $4.25 per hour of use for their parts cleaning operations.

If you have any hand held blow gun or lance operations in your facility that are using non engineered solutions, feel free to contact us and we’ll help you select the right Safety Air Gun along with assistance to calculate how much air and money you’ll be able to save by implementing the engineered solution.

Brian Farno
Application Engineer
BrianFarno@EXAIR.com
@EXAIR_BF