Laminar Flow vs. Turbulent Flow – Calculations and Examples

Super Air Knife

What is laminar flow and turbulent flow?  Osborne Reynolds popularized this phenomenon with a dimensionless number, Re. This number is the ratio of the inertial forces to the viscous forces.  If the inertial forces are dominant over the viscous forces, the fluid will act in a violent and chaotic manner.  The formula to determine the Reynolds number is as follows:

Equation 1:  Re = V * Dh/u

Re – Reynolds Number (no dimensions)

V – Velocity (Feet/sec or Meters/sec)

Dh – hydraulic diameter (Feet or Meters)

u – Kinematic Viscosity (Feet^2/sec or Meter^2/sec)

The value of Re will determine the state in which the fluid (liquid or gas) will move.  If the Reynolds number, Re, is below 2300, then it is considered laminar (streamline and predictable).  If Re is greater than 4000, then it is considered turbulent (chaotic and disarrayed).  The area between these two numbers is the transitional area where you start to get small eddy currents and velocities in a non-linear direction.  When it comes to effective blowing, cleaning and lower noise levels, laminar flow is optimal.

Let’s do a comparison of Reynolds numbers between the EXAIR Super Air Knife and a blower-type air knife.  Both products are designed to clean and blow off wide areas like conveyor belts.  The EXAIR Super Air Knife is powered by compressed air, and the blower-type air knife is powered by an air blower.  The main difference between the two products is the dimension of the slot opening.  The Super Air Knife has a gap opening of 0.002″ (0.05mm).  It uses the force of the compressed air to “push” it through the small opening to create a strong velocity.  A blower does not generate a high force, so the opening of the blower-type air knife has to be larger to overcome any back pressure the opening creates.  The gap opening is typically 0.5″ (13mm).  From Equation 1 above, the gap opening helps determine the hydraulic diameter, Dh.  The hydraulic diameter is an equivalent tube diameter from a non-circular flow area.  Since both the Super Air Knives and blower-type air knives have rectangular cross sections, the Dh can be calculated as follows:

Equation 2: Dh = 2 * a * b/ (a + b)

Dh – Hydraulic Diameter (feet or meter)

a – Gap Opening (feet or meter)

b – Gap Width (feet or meter)

If we compare for example a standard 12″ wide air knife, we can calculate the hydraulic diameter, Dh, by using Equation 2:

Hydraulic Diameter Calculations

 

The exit velocity of the Super Air Knives can be changed by regulating the air pressure.  The higher the air pressure, the higher the velocity.  The blower type air knives can use a blower with a variable frequency drive (VFD) to change the exit velocity .  A reasonable air pressure for the Super Air Knife is 80 PSIG, and the exit velocity is near 540 ft/sec (164 m/s).  To equate this to a blower system, the size of the blower will determine the maximum velocity.  To do this comparison, I will use the same velocity as the Super Air Knife.  With the kinematic viscosity of air, it has a value of 0.000164 ft^2/sec (0.000015 m^2/sec) at 70 deg. F (21 deg C).  Now we have all the information for the comparison, and we can now find the Reynolds number from Equation 1:

Reynolds Number Calculation

As you can see from the above calculations, the Super Air Knife has a Reynolds number, Re, below 2300.  The flow characteristic is in the region of laminar (predictable and streamline).  The blower air knife has a Reynolds number, Re, above 4000.  The flow dynamic coming out of the blower-type air knife is turbulent (chaotic and disoriented).  To better show the difference in laminar flow and turbulent flow, I have a picture below that shows both states with water as a fluid (being that air is an invisible fluid).   Here is an example of water  coming out of a drain pipe at Cave Run Lake (first picture below).  With the inertial forces much higher than the viscosity of the water, it is in a turbulent state;  loud and disorderly.  Reynolds number is greater than 4000.  The water is traveling in different directions, even upstream.  As the water flows into the mouth of the river after the channel (second picture below), the waves transform from a violent mess into a quiet, calm stream flowing in the same direction.  This is laminar flow (Re is less than 2300).

Turbulent Water from Pipe
Turbulent Water from Pipe

 

From Channel to River
From Channel to River

With the engineered design of the Super Air Knife, the thin slot helps to create that laminar flow.  All the air is moving in the same direction, working together to give a higher force to remove debris.  If you have turbulent flow like that of a blower air knife, the noise level is much higher, and the disoriented forces are less effective in blowing.  Turbulence is useful for mixing, but horrible for trying to clean or wipe a conveyor belt.  If you have any open pipes, drilled pipes or blower-type air knives in your application, you should try an EXAIR product to see the difference.  An Application Engineers can help you take advantage of laminar airflow.

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

Glass Filled PEEK Super Air Knife w/ Brass Hardware & PTFE Shim? No Problem!

That’s right folks, we’ve gone and done it again.  When a customer calls for custom product because their environment calls for it or due to dimensional requirements, EXAIR has the ability and flexibility to meet those needs!

This time around it was a customer with specific material requirements due to their environment. I had a customer contact me recently that was using an aluminum Super Air Knife near a high voltage operation and was getting ground interference due to the aluminum air knife.  They asked if it was possible to make them a custom knife out of PEEK plastic.  After some light discussions about the form of the knife and what other materials are safe for their environment we settled on a 30% glass filled PEEK plastic for the knife, brass bolts and pipe plugs, with a PTFE shim installed.   The form factor of the knife would follow the same shape as our PVDF Super Air Knives that are available from stock.   The customer could not use PVDF due to high temperature and potential off-gassing in the process.

The results are shown below.

IMG_6590
EXAIR 6″ Super Air Knife in 30% Glass Filled PEEK Plastic w/ Brass Hardware and PTFE Shim
End View – 6″ Super Air Knife in 30% Glass Filled PEEK Plastic w/ Brass Hardware & PTFE Shim

Whether you are looking for a one off product that is tailor made to your application or want to have a simple feature like hardware material changed in a stock EXAIR product that you are incorporating into thousands of machines, we have the solution for you.

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

 

 

 

 

Super Air Knife Replaces Homemade Manifold

I recently worked on an application with a manufacturer who was having issues with their labeling process. The sticker label is applied to the side of their container by a print roller and then passes by a 6” homemade manifold system with 3 nozzles to help permanently affix it n(see below). They were experiencing irregularities/air bubbles in the label and realized they were getting an uneven airflow which was stronger at each end nozzle but the middle nozzle had very little flow. They were operating at around 80 PSIG and previously tried to lower the pressure but the label would start peeling off. If they increased the pressure they were experiencing tearing and ripping in certain areas of the label. Another issue was the loud noise level. They were having to stop the line and turn off the air so an operator could manually replace the label. They emailed me a picture of the manifold and asked if EXAIR could improve their process.

Homemade Manifold

After reviewing the picture and further discussing their application, I recommended using one of our 6” Aluminum Super Air Knives. The Super Air Knife , with a 40:1 amplification rate (surrounding ambient air to compressed air), provides a high velocity laminar sheet of airflow the entire length of the knife. By continuing to operate at 80 PSIG, the Super Air Knife will produce a velocity of 11,800 feet per minute (6” away from target object) and consume only 17.4 SCFM (2.9 SCFM per inch of knife) with a low noise level of only 69 dBA.

SAK

By replacing the manifold, the customer was able to improve their process, decrease their air consumption and increase their personnel’s safety.

If you are experiencing a similar issue or need help with a different compressed air application, please give us a call.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

Lost In The Din? Not With An Ultrasonic Leak Detector!

Have you ever found yourself in a noisy environment, trying to hear what someone is saying to you? They could speak up, but sometimes that’s not enough. You might find yourself cupping your hand to your ear…this does two things:

*It blocks a lot of the noise from the environment.  This could also be called “filtering” – more on that in a minute.
*It focuses the sound of the speaker’s voice towards your ear.

IMG_1339
“What? They’re ALL still RIGHT behind me?”

Now, this isn’t a perfect solution, but you’ll likely have much better luck with this in a busy restaurant than, say, at a rock concert. Especially if it’s The Who…those guys are LOUD (vintage loud). If you’re at one of their concerts, whatever your friend has to say can probably wait.

You know what else can be loud?  Industrial workplaces.  Heavy machinery, compressed air leaks, cranes, forklifts, power tools, cranky supervisors/personnel…there are lots of unpleasant but necessary (mostly) sources of sound and noise, right here, where we work.

In the middle of all this, your supervisor might just task you with finding – and eliminating – compressed air leaks…like the person I talked to on the phone this morning.  This is where our Ultrasonic Leak Detector comes in: in places with high noise levels, it could be difficult (if not downright impossible) to hear air leaks.

Most of that noise from the machinery, cranes, etc., is in the “audible” range, which simply means that it’s of a frequency that our ears can pick up.  In a quiet room, you could likely hear an air leak…all but the very smallest ones will make a certain amount of noise…but when a compressed fluid makes its way out of a tortuous path to atmospheric pressure, gets turbulent, and creates an ultrasonic sound it is a frequency that our ears CAN’T pick up on.

Not only does the Ultrasonic Leak Detector pick up on this ultrasonic sound, it can also block (or “filter”) the audible sound out.  It comes with a parabola and a tubular extension so you can hone right in on the area, and then the exact location, of the leak.

If you’d like to find out more about compressed air leak detection, how much you might be able to save by fixing leaks, or how this could make your supervisor a bit less cranky (no guarantees on that last one,) give us a call.

Russ Bowman
Application Engineer
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IMG_1339 courtesy of Rich Hanley  Creative Commons License

How To Choose An Air Knife

The EXAIR Super Air Knife has a prominent place near the front of our catalog, and THIRTEEN pages of photos, application details, performance data & specifications. It’s the most efficient and quietest product of its kind on the market, and our most diverse product offering in terms of size range, operational adjustability, materials of construction, and accessories available. For almost any general industrial air blow off application, the EXAIR Super Air Knife is the superior choice in terms of air usage, sound level, and capability.

EXAIR Super Air Knives come in a wide range of lengths, for a wide range of possibilities.
EXAIR Super Air Knives come in a wide range of lengths, for a wide range of possibilities.

As tireless champions of the causes of reducing air consumption and noise, we’re always going to promote these benefits of the Super Air Knife. Still, a caller asked me the other day, “Well, why do you still make the others?”…meaning, of course, our Standard and Full Flow Air Knives. Why, indeed:

*Given the same air supply pressure, the Standard Air Knife generates the highest force of our three styles. The amount of force applied isn’t always a prime consideration…if you think about one of the more “textbook” applications for an Air Knife, it doesn’t take a great amount of force to blow off dust and light debris from a conveyor belt…certainly this is a case where efficiency factors in: the lower air consumption of a Super Air Knife can pay for the cost difference between it and a Standard Air Knife in as little as three months of operation.

The Standard Air Knife has the highest force, for when it's needed.
The Standard Air Knife has the highest force, for when it’s needed.

Of course, if you’re blowing stubborn debris out of tight spaces, like gummy, greasy dirt that’s accumulating in the recesses of a finned tube heat exchanger, that extra force can make all the difference. No; the Standard Air Knife isn’t as efficient or quiet as the Super Air Knife, but it’s still a far cry better than a drilled pipe.

*While the Super Air Knife is pretty compact – you only need a few square inches of profile area to successfully mount it – the Full Flow Air Knife is even smaller, requiring not much more than one square inch of profile for mounting. With ports on the rear face (instead of the ends & bottom for the Super Air Knife,) they can fit in very tight quarters.

Low profile and lightweight, the Full Flow Air Knives are a great fit for tight quarters.
Low profile and lightweight, the Full Flow Air Knives are a great fit for tight quarters.

The Full Flow Air Knife is also the lightest weight for a given length. A 36” Aluminum Super Air Knife, for instance, weighs about 8lbs. The 36” Aluminum Full Flow Air Knife weighs less than 4lbs. Most of the time, 8lbs is a very manageable amount of mass to support, but there are situations where every ounce matters, and if yours is one of them, you’re looking at the Full Flow Air Knife all the way.

*The biggest (in the most literal sense) factor in Air Knife selection is, well…size. We make the Standard Air Knives in lengths to 48”, and the Full Flow Air Knives come as long as 36”. The Super Air Knives, however, are stocked in lengths from 3” to 108”, and can be coupled together for as long of an uninterrupted, steady, laminar air flow as you need.

At the end of the day, a majority of blow off applications can be handled just fine with any of our Air Knives. If you’d like to discuss your application and see which one is best for you, give us a call.

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