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

Super Air Knives vs. Other Alternatives

There are many ways to blowoff, cool, and/or dry materials.  A few of these methods are the drilled pipe, an array of flat nozzles, using a blower driven air knife and the EXAIR Super Air Knife.  We’ll examine each in further detail, for blowoff of water after a bottling cleaning operation.  Testing was done at 60 PSIG of supply pressure.  The blower utilized a 10 hp motor and was a centrifugal type spinning at 18,000 RPM.  Sound levels were taken with product not present to test the sound of each of the blowoff types.

SAK black1 (2)

pipe-black (2)Drilled pipe is a common blowoff because it is very inexpensive and easy to make.  But drilled pipe performs poorly.  The low cost to make the drilled pipe is quickly outpaced by the inefficiency and high compressed air costs.  The holes are easily blocked and the noise level is excessive, both of which violate OSHA requirements.  Also, the air pattern across the length can be very inconsistent, with areas of low flow and areas of turbulent flow.

flatnozzle (2)Flat air nozzles installed along a length of pipe is another inexpensive option, but it can be a poor performer.  The flat nozzles are available in many materials, from many manufacturers.  The flat nozzles do offer some efficiencies, but similar to drilled pipe, the operating costs and noise levels are high. Air pattern across the length can be inconsistent with areas of high and low flows, leading to incomplete drying or cooling. Also, many of these nozzles are made from plastic material which breaks or cracks when it it hit which causes additional expense and maintenance to replace broken nozzles.

blower (2)A blower air knife can prove to be an expensive and noisy option.  Typically, the initial purchase price is high.  Operating costs are lower than the drilled pipe and flat nozzles and in line with the Super Air Knives.  The blowers can be very large and space for two 3″ diameter hoses requires extra mounting space compared to low profile other options. Noise levels are high, at 90 dBA.  Annual costs for bearing and filter maintenance can be significant.

gh_SAK_750x696EXAIR Super Air Knives performed exceptionally well in removing the water in one pass due to the strong, laminar flow of air.  Sound level was low at just 69 dBA, well within OSHA requirements for an hour 8 hour exposure time. Safe operation is assured, as the Super Air Knife design cannot be dead-ended.  Maintenance costs are low, as the Super Air Knife has no moving parts to breakdown or wear out.

Air-Knife-Blowoff-Comparison
** A pair of 12″ Super Air Knives was used for this comparison

Ultimately, the Super Air Knife is a low cost way to blowoff, dry, clean and cool.

If you have questions about Super Air Knives, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_BB

EXAIR Super Air Knife: Overview

 

Loud and inefficient homemade blowoff

Drilled pipes, like the one shown above, are all too common in industrial settings for processes where a wide surface area needs to be treated. Their popularity can be attributed to how cheap and easy they are to make but in actuality they are very expensive to operate, as they waste large amounts of compressed air, and are very dangerous to operate.

We frequently take calls from customers looking for a more energy efficient, safer solution to replace these types of blowoffs. EXAIR manufactures 3 different styles of Air Knife – the Super, Standard and Full-Flow – that are the ideal solution for wide coverage applications. Today, I would like to provide an overview of our award wining Super Air Knife.

The Super Air Knife

The Super Air Knife is our most efficient air knife in regards to compressed air usage, using only 2.9 SCFM per inch of knife length @ 80 PSIG. It is also the quietest on the market today at only 69 decibels. The Super Air Knife provides the highest air velocity of the 3 styles offered by EXAIR and produces 2.5 ounces of force per inch at 80 PSIG operating pressure. We offer stock lengths from 3” up to 108” in single piece construction with available materials of aluminum, 303ss and 316ss. We also offer PVDF (Polyvinylidene Fluoride) up to 54” for harsh environments.

The Super Air Knife provides a laminar airflow across the entire length with hard-hitting force. They also give a 40:1 amplification rate meaning they entrain 40 parts of the surrounding room air for every 1 part of compressed air used, producing a large volume outlet flow.

Coupling Kit for the Super Air Knives

For applications requiring an air knife length longer than 108″, we offer a coupling bracket kit that allows you to connect two Super Air Knives together for a seamless, uninterrupted flow. Kits are available in aluminum, 303ss or 316ss to match the construction of the knife.

In addition, we also offer plumbing kits as an accessory item. For aluminum Super Air Knives, we offer cut to length nitrile/PVC hose and brass fittings and for stainless steel and PVDF knives we offer 316ss cut-t0-length pipe and fittings.

If you have any questions on how the Super Air Knife might fit into your process, please contact an Application Engineer.

Justin Nicholl
Application Engineer
justinnicholl@exair.com
@EXAIR_JN

Super Air Knife – Free Air Via a 40:1 Amplification Ratio

Intelligent Use of Compressed Air – Most industrial facilities have at least one air compressor.  The compressor is used to power anything from pneumatic tools, air powered equipment, compressed air cylinders, blowoffs and many more types of operations.  Improper use of compressed air can lead to unnecessary energy costs, high noise levels and dangerous exposure of personnel to high pressure air.

The EXAIR Super Air Knife uses only 1/3 of the compressed air of typical blowoffs.

LSAKpr_2mb
Long Super Air Knife with Plumbing Kit Installed and using the model 9060 Universal Air Knife Mounting Kit to Blowoff Laser Cutting Debris

By taking advantage of the Super Air Knife’s highly efficient design and the action of air entertainment, the Super Air Knife draws in large amounts of surrounding free outside air into the air stream. The result is a strong powerful air flow made up of a small amount of compressed air and a large amount of ambient air.

Capture

  • Compressed air flows through an inlet (1) into the plenum chamber of the Super Air Knife. The flow is directed to a precise slotted orifice. As the primary airflow exits the thin slotted nozzle (2), it follows a flat surface that directs the airflow in a perfectly straight line.  This creates a uniform sheet of air across the entire length of the Super Air Knife. Velocity loss is minimized and force is maximized as room air (3) is entrained into the primary air stream at a ratio of 40:1.  The result is a well defined sheet of laminar airflow with hard-hitting force and minimal wind shear is delivered.

By using a Super Air Knife – and the advantage of the high amplification via air entertainment – for part blowoff, cooling, or drying you can reduce energy costs, reduce noise levels, and eliminate harmful dead end pressures. Other air knives typically entrain surrounding air at a ratio of 30:1 or less.

EXAIR offers the Super Air Knife with materials of construction of aluminum, Types 303 and 316 Stainless Steel, and PVDF to cover a wide variety of application temperatures and environments. Other materials may be possible, pending review by our Product Design Engineers. The Super Air Knives are offered as the knife only, as part of a full kit, which also includes a shim set, auto drain filter separator, and pressure regulator.  The Super Air Knife can be fitted with Plumbing Kits and/or Electronic Flow Control making installation easier and help to save on air usage.

If you have questions about Super Air Knives, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_BB

EXAIR Super Air Knife Promotion Runs Through 12/31/17

EXAIR will be giving away a free Soft Grip Safety Air Gun with the purchase of any EXAIR Super Air Knife from November 1st through December 31st, 2017.  This special promotion will apply to all Super Air Knife orders, whether aluminum, stainless steel, or PVDF.  Use the promotional model number during the promotional period and receive a free model 1210, a value of $81.00! The Soft Grip Safety Air Gun is a Top Pick, and provides the ideal combination of forceful flow, quiet noise level and low compressed air usage.  To get the special promotional item, use the special part numbers as detailed in the link below.

sgsagpromo

The Super Air Knife is the latest generation of the EXAIR engineered air knives with dramatically reduced compressed air usage and noise compared to other types of blowoffs.  The Super Air Knife is truly a more efficient tool for cleaning, drying or cooling parts, webs, or conveyors.  The air flow is a uniform sheet of laminar air across the entire length with a hard-hitting force.

Replacing a noisy blowoff with a Super Air Knife results is a significant reduction in sound.  Operation at a high pressure of 80 PSIG (5.5 Bar) leads to a sound level of just 69 dBA for most applications. Air amplification ratios (entrained air to compressed air) of 40:1 is produced. It also meets OSHA maximum dead-end pressure and noise requirements, for safe operation in the vicinity of plant personnel.

Typical applications for use include-

  • Post wash part drying
  • Sheet cleaning in strip mills
  • Conveyor cleaning
  • Part or component cooling
  • Web drying or cleaning
  • Environmental separation (air curtain)
  • Pre-paint blowoff and cleaning
  • Bag opening/filling operations
  • Scrap removal on converting operations

Take advantage of the Promotional Offering by clicking on the link at the top of the page.  Qualified orders receive the free model 1210 Soft Grip Safety Air Guns as shown below.

1210
FREE! EXAIR Model 1210 Soft Grip Safety Air Gun

If you have questions about Super Air Knives, Safety Air Guns, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

Send me an email
Find us on the Web 
Like us on Facebook
Twitter: @EXAIR_BB

Super Air Knife Makes EVERYTHING Better

When we compare the EXAIR Super Air Knife to other methods of providing a curtain or sheet of air flow in terms of operating cost, efficiency, safety, and sound levels, the Super Air Knife is ALWAYS the clear choice.

The EXAIR Super Air Knife is the most efficient and quietest compressed air blow off product on the market today.

The Super Air Knives successfully replace these, and many other methods of providing a curtain or sheet of air flow all the time, while saving compressed air and decreasing noise.  The word “replace” oftentimes means “do the same job as.”

What you’re about to read is NOT one of those times.

A paper products manufacturer has a machine that treats a specialty product, and the process generates ozone (O3) at levels that would exceed personnel exposure limits, so they need to be contained.  They installed a long piece of drilled pipe to blow an air barrier, but they could only run the machine at about 65% of their desired capacity before the ozone level in the operators’ area exceeded their limits.

This company was familiar with several of our product lines already…they had several Cabinet Cooler Systems, a Reversible Drum Vac, and Super Air Knives in a variety of applications, so they knew how they worked.  Since the barrier needed to be 120″ long, though, this was going to be a much larger scale than they were used to.

Not only was the drilled pipe loud and inefficient, it was not particularly effective either.

Still, the installation of two Model 110060 60″ Aluminum Super Air Knives, coupled with our Model 110900 Air Knife Coupling Kit, was quick and easy.  Then came the good part: they found they were able to operate the machine at 100% capacity, while keeping the ozone at a safe level in the operators’ area.

EXAIR Super Air Knives provided a total solution: quiet, efficient, and most of all, EFFECTIVE.

Then came the better part:  The machine was pretty loud (we couldn’t do anything about that,) at 93dBA when it was running.  With the drilled pipe in operation, it was 94.5dBA.  When they took that out and installed the Super Air Knives, there was no net increase in noise level…it remained at 93dBA.

THEN came the even better part: Compressed air consumption was reduced to about 30% of what the drilled pipe was using.  Right in line with our table above.  Just another validation of the trustworthiness of our published data.  As EXAIR’s President is fond of saying, “Claims are easy, proof is hard.”

If you’re looking for a quiet, efficient – and effective – solution for a compressed air product application, give me a call.

Russ Bowman
Application Engineer
Find us on the Web
Follow me on Twitter
Like us on Facebook

Super Air Knife Plumbing Kit Allows Installation In Tight Quarters

I recently had the pleasure of helping a long-time user of our Super Air Knives with a challenging application. They already use quite a few of our Model 110012SS 12″ Stainless Steel Super Air Knives to clean & dry their nonwoven material as it’s being rolled for packaging. They like them because they’re quiet and efficient, but also because they’re durable…this particular product off-gasses a mildly corrosive vapor, which used to corrode other equipment in the area. Not only does the Stainless Steel Super Air Knife resist corrosion itself, the air flow keeps these vapors contained. Two birds, one stone.

They have a new product…same kind of material, but much wider…that needed to be blown off, and the identified the Model 110060SS 60″ Stainless Steel Super Air Knife as a “no-brainer” solution. Thing is, it had to be a pretty even air flow across the length, and a 60″ Super Air Knife has to get air to four ports across its length for optimal performance. And, they wanted to install it at a point where it would serve not only as a blow off, but as a vapor barrier, just like the 12″ Super Air Knives they’re already so fond of. The space was a little limited, though, so they opted for the Model 110060SSPKI 60″ Stainless Steel Super Air Knife with Plumbing Kit Installed, which allowed them to simply run an air supply line to both ends.

EXAIR SS Super Air Knives can be ordered with a Plumbing Kit installed, or you can easily install a Plumbing Kit on your existing Super Air Knife.

If you want to find out more about an engineered solution for your compressed air application – cleaning, drying, vapor barrier, or all of the above – give me a call.

Russ Bowman
Application Engineer
Find us on the Web
Follow me on Twitter
Like us on Facebook