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

The Strength of the 1” High Power Flat Super Air Nozzles

1″ Flat Super Air Nozzles

A casting company used a die casting process to make large aluminum panels. In their operation, a two-part die would clamp together and be filled with hot liquid aluminum. Once the panel was formed and cooled, the die would open to release the part. Before the next panel was die casted, they would use a home-made cart to cool and clean the dies. The cooling was done first by spraying water onto the surface, then compressed air was used to dry the dies. When they started to use their home-made cart in their process, they noticed that the air pressure would begin to drop in their facility. Other locations in the plant started having problems with their pneumatic equipment.   They were using too much compressed air during the drying period; so, they contacted EXAIR to see if we could help reduce the amount of compressed air to dry the dies.

To explain a little more about the home-made cart, it was made from a 1” square piece of tubing that was bent in a U-shape. The dimension of the cart was about 40” long and 24” high. Across the top was a piece of extruded aluminum spanning the two ends of the U-shape tubing. This portion of the cart would supply the water to the liquid nozzles. The liquid nozzles hung vertically down from the extruded aluminum at designated heights to target certain areas of the dies. The U-shaped square tubing was used to supply the compressed air to the blow-off nozzles. The compressed air inlets were welded onto each end of the 1” square tubing. Across the bottom of the cart, the 1” square tubing had 38 holes that were drilled and tapped to 1/8” NPT (19 tapped holes on each side). The blow-off nozzles were 1/8” pipes with the ends smashed (reference picture below). They were made to different lengths to get as close to the die for maximum blowing force. The entire home-made assembly was attached to a robotic fixture with a cam to move the large cart between the dies. In applications using “smashed” pipes, they are very easy and inexpensive to make. But, as this customer found out, they use way too much compressed air and they are not as effective in blowing-off or drying.

Part of cart with 1/8″ flattened pipe

The customer above was limited to modifications to the home-made cart. It was already configured with the robot features and cam to hit the targeted areas. So, I recommended the model HP1126, 1” High Power Flat Super Air Nozzle. It has a 1” wide air stream that is very similar to the flow pattern of the 1/8” smashed pipe. But unlike the smashed pipe design, the model HP1126 nozzle can accomplish so much more. One of the biggest differences is that the EXAIR nozzles use much less compressed air. (The initial reason for contacting EXAIR). With the engineered design of the nozzle, it can entrain large amounts of ambient air which means that less compressed air is required. For a 1/8” NPT smashed pipe, it can use close to 70 SCFM of air at 80 PSIG – each!

The model HP1126 only requires 17.5 SCFM at 80 PSIG. That is a difference of 52.5 SCFM per nozzle. With 38 nozzles being used on this home-made cart, that equates to a total savings of 1,995 SCFM of compressed air. By simply replacing the 1/8” smashed pipe to a model HP1126 with a shorter nipple, their facility was able to save much compressed air and maintain the pneumatic requirements in the other work areas.

The customer was extremely happy with the air savings, but they asked about the amount of force that the model HP1126 can supply. It was important in their process to remove any residual water from the dies. The reason for the blow-off pipes to be so close to the die was to try and increase the blowing force. The best way that I could explain to them was by using an example of a garden hose. (Reference a blog by Neal Raker “Sometimes Back Pressure is Good; Sometimes it is Bad“).  The garden hose is attached to a spigot outside your house. As you open the spigot to supply water through the hose, the water will flow out of the hose at a slow velocity; not very strong. When you place your thumb partially over the end of a garden hose, you restrict the flow and increase the force. Now, you can reach the second-floor windows of your house to clean. With a lack of restriction at the end of the pipes, the air pressure will drop quickly as it travels through the long square tube and through the 1/8” pipe extensions. By the time the compressed air reaches the blow-off site, the pressure is much lower; thus, reducing the effectiveness of removing the water.

The EXAIR nozzles work like your thumb on the hose. The usable pressure is increased at the HP1126 nozzle, instead of a point much further upstream. By increasing the pressure at the point-of-use, the effective velocity and force is much stronger. In addition to this, they can now move the nozzles away from the die surface; in case of any “hiccups” in moving the cart in and out of the dies and eliminating any marring of the surfaces.

Once they installed the 38 pieces of the model HP1126 nozzles onto their cart, the first thing that they noticed was the amount of noise reduction. The model HP1126 only has a noise level of 82 dBA at 80 PSIG, compared to a noise level of an open pipe which is over 100 dBA. By replacing the flattened nozzles with the EXAIR nozzles, this company was able to…
1. reduce air consumption
2. keep the other areas of the plant operating by conserving compressed air at this location
3. reduce the noise level and
4. increase the effective blowing force

If you find that by using your blow-off/drying system, your pneumatic machines under-perform, or the low-pressure alarms are triggered, or you have to turn on an auxiliary compressor, you should contact an Application Engineer at EXAIR to see if we can optimize your compressed air devices. These EXAIR engineered nozzles can remove many issues in your system as it did with the casting company above.

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

Replacing Liquid Nozzles with Engineered Air Nozzles

I wrote a blog a few weeks ago about increasing efficiency with EXAIR Super Air Nozzles.  In the application for that blog we used engineered nozzles to place open pipes, resulting in an efficiency increased of ~65%.  This week’s installment of efficiency improvements boasts similar figures, but through the replacement of misused liquid nozzles rather than open pipe.

The image above shows a compressed air manifold with a number of nozzles.  BUT, the nozzles in this manifold are not compressed air nozzles, nor do they have any engineering for the maximization of compressed air consumption.  These are liquid nozzles, usually used for water rinsing.

In this application, the need was to blow off parts as they exit a shot blasting machine.  When the parts exit the shot blasting process they are covered in a light dust and the dust needs to be blown away.  So, the technicians on site constructed the manifold, finding the liquid nozzles on hand during the process.  They installed these nozzles, ramped up the system pressure to maintain adequate blow off, and considered it finished.

And, it was.  At least until one of our distributors was walking through the plant and noticed the setup.  They asked about compressed air consumption and confirmed the flow rate of 550 m³/hr. (~324 SCFM) at 5 BARG (~73 PSIG).

The end user was happy with the performance, but mentioned difficulty keeping the system pressure maintained when these nozzles were turned on.  So, our distributor helped them implement a solution of 1101SS Super Air Nozzles to replace these inappropriately installed liquid nozzles.

By implementing this solution, performance was maintained and system pressure was stabilized.  The system stabilization was achieved through a 61% reduction in compressed air consumption, which lessened the load on the compressed air system and allowed all components to operate at constant pressure.  Calculations for this solution are shown below.

Existing compressed air consumption:  550 m³/hr. (324 SCFM) @ 6 BARG (87 PSIG)

Compressed air consumption of (9) model 1101SS @ 5.5 BARG (80 PSIG):  214 m³/hr. (126 SCFM)

Total compressed air consumption of 1101SS Super Air Nozzles:

Air consumption of 1101SS nozzles compared to previous nozzles:

Engineered air nozzles saved this customer 61% of their compressed air, stabilized system pressure, improved performance of other devices tied to the compressed air system, and maintained the needed performance of the previous solution.  If you have a similar application or would like to know more about engineered compressed air solutions, contact an EXAIR Application Engineer.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

Adjustable Spot Cooler: How Cold Can You Go?

I had the pleasure of discussing a spot cooling application with a customer this morning. He wanted to get more flow from his Adjustable Spot Cooler, but still keep the temperature very low.  He machines small plastic parts, and he’s got enough cold flow to properly cool the tooling (preventing melting of the plastic & shape deformation) but he wasn’t getting every last little chip or piece of debris off the part or the tool.

After determining that he had sufficient compressed air capacity, we found that he was using the 15 SCFM Generator. The Adjustable Spot Cooler comes with three Generators…any of the three will produce cold air at a specific temperature drop; this is determined only by the supply pressure (the higher your pressure, the colder your air) and the Cold Fraction (the percentage of the air supply that’s directed to the cold end…the lower the Cold Fraction, the colder the air.)

Anyway, the 15 SCFM Generator is the lowest capacity of the three, producing 1,000 Btu/hr of cooling. The other two are rated for 25 and 30 SCFM (1,700 and 2,000 Btu/hr, respectively.)

He decided to try and replace the 15 SCFM Generator with the 30 SCFM one…his thought was “go big or go home” – and found that he could get twice the flow, with the same temperature drop, as long as he maintained 100psig compressed air pressure at the inlet port.  This was more than enough to blow the part & tool clean, while keeping the cutting tool cool, and preventing the plastic part from melting.

Model 3925 Adjustable Spot Cooler System comes with a Dual Outlet Hose Kit, and three Generators for a wide range of cooling performance.

If you’d like to find out how to get the most from a Vortex Tube Spot Cooling Product, give me a call.

Russ Bowman
Application Engineer
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Increasing Efficiency With EXAIR Super Air Nozzles

Earlier this morning I received a phone call from a gentleman in search of a more efficient compressed air solution.  The application was to remove thermoformed plastics from a mold immediately after the mold separates.  In the current state, the application is consuming ~40% of the available compressed air in the facility through the use of (9) ¼” open pipes, consuming a confirmed 288 SCFM at 60 PSIG.  Due to the use of an open pipe, this customer was facing a safety and noise concern through the existing solution.

After discussing the application need and the desire to reduce compressed air use, reduce noise, and add safety, we found a suitable solution in the 1101 Super Air NozzleInstalling (9) of these EXAIR nozzles will reduce the compressed air consumption by over 65%!!!  Calculations for this savings are below.

Existing compressed air consumption:  288 SCFM @ 60 PSIG

Compressed air consumption of model 1101 @ 60 PSIG:  11 SCFM

Total compressed air consumption of  (9) 1101 nozzles:

Air savings:

This is the percentage of air which the new EXAIR solution will consume.  To put it another way, for every 100 SCFM the current solution consumes, the EXAIR solution will only require 34.38 SCFM. Installing these EXAIR nozzles will result in lower operational cost, lower noise levels, and increased safety for this customer – all while maintaining or improving the performance of the blow off solution in this application.

EXAIR Application Engineers are well versed in maximizing efficiency of compressed air systems and blow off needs.  If you have an application with a similar need, contact an EXAIR Application Engineer.  We’ll be happy to help.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE

2” Flat Super Air Nozzles Separate Sheets of Metal Film

Heat Exchanger plates

An overseas company manufactures brazed plate heat exchangers. This type of heat exchanger has a series of corrugated plates that are stacked onto each other. It is designed to create a turbulent flow for better heat transfer in a very compact size. The plates inside the heat exchanger are made of 321 stainless steel which is basically a 304 type of stainless steel but with a titanium stabilizer. This company would receive plain sheets of stainless steel material that were stacked on each other in a column. The dimensions of the plates were as follows: 305mm wide by 520mm long with a thickness of 0.5mm (12” Wide X 20.5” Long X 0.02” thick respectively). Each sheet weighed 635 grams (1.4 lbs.). They would set a stack of the stainless-steel sheets at the beginning of a press machine. The press machine would form the corrugated design into the face of the sheet. They were using a pick-and-place vacuum system to lift one sheet at a time to place inside the press. They started having problems with their process when occasionally two or three sheets would stick together. The underlying sheet could either fall onto the floor which would bend the sheet or be stacked inside the press which would cause an improper corrugation. Both issues were causing much scrap as well as downtime in their process .

They contacted EXAIR to find a way to improve the efficiency of their process. They wondered if static could be causing the “sticking” issues. Generally, static forces are really noticed with sheets made of plastic or non-conductive materials. The stronger the static force, the more issues with sticking and misalignment. EXAIR does offer Static Eliminators to remove static forces in applications just like this. But, with plain metal sheets, static is not a problem as the ions are able to balance themselves.

Typically, the main cause for metal sheets to “stick” together is surface tension. Liquid like water has a strong affinity to itself within the molecular structure, called cohesion, and to the surface that it lies on, called adhesion. The cohesion plus the adhesion to the metal surface can have a strong enough force to overcome the weight of the sheets. To break the surface tension, an additional force is required.  An example of surface tension is with nylon tent material. The surface tension of water is strong enough to keep rain drops from penetrating the fabric. If you break the surface tension by touching the tent material, the surface will start to leak water. The same goes for the thin sheets of metal. We just need to break the surface tension to allow the sheets to separate.

2″ Flat Super Air Nozzle

I recommended two pieces of the model 1122, 2” Flat Super Air Nozzles. This nozzle gives a flat air pattern to force air between the sheets. Surface tension is based on force over length. Once the sheets start to separate, the contact length will decrease thus reducing the “sticking” force caused by surface tension. In this application, the amount of cohesion and adhesion forces caused by surface tension were unknown. Oil, water, and other liquids have different surface tensions which would require different amounts of blowing forces. To ensure the proper amount to separate the sheets, I recommended the shim set, model 1132SS.

The shims have different thicknesses that can be installed easily into the 2” Flat Super Air Nozzle to change the amount of blowing force.  In conjunction with a regulator, this customer could “dial” in the proper amount of force required to counteract the surface tension from any type of liquid that may be on the surface of the sheets.  I had them mount one nozzle at two different corners to help “peel” the sheets apart. The customer also tied in a solenoid valve into the compressed air system to cycle on the 2” Flat Super Air Nozzles only during the time when the vacuum system wanted to grab the top sheet. This reduced the amount of compressed air needed for their operation.  After the installation, the procedure ran smoothly without downtime and scrap waste.

If your application is creating scrap and downtime caused by sheets sticking together, EXAIR has many types of products to help eliminate this. Whether the “stickiness” is caused from static or liquid adhesion, an Application Engineer can direct you to the best product to eliminate the “stickiness”. For the overseas company above, we were able to apply a sharp flat burst of air to overcome the surface tension between the sheets.

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

 

Heat Exchanger Plates by epicbeerCreative Common by 2.0

 

Engineered Air Nozzles and Jets Outperform – Save Air, Increase Safety, Save Money

EXAIR’s Engineered Air Nozzles and Air Jets provide a simple solution to lower compressed air usage and reduce noise levels for compressed air blowoff operations.

Why Air Nozzles and Jets – When compared to commonly used open copper tubes or pipes, compressed air savings can be as high as 80%. And with less compressed air, sound levels are greatly reduced.  A 10 dBA noise level reduction is typical.  All EXAIR Air Nozzles and Jets meet the Occupational Safety and Health Administration (OSHA) maximum dead end pressure and sound level exposure requirements. They also carry the CE mark.

nozzlescascade2016cat29_559
The largest selection of engineered Air Nozzle sizes and materials are only available form EXAIR 

EXAIR Nozzles are engineered to take advantage of the Coanda effect to amplify the airflow up to 25 times or more. Compressed air is ejected through the small orifices and surrounding air is entrained into the main stream. The resulting air stream is a high volume, high velocity blast of air at minimal consumption.  EXAIR manufactures many styles, from the very small, but powerful Atto Super Air Nozzles, to the largest 1-1/4 NPT Super Air Nozzle.  Also offered are 1″ and 2″ wide Flat Super Air Nozzles, and the Back Blow style for cleaning out tubes, pipes, channels or holes from 1/4″ to 16″ in diameter.
EXAIR Air Jets utilize the Coanda effect (wall attachment of a high velocity fluid) to produce air motion in their surroundings.  A small amount of compressed air (1) is throttled through an internal ring nozzle above sonic velocity.  A vacuum is produced, pulling in large volumes of surrounding, or ‘free’ air, through an around the jet (2).  The exit flow is the combination of the two air sources (3).

air-jet
How an Air Jet Works

EXAIR manufactures Air Jets in two types, High Velocity, and Adjustable with materials of construction of brass and Type 303 Stainless Steel.  The High Velocity Air Jet uses a changeable shim to set the gap, controlling the force and flow of the air.  The Adjustable does not use a shim, and has a micrometer gap indicator and locking ring to allow for varying force and flow performance.

AirJetFamily
EXAIR Air Jets – High Velocity type on the left, Adjustable type on the Right

If you have questions about Air Nozzles and Jets, 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

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