Boundary Layer: Laminar and Turbulent flow

Fluid mechanics is the field that studies the properties of fluids in various states.  Fluid dynamics studies the forces on a fluid, either as a liquid or a gas, during motion.  Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Re. This number determines the state in which the fluid is moving; either laminar flow, transitional flow, or turbulent flow.  For compressed air, Re < 2300 will have laminar flow while Re > 4000 will have turbulent flow.  Equation 1 below shows the relationship between the inertial forces of the fluid as compared to the viscous forces. 

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)

To dive deeper into this, we will need to examine the boundary layer.  The boundary layer is the area that is near the surface of the object.  This could refer to a wing on an airplane or a blade from a turbine.  In this blog, I will target pipes, tubes, and hoses that are used for transporting fluids.  The profile across the area (reference diagram below) is a velocity gradient.  The boundary layer is the distance from the wall or surface to 99% of the maximum velocity of the fluid stream.  At the surface, the velocity of the fluid is zero because the fluid is in a “no slip” condition.  As we move away from the wall, the velocity starts to increase.  The boundary layer distance measures that area where the velocity is not uniform.  If you reach 99% of the maximum velocity very close to the wall of the pipe, the air flow is turbulent.  If the boundary layer reaches the radius of the pipe, then the velocity is fully developed, or laminar. 

Boundary Layer Concept

The calculation is shown in Equation 2.

Equation 2:

d = 5 * X / (Re1/2)

d – Boundary layer thickness (feet or meter)

X – distance in pipe or on surface (feet or meter)

Re – Reynolds Number (no dimensions) at distance X

This equation can be very beneficial for determining the thickness where the velocity is not uniform along the cross-section.  As an analogy, imagine an expressway as the velocity profile, and the on-ramp as the boundary layer.  If the on-ramp is long and smooth, a car can reach the speed of traffic and merge without disrupting the flow.  This would be considered Laminar Flow.  If the on-ramp is curved but short, the car has to merge into traffic at a much slower speed.  This will disrupt the flow of some of the traffic.  I would consider this as the transitional range.  Now imagine an on-ramp to be very short and perpendicular to the expressway. As the car goes to merge into traffic, it will cause chaos and accidents.  This is what I would consider to be turbulent flow.      

EXAIR Digital Flowmeter

In a compressed air system, similar things happen within the piping scheme.  Valves, tees, elbows, pipe reducers, filters, etc. are common items that will affect the flow.  Let’s look at a scenario with the EXAIR Digital Flowmeters.  In the instruction manual, we require the meter to be placed 30 pipe diameters from any disruptions.  The reason is to get a laminar air flow for accurate flow measurements.  In order to get laminar flow, we need the boundary layer thickness to reach the radius of the pipe.  So, let’s see how that number was calculated.  

Within the piping system, high Reynold’s numbers generate high pressure drops which makes the system inefficient.  For this reason, we should keep Re < 90,000.  As an example, let’s look at the 2” EXAIR Digital Flowmeter.  The maximum flow range is 400 SCFM (standard cubic feet per min).  In looking at Equation 2, the 2” Digital Flowmeter is mounted to a 2” Sch40 pipe with an inner diameter of 2.067” (52.5mm).  The radius of this pipe is 1.0335” (26.2 mm) or 0.086 ft (0.026m).  If we make the Boundary Layer Thickness equal to the radius of the pipe, then we will have laminar flow.  To solve for X which is the distance in the pipe, we can rearrange the terms to:

X = d * (Re)1/2 / 5 = 0.086ft * (90,000)1/2 / 5 = 5.16 ft or 62”

If we look at this number, we will need 62” of pipe to get a laminar air flow for the worse-case condition.  If you know the Re value, then you can change that length of pipe to match it and still get valid flow readings.  From the note above, the Digital Flowmeter will need to be mounted 30 pipe diameters.  So, the pipe diameter is 2.067” and at 30 pipe diameters, we will need to be at 30 * 2.067 = 62”.  So, with any type of common disruptions in the air stream, you will always get good flow data at that distance. 

Why is this important to know?  In many compressed air applications, the laminar region is the best method to generate a strong force efficiently and quietly.  Allowing the compressed air to have a more uniform boundary layer will optimize your compressed air system.  And for the Digital Flowmeter, it helps to measure the flow correctly and consistently.  If you would like to discuss further how to reduce “traffic jams” in your process, an EXAIR Application Engineer will be happy to help you.

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

Customer Saves Nearly $7000 by Installing Super Air Knives on Converting Machine

EXAIR proves often that we’re able to work with you to create a customized solution that best serves your application. Recently I had the pleasure of working with a customer who wanted a better solution on their tissue paper converting machine. What they currently had was too loud, too inefficient, and they knew there was a better way.

The machine was an old rewinder used to convert webs of tissue paper ranging from 99-115” in width. Installed on the old machine was a 115” drilled pipe with 1/16” drilled holes spaced out every ½” along the length of the pipe. This was using a substantial amount of compressed air and was significantly louder than they would’ve liked. They purchased a new machine that had an EXAIR Super Air Knife already installed and working great, so they reached out to us for some help.

The customer conducted some time studies to determine exactly how much air this application required. The air blast ran for 500 seconds per hour, equating to 8.3min/hr of air usage. The operation runs 24/7, but with time spent doing changeovers the actual run time is closer to 20hrs.

20hrs x 8.2min = 166 min/day of air usage

166min x 365 = 60,590 min per year

A 1/16” unpolished, drilled hole will consume 2.58 SCFM at a pressure of 60 PSIG. With a total of 228 holes across the full pipe, this is quite a bit of compressed air.

2.58 SCFM x 228 = 588 SCFM of compressed air

588 x 60,590 min = 35,626,920 SCF

Considering the lightweight nature of the material, we recommended that the customer use our .001” shim to cut the flow from our stock Super Air Knives to their minimum. We recommended our Model 110054-.001 and Model 110060-.001. At 60 PSIG, a Super Air Knife with .001 shim installed will consume 1.15 SCFM/inch of knife length.

114 x 1.15 SCFM = 131 SCFM of compressed air

131 x 60,590 min = 7,937,290 SCF

Installing the Super Air Knives with .001” shim reduced their air consumption by 77% for a total air savings of 27,689,630 SCF each year. But, what does this mean in terms of money? To determine the cost of compressed air, we use the approximate value of $0.25/1000 SCF.

27,689,630 SCF x $0.25/1000 = $6,922.41

In just one year, on this one single machine, this customer was able to save almost $7k per year. These knives quickly pay for themselves, then begin to contribute to your bottom line. All of this in addition to lowering the sound level and providing a safer working environment for their operators.

If you have areas in your facility that are using air inefficiently, contact an EXAIR Application Engineer today.

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@EXAIR.com
Twitter: @EXAIR_TD

Stainless Steel Super Air Knife removes lime from conveyor belt.

An aggregate company crushed limestone into powder, and they were having issues in losing falling off of their conveyor belts.  The conveyor moved the lime from their processing plant to an outside silo for storage.  The conveyor was 60” wide and had a “cracked” pattern for gripping and moving the powder.  So, the surface of the belt was very rough.  As the conveyor went around the end to dump the powder in the silo, some of the material would stick in the valleys of the “cracked” pattern.  At the head pulley, as the belt made its return, the material would collect on the floor underneath.  For safety concerns, they would have to stop the operation to clean up the floor.  The customer wondered if the EXAIR Industrial Housekeeping Product would help them in their cleanup efforts.

EXAIR Heavy Duty Dry Vac Family

Yes, it will.   When it comes to industrial vacuums, wouldn’t it be nice to have a durable, long lasting vacuum that is quiet?  There is such a vacuum that has these qualities; the EXAIR Heavy Duty Dry Vac System.  This vacuum system has no moving parts, no motors to fail, or bearings to wear.  They only require compressed air to operate.  The Heavy Duty Dry Vac is used to vacuum solid materials like plastic, sand, and even denser materials like steel shot.  So, it would work great with the lime material.  The vacuum pumps are made from a hardened alloy construction to resist wear and abrasion, and it is designed to generate a strong vacuum level.  This would have been a great solution for cleaning for this company above.  But, as an Application Engineer, I was able to provide a better solution, the EXAIR Super Air Knife System.  Instead of cleaning the mess afterwards, we can stop the mess from happening.

Super Air Knife Kit

EXAIR can create a non-contact wiping process at the dumping end of the conveyor.  We have that ability with our Super Air Knife System.  At EXAIR, we pride ourselves in energy efficiency.  Compressed air is expensive to make, so why not use it as effectively as you can?  The Super Air Knife has a 40:1 amplification ratio which allows 40 parts of ambient “free” air for every 1 part of compressed air.  I recommended a model 110260SSPKI, 60″ Stainless Steel Super Air Knife Kit for this application.  This Super Air Knife is made from Stainless Steel for chemical compatibility with lime. 

EXAIR stocks the Super Air Knife at the 60” size for a continuous even blowing force across the entire length.  The kit included a filter, a regulator, and a shim set to “dial” in the minimum amount of force to remove the material without blowing the lime powder into the ambient air.  The “PKI” suffix at the end of the model number indicates our Plumbing Kit.  This option is Installed on the Super Air Knife to allow for easy installation and to allow for the proper inlet airflow to get a consistent blow-off device.  After installing the model 110260SSPKI, the powder was no longer collecting on the floor underneath.  If we look at the cost of the increase in lime material and the removal of custodial duties, the Return on Investment, ROI, was only 27 days.  The photo below shows the floor before installing the Super Air Knife.

Lime powder collection

Spillage is wasteful, costly, and time consuming to clean up.  If you have excess waste from your conveying system, EXAIR will have the product to help you.  For the aggregate facility above, the Super Air Knife Kit made it possible to increase production efficiencies with a short ROI.  And, since they liked the durability of our Industrial Housekeeping Products, they still purchased the Heavy Duty Dry Vac for cleanup in other areas in their facility.  If you want to improve your aggregate conveying operations with non-contact blowing or housekeeping, you can contact an Application Engineer to review your possibilities.   


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

Managing Pneumatic Sound

I recently took my daughter to a basketball game to watch Xavier University’s Lady Musketeers play. Due to Covid the arena felt empty and we could hear the players on the court and also the the coaches from the opposite side. If this was a regular season game we could barely hear ourselves let alone the teams and coaches. The obvious reason why there was not much noise is that there wasn’t much of a crowd and the crowd makes a lot of sound. So, what is sound?

Sound can be defined as “vibrations that travel through the air or another medium and can be heard when they reach a person’s or animal’s ear”. Sounds hit our ears at different pressure levels depending on its strength (“loudness” or volume) and is measured in decibels (dB).

When sound travels and comes into contact with a surface, a portion will be absorbed and another portion will be reflected. Manufacturing environments obviously can be the source of a lot of sound and personnel near the sources should be protected as much as possible. One extremely effective way to do this is to substitute a loud noise source with a quieter one or remove the source all together. PPE can be effective but is much less reliable due to people forgetting to use PPE, using PPE improperly or even deciding for themselves they do not need it.

To substitute or eliminate means something like the strategic placement of air compressor which is not near personnel or recognizing the type of product you choose to use is vital to sound management and the health and safety of people working near the point of use. Many EXAIR products can help you reduce the sound level of your current point-of-use compressed air by replacing commercial air nozzles, open pipes and homemade blowoff solutions with our Engineered Air Nozzles, Safety Air Guns, Air Amplifiers, or Super Air Knives. These products are all designed to minimize compressed air noise and can contribute to lowering the overall noise exposure of your personnel. The additional benefit is that you customers will also typically see a reduction in air consumption which saves money on generating compressed air.

EXAIR’s Digital Sound Level Meter is a tool used to identify and quantify the particular noise levels within an area. The source of loud noises can be quickly identified and isolated so corrective measures can be implemented.

Compressed air noise levels often exceed OSHA (Occupational Safety and Health Administration) noise level exposure requirements. EXAIR pneumatic products meet or exceed the OSHA Standard 29 CFR-1910.95(a) and can be used to reduce sound levels in your compressed air environment.

EXAIR has has many engineered compressed air products that can help reduce your sound levels. Our Application Engineers are ready and eager to help assist your sound level decrease projects. Please contact us at www.EXAIR.com so we can be a vital part of your successful sound reduction program.

Eric Kuhnash
Application Engineer
E-mail: EricKuhnash@exair.com
Twitter: Twitter: @EXAIR_EK