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

Summer is Coming: EXAIR Cabinet Coolers Save Your Electrical Cabinets from Overheating

The pool is open, the garden is tilled, it must mean summer is coming. This year, I’m gearing up for a hot one. My love for spicy food is already well documented here on the EXAIR Blog, and this year it’s going to be the hottest one yet! In the garden for this year we have Purple Ghost Peppers, “regular” Bhut Jolokia (Ghost peppers), White Moruga Scorpion, Purple Ghost Scorpion, Bhutlah Scorpion, and a selection of “milder” peppers and other veggies for when I’d rather not melt my face off while eating. Needless to say, you should proceed with caution when eating salsa at my house. It’s going to be heating up in my kitchen at the same time your electrical cabinets also begin to overheat. Not to worry, EXAIR has the tools you need to keep things cool.

Some of what is in store for this year’s garden, acclimating to the “real’ sunshine in late April

EXAIR’s Cabinet Cooler Systems were designed specifically to rectify these issues within your facility. Utilizing Vortex Tube technology, the Cabinet Cooler produces cold air from an ordinary supply of compressed air. This cold air keeps the enclosure free of debris and moisture and is easily installed in minutes through a standard electrical knockout. Here is a short video that shows just how simple it really is. The Cabinet Cooler Systems are available with Nema 12 (IP54) ratings and are also available in Aluminum, 303 Stainless Steel, and 316 Stainless Steel construction for Nema 4/4X (IP66) rated enclosures. For systems that are not able to be mounted on top of the cabinet, we also have Side Mount Kits available in Aluminum, 303 Stainless, and 316 Stainless. This year, EXAIR also introduced a new line of Hazardous Location Cabinet Coolers for use in classified areas.

Many of our customers experience seasonal overheating problems with their cabinets. Cabinet Cooler systems are a perfect solution for electrical panels which start to fault out in may or June and continue to cause chaos through September or October. They are quick to install, maintenance free and can be purchased with a thermostat control so they turn off in October and back on in May. You’ll hardly even remember its there and you won’t miss the electrical problems normally associated with a hot summer.

These systems are available with cooling capacities of anywhere from 275-5,600 Btu/hr. To make things much easier for you, we offer a Cabinet Cooler Sizing Guide that will allow us to recommend the most suitable model for your cabinet. With a few quick measurements, we’ll be able to determine the exact heat load that we’ll need to dissipate and offer you a quick and easy solution.

If you experienced heat related issues on electrical panels last year, or just want to talk about spicy food and gardening, contact an Application Engineer today and we’ll be happy to help. Don’t wait until it’s too late, EXAIR’s Cabinet Cooler is the simple solution for maintaining the temperature inside of your electronic enclosures.

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

The Case For The High Lift Reversible Drum Vac

If you need to fill -and empty – a drum with water, or water-like liquid quickly and easily, there’s really no quicker – or easier – way to do it than the EXAIR Reversible Drum Vac.  They’ve been around for decades, and are successfully used in a wide variety of applications.  The Reversible Drum Vac pulls a -96″H2O suction head on a 5, 30, 55, or 110 gallon drum, which makes it ideal for a number of typical Industrial Housekeeping or fluid handling applications.

EXAIR Reversible Drum Vac Systems come in a variety of sizes and configurations to meet the needs of most any application.

In cases where the liquid has a higher viscosity, or is below grade, the High Lift Reversible Drum Vac was developed with those exact situations in mind.  It generates a -180″H2O suction head, and comes with a 20ft vacuum hose.  Since its introduction, the High Lift Reversible Drum Vac has been successfully implemented in numerous uses where the extra suction head has been key:

  • A maker of bottled condiments (think barbecue sauce- or ketchup-like consistency) uses them to clean up accidents when bottles are overfilled, or a conveyor malfunction results in dispensing a bottle’s worth of condiment when there’s not a bottle under the nozzle.  The 20ft hose gives them the reach to service several production lines from one centrally located drum, and the two way pumping action allows them to easily pump the drum into their waste collection system.
  • A precast concrete company uses one for various cleanup applications.  The High Lift RDV‘s suction head is needed, in particular for their hydraulic oil leaks & spills.
  • A ferry operator uses one to clean out the engine room bilge.  They put the High Lift RDV on a deck above the engine room…the 20ft hose extends down to the bilge to pump it out, and when the drum is full, it reaches to the main deck so the drum can be emptied into their waste recycling company’s receptacle.
  • A construction company uses one to clean up the slurry created during concrete cutting operations.  The High Lift RDV is able to keep up with the slurry from even their largest saws, and the 20ft hose allows them to keep the drum conveniently out of the cutting area.

EXAIR High Lift Reversible Drum Vac Systems come in different configurations for 30, 55, or 110 gallon drums.

EXAIR Corporation has a variety of Industrial Vacuums to meet most any cleanup need.  They’re all compressed air operated, which means they have no electric motors to burn out, or moving parts to wear out.  If you’d like to find out more, give me a call.

***Order before 5/31/2021 and get a FREE Vac-u-Gun with any qualifying Industrial Vacuum System purchase***

Russ Bowman, CCASS

 

 

 

Application Engineer
EXAIR Corporation
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EXAIR Product Overview: Mufflers

Noise, we all hate it to some extent. From the hustle and bustle of crowed streets to the whine of a jet engine noise has plagued the world for eons leaving people to search for a way to escape into a moment of peace and quiet. The majority of people that I know pack their massive over sized backpacks and head deep into the mountains for days on end to escape the noise sometimes traveling for 10+ miles at a time. But how can we help eliminate this monstrosity that we have created in our manufacturing environments? The answer is mufflers, and no I don’t mean your car muffler (although they do the same thing) I mean compressed air mufflers. Compressed air can be a loud utility inside of a plant environment and exceed the OSHA guidelines for personnel noise exposure. But this noise can easily be mitigated with the use of Intelligent compressed air products and mufflers.

Big Sandy Lake Trail – Wind River Range State Park, WY

OSHA Standard 29 CFR 1910.95(a) outlines the total noise exposure to a particular noise level per day and dictates that noise exposure at or above 85 decibels require ear protection. By placing a muffler on the end of the pipe one can reduce the sound level significantly to the point it could be the difference between having to wear ear protection and not having to. With that being said EXAIR offers four different types of mufflers to choose from and they are Reclassifying, Sintered Bronze, Straight-Through, and Heavy Duty.

Reclassifying mufflers offer the best noise reduction at 35 dB and have the added benefit of removing oil mist from the air line. This means that the Reclassifying mufflers are ideal for pneumatic cylinders. Per OSHA Standard 29 CFR 1910.1000 worker shall not be exposed to more than 5mg/m3 of oil by volume in a 40-hour work week. The patented design of the removable element separates oil from the exhausted air and meets or exceeds the OSHA Standard.

Sintered Bronze Mufflers are an excellent low-cost solution which can be easily installed into your current existing ports. These mufflers also come in the largest variety of different sizes ranging from thread sizes of #10-32 to 1.5” NPT. Also, the Sintered Bronze Mufflers are specifically designed to provide the minimal amount of back pressure and restriction. The main difference between these mufflers and the reclassifying is that the Sintered Bronze Mufflers cannot collect oil out of the exhaust.

The quick pick chart for easily choosing which muffler you need

If the process air needs to be directly plumbed away from personnel, then the Straight-Through Muffler is the way to go. Straight-Through Mufflers are ideal for situations that require both a threaded inlet and exhaust. In most applications you will see the Straight-Through Muffler pair with our E-Vac vacuum generators or Vortex Tubes to provide noise reduction of the unit. All in all, the Straight-Through Muffler can reduce noise levels up to 20 dB.

Model 3913 Straight-Through Muffler

Lastly, the Heavy Duty Muffler provides a corrosion resistant aluminum outer shell with a stainless steel inner screen. This design allows the muffler to catch any contaminants such as rust from being ejected potentially causing harm or quality defects. Typically, this muffle will reduce noise levels up to 14 dB.

Model 3903 Heavy Duty Muffler

If you have any questions or want more information on EXAIR’s E-Vacs and their Accessories. Give us a call, we have a team of application engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
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