Fluid Mechanics – Boundary Layer

Fluid mechanics is the field that studies the properties of fluids in various states.  Fluid dynamics studies the forces on 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; laminar, transitional, or turbulent.  For compressed air, a value of Re < 2300 will indicate a laminar air flow while the value of Re > 4000 will be in the range of 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 the fluid dynamics, we can examine the layer which is next to the surface; the boundary layer.  This could refer to a wing on an airplane, a blade in a turbine, or inside compressed air lines.  In this blog, I will target the boundary layer inside pipes, tubes, and hoses that are used to transport compressed air.  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 you move away from the wall, the velocity starts to increase.  The boundary layer thickness 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.  Mathematically, laminar flow can be calculated, but turbulent flow requires theories and experimental data to determine. 

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.  Now imagine an on-ramp to be 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.     

In a compressed air system, similar things happen within the piping scheme.  Valves, tees, elbows, pipe reducers, filters, etc. are common items that will disrupt 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 where 99% of the air speed is represented at the center. 

Why is this important to know?  In many compressed air applications, the laminar region is the best flow 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 accurately 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

Photo: Smoke by SkitterphotoPixabay license

Compressed Air Digital Flow Meters Monitor Your System Demand

In the times we live in, efficiency is key. To offset the rising costs of goods, services, and energy we must be purposeful in lowering costs wherever we can. Every company has large utility expenses such as electricity, water, gas, sewer, waste, and recycling. Many companies have policies and systems in place to help control these expenses. One major utility that gets overlooked is your compressed air. Many companies just loop this expense into the gas and or electricity funnel and move on. But that can be a costly mistake. Assuming you utilize compressed air in your facility, it is most likely your 3rd-4th highest utility expense. The good news is there are many ways to make this utility much more efficient.

We have several EXAIR blogs on how to improve your compressed air efficiency from mitigating leaks, sizing pipe properly, flow control, pressure regulators, engineered nozzles and tools, and even receiver tanks. These are all very effective ways to reduce this expense. There is another tool that I would like to share with you; our Digital Flow Meters. What they are, how they work, and how they can save you money…

With a rough cost of $0.25 per 1000 SCFM, wouldn’t it be nice to know how many SCFM you are using? (Please click here for a great blog on how to calculate your SCFM cost) More importantly how much air are you wasting… Your compressor information already tells you how much air it is producing and with EXAIR’s Digital Flowmeter, you will know the exact amount of compressed air that is being used, making it very easy to identify loss. These losses are primarily found in leaks or inefficient air products. A best practice is to install one of these on each leg of the air distribution system and monitor and and benchmark the compressed air usage.

EXAIR Digital Flowmeter

The EXAIR Flowmeters work by measuring the temperature differential between two probes that are inserted into the compressed air pipes. One probe is kept warmer than the other, and the mass flow rate is determined by the amount of heat required to maintain the temperature differential. The flow rate, or SCFM is displayed on the large digital display

To install the Flowmeters, you will drill 2 holes into your pipe for the probes. Included with the meter is the drill bit and the Drill Guide to quickly install the meter. We have these available in many sizes from 1/2″ to 4″ iron pipe, We also offer these for Copper Pipes, and can make / calibrate them for many others, we will just need the information.

In addition to our standard version we offer several upgrades. One is a Data Logging version. You can download our software and then set the data logging to record from once a second for about 9 hours of data to twice a day for over 2 years worth of data. You simply plug the Data Log Stick this into your computer, download and repeat.

We also offer a wireless option that will run through a ZigBee mesh Network. A radio module within each meter transmits data to an ethernet connected gateway. You can also piggyback meter to meter to extend the range for this wireless solution. Each meter has a range of about 100 feet.

Many people ask if they have to shut down their system to install the Flowmeters. Understanding that this could cause systemic issues, we have a Hot Tap option that will allow you to install while the pipe is still under pressure. It incorporates 2 valves that the probes pass through as well as a muffler that collects the chips from the drilling process. This is only available on the 2″ or larger units.

Speaking of pressure, we also offer a Pressure Sensing Digital flow meter. On this, there is a pressure sensor that is mounted between the two flow sensor probes. The pressure is sent via a second milliamp output. The display can be configured to show either pressure or flow. You can set this to send alarms if the pressure falls under 50 psi. Also only available on 2″ pipes and above.

Finally, we also offer Block Off Rings. These are simply rings to block off holes where the flowmeter was, in case you need to use the same one in a different location.

Please fee free to reach out with any questions, or for more information. Let us help you save air and money…

Thank you for stopping by,

Brian Wages

Application Engineer

EXAIR Corporation
Visit us on the Web
Follow me on Twitter

Digital Flowmeters: Converting Them to Serial Communication

Model 9090-DAT

An international customer purchased three Digital Flowmeters, model 9090-DAT in yr2018.  This model number refers to the EXAIR Rev3 Digital Flowmeter with a Datalogger (photo above) for ½” Schedule 40 black pipe.  They used the Datalogger to log flowrates for their blow-molding machines.  A new engineering manager joined their team and was looking for a more “real time” reading of the flowrates.  They could compare flow data results from prior setups to monitor the program for their current blow-molded products with certain materials and sizes.  It was important for quality control to measure and compare the results to verify the proper machine set points.  They contacted EXAIR to see what we could offer.

EXAIR offers a variety of Digital Flowmeters for black pipe, aluminum tubes, and copper pipes.  They can accurately measure compressed air and nitrogen flow by measuring the temperature difference between the two probes.  The display is large for easy reading, and it can be programed for different units like SCFM, M3/hr, and M3/min.  For software data collection, we have the Wireless communication, USB Datalogger, and wire communication.  We also have Hot Tap versions, Pressure measuring, and Remote Display. 

In describing their current programming system, they were using a Modbus RS485 network.  This type of network system uses a “master” device to communicate with multiple “slave” devices.  In this instance, they were going to use a computer as the “master” device to communicate with the Digital Flowmeters.  With this type of serial communication, each Digital Flowmeter would have a particular and exclusive node address.  The master device can “open” the serial communication with that particular Digital Flowmeter and transfer data points of flow measurements.  The serial boards that EXAIR uses can connect to over 100 meters with a total length of 4,000 feet (1,212 meters).

Now, for our customer above, they did not need to purchase different Digital Flowmeters.  EXAIR offers a model 901785 serial communication board to convert the standard Rev3 Digital Flowmeter to connect to Modbus RS485.  They purchased three of them and converted each unit that they had without removing them from the compressed air system.  Since they were using the serial communication to connect to a computer, they requested an Ethernet connection.  EXAIR is able to convert serial communication to either Ethernet or USB connection with a converter.  With the Ethernet converter, they were able to connect directly to their computer.  They downloaded a software program at no charge to start monitoring and collecting flow information from all the digital flowmeters.  This improved the setup times for each machine.  As an extra bonus, they could also determine if they had some pneumatic issues with valves, cylinders, or leaks with the readings from the Digital Flowmeters. 

EXAIR Digital Flowmeters for large pipe

When you need to analyze your pneumatic components, flow is an important point.  The EXAIR Digital Flowmeter can give you that important data point.  You can cut energy consumption, improve pneumatic efficiencies, reduce setup times, and save yourself money.  If you are missing that detail with your pneumatic system, an Application Engineer at EXAIR can help you select the best Digital Flowmeter.  And if in the future you wish to upgrade, we may have a simple solution for you as seen with our international customer above. 

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

Video Blog: How To Remove, or Move, A Digital Flowmeter By Using Block-off Rings

If you need to remove (or move) a Digital Flowmeter, EXAIR has Block-off Rings that are used to safely cover & seal the holes that were drilled in the pipe for installation. Here’s how they work:

If you’ve got any questions about Block-off Rings, Digital Flowmeters, or would like to find out more about EXAIR Corporation can help you get the most out of your compressed air system, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
Visit us on the Web
Follow me on Twitter
Like us on Facebook