Laminar flow is an fundamental component of compressed air efficiency. Believe it or not, laminar flow is controlled exclusively by the airline used in a compressed air system. To fully understand the effects of laminar flow in a compressed air system, we need to explain exactly what it is.
Fluids & gases are unique in their ability to travel. Unlike solid molecules that remain stationary whose molecules tend to join others of the same kind; fluid molecules aren’t so picky. Fluid molecules, such as gases and liquids, partner with different molecules and are difficult to stop.
Laminar flow describes the ease with which these fluids travel; good laminar flow describes fluid travelling as straight as possible. On the contrary, when fluid is not travelling straight, the result is turbulent flow.
Turbulent air flow results in an inefficient compressed air system. This may not seem like a major concern; yet, it has huge impacts on compressor efficiency. Fluid molecules bounce and circle within their path, causing huge energy wastage. In compressed air systems, this turbulent airflow results in a pressure drop. How do you avoid this from happening? It all comes down to compressed air system design.
The design and material of the air pipe, as well as the positioning of elbows and joints, has a direct connection to laminar flow and pressure drop. To avoid high energy consumption of your compressed air system, reducing pressure drop is key.
If your system is experiencing high pressure drop, your compressor has to work overtime to provide the needed air pressure. When your compressor works overtime, it not only increases your maintenance costs, but also your energy bills.
A drilled pipe has been used for many years to blow compressed air across a span for cleaning, cooling, and drying. They are a simple tool that was created from spare parts and many holes. The cost to make this type of product is not expensive, but to use this product in your application is very expensive. Similarly, an incandescent lightbulb is inexpensive to purchase, but it will cost you much more in electricity than a LED light bulb. Since 1983, EXAIR has been innovating safe and efficient products to be used in compressed air systems. In this blog, I will compare the drilled pipe with the Super Air Knife.
Even though you can find the components relatively easily to design your own drilled pipe, this blow-off design is very costly and stressful to your compressed air system. Typically, the holes along the pipe are in a row next to each other. As the airstream leaves from each hole, it will hit the airstream from the one next to it. This will cause turbulent air flows which has inconsistent forces and loud noises. Also, with turbulent air flows, the ability to entrain the surrounding ambient air is very small. We call this the amplification ratio. The higher the amplification ratio, the more efficient the blow-off device is. For a drilled pipe, the amplification ratio is near 3:1 (3 parts ambient air to 1 part compressed air).
A colleague, Brian Bergmann, wrote a blog about the amplification ratio of the EXAIR Super Air Knife. (Read it HERE.) This blog demonstrates how EXAIR was able to engineer an efficient way to blow air across a span. The unique design of the Super Air Knife creates an amplification ratio of 40:1 which is the highest in the market. Unlike the drilled pipe, the gap opening runs along the entire knife for precise blowing. This engineered gap allows for laminar air flow which has a low noise level, a consistent blowing force, and maximum amplification ratio. With these benefits, the Super Air Knife can reduce the amount of compressed air required, which will save you money and save your compressed air system.
In comparing the drilled pipe to the Super Air Knife, I will relate both products in a simple cooling application. Thermodynamics expresses the basics of cooling with an air temperature and an air mass. Since both products are represented in the same application, the air temperature will be the same. Thus, the comparison will be with the amount of air mass. In this example, the customer did some calculations, and they needed 450 Lbs. of air to cool the product to the desired temperature. At standard conditions, air has a density of 0.0749 lbs/ft3. To convert to a volume of air, we will divide the weight by the density:
450 lbs. / (0.0749 lbs./ft3) = 6,008 ft3 of air
To meet this requirement, reference Table 1 below. It shows the volume of air required by your compressed air system to meet this demand. As you can see, your compressor has to work 13X harder to cool the same product when using a drilled pipe. Just like the LED light bulbs, the Super Air Knife has more efficiency, more innovation, and uses less compressed air. In turn, the Super Air Knife will save you a lot of money in electrical costs. If you would like to see how much the Super Air Knife can save compared to the drilled pipe, we have that information in this blog. (Read it HERE.) For my reference, it will reduce the stress of your compressed air system.
if you would like to compare any of your current blow-off devices with an innovative EXAIR product, you can contact an Application Engineer. We can do an Efficiency Lab to shine an LED light on saving energy and money with your compressed air.
Fluid mechanics is the field that studies the properties of fluids in various states. There are two areas, fluid statics and fluid dynamics. Fluid dynamics studies the forces in a fluid, either as a liquid or a gas, during motion. Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Reyonlds number. This number can indicate the different states that the fluid is moving; either in laminar flow or turbulent flow. The equation below shows the relationship between the inertial forces of the fluid as compared to the viscous forces. Reynolds number, Re, can be calculated by Equation 1:
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 mark the region in which the fluid (liquid or gas) is moving. If the Reynolds number, Re, is below 2300, then it is considered to be laminar (streamline and predictable). If Re is greater than 4000, then the fluid is considered to be turbulent (chaotic and violent). The area between these two numbers is called the transitional area where you can have small eddy currents and some non-linear velocities. To better show the differences between each state, I have a picture below that shows water flowing from a drain pipe into a channel. The water in the channel is loud and disorderly; traveling in different directions, even upstream. With the high speed coming from the drain pipe, the inertial forces are greater than the viscous forces of the water. The Reynolds number is larger than 4000 which indicates turbulent flow. As the water travels into the mouth of the river after the channel, the waves transform from a disorderly mess into a more uniform stream. This is the transitional region. A bit further downstream, the stream becomes calm and quiet, flowing in the same direction. This is the laminar flow region where Re is less than 2300. Air, like the water in the picture, is also a fluid, and it will behave exactly in the same way depending on the Reynolds number.
Why is this important to know? In certain applications, one state may be better suited than the other. For mixing, particle suspension and heat transfer; turbulent flows are needed. But, when it comes to effective blowing, lower pressure drops and lower noise levels; laminar flows are required. In many compressed air applications, the laminar flow region is the best area to use compressed air. EXAIR offers a large line of products, including the Super Air Knives and Super Air Nozzles that uses that laminar flow to generate a strong force efficiently and quietly. If you would like to discuss further how laminar flows could benefit your process, an EXAIR Application Engineer will be happy to assist you.
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:
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:
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).
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.
A company had a small converting machine that was winding a plastic film onto a roll. The width of the plastic film was only 3” across, and the amount of tension required for a consistent roll was small. The maximum amount of tension without damaging the plastic film was 16 ounces of force. In converting media onto rolls, it is very important to control the tension on the web to reduce defects like wrinkles, out-of-round rolls, or stretching.
They explained the setup that they were trying. They had a 4” manifold with two 2” wide “duck-foot” nozzles attached. They sent a hand drawing to better describe what they were using. (See below). The issue that they were seeing was too much variation in the blowing force being applied to the film. To get near the correct blowing force, they had to start at an air pressure of about 18 PSIG. As they ran the process, the operator would have to adjust the pressure continuously to evenly roll the film onto the core. The process was out of control, and they wondered if EXAIR had a better way to evenly exert this force.
In analyzing the drawing and their setup, I noticed a couple of things that could cause the variations. I modified his drawing to better explain the situation (Reference below). As compressed air leaves the two flat nozzles, the center section will overlap. This overlap will cause turbulence in the air flow pattern. In order to get an even distribution of forces across the width of the product, turbulence cannot exist. Turbulence is a mixing pattern where the velocity is not linear; thus, causing high and low pressure points on the target. The other thing that I noticed was the low air pressure that they could not go above. This limited the precision of the incremental forces. Because of the fixed openings of the two nozzles, they had to have a ceiling with the air pressure at 18 PSIG for 16 ounces of force. If they had to “bump” the force level, the change was difficult to hit exactly. If we divided the 16 ounces of force between 0 – 18 PSIG, we would get roughly 0.9 ounce of force per PSIG. You lose the accuracy to make fine adjustments.
I recommended our model 110003, 3” Super Air Knife and a model 110303 Shim Set. The Super Air Knife blows compressed air across the entire length. Without any overlap, the flow is laminar, and the velocity profile is moving in the same direction. Thus, an even force across the entire 3 inches. The Shim Set comes with additional shim thicknesses of 0.001”, 0.003”, and 0.004” thick (the standard thickness of 0.002” is installed in the Super Air Knife). In working with such a precise force requirement, they needed additional options for more control. They could change the shims as a coarse adjustment and adjust their pressure regulator as a fine adjustment. This combination gave them the best results to accurately dial in the correct force and not damage the material. With the maximum requirement of 16 ounces across 3 inches of film, they were able to change the shim to the 0.004” thickness. For the model 110003 Super Air Knife, it put them at a maximum pressure of 86 PSIG, not 18 PSIG. Thus the increment was now 0 – 86 PSIG for 16 ounces of force, or 0.19 ounces per PSIG. There was much more resolution to make smaller changes to the force levels thus optimizing their adjustment range.
In replacing the competitor’s product with a Super Air Knife, our customer had all the necessary control to wrap rolls of film without issue. The setup with the nozzles on a manifold design resulted in turbulence, which was noisy and produced inconsistent results. It also restricted their adjustment resolution in changing forces, as they do not use shims. If you would like to exert a greater degree of precision blowing with products like the Super Air Knife, please contact us. We would be happy to discuss your application and help you meet such goals.