Fluidics is an interesting discipline of physics. Air, in particular, can be made to behave quite peculiarly by flowing it across a solid surface. Consider the EXAIR Standard and Full Flow Air Knives:
If you’ve ever used a leaf blower, or rolled down the car window while traveling at highway speed, you’re familiar with the power of a high velocity air flow. Now consider that the Coanda effect can cause such a drastic redirection of this kind of air flow, and that’s a prime example of just how interesting the science of fluidics can be.
As fascinating as all that is, the entrainment of air that these products employ contributes to another principle of fluidics: the creation of a boundary layer. In addition to the Coanda effect causing the fluid to follow the path of the surface it’s flowing past, the flow is also affected in direct proportion to its velocity, and inversely by its viscosity, in the formation of a boundary layer.
This laminar, lower velocity boundary layer travels with the primary air stream as it discharges from the EXAIR products shown above. In addition to amplifying the total developed flow, it also serves to attenuate the sound level of the higher velocity primary air stream. This makes EXAIR Intelligent Compressed Air Products not only as efficient as possible in regard to their use of compressed air, but as quiet as possible as well.
If you’d like to find out more about how the science behind our products can improve your air consumption, give me a call.
The following short video explains the differences between the 3 styles of Air Knives offered by EXAIR – The Super, Standard and Full-Flow. All of these Models are IN STOCK, ready to ship, with orders received by 3:00 PM Eastern.
If you need additional assistance choosing your EXAIR Air Knife, please contact an application engineer at 800-903-9247.
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.