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Understanding the challenge

Drying Bad

Why your functional films just won’t dry right 

Lab scale impingement dryers usually are just smaller batch-process versions of the impingement dryers used in roll-to-roll production plants. 

Unfortunately, standard impingement dryers with arrays of slot or round nozzles suffer from large deviations of the local heat and mass transfer coefficients, which in static applications aren’t evened-out through the lateral movement of the coated web. 

That’s why we end up with surface deformation, particle agglomeration or cracking and spallation of our lab-made coatings that finally lead to local deviation of product performance.

Who doesn't like to have a little edge?

"Research engineering without solid experimental data 
is like cereal without milk."

“The goal is to turn data into information, and information into insight.”
Carly Fiorina

Off-Center Nozzle: low HTC

Fluid from originating from center jets causes deflection of jets closer to the edges of the array, hence no impingement occurs any more. The flow-field resembles channel flow.

Wall Jet Center Nozzle: lower HTC

After deflection of the jet at target plate the boundary layer starts to grow resulting in decreased HTC.
Interaction effects between neighbouring jets
create complicated flow structures.

Center Nozzle: high HTC

Stagnation region resulting in minimum boundary layer thickness and maximum HTC.  Constant supply of non-contaminated drying fluid results in high drying rates.

* Experimentally determined distribution of heat coefficients for an array of round nozzle impingement jets without local fluid removal

Impinging jets come with the advantage of drastically enhancing transfer coefficients but with the downside of the enhancement being locally very limited in range. Impinging jets, slot or round nozzles grouped into arrays interact creating disturbances in the flow field.

Not controlling the removal of the spent fluid post-impingement ithe interaction effects make it impossible to realize high-quality batch-drying.

* Experimentally determined distribution of heat coefficients for an array of round nozzle impingement jets without local fluid removal

Impinging jets come with the advantage of drastically enhancing transfer coefficients but with the downside of the enhancement being locally very limited in range. Impinging jets, slot or round nozzles grouped into arrays interact creating disturbances in the flow field.

Not controlling the removal of the spent fluid post-impingement ithe interaction effects make it impossible to realize high-quality batch-drying.

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Drying Blind

You don’t know what you are doing…

Not even your fault. You just can’t. Because with your average lab-impingement dryer the only parameters you control are nozzle distance, time exposure and—if you are lucky—power input.

So much for working in the exact sciences in the 21st century...

Of course, eventually we get it all dry and kind of smooth. We are engineers after all. But how? Wouldn’t it be nice to know exactly what’s going on? That’s what we thought.

How we solved it

Step I.: Inspired by Nature

Re-assessing the nozzle
structure

To design a nozzle pattern offering a maximum balanced heat transfer distribution under batch-process impingement jet arrays the first step was to get the fluid jets hitting the substrate to cover the space on the substrate as efficiently as possible. There is a reason why many crystals, molecules and even honeycombs have a hexagonal structure:

It allows maximum use of space for equally tiled cells while minimizing the total perimeter of the compounds structures. Naturally, working against nature isn’t the smartest thing to do. So we followed its lead instead.

Unfortunately,—even with hexagonal nozzles—grouping jets at high array-densities comes at the price of messy interaction effects.

To solve this problem we created little air vents—they remove the spent fluid after it hits the substrate—and distributed them evenly around each comb nozzle. 

We coupled numerical investigation and experimental testing to find the best configuration that can be manufactured with current state-of-the-art SLM technology. Not only does it look slick, but it keeps interaction effects between neighbouring jets at a maximal minimum. And, in case you couldn’t tell, we are little proud of ourselves.

More Fluid Vents

There are a lot of them. Every nozzle has to share one air vent for each of his sides with a neighbouring nozzle. Thus, there are three times as many air vents as there are nozzles because our nozzles are combs. All working in perfect sync to remove exactly the right amount of spent drying fluid at their very location.

Did we mention they are living at the edge of manufacturability?

Trust us, they don't come any smaller than this.

Fluid Vents

The purpose of fluid vents is to remove all of the spent drying fluid, before it can mess with all the nicely calibrated jet streams of the surrounding nozzles.

Fluid vents are tiny, but really important.

One Comb Nozzle

with all his friends.

The actual size of the nozzles is pre-determined by the size of the fluid vents. The size of the fluid vents again is predetermined by  the smallest diameter that still lets air through and can be manufactured—it's bigger than you think.

Yes, it‘s this size.

* Our comb nozzle arrays, living at the edge of manufacturability, get 3-d printed in one piece from aluminum.

Fun fact:
 Honey bees don’t actually build the hexagonal honey combs in their bee hives either. They build round tubes (just like the impingement dryer engineers of the last century used to do).
However, the activity of all the bees buzzing around in their honeycombs doing their bee things creates so much heat (around 40° C) that the wax starts to soften up a little and then takes on the most geometrically efficient form it can have: Hexagonal structures.

* Our comb nozzle arrays, living at the edge of manufacturability, get 3-d printed in one piece from aluminum.

Fun fact:
 Honey bees don’t actually build the hexagonal honey combs in their bee hives either. They build round tubes (just like the impingement dryer engineers of the last century used to do).
However, the activity of all the bees buzzing around in their honeycombs doing their bee things creates so much heat (around 40° C) that the wax starts to soften up a little and then takes on the most geometrically efficient form it can have: Hexagonal structures.

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Step II.: Adding Control

Automatizing the drying process

Lastly, we had to create the necessary infrastructure that allows us to balance the inlet pressure for each jet with the outlet pressure of the surrounding air vents as well as making sure that each individual nozzle actually receives the same amount of air at the same temperature. The reason for all this…  should be fairly obvious.

Not as obvious is the fact that in order to create this steady, yet fully adjustable and easy controllable equilibrium we are employing a chain of hand-picked high-class components.

We would love to brag about the details of how all of these components play together and how our correlation based control algorithm enables you to simply set the drying conditions you need, but that could make the wrong people rich, i.e. Not us. So we'll practice moderation here.

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