What is Fluidization?
The term fluidization is used to describe what happens when a finely ground powder is subjected to air and vibration in prescribed quantities. With accurate adjustments, the powder performs in the fluid bed equipment similarly to a liquid or fluid. In other words, fluidization is a process where a granular material is converted from a static solid-like state to a dynamic fluid-like State.
Fluidization can occur either with a gas or with a liquid. There are some majors differences that we won’t go through because they are out of scope, but particulate or smooth fluidization can only occur with liquids while aggregative or bubbling fluidization can occur with gas.Gas will be the main focus of this article since this aggregative state is what we are using to coat epoxy coating powders with busbar dipping.
Gas fluidization generally occurs in two separate phases, one being a dense, continuous phase and the other being the bubble or discontinuous phase that we also call aggregative fluidization.
Aggregative fluidization as a gas-exclusive attribute has traditionally been is defined by the Froude number:
When the Froude number is <1 the fluidization is particulate while when it is >1 it is considered aggregative.
Froude Number = u2mf/gd
Froude number is a function of flow velocity, gravity and particle diameters. While there are newer and better methods to calculate aggregative and particulate fluidization we’ll leave them for university classes.
What are the principles of a fluidized bed?
Fluidized beds are used in industrial applications such as powder coat painting or, for example, in grain silos to make sure that the grain flows smoothly to the outlet.
A fluidized bed is created when a gas flow is introduced at the bottom of a bed. This process occurs when a fluid is passed up upwards through the granular material such as epoxy coating powder. The gas flow will move upwards through the grains via the empty spaces between the particles and when the velocity is high enough it will begin to counteract the gravitational forces. The weight of the grains is then compensated through the upward flow of the gases. This is due to the frictional drag on the particles becoming equal to the apparent weight (the actual weight minus the buoyancy force)
This will increase the space between the grains allowing them to move among each other more freely. The internal friction between the powder particles gets reduced and as a result, they are starting to swim around like particles in a liquid.
This process continues with an increase in velocity with the total frictional force remaining equal to the weight of the particles until the bed has assumed its loosest stable form of packing. If the velocity is then increased even further, the individual particles separate from one another and become freely supported in the fluid. And that’s when we have a fluidized bed!
To simplify it, it is a fight of wit between gravity and airflow. After you reach an equilibrium by applying air pressure, anything above that will make the powder float. This is a big pressure gradient that has to be optimized in order to push fine particles upwards while leaving large, high volume chunks at the bottom of the bed.
Factors contributing to optimum fluidization are:
Vibration: Sufficient vibration is needed to break up geysers (columns of air) that may build up in the powder.
Air: Insufficient air results in a dense bed of powder. Too much air causes geysers, excessive dust and erratic bed settling times.
Particle Size Distribution: To assure consistent coating, particle size distribution must be maintained. Fines depletion can cause problems in consistency from dip to dip. Bed density will increase as the fines are lost, either through non-recirculating dust collection systems or selective deposition which is caused by the vibration not being set correctly. This causes the powder to segregate, with the coarser particles going to the outside and the fines moving toward the middle where they are picked up by the heated part. As the fines are lost the bed becomes denser requiring more air and causing inconsistent coatings.
You can read more about fluidized bed troubleshooting in Application of Powder Coatings for Electronic Component Protection.
How can I make a fluidized bed in my lab?
There are industrial fluidized beds as well as “ghetto” fluidized beds that you can make with simple materials in your lab. You can achieve this by putting tubes in a spiral shape on the bottom of your container. These tubes need to have small holes and then be connected to a high air pressure source. You can’t just have one big inlet. It’s not going to work. You need multiple spread out holes or ideally a membrane that manages the air flow into the bed. Holes need to be angled sideways so that they won’t get stuffed with powder when the air flow stops.
That’s pretty much it. Of course there’s no optimization to be done but this is a way to play around with the idea. You can contact us for some detailed designs if needed or we can coat stuff for you if you want to test some materials.
We also offer multiple electrically insulating coating powders that can be used for coating low and medium voltage busbars. These busbars are typically dipped into an fluidized bed (electrostatic or not) and after multiple dips you are able to achieve the desired thickness.
Powders such as LINQSOL BCP1507 and LINQSOL BCP1509 have been proven to coat thick under various temperatures. You can read more about their performance in the busbar application pages or in the product pages.
Do you have a busbar that needs coating? Do you need to select a electrically insulating powder for your busbar application? Contact us with your process and application requirements and we’ll make sure to find something that fits your needs