Principles of fluidizations and fluidized bed for coating powders

Fluidized bed for epoxy coating powders

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)

Epoxy coat­ing pow­der flu­idiza­tion states

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.

Pres­sure drop = weight of par­ti­cles — buoy­an­cy force on particles/bed cross-sec­tion­al area

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

About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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