Centrifuge dryer for plastic flakes

If we make too long you will not reading it but, if you pay some attention to this, you may save a lots of time and money.
And everybody already running a washing line knows what we are talking about.
Drying, like washing, is very related to the thickness of flakes so, if you're running a washing line for ground crates or battery cases, drying is not big deal because thickness is in the range of some millimeters that, for us, is a huge thickness.
Going down to microns, the matter is completely different.
There are four of five common drying systems available on the market:
- Centrifugal dryers (spin dryers)
- Squeezing presses
- Hot air systems
- Rotating screens with hot air crossing
- Agglomerators
Plus some more not very common
- Fluidizing beds
- Microwave continuous owens
- Infrared lights continuous owens
- Etc.
Among these, first drying step is anyway the removal of most of water when flakes come out from a sink float tank or any other dewatering system of any line, meaning when material has 50% or more water content.
And the only two ways we know of, and they are spin dryers and squeezing presses.
Our choice went to spin dryers for the following reasons:
- A squeezing press has an enormous torque on main shaft therefore to build it mechanically right one makes it very expensive.
- The hydraulically operating counter-pressure doesn't react in real time and this gives quite many problems. Too long to list.
- While water goes out, dirt, if any left, stays together with material.
- Sometimes presses melt some or all material so it needs to work together with a granulator or an agglomerator.
- Most of the time capacity is limited or to say it better, if throughput is high, drying effect is low.
- Very high maintenance.
- It works only with soft materials so the range of application is limited.

So, our choice went to spin dryers only, not because we say it's easy; it is just easier and more effective.
A spin dryer behave differently according to the following parameters:
RPM of the rotor.
Size of the holes of the screen basket.
Number of holes per square unit.
Residence time of material.
Production rate doesn't effect much the drying curve.
Here is a diagram of one of our standard spin dryer:


As we said before, performance can vary a little bit according to some parameters but this gives you a fair idea of what we are talking about.
For example, changing the holes size to a bigger dimension, certainly means increase the drying effect but also to increase the amount of fines coming out from the screen itself, so it is always matter to find the right compromise.
Another important matter is the cleanness of the screen basket; if holes become jammed by material, the drying effect decreases a lot, therefore continuous cleaning is strongly suggested.
As we said before, residence time, that means how long flakes stay into the basket or, in other words how many times flakes hit the screen, has a lot of importance.
And, believe it or not, there are way ways to control the residence time.