Drying of materials Part 5.

In the previous tips on the subject of drying of materials the factors effecting dryness have been brought up. The only factor left is residence time in the hopper, or better yet have we applied the airflow, temperature, dryness of air to the material for the correct amount of time to allow it to become dry enough to process.

We have previously used an equation to determine CFM, now we need to look at time at condition. Basically it is straight forward in that if we process 100 pounds of material an hour our through put on the hopper is 100 pounds per hour we would than need a 400 pound hopper, if our residence time is 4 hours.

The key here is that our material must see the conditions for 4 hours. Now if we drop the material into the hopper and turn the dryer systems on, we than have approximately 4 hours plus before we can process material, please understand the plus is waiting for the temperature to get there.  Please note that if our drying system has not been used in a while we may need to dry cycle the unit to regenerate the desiccant beds if that is our dryer of choice. This is typically done with an empty hopper.

Now there are some common factors and those whom have attended seminars I or other have given may have seen the hopper model. One side of the model flows correctly and the other side of the model flows down the center of the model hopper. What this shows is that hopper design now comes into play as far as residence time is concerned. If the hopper is not correctly designed than no matter what we do we may not dry the material correctly.  Within your plant if you wish to test your hopper through put, / time you can use different color same material and time the time it takes to show up in parts. This is a simple test and will show how the hopper is working. The question here is it the 4 hours or did it come through in 45 minutes?

If your hopper does not have the capacity for your through put can you use two hoppers in tandem? Yes this is a common practice, but we must watch that everything is sealed with hoses and connections.

A final point here is loading the hopper on the machine. Understand that at 50% relative humidity that we can undo what just took 4 hours to achieve, that is dry the material. It only takes as little as 15 minutes at 50% RH to adsorb enough moisture to destroy our efforts of drying the material.

Thus loading material into the hopper with a besides the press dryer we may want to only load less than 15 minutes supply of material into the hopper. If using the bucket method, stay at 15 minutes or less, otherwise fill the hopper full and put an airtight lid that seals the hopper.

I have run into cases where in loading with vacuum loaders that enough air was sucked into the material stream that is became wet again before it was used in the process and the bucket method yielded a similar result in that every ½ hour we achieved brittle parts in a PC job.

Thanks for the time, have a great summer!

TA
Steven L Silvey
Silveys Plastic Consulting
silveysplastics@aol.com
360-882-3183