Drying of materials Part 3.

In the previous installment we gave the basis of the drying process, in that we need to heat the material up, run a dry air stream over it and allow the moisture in the pellet to jump into the air stream passing over it.

So let us start as previously stated with factor “b” temperature of our pellets.

When we talk about temperature we are discussing the actual temperature of the pellets inside our hopper. Now this is a where we have issues in that we have no way of actually measuring the temperature of the pellets in the hopper. What we rely on is to measure the air temperature going into our hopper.

Given that we are measuring the air temperature and correlating this to the temperature our pellets see, where we take the temperature is very important. This is why we should be taking the temperature at the air inlet to the hopper. Some newer dryers on the market already have a thermocouple located at this junction. Older models may have a dial thermometer at this point to read but not to set, and yet many I run across have a cap head bolt, pipe plug or other means of blocking the port so as not to be able to take temperature.

Please note that the set temperature and actual temperature at the inlet can vary with length of hose feeding it along with the airflow of material. If you have a new dryer than you may be okay, or maybe not dependant on the unit and logic. I have been known to run the dryer set point upwards of 100º F above what I want so as to achieve the correct temperature at the inlet.

Why is temperature so important? We need to get the pellet to thermally expand so that the molecules of water inside the pellet have room to move. This is why we will say it is a minimum temperature necessary to dry the material. If we do not reach the minimum temperature the pellet does not expand enough and the water will never come out of it when we pass dry air over it. Some material may have a range, or the drying temperature varies per grade of that type of material from a manufacture, an example would be PC/ABS, due to different percentages of components for different grades.

Can we go too high on temperature? YES... This yields what has lovingly been called the Texas pellet. Can we go too cold? Yes, with the results that we never dry the material.

In the previous article we talk about airflow and non full hoppers. Now add heat to the situation and even if you are at proper temperature you may not dry material as not all material in the hopper is seeing the airflow and thus the temperature.

Thanks for the time, to be continued.

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