16 - What types of drying are there?
Plastics are generally dried by the transfer of heat by convection or radiation. Typical convection drying methods include hot-air, dehumidifying, compressed air and vacuum; infrared dryers are an example of the radiation method.
A convection dryer transfers heat to plastic via air, driving entrained moisture to the surface, where it is removed by an airflow. Radiation dryers, on the other hand, use electromagnetic waves to transfer energy to the plastic, thereby heating it. However, here as well, airflow is employed to wick away the moisture.
The plastic’s behaviour in relation to air humidity is key to the choice of dryer type. Is the material hygroscopic or non-hygroscopic? Does the plastic contain fillers or reinforcing materials that may absorb moisture?
Hot-air dryers are suited to non-hygroscopic or slightly hygroscopic plastics. They are used to pre-warm the material and remove surface moisture.
As they transfer heat via ambient air, results are dependent on ambient humidity – and therefore current weather conditions. Consequently, drying performance differs between summer and winter, even when drying temperature is unchanged.
Dehumidifying dryers are suitable for all plastics. In practice, there are two methods. The first employs dehumidified air from a dry-air generator (adsorption drying), while the second uses dry compressed air. Adsorption drying takes place within a closed loop. Pre-dehumidified hot air flows through the drying hopper, extracting moisture from the granulate. The air, now entrained with moisture, subsequently passes through a desiccant bed, which adsorbs the moisture. A separate process extracts the moisture from the desiccant bed, regenerating it.
Water’s boiling point is a direct function of air pressure. This is the principle underlying vacuum or low-pressure dryers.
You need help with drying equipment? Find our full range of different models on www.motan.com