Dual Bed (also called Twin Tower) dryers used to be the most commonly purchased type of resin dryer. In fact, it is estimated that there are between 10 and 20,000 of these dryers still in use today despite their disadvantages.
Dual Bed dryers consist of two beds (or towers) of desiccant beads, process and regenerative blowers, process and regeneration heaters and valves to re-direct the airflow. While one bed of desiccant supplies dry process air to flow through the drying hopper, the other bed – with saturated desiccant – is regenerated, by forcing hot air through it. When the regeneration is complete, that bed becomes the one supplying the dry process air and the first bed goes into the regeneration mode.
Dual Bed dryers are commonly used because they attained a -40˚ dew point and generally do a good job of drying most resins.
They do have their disadvantages:
High energy usage
Relatively high maintenance
Spikes and deviations in temperature and dew point during bed changeover
In recent years, there have been great advances made in drying technology and several types of dryers now overcome the disadvantages of Dual Bed dryers. These new dryer types include Desiccant Wheel, Membrane, Vacuum and Infrared versions.