The Need for Advanced Drying Technologies

A conventional dryer may be admirably suited for a specific application while another may need one to look outside the conventional set of dryers. One must set the criteria for selection and then see which one meets them better and more cost effectively. There is a cost associated with the risk accompanying the technology not verified in pilot scale. Most companies shun this and are prepared to pay a higher cost for a conventional technology—the premium is often considered an insurance premium rather than a cost. 

In some cases new drying technologies are sought simply because the current technologies have a limit in terms of the production rates possible. For example, today s modern newsprint machine is limited by the dryer speed. One can make the wet paper sheet faster than it can be dried cost effectively on the current multicylinder dryers. For higher speeds entirely new drying concepts are being evaluated. 

In the following sections we will review two evolutionary types of advances in drying technologies, specifically the intensification of drying rates and multistaging of convective dryers. 

It is obvious that reduction of the size of the dryer will lead to a reduction in initial capital cost. Although this should not be a deciding factor in the selection of an individual dryer, since only 10 to 15 percent of the life-cycle cost of a direct dryer is due to the initial capital cost of the drying system, it is still an important consideration as it can reduce the space requirement, duct sizes, size of ancillary equipment, etc., as well. One must intensify the drying rates without adversely affecting product quality in order to make the equipment smaller. 

Finally, since particle-to-particle heat transfer is more efficient (provided sufficient contact area) than between a gas and particles, the use of immersion drying (e.g., mixing hot inert particles with wet particles) can yield very high 

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