Schematic of conductive drying in a tray

We know that at t = 0 y = I then integrating Equation 11.23 by substituting 

The drying is completed when y = 0, which gives the time for drying as 

Another type of dryer where heat conduction is used for drying is vacuum rotary dryers also called a vacuum tumble dryer. Heat is supplied by warm water or any other heat exchange medium circulating through a jacket of rotating-cone drum. Vacuum is applied to be able to dry at lower temperatures and more rapidly. 

For scale up of these dryers, small scale dryers are used to obtain the time for drying and the following scale up equation is used. 

There are three steps involved in freeze drying, freezing, primary drying, and secondary drying. 

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