Basically, the drying procedure depends on two families of variables:

• Design variables such as the dryer type, actuators, sensors, equipments and dimensions.Such variables appear when the dryer does not exist. The choice and the value of such variables are the first to be discussed in order to design and build the dryer.
• Manipulated variables (or decision variables or control actions) such that heating power, air flow rate and air humidity. After the design and the building of the dryer, the value of such variables has to be tuned off-line (before the drying) or on-line (during the drying) either manually or automatically.

In terms of control engineering, tuning of both design variables and manipulated variables needs to be discussed. But, due to lack of a multidisciplinary approach, usually control engineers are not involved in the decisions concerned with the design variables, even if these variables also have a large impact on the performance of the drying operation. For example, the type, location and number of sensors, that are very important for on-line control, are usually not considered seriously for this purpose during the process design stage. In this paper, our discussion deals only with the tuning of the manipulated variables, which is the
fundamental issue in control.