4.1 The exchange of mass and heat.
The exchange of mass and heat between dried sludge and air (material and factor) is of essential
significance for the drying process. [1] The heat exchange is achieved through radiation, convection
and conduction. Moisture mass goes from the area of higher concentration to the area of smaller
concentration as a result of diffusion.
While being in contact with the heated factor moisture from the surface of sludge evaporates to the
air. The rate of moisture evaporation differs for materials depending on their properties. It is highest
in the first phase of drying when water content in sludge is greatest. Then it diminishes. The rate of
moisture evaporation depends also on the contact surface of a drying medium and a dried material.
[1] The more extended the contact surface is the higher rate of moisture evaporation can be
obtained.
In case of sludge the extension of the contact surface can be achieved through granulation.
Granulation extends the contact surface of the drying medium and the dried material which keeps
the drying rate at a high level what is the basic condition for making the drying process
economically rational. [1]