Example of Drying Process Calculation

The problems experienced in drying process calculations can be divided into two categories the boundary layer factors outside the material and humidity conditions, and the heat transfer problem inside the material. The latter are more difficult to solve mathematically, due mostly to the moving liquid by capillary flow. Capillary flow tends to balance the moisture differences inside the material during the drying process. The mathematical discussion of capillary flow requires consideration of the linear momentum equation for water and requires knowledge of the water pressure, its dependency on moisture content and temperature, and the flow resistance force between water and the material. Due to the complex nature of this, it is not considered here. 

We will cover a simple drying model to examine the radiation drier of coated paper. We assume there are no major temperature or humidity variations in the direction of the paper web thickness, and that temperature T and humidity u are constant in the direction of thickness. This assumption requires that the capillary action be ignored, and the pressure gradient of water is zero on the assumption hu/dx = dT/dx = 0. How is it possible that the humidity distribution remains uniform  

The only approach is to ignore the capillary flow and to assume water vaporization takes place evenly in the thickness of the paper web. With a 

To derive formula (4.318) for vapor flow in a porous material, we approximate the pressure gradient in Eq. (4.318) with 

FIGURE 4.38 (a) The uniform vaporization of water in paper, (b) Resistance web analogy for 

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