The items to be considered prior to selection of dryers are explained, and a simple method for a rough estimation of dryer sizes was proposed based on data obtained from operating industrial dryers. The equations of basic design for batch or continuous type dryers were derived. The heat was supplied to materials by convection and/or conduction. The equations were simplified to the case when the falling rate of drying is proportional to the moisture content of materials under the constant drying conditions. The heat transfer coefficient used in the equations can be determined based on the calculations or the data obtained from the experimental or industrial dryers. The equations are useful for estimating the scale-up effect of dryers.

Conventional basic design for convection or conduction dryers

Chapter 6 – evaporation and humidification

The amount of moisture in the surrounding air has a crucial bearing on the rate at which a piece of wet wood will dry out. The ratio of water vapour to dry air, on a mass basis, is called thehumidity. This quantity is dimensionless, but at low moisture-vapour levels it is often convenient to record humidities in grams water vapour per kilogram of perfectly dry air. Sometimes values of the absolute humidity are reported on a volume basis. However, it is easier to follow changes in humidity throughout a kiln on a dry-air basis, since the mass of circulating moisture-free air is not altered by the drying process.

Evaporation and Humidification

Establishment of Manufacturing Technology for Consumer Ferrite Core

I. On the stational drying conditions:The drying rate curves for various materials and methods from which Eq. 1 is derived are shown in Figs. 1-4. It is clear from these that the decreasing drying rate of granular material is proportional to the water content of the material. The heat transfer between air and material is shown by Eq. (2). When the temp. gradient of the material is negligibly small, Eq. (3) is obtained. As shown in Figs. 5 and 6, the temp. gradient can be neglected for the granular material whose diameter is below 2-3mm. Eq. (4) derived from Eqs. (1) and (3) may be solved numerially, e.g., by Runge-Kutta's method. When the sensible heat of water (wc·cw) contained in the material is small as compared with the specific heat of the dried material and rm≅rw, Eq. (5) can be solved analytically:(6)In case (wc·cw) has a value comparable to the specific heat of the material, Fcrw/(c+cww )(t-tw)>>1 and rm≅rw, the following approximate equation, Eq. (8), can be obtained.(8)Eqs. (6) and (8) give the relation between tm and w.II. On the unsteady drying conditions:The unsteady drying which takes place in a continuous (parallel or counter current) dryer, such as a rotary, pneumatic conveying, spary or fluidized-bed dryer, can be presented by Eqs. (9)-(12). From these are derived Eqs. (13) and (14), whose solutions show the relations among t, tm and w in the dryer.These equations could be solved numerially. The calculated examples are shown in Table 1 and Figs. 9 and 10.The drying rate and the drying time in the continuous adiabatic dryer which can be easily calculated by using these relations may help to decide the dryer volume.The application of this calculation method to the dryer design would be expatiated upon in our report.

R. Toei

Papers

Drying Process Analysis of the Decreasing Drying Rate Period of Granular and Powder Material