Main drying time heat transfer

The possible drying times for the main drying are estimated in Section 1.2.1 and complemented by examples. The decisive qualities are the heat transfer coefficient from the shelf to the sublimation front of the ice (Ktot). The heat conductivity in the product does normally not play an important part (see Fig. 1.67), except that a granulated product is dried from the surface to the center (see Fig. 1.68). The shortest possible main drying time can be estimated with 5 or 10 % error, if the dimensions of the product and the maximum tolerable TKe (e. g. -10 °C) are given (Eq. (12), (12 a-c) in Section 1.2.1). 

Table maximum water vapor flow-rates given by the hydrodynamic data of the plant. The maximum sublimation rates depend also on the heat transfer from the brine to the sublimation front of the ice [see Eq. (12)]. The main drying time will generally be governed above 0.06 mbar by the heat transfer and below 0.06 mbar by the water vapor transport, as shown in the example below ... 

The specifications of the plant are given in the caption of Figure 2.58. As can be seen from the tables in the caption, the main drying time depends on the heat transfer and the vapor flow rate. To optimize a freeze-drying cycle it is helpful to calculate a table as shown in Figure 2.58 for mns 1-7, which can be commented upon as follows ... 

Main drying time calculated from heat transfer data (h) 11 9... 

The main advantages of fluidised dryers are rapid and uniform heat transfer short drying times, with good control of the drying conditions and low floor area requirements. The power requirements are high compared with other types. 

From the drying kinetics it is clear that drying of pasty pigments which belong to the class of capillary-porous materials takes place mainly in the constant drying rate period. Hence, the drying time can be calculated from the heat transfer equation... 

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