Vacuum paddle dryer

FIGURE 23.16 Continuous vacuum paddle dryer (1) vapor filter (2) steam jacket (3) shaft with paddles (4) paddles (5) valves (6) shaft drive (7) shaft oscillator. 

In the vacuum batch dryer, approximately 60% total volume is required for optimum drying result. A very simple and approximate equation can be used for the scaleup of vertical pan and horizontal paddle dryers ... 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

For the first group the following types of dryers are generally used convective (tray, band, fluid bed, flash dryers, and their modifications) or contact (vacuum dryers such as double-cone dryer-blender, conical dryer with rotating helical mixer, paddle dryer). Pastelike materials are dried in tray dryers, band dryers equipped with extruding devices, and spin-flash dryers. Finally, thin pastes can be dried in spray dryers or on fluid beds or spouted beds of inert particles. Small amounts of solutions and suspensions are generally freeze-dried, especially if the product is thermolabile. 

With a paddle dryer (Fig. 5-40) the liquid, pasty, or free-flowing wet product is evenly distributed by rotating paddles over the heated areas (jacket, hollow shaft, and paddles) and discharged at atmospheric pressure or under vacuum. 

Typically, indirect dryers are used for small- or medium-size production. The product from such a unit has a higher bulk density than the same material processed in direct dryers. Particle size degradation usually can be minimized by proper selection of agitator speed or design. The common indirect heated dryers are tubular dryers (with or without vacuum), drum dryers (atmospheric, vacuum, horizontal or rotary vacuum, and others), hollow disk dryers, paddle... 

FIGURE 12. Popular vacuum dryers with a mechanical paddle. ... 

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