Plate type dryer

Batch units are frequently cylindrical shelf-type dryers equipped with heating and cooling coils or plates. Designs for belter heal input, that is, spikes or expanded metal sheet that penetrate the frozen mass or movable heated shelves compressing the frozen materials, have met with some success. 

The functional relationship between product temperature, on the one hand, and shelf temperature and chamber pressure, on the other hand, is affected by many factors including the size and design of the lyophilizer, the characteristics of the product, and the time evolved since the start of primary drying. With a sucrose formulation in vials, we have observed a maximum primary drying product temperature rise of -i-5°C when the shelf temperature was varied from -15 to -i-30°C, whereas a pressure variation from 30 to 250 microbars generated an increase of around -i-2.5°C. With a lactose formulation in ampoules lyophilized in a larger freeze-dryer equipped with a plate-type condenser, the effect of pressure was found to be predominant -i-6.5°C for a pressure move from 50 to 300 microbars, versus -t-l°C for a shelf temperature move from 0° to 25°C. 

These models use experimental data from drying kinetics tests in a laboratory, pilot-plant or full-scale dryer, and are thus more accurate and reliable than methods based only on estimated drying kinetics. They treat the dryer as a complete unit, with drying rates and air velocities averaged over the dryer volume, except that, if desired, the dryer can be subdivided into a small number of sections. These methods are used for layer dryers (tray, oven, horizontal-flow band, and vertical-flow plate types) and for a simple estimate of fluidized-bed dryer performance. For batch dryers, they can be used for scale-up by refining the scoping design calculation. 

Steam dryers These provide closely spaced corrugated plates through which steam must pass. Chevron dryers and other dryer types reduce the velocity at which the steam passes, causing the impurities to drop out. 

Pressure filters or filter presses are commonly of the batch type, and are characterized by smaller floor area, high filtration rates, and lower capital cost. Dryer cakes are produced. The chemical industry uses these filters more widely than mineral processing industries, mainly because of its batch operation. The most common types of pressure filters used are the plate and frame presses. These comprise a series of vertical, alternating parallel frames and plates, with the filter cloth being held against the plate and the formation of cake occurring in the hollow frame. 

The principles of fluidisation, discussed in Chapter 6, are applied in this type of dryer, shown typically in Figure 16.25. Heated air, or hot gas from a burner, is passed by way of a plenum chamber and a diffuser plate, fitted with suitable nozzles to prevent any back-flow of solids, into the fluidised bed of material, from which it passes to a dust separator. Wet material is fed continuously into the bed through a rotary valve, and this mixes immediately with the dry charge. Dry material overflows through a downcomer to an integral after-cooler. An alternative design of this type of dryer is one in which a thin bed is used. 

The handling of sticky materials can present difficulties, and one type of dryer which is useful for this type of material is the turbo-dryer. As shown in Figure 16.30, wet solid is fed in a thin layer to the top member of a series of annular shelves each made of a number of segmental plates with slots between them. These shelves rotate and, by means of suitably placed arms, the material is pushed through a slot on to a shelf below. After repeated movements, the solid leaves at the bottom of the dryer. The shelves are heated by a row of steam pipes, and in the centre there are three or more fans which suck the hot air over the material and remove it at the top. 

The most suitable equipment for the production of dried fruit is the chamber or shelf dryer. Smaller types have strips inside, along which the drying screens (shelves) are moved like shutters. Bigger dryers are designed in such a way that a mobile stand is filled with these shelves. The stand is then pushed into the dryer. Electrical heating elements are usually employed to heat the dryers. Built-in fans provide the necessary air circulation. The better dryers are fitted with air baffle plates which distribute the warm air evenly throughout the dryer. 

The conventional top spray method shown in Figure 2 has been used for layering and coaling for decades. It evolved from the fluidized bed dryers commercialized more than 40 years ago. The substrate is placed in the product container, which is typically an unbaffled. inverted, truncated cone with a fine retention screen and an air or gas distribution plate at its base. Perforated plates such as a Conidur or Gill plate may also achieve air distribution and product retention. These types of plates may have directed holes for guiding the airflow horizontally in the product container (for side discharge as an example). Process air is drawn through the distribution plate and into the product. 

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