Types of Dryers

Dryers can be continuous or batch operated. Batch dryers are less capital cost intensive and flexible. Major categories of dryers fall under four categories tray or shelf dryers, batch vacuum rotary dryers, freeze dryers, and spray dryers. The following paragraphs describe these different dryers, design and operation. 

Multi-pass tray dryer operation heated by air 

For the systems where the heat is conducted through the shelves to the solids, the following analysis could be used for design and operation of a dryer. For heat conduction drying Fourier s law of heat conduction is applicable. 

The heat required to vaporize the water is equal to heat conducted resulting in the following equation. 

We know that at t = 0 y = I then integrating Equation 11.23 by substituting 

In other cases not the vessel is rotated, but revolving rotors are placed inside it. An example is the paddle dryer shown in Fig. 10.1-3. For sticky drying goods, a selfcleaning twin screw such as in Fig. 10. 1-4 can be employed, where the heat is supplied via the hollow screws as well as via the outer jacket. 

In a multistage rotaiy jacketed shelf diyer such as in Fig. 10.1-5, the diying good consecutively moves over various heated trays, where it is heated and dried. The diying process may be improved by passing hot gas over the diying good. 

Finally it should be kept in mind that, particularly, in the paper and textile indirstry, in food technology, and in the ceramic industry scores of specific dryers are applied, which are not discussed here. 

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The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

Other types of dryers may be employed for special products or situations. For example, the Yankee dryer, a steam-heated cylinder, 3.7—6.1 m dia, dries the sheet from one side only. It is used extensively for tissues, particularly where creping is accompHshed as the sheet leaves the dryer, and to produce machine-gla2ed papers where intimate contact with the poHshed dryer surface produces a high gloss finish on the contact side. 

Dryers. Drying, another type of evaporation technique, is suited for waste streams of very high soHds content. Several common types of dryers are vacuum rotary dryers, dmm dryers, tray and compartment dryers, and pneumatic conveying dryers. 

Results of diying tests can be correlated empirically in terms of overall heat-transfer coefficient or length of a transfer unit as a function of operating variables. The former is generally apphcable to all types of dryers, while the latter applies only in the case of continuous diyers. The relationship between these quantities is as follows. 

Initial. selection of diyers. Select those diyers which appear best suited to handhngthe wet material and the diy product, which fit into the continuity of the process as a whole, and which will produce a product of the desired physical properties. This preliminaiy selection can be made with the aid of Table 12-9, which classifies the various types of dryers on the basis of the materials handled. 

Type of dryer tions, extracts, milk, blood, waste liquors, rubber latex, etc. gents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. trifuged sobds, starch, etc. dry. Examples centrifuged precipitates, pigments, clay, cement. ores, potato strips, synthetic rubber. objects, rayon skeins, lumber. sheets. her sheets. 

Type of dryer 2-tnick 16-tray dryer 16-tray 3-truck 2-tnick... 

Drying Fhiidized-bed units for drying solids, particularly coal, cement, rock, and limestone, are in general acceptance. Economic-considerations make these units particularly attrac tive when large tonnages of solids are to be handled. Fuel requirements are 3.3 to 4.2 MJ/kg (1500 to 1900 Btu/lb of water removed), and total power for blowers, feeders, etc., is about 0.08 kWh/kg of water removed. The maximum-sized feed is 6 cm (IV2 in) X 0 coal. One of the major advantages of this type of dryer is the close control of conditions so that a predeterminea amount of free moisture may be left with the solids to... 

The choice between different types of dryers is often guided by the chemicals involved and their physical properties, particularly heat sensitivity. As when selecting other equipment, the designer should first ask if the step is necessary if so, whether this is the correct or safest process step. Does the material being processed have to have all of the liquid removed Can the downstream step or customer use the material in a liquid, slurry or paste form ... 

The theory of drying processes is discussed in Volume 2, Chapter 16. Full descriptions of the various types of dryer and their applications are given in that chapter and in Perry et al. (1997) and Walas (1990). Only brief descriptions of the principal types will be given in this section. 

In this type of dryer, the drying gas is passed through the bed of solids at a velocity sufficient to keep the bed in a fluidised state which promotes high heat transfer and drying rates. 

Thermal drying can be used to remove moisture from solids into a gas stream (usually air) by heat. Many types of dryers are available and can be compared on the basis of their thermal efficiency. 

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. 

In conveyor and tray types of dryer, air is often recirculated inside the drying vessel in an attempt to save energy or to maintain a relatively high moisture content in the drying air. In other direct dryers such as flash, fluidised-bed, rotary and spray units, any recycle of exhaust air must be returned to the dryer using external ducting, the cost of which is offset by the net savings from the lower volumes of exhaust streams which have to be handled. 

Viscosity (m.Pa.s) Drying time (min) Temperature (°C) Type of dryer Groove pattern... 

In order to aid in the design of dryers by analogy, examples of dimensions and performances of the most common types of dryers are cited in this chapter. Theory and correlation of heat and mass transfer are treated in detail elsewhere in this book, but their use in the description of drying behavior will be indicated here. 

Figure 9.4. Types of dryers cited in Tables 9.1 and 9.2. (a) Tray or compartment, (b) Vacuum tray, (c) Vertical agitated batch vacuum drier, (d) Continuous agitated tray vertical turbo, (e) Continuous through circulation, (f) Direct rotary, (g) Indirect rotary, (h) Agitated batch rotary (atmos or vacuum), (i) Horizontal agitated batch vacuum drier, (j) Tumble batch dryer, (k) Splash dryer. (I) Single drum, (m) Spray, (n) Fluidized bed dryer, (o) Pneumatic conveying (mostly after Nonhebel and Moss, 1971).

Figure 9.5. Residence time distribution in particle dryers, (a) Four types of dryers (McCormick, 1979). (b) Residence time distribution cf air in a detergent spray tower example shows that 27% (difference between the ordinates) has a residence time between 24 and 32 sec [Place et al., Trans. Inst. Chem. Eng. 37, 268 (1959)]. (c) Fluidized bed drying of two materials (Vanacek et al., Fluidized Bed Drying, 1966).

Type of Dryer and Feed Size by Letter, A, B, or C Drum Speed (rev/min) Steam Press (bar, g) Type of Material Physical Form of Feed Solids in Feed (%) HzO in Product (%) Output of Dried Product (g/secm2) Evaporation Rate of Watar (g/sec m2)... 

Powder handling—gentle treatment reduces the level of free fat hence, avoid pneumatic transport, use a type of dryer with a low cyclone fraction, operate with a low pressure drop over cyclones, allow adequate powder cooling in fluid bed, and avoid high powder moisture from first drying stage. 

Tray dryers, the simplest type of dryer, are commonly used for batch drying of biological materials, where the wet solids are placed on trays which are then transferred into a chamber. The chamber may have a heating jacket, heated tray supports, or a hot air supply. Vacuum may be applied to reduce the temperature at which the liquid evaporates, preserving heat labile products. These are well suited to low-volume products or flexible plants where a number of different products with different characteristics must be dried. They are relatively inefficient to operate, difficult to clean, and labor intensive to operate. The product is exposed when being loaded and unloaded, so the dryer may need to be located in a clean room or area for pharmaceutical products. 

Are fines created as a result of product drying If so, select a different type of dryer. High air velocities or thermal degradation of solids lead to fine formation in drying equipment. These can be avoided by choosing the proper type of dryer depending on the properties of the solid. 

The physical nature of the material to be handled is the primary item for consideration. A slurry will demand a different type of dryer from that required by a coarse crystalline solid, which, in turn, will be different from that required by a sheet material (Table 12-9). 

Performance data on some typical tray and compartment dryers are tabulated in Table 12-10. These indicate that an overall rate of evaporation of 0.0025 to 0.025 kg water/(s m ) of tray area may be expected from tray and tray-truck dryers. The thermal efficiency of this type of dryer will vary from 20 to 50 percent, depending on the drying temperature used and the humidity of the exhaust air. In drying to very low moisture contents under temperature restrictions, the thermal efficiency may be in the order of 10 percent. The major operating cost for a tray dryer is the labor involved in loading and unloading the trays. About two labor-hours are required to load and unload a standard two-truck tray dryer. In addition, about one-third to one-fifth of a... 

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