Drying equipment dryers

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

Pneumatic dryers, also called flash dryers, are similar in their operating principle to spray dryers. The product to be dried is dispersed into an upward-flowing stream of hot gas by a suitable feeder. The equipment acts as a pneumatic conveyor and dryer. Contact times are short, and this limits the size of particle that can be dried. Pneumatic dryers are suitable for materials that are too fine to be dried in a fluidised bed dryer but which are heat sensitive and must be dried rapidly. The thermal efficiency of this type is generally low. 

Sludge drying equipment does not always have to be expensive. Valley Plating Works Inc. of Los Angeles could not justify the expense of commercial dryers that it examined, which... 

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. 

Drying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is... 

A classification chart of drying equipment on the basis of heat transfer is shown in Fig. 12-45. This chart classifies dryers as direct or indirect, with subclasses of continuous or batchwise operation. 

These tests should establish the optimum operating conditions, the ability of the dryer to handle the material physically, product quality and characteristics, and dryer size. The principal manufacturers of drying equipment are usually prepared to perform the required tests on dryers simulating their equipment. Occasionally, simple laboratory experiments can serve to reduce further the number of dryers under consideration. 

Changes and advances in mechanical design of freeze-drying equipment and control systems have had a strong impact. Modem freeze-dryers are easier to use, require less operator intervention and are applicable to a wide variety of products. Current, automated freeze-dryers allow the initial steps of the protocol to be implemented, thereby providing the operator with more data of interest also, they are safer and easier to operate than previous models. 

Another useful classification is whether or not a dryer is a direct or indirect dryer. A direct contact dryer is one in which the material is dried by exposure to a hot gas, whereas in an indirect contact dryer, the heat required for evaporation is transferred from a heating medium through a metal wall to the material. Generally, direct heat dryers are more efficient. Dryer efficiency is defined by the fraction of energy supplied to the drying equipment which actually causes the evaporation of the liquid. As we shall see later in the chapter, heating is not always necessary to achieve drying. 

Drying equipment may be classified in several ways. Effective classification is vital in selection of the most appropriate dryer for the task and in understanding the key principles on which it operates. The main categories are as follows ... 

Dust Explosions Dispersion dryers can be more hazardous than layer-type dryers if we are drying a solid combustible material which is then dispersed in air, particularly if the product is a fine particle size. If this finely dispersed product is then exposed to an ignition source, an explosion can result. The following conditions (van t Land, Industrial Drying Equipment, Marcel Dekker, New York, 1991) will be conducive to fire and explosion hazard ... 

Clean design Care should be taken in the design of both the dryer and dryer ancillary (cyclones, filters, etc.) equipment to eliminate ledges, crevices, and other obstructions which can lead to dust and product buildup. Smooth drying equipment walls will minimize deposits. This can go a long way in prevention. No system is perfect, of course, and a routine cleaning schedule is also recommended. 

A hypothetical example serves to illustrate this situation (Figure 15.9). Let us suppose that a reaction is carried out in reactor R1 over a period of about 8 hours. When the reaction is complete, the reaction mixture is transferred to vessels R2 and R3 for an extractive work-up that requires about 6 hours. Then the rich extract is transferred to R4 for concentration and crystallizing, which takes about 16 hours. Filtration on centrifuge Cl requires 4 hours, and the product is dried in dryer D1 for 8 hours. When the reaction mixture has left Rl, the vessel and staging area can be cleaned. If the equipment is to be used to make the same material that was generated in that vessel in the prior run, simple rinsing of Rl may suffice to ready the vessel for the second batch. 

CLASSIFICATION OF DRYERS. There is no simple way of classifying drying equipment. Some dryers are continuous, and some operate batchwise some agitate the solids, and some are essentially unagitated. Operation under vacuum may be used to reduce the drying temperature. Some dryers can handle almost any kind of material, while others are severely limited in the type of feed they can accept. 

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