Drying contact dryer

Contact Dryers. Coatings on webs as well as sheets of newly formed paper can be dried by contacting the web around the surface of a heated dmm. Thus conduction is used to transfer the heat. Dmms can also be used to rapidly cool warm extmded films, to increase the viscosity, and soHdify the film. 

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

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. 

Contact dermatitis, from nickel, 17 119 Contact dryers, coatings, 7 29 Contact drying, 9 105-107 Contact icing, of food, 21 561 Contacting, differential, 10 760-762 Contact mechanics, 1 515-517 Contact mode atomic force microscopy, 3 320-325 17 63 Contact nucleation, 8 105 Contactors ozone, 17 801-802 selection of, 10 767-768... 

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. 

TABIE 12-26 Comparative Dimensions and Drying Times for Various Batch Contact Dryers... 

Particles and powders below 1 mm are effectively dried in dispersion or contact dryers, but most through-circulation units are unsuitable. Conversely, for particles of several millimeters or above, through-circulation dryers, rotary dryers, and spouted beds are very suitable. 

A major division may be made between (1) dryers in which the solid is directly exposed to a hot gas (usually air) and (2) dryers in which heat is transferred to the solid from an external medium such as condensing steam, usually through a metal surface with which the solid is in contact. Dryers that expose the solids to a hot gas are called adiabatic or direct dryers , those in which heat is transferred from an external medium are known as nonadiabatic or indirect dryers. Dryers heated by dielectric, radiant, or microwave energy are also nonadiabatic. Some units combine adiabatic and nonadiabatic drying they are known as direct-indirect dryers. 

Comparing to their direct-contact counterparts indirect-contact dryers possess some advantages such as the reduced deterioration of the drying material due to an excessive exposure of the material to the high-temperature drying medium. In addition, adsorption of such toxic substances in the drying medium (in the case where direct combustion gases are used) as NO j is eliminated. 

Once the drying rate is obtained it is possible to determine the size of a contact dryer by finding the value of the following integral ... 

Lecomte D, Eudym O, Carrere-Gee C, Arlabosse P, Vasseur J. Method for the design of a contact dryer—Application to sludge treatment in thin film boiling. Drying Technology, 22, 2151-2172, 2004. 

Oakley D. Contact dryers. In CGJ Baker, ed.. Industrial Drying of Foods. London, U.K. Blackie Academic Professional, pp. 115-133,1997. 

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