General References

American Dairy Products Institute, Standards for Grades of Dry Milks Including Methods of Analysis, Bulletin 916 (Rev.), American Dairy Products Institute, Chicago, 111., 1990. American Dairy Products Institute, Dry Milk Products— Utilization and Production Trends, 1997, Bulletin 1000, American Dairy Products Institute, Chicago, 111., 1999. C. E. Beardslee, Dry Milks—The Story of an Industry, American

Dry Milk Institute, Chicago, 111., 1948. C. W. Hall and T. I. Hedrick, Drying of Milk and Milk Products, 2nd ed., AVI Publishing, Westport, Conn., 1971.

Warren S. Clark, Jr. American Dairy Products Institute Chicago, Illinois

DRYERS: TECHNOLOGY AND ENGINEERING

Throughout the food processing industries, there are many and varied requirements for thermal drying. Some involve the removal of moisture or volatiles from various food ingredients or products that differ in both chemical and physical characteristics. Others involve the drying of solutions or liquid suspensions and different approaches to the problem. To assist manufacturers in arriving at a reasonably accurate first assessment of the type, size, and cost of element for a particular duty, this article describes the most widely used types of both batch and continuous dryers in the food industries and gives an indication of approximate sizes and capital costs for typical installations.

Three basic methods of heat transfer are used in industrial dryers in varying degrees of prominence and combinations, specifically, convection, conduction, and radiation.

In the chemical processing industry, the majority of dryers employ forced convection and continuous operation. With the exception of the indirectly heated rotary dryer and the film drum dryer, units in which heat is transferred by conduction are suitable only for batch use. This limitation effectively restricts them to applications involving somewhat modest production runs.

Radiant, or "infrared," heating is rarely used in drying materials such as fine chemicals or pigments. Its main application is in such operations as the drying of surface coatings on large plane surfaces since for efficient utilization, it generally is true that the material being irradiated must have a sight of the heat source or emitter. There is, however, in all the dryers considered here a radiant component in the heat-transfer mechanism.

Direct heating is used extensively in industrial drying equipment where much higher thermal efficiencies are exhibited than with indirectly heated dryers. This is because there are no heat exchanger losses and the maximum heat release from the fuel is available for the process. However, this method is not always acceptable, especially where product contamination cannot be tolerated. In such cases, indirect heating must be used.

With forced-convection equipment, indirect heating employs a condensing vapor such as steam in an extended surface tubular heat exchanger or in a steam jacket where conduction is the method of heat transfer. Alternative systems that employ proprietary heat-transfer fluids also can be used. These enjoy the advantage of obtaining elevated temperatures without the need for high-pressure operation as may be required with conventional steam heating. This may be reflected in the design and manufacturing cost of the dryer. Furthermore, in addition to the methods listed above, oil- or gas-fired indirect heat exchangers also can be used.

In general, dryers are either suitable for batch or continuous operation. A number of the more common types are listed in Table 1, where an application rating based on practical considerations is given. In the following review, some of the factors likely to influence selection of the various types are discussed for particular applications.

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