Microwave dryer systems

Capital equipment costs refer to the cost of the equipment and are sometimes a bit difficult to define. For example, a microwave drying system may increase the throughput of a product dramatically and thereby necessitate the purchase of additional packaging equipment, conveyors, feed systems, and the like. In that case, the overall capital outlay would be much higher than for the microwave dryer alone. 

Thermal dryers are sometimes used for predrying, but are almost always used for final drying because of the limitations of mechanical dryers. After mechanical predrying, much of the remaining water is chemically bonded to the fiber and must be evaporated. This is accomplished using several types of thermal system such as heated cans, convection ovens, and radio frequency (RF) dryers. Recently, microwave dryers have been designed for drying textiles and carpet tiles [5]. 

As an example, consider the drying of bread crumbs from 27 to 5% moisture, at a rate of 1000 lb of wet bread crumbs per hour. In such a system, it would be necessary to evaporate 231.6 lb of water per hour (1000 lb of bread crumbs contain 270 lb of water, but when dried to 5% contain only 38.4 lb of water). This would require about 77 kW to dry (231.6/3.0), plus an additional 20 kW to heat to the drying temperature, for 97 kW, neglecting all heat losses. If we assume this system to have a coupling efficiency of 75% (that is, the efficiency of coupling microwaves into the product), then a system of 130 kW is required, which would cost approximately one million dollars. On the other hand, if a conventional hot air dryer is used to reduce the moisture from 27 to 12% and the microwave dryer to finish drying it to 5%, then the amount of water to be evaporated by the microwave system is only 61.6 lb, which requires 20.4 kW (61.1/3.0). Since the product is already hot, we need to only increase the output to 27 kW to account for the 75% coupling efficiency (20.4/0.75). Such a system would cost of the order of 200,000 plus the cost of the less expensive conventional dryer. 

Fig. 9.n Scheme of microwave-assisted inert medium fluidized-bed dryer system. Reproduced with permission from Souraki etal. (2009). 

Microwave radiation is emitted by dryers, ovens, and heaters normally used in the home. The high-power radars used for military purposes, communication equipment, alarm systems, and signal generators are other sources of microwave radiation. The low-power microwave radiation can cause heating and skin redness whereas high-power microwave radiation can cause inductive heating of metals and induced currents that can produce electric spark. Containers with flammable materials may catch fire if they are placed in the microwave fields. Rings, watches, metal bands, keys, and similar objects worn or carried by a person in such a field can be heated vmtil they burn the bearers. 

Closed-system dryers, spray dryers, fluidized bed dryers Batch or continuous tray, vibrated bed dryers Use dielectric heating (microwave or radio frequency) over part of drying... 

Another problem in trying to compare the costs of micro-wave and dielectric dryers is that the latter often include hydraulic or pneumatic presses, materials-handling devices, and the like, built into the system, whereas this is unusual for a microwave system [8]. Also, suppliers express their costs in many different ways. 

The idea for the microwave oven was discovered by engineer Percy LeBaron Spencer of the Raytheon Corporation in 1946 while working on a radar system. He later developed the first micro-wave oven. Infrared heating elements were used in many appliances, such as hair dryers by the 1990 s, demonstrating another novel use of electromagnetic energy. 

To date, only microwave hot-air dryers used principally in flnish drying processes to level-off the moisture content in pasta, crackers or chips, and microwave vacuum dryers custom-made to specific product requirements, are available commercially (Ratti, 2008). Good microwave-assisted drying equipment should have a uniform distribution of the microwave field, provide a movement of the material to be dried, keep the microwave field strength in the drying chamber lower than the dielectric breakdovm strength, and possess an economic dehumidification system. 

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