Temperature microwave power drying

The silicon carbide-based ceramic layer is obtained by CVD activated by a microwave plasma (2.45 GHz) on the tantalum previously cleaned by an argon plasma (56 min, 133 Pa, 250 W) at a temperature identical to the deposition temperature (T = 570°C), lower than the substrate annealing temperature. After readjustment of the total pressure and microwave power by introduction of a precursor (TMS) in the argon flow, the SiC coating is produced under selected conditions (66 Pa, TMS/Ar = 0.2/5.5 l.h-, T = 570°C, 350 W) derived from a previous parametric study. Coatings obtained then presented low dry friction coefficients. The mechanical properties, which are essential in order to be able to appreciate the mechanical stability, were acquired by several methods ... 

The influence of microwave power input and inlet temperature on microwave fluid-bed drying. (From... 

The limitations on heat transfer rates in conventionally conducted freeze drying operations have led early to the attempt to provide internal heat generation with the use of microwave power [21,22]. Theoretically, the use of microwaves should result in a very accelerated rate of drying because the heat transfer does not require internal temperature gradients and the temperature of ice could be maintained close to the maximum permissible temperature for the frozen layer without the need for excessive surface temperatures. 

Figure 7 Relationship between microwave forward power, microwave reflected power, and product temperature during microwave vacuum drying.

Figure 9.14 presents typical temperature curves of microwave-assisted drying for food. Here, the shape of the temperature curve of microwave-assisted freeze-drying can be seen to differ from that of the three other types of microwave-assisted drying, due to different drying mechanisms. Temperature changes are heavily dependent on the microwave power, microwave emission mode, type of dryer used, and the presence or absence of a temperature control unit. 

Three different temperature stages are observed at constant microwave power in most microwave-assisted drying processes (Clary et al., 2005 Feng et al., 1999 Ahme et al., 2007 Duan et al., 2010b) ... 

Variable microwave power means that the microwave energy is emitted in a pulsed mode by turning on-off the microwave power during the drying process, or that the microwave emission is declining continually with time. Compared with the constant power mode, the variable microwave power mode is able to stabilize the temperature of the sample at a value that depends on the microwave power used, the duration of the power-on and power-off periods, and the other operating conditions, which indicates that thermal runaway can be inhibited in the variable microwave power mode. 

The best temperature control is achieved with a phase controller, which can continuously and automatically adjust the variable microwave power, based on predefined power profiles (Li et al., 2010). It is worth emphasizing here that maximum power levels must be selected, based on trial and error, for a designated temperature. If the power was too high, the temperature fluctuations were too large, but if the power was too low, then during certain drying stages the temperature could not reach the preset temperature. 

Pereira, N., Marsaioli, A., Ahme, L., 2007. Effect of microwave power, air velocity and temperature on final drying of osmotically dehydrated bananas. J. Food Eng. 81 79-87. 

Esveld et a/.81,82 developed a continuous dry media reactor (CDMR) for pilot-scale applications. It consisted of a multi-modal tunnel microwave cavity operating at a frequency of 2.45 GHz with a power range from 0 to 6 kW irradiated on a surface of 0.6 m2. Temperatures of up to 250°C were achieved. A web conveyor travelling at 17 cm min-1 transported the solid-phase reaction mixture to the oven in low, open Pyrex supports closely packed on a polytetrafluoroethylene (PTFE)-coated glass fibre. An open flat bed process was employed to facilitate easy evaporation. 

The second test, developed by the Society of the Plastics Industry, involved preparing an amount of bacon that covered at least 50% of the dinner plate surface area [2], The plate with the bacon layer was heated in a microwave oven set at full power until the temperature at the center of the plate area reached 121°C ( 5°C). The time period required to reach this temperature within the bacon fat was 5 min on the microwave unit used for this test Immediately after the cooking cycle, the plate was cleaned on the lower rack of an automatic dishwasher. The dishwasher cycle was set at normal and included a heated dry cycle. The water temperature reached 66°C ( 5°C). Once the cycle was complete, the plate was inspected for surface blemishes such as cracks or blisters. This test is considered the most severe of the three due to the extreme localized heat generated, which causes the difference between the thermal coefficient of expansion for the two materials to shear the interface resulting in delamination. 

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