Microwave Hot Air Systems

Keywords Microwave hot air systems Life cycle assessment Analytic hierarchy process ... 

Based on the concept mentioned above, the objective of this research was, on one hand, to study the assessment model of microwave hot air systems using life cycle assessment, on the other hand, to characterize and analyze the green degree of assessment model using analytic hierarchy process and fuzzy decision-making method. 

In this study, an analytic hierarchy structure was employed for evaluating product process and analyzing the process results. The effects of tangible aspects and objectives on the evaluation goal were systematically investigated using the second levels of tlie life cycle assessment model. The hierarchical structure of life cycle assessment of microwave hot air systems was shown m Figure 1. 

Figure 1 The hierarchical structure of life cycle assessment for microwave hot air systems...

The microwave hot air systems were carried out in an industry-made microwave hot air system, and die schematic diagram of the microwave hot air systems was shown in Figure 2. 

Figure 2 The schematic diagram of the microwave hot air systems Comparison of Pairwise Comparison...

Table II Life Cycle Assessment Model of Microwave Hot Air Systems...

The usability of microwave hot air systems is evaluated under established the life cycle assessment system with the analytic hierarchy process and fuzzy comprehensive evaluation. The aspects, objective and fuzzy assessment matrices of microwave hot air systems are shown in Table 11. 

The results show that the microwave hot air systems have more advantages than conventional heating of hot air systems, with respect to energy-consumption, processing time and environmental protection, respectively. Therefore, the microwave hot air systems is very rapid, uniform and highly energy efficient. Thus, microwave hot air system is better than conventional hot air system, which fulfill with the aim of life cycle and green manufacture. 

In this study, the life cycle assessment of microwave hot air systems were developed using the analytic hierarchy process and the fuzzy comprehensive evaluation. The hierarchical structure consists of assessment aspect and assessment objective were established. The fuzzy assessment matrices of the assessment mathematical model were calculated using eigenvalue method and Gauss-Seidel iterative matrix. The life cycle assessment results show that the microwave hot air systems have a good green degree. 

Vulcanisation using hot air systems remains the most important production system for profile production. Hot air can be used alone but can also be supplemented by the addition of infrared heaters. The systems usually consist of modular units which can be built up into the required length. Air speed can usually be controlled and the compound throughput can be adjusted from speeds of 2.5 m/sec up to 20 m/sec. These systems are sufficient for thin section profiles, but thicker articles will require slower transport speeds. Addition of microwave systems to these units... 

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. 

The temperature in the extruder rises to 80-90 °C and microwave preheating, placed before the input of the vulcanization tunnel, is set approximately to 130 ° C. After entering the tunnel, the material is quickly heated to the vulcanization temperature (e.g., 180 °C) depending on the mix composition. After passing through the microwave tunnel, the material is maintained for 60-90 s at the required temperature by hot-air system and cooled. Microwave vulcanization under atmospheric pressure allows universal design and processing of various profiles without fundamental modification of lines, the construction of which is mostly similar. 

The forming line may also include the means for heating the mat ahead of the press. Microwave heaters have been used. Other systems apply steam or a steam/hot air mixture to heat the mat. Typically the mat can be heated to 50-60°C before the press. Temperatures in excess of this are likely to result in some resin cure ahead of the press. 

When drying with dielectric heating, it is usual to combine hot air with the system, particularly with microwave systems. This is because it usually improves the efficiency and the economics of the drying process [7]. Hot air is, by itself, relatively efficient at removing free water at or near the surface, whereas the unique pumping action of dielectric... 

Microwaves have been used to dry pasta products, and there were over a dozen operational industrial systems. The systans utilized microwaves and hot air of controlled humidity to dry pasta and macaroni products in less than 1 h instead of the conventional 8 h drying time. These systems handled approximately 3000 lb of product per hour with 60 kW of miCTowave energy at 915 MHz. These systems offered substantial savings in energy, operation, and maintenance. They also provide bacteriologically more acceptable product, with reductions in microbial contamination and insect infestation [14]. 

The application of microwave irradiation to cure isocyanate/epoxy resins in the presence of N-(2-hydroxyalkyl)trialkylammonium halides was claimed to impart accelerations to both curing and post-curing kinetics with respect to conventional hot-air heating [122]. More recently, Parodi et al. presented further development of new class of catalysts that endow aromatic isocyanate/epoxy and aliphatic or cycloaliphatic epoxy/anhydride systems with a particular efficiency for microwave processability [123]. The catalysts belong to the family of N-(cyano-alkoxy-alkyl)trialkylammonium halides, of the general formula ... 

The data of Table 17 show the strong reaction enhancements of the specific catalysts impart under microwave heating to all of reactive systems examined. The gelation and vitrification times were lowered to one-eighth to one-tenth of those under hot-air heating with the same catalyst and its concentration. An ion-hopping conduction mechanism was recognized as the dominant source of the microwave absorption capacities of these catalysts [2]. 

The use of microwaves in the foundry industry for drying and polymerizing the sand molds used for casting is very important. This allows the complete recovery of the sand and provides a great increase in speed in making the mold, which otherwise must be slowly dried and cured with hot air. An example of this application is the manufacture of the internal castings for automobile engines. Many RF systems are also used for this process. 

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. 

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