However, all microwave dryers are not created equal, and all products might not be suitable for a microwave environment. Determining whether a microwave dryer can provide benefits in a particular application requires an understanding of how the technology works, as well as how different materials react when subjected to microwave energy.
Three basic interactions occur between a material and a microwave field:
• Transparent materials, such as air, quartz glass and water-free ceramic bodies, allow the waves to pass through unhindered, as glass does with light (see Figure 1a). In the microwave field, these materials remain cold.
• Reflecting materials, such as metals or graphite, ideally permit no rays to penetrate them. The waves hit the surface and are thrown back almost unchanged into space (see Figure 1b). These materials also remain cold in the microwave field.
• Absorbing materials, such as foods, fresh wood and moist ceramic bodies, are able to absorb the microwave energy and convert it into heat (see Figure 1c). How deeply the rays penetrate the interior varies, depending on the material and its specific dielectric loss. If a material consists of several components, and at least one component is a good absorber, the material can be heated well.
In all cases, the temperature can cause these interactions to change. The ideal microwave dryer enables the temperature profile to be adjusted to optimize drying for each material.
When exterior heating (such as with hot air) is used, the energy is applied to the surface, where it is transported to the interior of the material with heat conductance. The main force behind hot air drying is the water vapor partial-pressure differential between the surface of the product being dried and the air—m(delta- pH2O). The temperature drop counteracts the transport of water, which creates a temperature curve that drops from the exterior to the interior.
The reverse is true when the heat is produced by microwaves and the surrounding air is not heated. Microwaves make drying possible even without the partial-pressure differential (see Figure 3). Here, the water transport is caused by the temperature differential—m(delta-T).
Hybrid drying combines both the partial-pressure differential and temperature differential to produce a balanced temperature curve and a particularly fast drying process. A comparison of hot air drying and hybrid drying is shown in the equations in Figure 4.
Measuring, open-loop control and closed-loop control components guide the process and link the dryer to the entire production system. Visualization systems increase user friendliness and make it easier to acquire and analyze the production parameters.
The two basic designs for microwave hybrid dryers are continuous-flow and chamber dryers. The continuous-flow dryer shortens drying times to as little as one hour and is ideal for automated production lines. With this type of system, the temperature curve generated inside the product corresponds to a performance profile set by the dryer length. The system must be open on both ends to ensure a continuous flow of material; however, the dryer’s construction and special absorber zones ensure a very low microwave leakage rate of 5 mW/cm2, which is comparable to the leakage rate permissible with a mobile phone.*
The chamber dryer is recommended for drying processes that require several hours of drying time and is a particularly good way to dry products with many different wall thicknesses (see Figure 5). The size and shape of the applicator area can be adjusted to fit a variety of product dimensions, enabling the dryer to handle even very large objects. In addition, this design provides good data acquisition and open- and closed-loop control of the process variables. Because this is a closed system, the microwave leakage rate is not critical.
Ceramic-Bonded Grinding Wheels.
The main problem with drying large ceramic-bonded grinding wheels in conventional climatic cabinets is that a moist core often remains inside the wheels. During firing, this moist core causes tension, resulting in tearing.
Because hybrid drying can achieve a higher degree of total dryness, it decreases tension in the wheels and thus the number of rejects due to tearing. It also shortens the drying time—in one application, in which the hybrid dryer was used to dry wheels with a diameter of 1.2 m and a thickness of 0.3 m, the drying time was reduced from 190 to 35 hours. The benefits also extend further downstream in the manufacturing process—hybrid-dried wheels can be fired immediately after drying and do not need to spend two days in climatic cabinets for moisture offset, which conventional drying methods require.
The higher degree of dryness also permits the production of grinding wheels with compositions or grain sizes that were previously impossible to produce economically with conventional dryers. As a result, hybrid drying provides a variety of new opportunities in product design and development.
Test results in actual applications have shown reductions in drying times of up to 85 percent, a significant reduction in rejects due to tearing, and the ability to use new material combinations without increasing drying times.
Many different types of honeycomb ceramics are used as catalyzers or filters in a variety of applications, and these products often have very complicated shapes. Due to the demand for increasingly fine structures (up to 800 cells per square in.), handling these products in a wet state can be very difficult. In addition, these structures have poor heat conductivity; in some cases, this characteristic makes drying by conventional means nearly impossible.
The microwave hybrid dryer is well suited to the task of effectively drying such structures in a very short time period. The process is typically implemented as a continuous-flow procedure so that handling errors are eliminated and the product can be moved immediately to the next production step (see Figure 6). The technology can be used successfully to dry even very fine structures or structures with large dimensions, providing the potential to develop new products.
Porcelain is typically dried with waste heat from the kiln in a batch operation, so energy costs are generally not a factor. However, market globalization has forced porcelain manufacturers to continue to look for ways to reduce labor and production costs. As a result, an increasing number of companies have begun implementing automated manufacturing systems. Hybrid drying provides an optimal framework for this because it enables rapid, continuous drying.
One porcelain factory in Sch?nwald, operated by a branch of BHS Tabletop AG in Germany, provides an example of how the microwave hybrid drying system can be implemented in automated production lines. After each product is shaped, it is automatically transported from the forming process to the microwave hybrid continuous-flow dryer, where drying takes only a few minutes. Four separate channels ensure uniform cycle times for drying the different items and create a production flow that can easily be handled by robots (see Figures 7 and 8). With the company’s previous batch dryers, this level of automation would not have been possible.
The microwave hybrid dryer also offers economical solutions for designing new porcelain products. Figurines, for example, often have extreme variations in wall thicknesses that result in wet areas where the material is thick. With conventional technology, dry edges are created in these areas, and these are very difficult and time-consuming for the residual water to penetrate. In contrast, the microwave goes to work directly where the water is, and the colder edges remain moist longer, enabling the residual water to escape quickly and easily. In some applications, drying times of six to nine months in the air dryer have been reduced to just three to five days in the microwave hybrid dryer.
Benefits of Microwave Hybrid Drying • Can be used to dry very fine structures and large dimensions
• Provides reduced drying times
• Can easily be integrated into automated systems
• Lends itself to the development of new products
• Provides increased productivity and
• Reduces handling errors and production costs