issue: April 2003 APPLIANCE Magazine Engineering: Dryer Technology Microwave Clothes Drying - Technical Solutions to Fundamental Challenges	Share  Printable format  Email this Article  Search
by John F Gerling, president, Microwave FabriCare Technology, a division of Gerling Applied Engineering, Inc. The following discusses the use of microwave heating technology in laundry appliance applications, and recent solutions that address the technology's historical challenges.

Technology Background

The primary mechanism of microwave drying is dielectric heating, which is similar to what occurs in microwave ovens. Electromagnet energy is radiated into the drying chamber where it couples to materials according to their dielectric and electrical properties. Power dissipates into a material according to,

P_v= 5.56E²ƒε"(10^-13)

where:

• P_v is the power dissipated per unit volume (W/cm³)
• E is the electric field strength (V/cm)
• ƒ is the microwave frequency (Hz)
• ε" is the dielectric loss factor of the volume.

Water molecules have a higher dielectric loss factor than most common fabric materials by several orders of magnitude, and thus will preferentially absorb the microwave energy. Furthermore, most fabric materials are relatively transparent to microwaves and will allow them to reach the water molecules wherever they are embedded in the fabric.

The first known documented conceptualization of a microwave tumble dryer for fabrics was in the mid-1960s by Levinson [1]. Shortly thereafter, General Electric Company [2] and Maytag expressed interest in the concept, but both companies declined to pursue product development, citing perceived high manufacturing costs and difficulties in overcoming the problem of arcing [3].

Throughout the years that followed, the potential advantages of microwave drying were well documented and easily demonstrated - faster drying, greater efficiency, lower drying temperature, and reduced fabric wear [4]. However, most of this work was conducted using clothing articles and materials that are well suited for microwave drying. The hazards relating to arcing and overheating of the less ideal clothing articles were also well known, but viable solutions had yet to be developed.

After several years of investigations focused on full-size dryers, The Electric Power Research Institute (EPRI) redirected its efforts in 1997 toward compact countertop dryers as a result of economic feasibility analysis and market surveys. Gerling Applied Engineering, Inc. was then contracted to design, construct, and test several prototype dryers by merging EPRI dryer technology with well-established microwave oven technology. Demonstrations conducted for several major appliance OEMs and subsequent delivery of prototypes for in-house evaluation have since led to negotiations for technology licensing [5] and indications of serious product development activity for a residential microwave clothes dryer appliance [6].

Performance Optimization

Parametric testing of the EPRI prototype dryer confirmed most of the earlier claims regarding increases in overall efficiency and reductions in cycle time and fabric temperature. Any one of these characteristics could be optimized by adjusting the process parameters, specifically the amounts of and the ratio between microwave and hot air energy input. Optimization was also dependent on the size and type of fabric load.

For example, a microwave-only cycle was found to increase efficiency while decreasing cycle time and temperature for small loads and delicate fabrics. By contrast, a combined microwave/hot air cycle improved efficiency while having no effect on cycle time or temperature for large loads of heavy fabrics.

These factors were then related to the differences between typical loads found in residential, commercial, and industrial markets. Residential dryers must accommodate a wide range of load size and types; thus, optimization necessitated a compromise between performance characteristics. By contrast, most industrial users handle a limited range of load sizes and types and, thus, may benefit from selective optimization.

Termination of the drying cycle is an important performance factor for any fabric dryer, but it is particularly important for microwave dryers. Once the load becomes dry, the amount of time the dryer may continue to operate without causing damage to the load is much shorter for a microwave dryer than a conventional dryer. This is due to the difference in the rate of fabric temperature rise at the end of the drying cycle. Under normal conditions in a conventional dryer, the fabric temperature will not rise beyond the incoming air temperature, even after the moisture has evaporated. In a microwave dryer, the absence of moisture will allow the microwave energy to couple more to the fabric material, causing its temperature to continue rising as long as energy is transferred. Furthermore, the dielectric loss factor of most fabric materials, especially synthetics, rises with increasing temperature, thus causing an exponential rise in fabric temperature.

Figure 1. Load temperature versus test cycle time. CLICK for full-size graphic.

Contact moisture sensors used in conventional dryers to detect the load dryness condition are not practical in microwave dryers due to the interaction between the electromagnetic field and metallic sensor contacts. Early microwave dryers relied on exhaust humidity sensing, but variations in ambient humidity conditions render this method unreliable. Other methods include magnetron anode temperature and exhaust temperature sensing, both of which proved to be unsatisfactory due to slow response and sensitivity to load size.

Two methods which proved to be the most reliable, especially when employed in tandem, are the sensing of microwave electric field strength and fabric temperature. Both parameters correlate well with moisture content, remaining relatively low until evaporation nears completion, as signaled by rising slopes of the measured signals. Their responses to variations in load size are predictable and provide a basis for corresponding adjustments in cycle control.

Figure 2. Field strength (normalized) versus test cycle time. CLICK for full-size graphic.

Figures 1 and 2 illustrate the responses of load temperature and field strength for a typical large load in a prototype compact 1-kW microwave-only dryer. In this case, the change in slope after roughly 1,500 sec into the cycle was relatively rapid, and the drop in field strength indicated an increase in the dielectric loss characteristics of the load. The combination of these two effects indicated the need to terminate the cycle quickly after reaching a threshold of 105¡F (40.6¡C).

Periodicity of the signals, particularly noticeable in Figure 2 between 1,000 and 1,400 sec, is due to variations caused by reversing the drum rotation. This effect was reduced in later configurations by more careful placement of the sensors.

Hazard Detection and Mitigation

The phenomenon of arcing and heating of metal objects in a microwave dryer is caused by the electric field, which induces voltage differential between metal objects and the current flow within them. Arcing may be induced depending on the strength of the electric field, spacing between objects, insulating barriers, and other factors. Similarly, the current flow in metals may result in excessive heating, depending on the electric field strength and resistivity of the metal. The shape of a metal object can also influence its propensity for arcing and/or overheating.

It is easy to demonstrate how objects such as coins in a pocket can survive a microwave drying cycle without damage to the coins or fabric, but it is also just as easy to demonstrate arcing and scorching of the fabric using the same coins under different fabric load conditions. Whether part of a clothing article or a "tramp" object left in a pocket, a great variety of metal objects may find their way into a microwave dryer. The possibility also exists for non-metallic materials to behave unfavorably in a microwave field.

Preventing a potentially hazardous condition from occurring is nearly impossible, so a more prudent action is to develop a means to prevent the hazard from causing damage or becoming a safety risk. The EPRI research developed a system of gas sensors to detect minute amounts of pre-combustion vapors in the exhaust stream. A suite of selective adsorbent sensors was chosen to cover a broad range of compounds commonly found in combustion byproducts. Figure 3 illustrates the response of one such sensor when butane was injected periodically into the dryer air intake. The rapid rise in signal level is easily detected in software, thus enabling rapid shutdown of the drying cycle upon detection.

Figure 3. Response of Figaro 842 sensor to short bursts of butane into the dryer air stream. CLICK for full-size graphic.

Ongoing work to further reduce the likelihood of damage to clothing has identified methods of controlling and/or preventing the conditions that result in fabric damage. These include techniques for suppression of arcing and burning by maintaining low electric field strengths under any load condition. But while it is possible to effectively and reliably prevent unsafe operation of a microwave dryer, the complete elimination of fabric damage in a microwave dryer is unrealistic.

Conclusion

Some experts in the microwave heating community have doubts about the long term viability of microwave clothes drying, while others express optimism by comparing the challenges to those overcome by the microwave oven in its early days. Noting how consumers have accepted and adapted to the microwave oven, the most likely scenario will be an evolution in consumer laundry habits and the birth of an entirely new industry of clothing and laundry products developed expressly for the microwave clothes dryer.

References

1. M. Levinson, "Microwave and Ultrasonic Apparatus," U.S. Patent 3,410,166, issued Nov. 12, 1968.

2. D. Heidtmann, "Microwave Dryer Control Circuit," U.S. Patent No. 3,439,431, issued April 22, 1969.

3. "Assessment of Residential Microwave Clothes Dryers," EPRI Report RP2034-35, February 1990.

4. M. Hamid, "Microwave Drying of Clothes," Journal of Microwave Power and Electromagnetic Energy, pg. 107-113, Vol. 26, No. 2, 1991.

5. "Countertop Microwave Clothes Dryer," EPRI Technical Brief No. 1006408, December 2001.

6. E. Spagat, "Whirlpool Goes Portable to Sell Dryers to Gen Y," Wall Street Journal, June 4, 2002.