The notion of applying microwave heating technology to consumer laundry appliances was first proposed more than a quarter of a century ago. For many years since, the economic and technical feasibility had been investigated by various groups, but major appliance producers were reluctant to pay any attention to these efforts until concerns regarding cost and safety were addressed. Recent advances in the enabling technologies for microwave clothes dryers have mitigated most or all of the concerns, resulting in serious interest in developing these appliances for the consumer market.
issue: April 2003 APPLIANCE Magazine
Engineering: Dryer Technology
Microwave Clothes Drying - Technical Solutions to Fundamental Challenges
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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.
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,
- Pv is
the power dissipated per unit volume (W/cm3)
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
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 . Shortly thereafter, General Electric Company  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 .
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 . 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  and indications of serious product development activity for a residential microwave clothes dryer appliance .
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
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
the fabric temperature will not rise beyond the incoming air temperature,
even after the moisture has evaporated. In a microwave dryer, the absence of
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
1. Load temperature versus test cycle time.
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
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.
2. Field strength (normalized) versus test cycle time.
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
dielectric loss characteristics of the load. The combination of these two
effects indicated the need to terminate the cycle quickly after reaching a
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
objects and the current flow within them. Arcing may be induced depending
on the strength of the electric field, spacing between objects, insulating
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
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
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
A suite of selective adsorbent sensors was chosen to cover a broad range
of compounds commonly found in combustion byproducts. Figure 3 illustrates
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.
3. Response of Figaro 842 sensor to short bursts of butane into
the dryer air stream.
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.
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
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.
1. M. Levinson, "Microwave and Ultrasonic Apparatus," U.S. Patent
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
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
6. E. Spagat, "Whirlpool Goes Portable to Sell Dryers to Gen Y," Wall
Journal, June 4, 2002.
John F. Gerling is president of Gerling Applied Engineering, Inc., a manufacturing
and consulting firm applying microwave heating technology to consumer, commercial,
and industrial applications. He has a background of more than 15 years in residential