Pro’s and Con’s of microwave technology

Advantages

  • Volumetric heating
  • Energy savings(up to 70%)
  • Reduced equipment size(down to 20%)
  • Instantaneous control
  • Selective heating
  • Clean energy transfer
  • High temperatures(+2000 C)
  • Drive chemical reactions

Volumetric Heating – The wave penetration into various materials has huge positive consequences in many applications. This volumetric heating gives rise to a very rapid energy transfer into the material being heated. In conventional heating, heat flow is initiated on the materials surface and the rate of heat flow into the centre is dependant on the materials thermal properties and the temperature differential. A conventional oven is required to be heated to temperatures much higher than is required by the material itself since their is asymptotical rise in workload temperature towards the required level.

Energy Savings – The rapid heating of the workload, along with the fact that in a properly designed applicator the majority of the available energy is dissipated in the workload, lower temperatures associated with the cavity surroundings mean that radiation, conduction and convection heat losses are reduced. This can represent energy savings of up to 70%.

Instantaneous Control – power can be controlled instantly giving better control of process parameters, rapid start-up and shut down.

Reduced Equipment Size – The rapid dissipation of energy, mainly into the workload and the high energy densities capable in small volumes allows equipment to be up to 20% the physical size of conventional systems.

Selective Heating – A materials ability to be heated by electromagnetic energy is dependant on its dielectric properties, this means that in a mixture containing a number of differing constituents the heating of each will vary. This can have profound positive consequences on energy usage, bulk reaction temperatures, moisture removal and process simplification.

High Temperatures – The energy transfer mechanism from electromagnetic to thermal energy is a function of a materials electrical properties. This allows a continuous dumping of energy into some materials and provided that heat losses can be controlled, very high material temperatures can be achieved with simple and relatively low power microwave generators.

Clean Energy Transfer – The electromagnetic nature of microwaves means that energy transfer to a material is usually via some form of polarisation effect within the material itself. This direct transfer of energy eliminates many of the problems associated with organic fuel usage for the end user.

Chemical Reactions Driven – Many chemical reactions can be accelerated using microwaves. Solvent free reactions are gaining popularity in many labs , thus reducing problems associated with waste disposal of solvents and other hazardous chemicals.

Disadvantages

Field Complexity – It can be very difficult to accurately predict the exact nature of electromagnetic field interaction with materials.

This difficulty is especially true when using multimode cavities, their are many different field patterns possible since variations in a materials volume, temperature, shape, moisture content, location and chemical structure all influence the cavities boundary conditions and hence ability to support various field patterns(modes). This unpredictability is often easily overcome and understood by experimentation and many successful industrial applications have been implemented without the need to know precise field behaviour.

Temperature uniformity – Many factors influence temperature uniformity but broadly speaking, uniformity is a function of either material characteristics or boundary conditions.

Material characteristics affect the field penetration depth, thermal runaway, high field edge effects, micro-arcing and high displacement currents around points of contact between particulate loads. The other factor influencing temperature uniformity, is the cavities boundary conditions. These determine the possible modes that may be excited inside the cavity, all of which superimpose to create an uneven field distribution within the cavity, resulting in temperature non-uniformity.
Some techniques used for improving heating uniformity are listed below;

  • Increase cavity size , thus increasing the number of modes and hence improving field uniformity,
  • Increase the number of microwave feed ports, this will excite a larger number of modes producing greater field uniformity.
  • Modulate the magnetron frequency, thus altering the amount of power coupled to various modes(see magnetron Reiki diagram).
  • Use rotating deflector plates to excite different modes and change the cavities boundary conditions thus providing a continuous moving field environment.
  • Move the material being heated, this is the most common way of overcoming non-uniform heating and the most effective when liquids are concerned.

Temperature measurement – The spot nature of most temperature measurement techniques and the inherent non-uniform heating of electromagnetic heating means that a spot measurement can be misleading.

Initial Capital Cost – When compared to conventional heating techniques, electromagnetic heating in industrial situations usually requires a higher level of initial capital outlay.

Largest Magnetron – The largest single microwave source suitable for industrial applications is 100kW. Applications requiring large amounts of energy would need multiple sources.

R & D – New applications usually require development of the microwave applicator and understanding of the fundamentals associated with the new process. This means that immediate implementation is not usually possible.