Technologies / Microwave

What is Microwave Heating?
Microwave (MW) heating is a type of electro-heat technique designed to heat electrically-insulating (dielectric) materials. The principle behind these techniques is that portions of the electromagnetic spectrum are utilised as the primary energy source to heat a material. The application of electromagnetic radiation may be direct (straight into the material) or indirect (via a heated appliance). This differs from conventional heating which is based on heat moving through the material.

Fundamentals of Microwave Heating
There are two principal mechanisms by which a dielectric material can be heated by a microwave electromagnetic-field:

  1. Electrical conduction.
  2. Dipole rotation.

Heating through electrical conduction is where small currents are induced within the dielectric material by the oscillating electric-field, dissipating power as heat in a process known as Joule heating.

Many dielectric materials have dipolar molecules. A common dipolar molecule in MW heating applications, for example in the food or drying industries is water. The formulation and dipolar moment for the water molecule is shown below.

Dipolar molecules within a dielectric material couple themselves electrostatically to the applied electromagnetic-field and tend to align themselves mechanically with the field polarisation. The applied electromagnetic-field is alternated in time and the dipoles attempt to realign themselves with the oscillating field, resulting in molecules which are in a state of mechanical oscillation at the applied frequency. The successive rotations generate heat through friction at the molecular level. When the applied field is removed, the dipolar molecules relax back into their original equilibrium state.


Practicalities of Microwave Heating
Microwave heating typically refers to heating in the 0.5-3GHz frequency range. The ISM (Industrial, Scientific and Medical) RFR bands classify the portions of the spectrum for microwave dielectric heating applications in the UK at 896MHz (±10MHz) and 2450MHz (±10MHz).

A typical microwave system utilises a magnetron as a source of energy which is physically connected to the cavity (usually a closed metal structure containing the microwave field around the material) via the waveguides (metal conduits).

The higher frequency of microwave fields means that the applied electric-field strength is less than at lower frequencies resulting in improved safety because of less risk of arcing and lower overall system voltages. Also, the penetration depth of the field is less, so microwave heating is better suited to smaller applications.

Typical applications of microwave heating are in the food industries, including:

  • Baking bread without crusts

  • Fast bake of meringues

  • Rapid defrosting / tempering of meats

  • Novel structure confectionary products

  • Low / zero fat potato snack cooking

  • Post baking of biscuits

  • Pre cooking of poultry with controlled colour development

  • Pasteurisation of meats / poultry

Microwave heating can also be augmented with conventional heating techniques, such as hot-air and steam, to assume the heating benefits of multiple heating techniques such as moisture containment, surface drying, and reductions in external evaporation.

Advantages of Microwave Heating
The advantages of microwave heating for industrial processes are:

  • Volumetric Heating – energy is transferred through the surface and into the material electromagnetically. The electromagnetic-field and material interact throughout the whole product, which greatly enhances heating uniformity. In contrast to conventional heating where the heat travels from the surface inwards.

  • Rapid Volumetric Heating – the ability to dissipate extremely high power densities within a material (up to 10W/cm3) resulting in very fast heating times. This is distributed evenly across the material (not confined to the edges as with conventional heating), preventing product damage.

  • Selective Heating – the electric-field is stronger in areas of the material which can’t convert this to heat as easily (e.g. wetter parts) This gives even moisture levels throughout a product as they balance out over time.

  • Controllability – instantly controllable, and the power supplied can be regulated very accurately. This allows safe and precise control of an applicator even when applying large power or rapid heating rates.

  • Efficiency –up to 80% of the total power consumed can be transferred electromagnetically (compared to less than 25% for conventional heating) because heat losses from the equipment are dramatically reduced.

Back to top of page