Pros and Cons of Microwave Heating
• 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. Volumetric heating gives rise to a very rapid energy transfer into the material being heated. In conventional heating, heat flow is initiated on the material’s surface and the rate of heat flow into the centre is dependant on the material’s thermal properties and the temperature differential. A conventional oven requires heating to temperatures much higher than is required by the material itself since there 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 – microwave heating 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 material’s ability to be heated by electromagnetic energy is dependent 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 microwave heating 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.
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 microwave 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 100 kW. 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.
Which Frequency Should I Use and How Many Magnetrons?
Both of the above questions are sometimes related. Some facts about each approach are listed below.
• Magnetron efficiencies are between 50-72% for 2.45 GHz compared to 80 to 87 % for high power 922 or 915 MHz.
• The lower the frequency the great the penetration depth into a material.
• Field uniformity is usually better when multiple magnetrons are used.
• There are no 915 or 922 MHz magnetrons less than 5 kW.
• At 2.45 GHz magnetron range in power from 1 kW to 30 kW, at 915 MHz the range is from 5 kW to 100 kW.
• Different frequencies can initiate different energy transfer mechanisms within the material being heated, which may impact on reaction thermodynamics or product quality.
• Applicator dimensions and complexity would be much greater using multiple magnetrons.
• The larger the applicator greater will be the thermal losses and reduced microwave efficiencies. A larger applicator would be required if multiple magnetrons were used.
• In multiple magnetron installations, magnetron life is reduced due to cross coupling of power between each.
• Multiple magnetron systems provide a high degree of redundancy.
• Multiple low power magnetron generators are simple in design allowing any technician microwave technician to repair them.
• In some situations the replacement cost of low power magnetrons per kW can be higher than that of high power magnetrons.
Microwave radiation is the same stuff being emitting from mobile phones and is completely different to radioactive radiation and is not cumulative. The only effect ever reported on the human body is one of heating.
Just as too much infra red heat can cause burning so to can high levels of microwave radiation.
Microwave equipment must be designed to comply with health and safety limits imposed by the local authorities. These levels allow radiated emissions of up to 10 mW/cm2 at a distance of 50mm from the equipment. It is quite common for installations to have radiated emissions less than 1 mW/cm2 .
Many installations install monitoring systems that measure the field intensity of all polarisations emitting from the machine and alarm if levels exceed a preset value.
The quickest way to arrive at a capital budget estimate is to establish the total microwave power required for the application. This can be achieved relatively simply by understanding how each of the following applies to the process under question.
• heat of fusion
• sensible heat
• heat of vaporisation
• conduction losses
• convection losses
• radiation losses
• transmission losses
• impedance mismatch
If you think a microwave heating system is right for you, contact AMT for a proposal and project evaluation now.