Introduction Treatment of materials with micro -waves has a variety of promisingadvantages compared to conven-tional heating technologies, e.g.better product quality, reduction ofprocess time, saving of energy andenergy costs by higher efficiency,environmental relief, lower invest-ment costs and higher flexibility ofthe furnace [1–5]. Microwave heat-ing is a process where electromag-netic energy with a frequency from300 MHz to 300 GHz gets into aheating product as electromagneticwave with wavelengths in the rangefrom 1 m to 1 mm and which istransferred into heat inside the prod-uct (Fig. 1).Basically, 4 ISM-frequencies (Fre-quencies for Industrial, Scientific andMedical radio-frequency equip-ment) are available for microwavetechnology which can also deviatedepending on country specific rules.The highest frequency is 28 000respectively 30 000 MHz althoughan industrial and cheaper use in alarger scale is not yet in sight. Thelow frequency of 915 MHz is subjectto a certain technical requirementwhich only justifies a use for certaincases. The most economic “frequen-cy” is 2450 ± 50 MHz which is usedfor household microwaves all overthe world. From the point ofmicrowave thermo-process technol-ogy, the SHF-band with the frequen-cy 5800 ± 75 MHz is used in theindustry [5].

Theoretical basic principlesBefore the detailed discussion of thephysical principle of microwavetechnology, the authors first give a

quick look at the conventional heat-ing process. Resistance- or infrared-heating elements serve as heatingsources whereby they are placednear the material to be heated. Byheat radiation and convection, theirenergy is transferred to the surface ofthe material and has to flow fromthere to the inner part to enable acomplete heating of the material.Heat conductivity, adsorption andthe specific heat capacity of thematerial hereby basically determinethe heating process.Sensitive materials may not allowhigh temperatures. And if the mater -ial also has low thermal conductivity,a long process is inevitable so thatthere are strong limits for the pro-duction of certain products regard-ing conventional heating technol -ogy. In order to avoid these limits,physics doesn’t have to be redevel-oped. “High frequency technologyrespectively radar engineering” onlyhave to attract more interest.Microwave heating differs from con-ventional heating systems regardingthe fact that heat doesn’t have to be

cfi/Ber. DKG 90 (2013) No. 4 E 41

brought into the heating product byheating the ambient gas and sub -sequent heat transfer but it can bedirectly coupled into the materialvolume. It has the potential of anextremely energy efficient heatingmethod so that there have alreadybeen published a variety of researchpapers concerning microwave heat-ing [1–3, 5, 7–8].The transformation from electro-magnetic energy to heating energyis realized on the basis of the elec-tromagnetic characteristics of thematerials and is basically indepen-dent of material, temperature and

Process Engineering

Ivan Imenokhoyev, Hans Windsheimer, Roland Waitz,Nadja Kintsel, Horst Linn

LINN HIGH THERM GmbH92275 Eschenfelden, Germany

Corresponding autor: I. ImenokhoyevE-mail: Imenokhoyev@linn.dewww.linn.de

Microwave Heating Technology:Potentials and LimitsAbstract: For approximately 50 years, microwave heating technology has been used in industry. In order to outlinethe advantages of this technology, the theory of microwave heating is presented in detail. In addition, the practicaleffects are shown by selected examples. Microwave heating devices meanwhile exist in many different designs.Despite the classic chamber system like microwave ovens in kitchens, microwave heating is also used in industriallycontinuously operated drying and heating furnaces. Long-time experience in microwave area and innovative ideasare the basis of LINN HIGH THERM’s microwave furnaces. This article shall give an overview of currently availabletypes of industrial low- and high-temperature microwave heating furnaces. The drying- and heating-processes whichare possible with these furnaces are as diverse as in the conventional thermoprocessing technology. Microwave heat-ing can often accelerate significantly the drying respectively heating process and thus can safe time, energy andmoney. Keywords: microwave heating, microwave technology, microwave furnace, furnace, numeric simulation, FEM, elec-trical heat, low-temperature application, drying, heat treatment, hardening, baking, debinding, sintering

Fig. 1 Spectrum of electromagnetic radiation [6]

the frequency range in form ofHelmholtz equations (4) and (5),which are the basis of microwavetechnology, is facilitated consider-ably. These equations are furthertreated for numeric simulation in thefrequency range [2, 7].

(4)

(5)

The energy which is transmitted bythe electromagnetic waves can bededuced by Maxwell’s equationsand leads to the famous Poyntingvector in the frequency range [7, 9]:

(6)

This vector says that the averageenergy Pin which flows into an en -veloping surface depends on theamplitude, the distribution and thecorresponding phase of the electricand magnetic field. If one transfersthe surface integral in eq. (6) to avolume integral following Gauss’theorem, the efficacy loss in anydielectric is determined:

(7)

This way, a 3D-heat source densitydistribution in case of non-magneticmaterial can be calculated. Fig. 2shows electromagnetic fieldstrength and heating source densitydistributions as an example of anumeric 3D-Finite-Element-Method(FEM) simulation. It deals with acylindrical multi-mode applicator(microwave chamber, Fig. 2, left)which has an axial arrangement ofthe rectangular waveguide andwhich is filled with modeling mater-ial. Low absorbing and highlyabsorbing materials with differentvalues of DC ε_ges are used as model-ing material [4].As the whole volume of the object isheated simultaneously, a highertemperature develops in the insideas the surface adjoins to the “coldenvironment” and is thus cooled.However, the inside insulates theheat as the neighbouring molecules

g

dVEP

V

r∫∫∫ ⋅⋅=2

//

0abs

2

1 rεεω

[ ] ∫∫ ⋅=⋅×=FF

dFnSdFnHEP00

*

ein

2

1

2

1 rrrrr

( ) ( )rErEgesges

rrrr⋅⋅⋅=∇ 22 εμω

( ) ( )rHrHgesges

rrrr⋅⋅⋅=∇ 22 εμω

have the same approximate tem -pera ture. Thus, the temperaturecurve is inverse compared to con-ventional heating methods. In manycases, this effect is desired as the sur-face is prevented from thermal dam-age and heat inside the object isgenerated faster.The diffusion speed of microwaves isthe velocity of light in vacuumrespectively in air. If microwavesource is activated, its energy isdirectly present in the object to beheated and immediately also startswith energy transformation. If thesource is switched off, the heatingprocess is stopped immediately.There are no long heating-up andcooling-down processes of the fur-nace.Nonpolar materials (e.g. air, Teflon,quartz glass) cannot transform en -ergy and thus cannot be heated.Microwaves penetrate these mater-ials and are not dissipated. General-ly, the material to be heated which isable to make the heat transform -ation can be seen as “heater” as thematerial itself represents the heatingsource. The metallic furnace hous-ing, i.e. the microwave chamber,only serves for reflecting back themicrowaves to the material so thatno energy gets lost and that theoperating personnel is not exposedto microwave radiation.Drying as example for industrial lowtemperature application has a spe-cial status in the production processand determines in certain cases eventhe production rate respectively thecycle time. In case of optimization ofthe whole process, thus the dryingprocess is examined carefully. Re -garding ceramic industry, dryingtimes of 10–14 days and in case ofbig parts even up to several monthsare no rarity. As shorter deliverytimes and smaller stock capacitiesare the aim, microwave drying willplay a more and more importantrole. The reason for that is thephysics of microwave technologyrespectively the propagation andcharacteristics of electromagneticwaves. In special cases, even a ma -ter ial improvement can be achievedby microwave treatment.In case of microwave drying, theinverse temperature profile is advan-tageous, as a higher vapour pressuredevelops inside the material and dry-ing is effected from the inside to theoutside. In the colder outer layers, apart of the steam condenses andkeeps the surface humid and perme-able until there is no more steamfrom the inside and the surface con-

E 42 cfi/Ber. DKG 90 (2013) No. 4

frequency. As generally only one fre-quency is used during the heatingprocess and as temperature depend -ence of electrodynamic characteris-tics is not clear, consideration is doneonly depending on material proper-ties.In order to describe any material, thethree parameters electrical conduc-tivity, permeability and permittivityare necessary. For the latter, the for-mer description dielectric constant(DC) respectively relative permittiv-ity and loss factor are used.A complex dielectric constant isdescribed by eq. (1). Eq. (2) is validfor the permeability constant:

(1)(2)

The influencing factor for energytransformation is the imaginary partof the permittivity constant ε //r = lossfactor (tanδ ) x relative permittivity(ε /r ). This part is often confused withthe loss factor by mistake. This con-nection is shown in eq. (3).

(3)

With: ε_ges = complex dielectric constant

(DC);μ_ ges = complex permeability con-

stant;ε0 = 8,85418 · 10–12 A � s

V · m=electric field constant;

μ0 = 4 · π · 10–7 V � s A · m =

magnetic field constant;

ε /r = real part of DC; μ /r = real part of permeability;ε //r = imaginary part of DC;μ//r = imaginary part of perme -

ability;ε_ r = relative dielectricity number;μ_ r = relative permeability;tanδ = loss factor and δ = loss angle.

With this description of the complexdielectricity constant and the com-plex permeability constant, thetreatment of Maxwell’s equations in

/

//

///

tanundtan

r

r

rr εεδεδε =⋅= and

[ ]rrrges

j0

///

0εεεεεε ⋅=−⋅=

[ ]rrrges

j0

///

0μμμμμμ ⋅=−⋅=

Process Engineering Fig. 2FEM simulation:distribution of theelectric field and ofthe heating sourcein cylindrical multi-mode-applicator [4]

sequently starts to dry. As water gen-erally transfers the most microwaveenergy due to its high loss factor,lower energy transformation (micro -waves go on without being weak-ened) is effected depending on dry-ing substance and drying rate in theinside although this energy can beused in other areas. That way, effective drying withremoving all water nests is possible.Because of different energy input ofthe materials to be dried, in principledifferent processes are possiblealthough there is no essential differ-ence above a humidity content ofapproximately 15 %. In this case,water determines the process. In therange from 5–15 %, the drying sub-stance itself can play a more andmore important role. If the materialitself can transform microwave en -ergy into heat, the temperature ofthe material can increase althoughthe temperature dependence of thedielectric constant determines theprocess. In case of certain chemicals,the chemically bonded water can besplit off that way. Below 5 %,microwave drying can become inef-fective with decreasing humiditycontent. However, it is recommend-ed to examine the material beforefor ensuring that the necessary tem-peratures can be reached.

Microwave continuousbelt dryer MDBT-rangeAt the beginning of the 1990s, LINNHIGH THERM GmbH (LHT) startedtheir activities in the area of micro -wave heating together with Ried-hammer GmbH. In order to meet the demand for industrial dryers, a modularly designed microwave con-tinuous belt dryer was developed[10]. Due to the easy and flexibleconcept like modular design, it waspossible to manufacture a cost-ef fect ive microwave continuous beltdryer (MDBT) which can be used for various applications. The mainapplication in many manufacturingprocesses is drying. The microwave continuous belt dry-er type MDBT offers the followingadvantages compared to conven-tional heating furnaces:• Faster, reproducible and homo -

geneous heating• Immediate readiness for operation

respectively control of the heatingpower without delay

• Good suitability for process auto -mation

• No storage heat losses

• Low specific energy consumption• Shorter production times• Faster heating of thicker layers.In case of thicker materials or biggerbulk densities, the penetration ofmicrowaves into the material andthe immediate transformation of themicrowave energy into heat result ina faster drying compared to conven-tional drying technology. The dryingtime of several materials of severalhours or even days is reduced considerably. In addition to that,there is the possibility to achieve animproved product quality. As theheating of the material depends onthe volume, overheating is mostlyavoided. In case of insulation mater -ials which have a low thermal con-ductivity, the drying process takeslong until the conventionally pro-duced heat has reached the insideand until drying starts there. In caseof microwave drying, heat conduct -ivity of the material only plays aminor role.The applications of microwave con-tinuous belt furnaces are of coursenot only limited to the drying pro -cess. In food industry, microwaveheating has been used for approxi-mately 50 years. An example for thisis the production of sliced bread.Microwave heating is used for pas-teurization [11]. Furthermore, bak-ing of bread without crust bymicrowaves has proved to be veryenergy-saving – more than 40 % lessenergy requirement by decreasingthe baking process compared to theconventional baking oven – bakingtime with microwaves approximate-ly 10 min and with conventionalbaking oven approximately 24 min.The production of fast-cooking riceby microwaves saves up to 90 % ofthe energy in case of the same gus-tatory characteristics and avoids theusage of water in the production(Fig. 3). As the subsequent drying isnot necessary, a lot of energy can besaved [12]. Moreover, this heatingprinciple is used for thawing, calci-nation, hardening, tempering andsynthesis acceleration.Older microwave continuous fur-naces are based on the concept ofless generators (magnetrons) withhigh power on a rectangularmicrowave chamber. This impliesthat it is difficult to generate a homo-geneous microwave field in such fur-naces. High power is transferred onthe supply points of the magnetronswhich, however, cannot disperseevenly in the chamber volume. Add -itionally, the reflection of the micro -waves is lead back to the magne -

cfi/Ber. DKG 90 (2013) No. 4 E 43

trons due to the rectangular designof the chamber. All these facts leadto a comparatively irregular micro -wave field distribution.LHT’s microwave continuous furnacesare based on the concept of manysmall magnetrons and a cylindricalmicrowave chamber. An equal supplyof many small microwave powersresults of the distribution of a numberof magnetrons on the chamber walls.Thus, a more homogeneousmicrowave distribution is achieved.This effect is furthermore supportedby the curved chamber which diffu-sively reflects the striking microwavesinto the chamber volume.An example for such a furnace withnew design is below microwave con-tinuous belt dryer MDBT 70+24/ 1040/210/16300 which is used forheating and drying pulses. This fur-nace has a heated length of approx-imately 16,3 m and a belt width ofapproximately 1 m (Fig. 4). In caseof this furnace, a modular construc-tion was chosen. This offers the pos-sibility to modify the furnace after-wards without much effort. Themicrowave generators (magnetrons)are installed spiral-like around thelongitudinal axis of the cylinderchamber so that a more equal fielddistribution is achieved. The convey-or belt is lead over bottom plateswhich are equipped with secondarybeams (slotted antennas) so that fur-ther field influence (concentration)takes place. The inlet and outletopenings are lined out with specialabsorbing materials for attaining thepermitted leak radiation values.Depending on the size of the open-ing, additional absorbing zones areintegrated which effect furtherreduction of the leak radiation. Incase of much bigger openings, add -itional absorber curtains, e.g. manu-factured from carbon fibers, areinstalled. The used magnetrons arecooled by air although the heatedcooling air flows into the furnaceand is able to take up humidity. Thehumid air is then exhausted from thefurnace by an exhaust system. Thismicrowave continuous belt furnacecan be equipped with a microwavepower of up to 100 kW.

Process Engineering Fig. 3MDBT-furnace in fast-cooking riceproduction (instal -led microwave power 21 kW,throughput approximately300 kg/h)

drying process (MIDD-process) is adrying process controlled by evapor-ation. The most important charac-teristics and advantages of the MIDDprocess compared to resistanceheated systems are:• Heat is directly produced in the

solution over the complete vol-ume.

• In this process, only minimal tem-perature gradients develop andthus the most possible homo -geneous drying/solidification.

• Compared to resistance heatedsystems, the process time is short-ened significantly.

The MIDD-furnace is operated semi-continuously. At the beginning, adefined quantity of liquid waste is pumped into the final storage container while it is pre-heated by in ductive heating. Subsequently,micro wave heating starts and liquidwaste is supplied continuously. Thefinal storage container and themicrowave applicator are held at anabsolute pressure of approximately900 mbar. The developing steam isabsorbed by a fan. A droplet separ -ator filters dust and carried awaywater drops from steam flow beforeit is condensed out in a plate heatexchanger. The condensate is col-lected in a separate container. At the end of the process, the supplyof liquid waste is stopped and theremaining liquid in the final storagecontainer is evaporated with adjust-ed microwave power. After contain-er has cooled down, it is replaced bya new final storage container andthe cycle starts again. The conden-sate can be used further on or it canbe recycled and the solid, dry re -sidual can be finally stored in thecontainer. During the process, acooling water recooler ensures thatthe cooling liquid of the plate heatexchanger is below a defined tem-perature limit for guaranteeing acomplete condensation. The MIDD-

furnace is operated by a program-mable logic controller (PLC). Apartfrom all mass flows, temperature, fill-ing levels as well as difference pres-sures are controlled and measured inorder to draw back the conclusionover the total mass balance to thethickness of the liquid layer abovethe already dried material. The layeris important for the automatic oper-ation of the furnace. All measureddata are visualized and documentedby a separate data recorder. The pro -cess is visualized by a touch panel, allnotifications are documented.As the result of numerous tests andfurther development work on thisprocess, LHT built a ready for seriesproduction prototype of a MIDD-furnace (Fig. 6).The new furnacewas developed and built for a per-manent industrial use. The safetydevices which are required bynuclear regulations, were imple-mented in collaboration with a part-ner of the German nuclear industry.For the process it is important toachieve the best possible efficiencyas well as the most homogeneouselectromagnetic field. This is realizedby nine 900 W standard magnetronswhich do not only have a long life-time but also significantly lowermaintenance and repair costs. Allmaterials which are in contact withthe components are made of stain-less steel, Teflon or silicone. Duringdesigning, it was also put emphasison the fact that all components can be cleaned, maintained andchanged easily [1].

Microwave chamber dryerMKT-rangeIn case of microwave drying, a ruleof thumb can be used for determin-ing the necessary microwave powerwhich says that a microwave powerof approximately 1 kW is necessaryfor the evaporation of 1 kg water/h.

E 44 cfi/Ber. DKG 90 (2013) No. 4

Microwave chamber vacuum dryer MKST-rangeThe patented microwave furnace ofthe MKST range (Fig. 5) is used forvacuum drying. It is a universal testunit which can be adapted to variousapplications e.g. for drying wood,ceramics, chemicals, food, buildingmaterials, for hardening of fiber re -inforced composite materials andmore. The microwave furnace con-sists of a cylindrical microwavechamber with an inner diameter ofapproximately 550 mm and alength of approximately 3485 mm.The microwave furnace is designedfor operation under normal pres-sure, rough vacuum (10 mbar) andslight protective gas overpressure. Itis equipped with 12 magnetrons á800 W/2,45 GHz (totally 9,6 kW).The power of these magnetrons canbe adjusted continuously in a rangefrom 15–100 %.

Microwave in drum dryingMIDD-rangeAs the result of a long collaborationwith the German nuclear industry(Nukem), LHT patented a processwhich vaporizes liquid waste bymicrowaves. The original applicationbackground was drying/crystalli za -tion of slightly radioactive salinesolutions, decommission sludge,cooling and washing liquids whichwere presented in numerous publi-cations [1]. The microwave in drum

Process Engineering

Fig. 4 Microwave continuous belt dryer MDBT 70+24/1040/210/16300 of LINN HIGH THERM in production (installed microwavepower 70 kW, hot air power 24 kW, throughput approximately 2000–3000 kg/h)

Fig. 5 Microwavechamber dryerMKST-9,6 200/2500

This rule is valid as long as there is asufficient starting humidity. Multi-mode batch furnaces consist of themicrowave chamber which is closedby a door. The microwave power ismostly introduced on the sides and/ or the ceiling and the rear wall. Asshown in the picture, the furnacescan be equipped with a roll transportsystem. In such furnaces, the insertgood is mostly not moved. That iswhy a very homogeneous micro-wave distribution is necessary in thechamber for avoiding an irregularheating. Such furnaces are mostlyused for drying or heating productswhich are too big, too heavy or toosensitive for being transported in acontinuous furnace. Even for longdrying times it is often advantageousto use a batch furnace. An examplefor a multi-mode batch furnace is thebelow shown microwave chamberdryer with a microwave power of30 kW and a chamber volume ofapproximately 21 m³ (Fig. 7).The microwave chamber dryer isused for drying industrial ceramic orresin bonded grinding wheels whichcan be moved into the furnace bymetal racks. The microwave energyis produced by 38 magnetronswhich are placed on both sides ofthe furnace for assuring a uniformheating. The ventilation system isalso installed on both sides so that ahomogeneous drying of all parts canbe ensured. For microwave tight-ness, the rolling door is pressed onpneumatically in case of microwaveoperation [13].Another application is heating ofnatural and synthetic rubber. So far,the state of the art for the cold sea-son of the year was the preheating ofnatural rubber based on big heatingchambers in which the standard rub-ber plates were preheated for daysuntil weeks. Especially during wintermonths, the pre-heating times arevery long as the plates exhibit lowtemperatures of –10 °C or lower. Thelow thermal conductivity of the rub-ber and the big volume of the plates

inevitably result in long preheatingtimes as heating is effected by warmair with maximum 60 °C.Microwave technique enables pre-heating of single plates within0,5–2 h depending on requirement.The advantage of microwaves is thatthey can penetrate into the materialand that heat is also produced in theinside of the rubber. That is why acomplete pallet can be heatedhomo geneous and fast. Thus a bigheat hall (several 1000 m³) can besaved when using two microwavefurnaces. Consequently, stock re-quirement and investment can bereduced considerably as rubber fore.g. one week production does nothave to be stored in the heat halls.

Microwave laboratory furnaces MKE-rangeDue to the variety of materials andproduction processes, it first has tobe investigated how the materialspecific microwave process can beused optimally. For this purpose,special test microwave furnaces areavailable with which the corres -ponding tests can be made on thepremises of LHT. The multi-mode furnace type MKE isequipped with two magnetrons.They can be operated individually ortogether and provide each 800 Wheating power in case of a frequen-cy of 2,45 GHz. The microwaves aredirectly introduced into the cylin -drical microwave chamber. Control

cfi/Ber. DKG 90 (2013) No. 4 E 45

of microwave power is effected bypotentiometers. For heat insulation, a rectangularhousing of fiber insulation plateswith a useful volume of one liter isused. The sample surface tempera-ture is measured contactless by apyrometer. The data is recorded andanalysed by software. The furnace isdesigned for use in laboratory espe-cially for low temperature range.With this furnace, first generalknowledge about microwave behav-ior of different materials can befound out (Fig. 8).

Microwave furnaces MEK- and MFH-rangeMicrowave flow furnaces are used asspecial furnaces for heating liquid,highly viscous materials. The liquidflows in a microwave transparentTeflon tube (PTFE tube) through theheating zone and is heated by

Process Engineering

Fig. 6 Schematic diagram of MIDD-process (l.) and new MIDD prototype (r.)

Fig. 7 Basic scheme of a multi-mode batch furnace (l.) and microwave (MW) chamberdryer MKT-30 (r.); 1 = MW chamber, 2 = MW inlet openings; 3 = heating good; 4 = door, 5 = transport rolls

Fig. 8 Microwave laboratory furnaceMKE-1,6

applicator

magnetrons

fibre insulationgeneral view of the furnace

1 2 3

4

5

the outside of the tube. As themicrowaves can penetrate into thecast resin, the whole volume of theresin is heated evenly. Apart fromreduction of the gel time, micro-wave heating had other advantages:the colour fastness and the strengthhad been increased. Another application is the hardeningof glass fiber composite rods. Rodsmade of glass fiber reinforced plasticare produced in a pultrusion processand are e.g. used as cores for glassfiber cable fishing rods. The glassfibers which are soaked with syn-thetic resin have to be hardened forachieving their final strength. It isespecially important to harden therods completely for achieving opti-mal product characteristics. With conventional methods, thiswas not always guaranteed as thecore of the rods was not completelyhardened due to its low thermalconductivity. With the help ofmicrowave heating, this problemcould be solved as microwaves heatthe rods from the inside such that itis guaranteed that they are heatedcompletely. The microwave furnaces which areused for this process are very com-pact and achieve the necessary heat-ing in the passage with a heatedlength of only approximately 30 cm.In case of load carriers for glass fibercables, for example, a conventionaltube furnace with a length of 6 m isreplaced by this unit.

Microwave rotary tubedryer MDRT-rangeMicrowave rotary tube furnaces oftype MDRT are used as special fur-nace for powder and granules [15].The material is lead through theheating zone by a rotating PTFE orquartz glass tube and is being heat-ed by microwaves. The furnace canbe operated under vacuum or pro-tective gas. The microwave rotarytube furnace can be used for heattreatment and coating of granules,powders and fibers. In case of thisfurnace, the material also doesn’tget in touch with the heating zone(Fig. 10).

Microwave hybrid furnaces MHT-rangeThis furnace is a microwave hybridfurnace which is designed for hightemperature operation. Heating canbe done by resistance heatingrespectively by microwaves or bothin combination. The microwavechamber is rectangular and is madeof stainless steel. Heating insulationis made of 3 respectively 4 layers ofspecial ceramic fiber plates and isinstalled inside the microwavechamber wall. The insulation of these microwavehybrid furnaces is suitable for anoperation up to maximum 1800 °C.It is equipped with 8 magnetronswhich provide each 900 W heatingpower in case of a frequency of2,45 GHz and which can beswitched on separately. The totalavailable microwave heating poweris thus 7,2 kW.The magnetrons are directly fixed onthe microwave chamber so thatthere is a direct radiation of themicrowaves into the chamber. Thereare each two magnetrons on the rearwall, on both side walls and on thefloor of the microwave chamber. Themicrowave inlets are arranged in ashifted way for improving the uni-

E 46 cfi/Ber. DKG 90 (2013) No. 4

microwaves. Thus, the liquid can beheated homogeneously in the com-plete volume without getting intouch with the walls of the micro-wave heating chamber (Fig. 9).Pre-heating of cast resin is anotherindustrial application of interest[14]. Plastic insulators for high volt-age furnaces are produced by bring-ing cast resin into a heated metalmould. The time the resin needs forhardening in the form, the so calledgel time, determines the productiv -ity of the furnace. For reducing thegel time and thus increasing the pro-ductivity, it is possible to pre-heatcasting resins with Al2O3 or SiO2 fillercontents of up to 50 mass-% beforebeing filled into the form mould.Heating up to approximately 100 °Cthereby proved to be optimal. Geltime could be reduced by up to40 %.For achieving this effect, it is neces-sary to heat up the cast resin evenlyto the temperature. If this is not thecase, the resin cannot harden com-pletely due to the reduced gel time.An equal heating could not beachieved with conventional heaterswhich heated a metallic tube linethrough which the resin was lead as only the parts of the resin whichget in touch with the tube are heat-ed. In case of microwave heating, castresin flows through a microwavetransparent PFTE-tube. Thus, themicrowaves can heat the resin from

Process Engineering Fig. 9 Microwaveflow furnace of typeMEK (l. and middle)and MFH (r.)

Fig. 10 Microwave rotary tube dry-er type MDRT (microwave power2,7 kW respectively 5,4 kW)

Fig. 11 Position of the magnetrons on themicrowave chamber (l.) and microwave hybridfurnace MHT-1600 (r.)

formity of the microwave field. Alter-natives with an insert temperature ofup to 1400, 1600 and 1800 °C re -spectively are available. The control of the complete micro -wave power can be made by either apotentiometer or a program con-troller. 6 resistance heating elementsserve for the conventional heating,e.g. made of molybdenum disilicidewith a total heating power of 9 kW(Fig. 11). In case of microwave hybrid heating,the microwave is combined with aconventional heating method. Themost common heating methods arehot air and resistance heating butothers are also possible, e.g. gas-fir-ing and infrared-heating. Microwavehybrid heating with an additionalhot air heating achieves high con-ventional temperature homogeneityby circulation of the air in the cham-ber. Insulation is necessary for protec -ting the metallic microwave cham berand for reducing heat losses.Hot air is mostly used in case ofmedium temperatures as they are forexample necessary for debinding ofceramic parts. In case of microwavehybrid heating with an additionalresistance heating, heating elementsare used for achieving high preheattemperatures, e.g. for sintering. Aninsulation of the chamber is neces-sary as well. Due to the additionalresistance heating, materials whichonly absorb microwave energy atelevated temperatures can be pre-heated. In the low temperaturerange, an almost conventional typeof heating with hot air or resistanceheating takes place, microwavesonly contribute to heating at ofhigher temperatures. The additional introduction of hotair can also be of advantage in caseof low temperature processes likedrying, e.g. in order to acceleratethe humidity extraction. The appli-cations for both hybrid heatingprocesses are mainly debinding andsintering of ceramics and powdermetals.

LHT manufactures specific specialmicrowave hybrid continuous beltfurnaces for recycling of carbonfibers, production waste and end oflife components in the high tem -pera ture range (Fig. 12).

Modeling of microwaveheating furnacesDuring the last two decades, hugeprogress was achieved in the devel-opment and application of numer -ical methods for calculating electro-magnetic field as well as tempera-ture distributions. Due to the simul-taneous rapid development of com-puter technology, it is currently alsopossible to calculate electromagnet-ic and thermal fields in technicalproblems even in case of compli -cated geometry and non-linear ma -terial behavior for three-dimensionalgeometries.The modeling of microwave heatingfurnaces has often been dealt with inliterature [2–4, 16]. However, thenumeric simulation of microwaveapplicators is still subject of manyworks, as the results of other authorscannot be transferred to other con-figurations which have to be investi-gated and thus are hardly suitablefor application. When developingmicrowave heating furnaces, the dis-tribution of the electromagnetic fieldis important. However, an exclusive-ly electromagnetic simulation willnot be sufficient for designingmicrowave heating furnaces. Apart

cfi/Ber. DKG 90 (2013) No. 4 E 47

from dissipation of electromagneticenergy, the thermal process plays animportant role in case of microwaveheating. In order to determine theheating process exactly, various heattransfer mechanisms as well as theeffects resulting thereof have to beconsidered as well.There are a number of excellent ref-erence books which treat the heattransfer mechanism with well-known mathematic methods. Thebooks of Metaxas [17] as well asKramer and Mühlbauer [18] have to be mentioned. The books ofMetaxas and Meredith [19], Zhaoand Turner [20] and Feher [3] areespecially dedicated to the topic ofheat transfer in the case of micro -wave heating.The thermal and electromagneticcharacteristics of the considered ma -terials which depend on tempera -ture can be found in literature [21].Measurements of material character-istics which depend on temperatureare very complex and often contra-dictory [22]. They are a huge scien-tific and technical problem whichcurrently is only solved unsatisfactor -ily [23].An example for the FEM validationby the experiment is a cylindricalmulti-mode applicator with a soot-loaded cordierite diesel particulatefilter (cordierite DPF). An open wave-guide is used for the transport of themicrowave power of a magnetroninto the applicator. In this type ofsupply, the tangentially installed,

Process Engineering

Fig. 12 Recycling of carbon fibres (l. and r.) and microwave hybrid continuous belt furnace (middle)

Fig. 13 Experimental setup (rear side) and test assembly for experimental validation ofthe 3D-FEM-simulation [4]

cylindrical multi-mode applicator[4]. Fig. 13–15 show very goodagree ment regarding quality andquan tity of numerically calculatedand experimentally measured tem-perature field distributions. Thereflected power is determined byeq. (8)

(8)

as difference between incident pow-er Pein and absorbing power Pabs.The efficiency can then be calculatedfollowing eq. (9):

(9)

The reflection factor is:

(10)

which leads to the so-called VoltageStanding Wave Ratio (VSWR) [7]:

(11)0

0

1

1

R

RVSWR

−+=

abseinrefPPP −=

ein

ref

ein

refein

ein

abs1P

P

P

PP

P

P −=−==η

η−=−== 11

ein

abs

ein

ref

0

P

P

P

PR

The achieved results of the experi-mental validation show the follow-ing:• The numerically and experimental-

ly determined temperature fielddistributions show very good con-sistency regarding quality andquantity.

• The experimentally tested micro -wave test unit is energy-efficient,the net efficiency is approximately95,3 %.

• However, field distribution stayshomogeneous (Fig. 14–15) whenusing only one magnetron.

• It is obvious that the concept ofLHT provides better field distribu-tion by using many magnetronsand that it is thus resulting in thehomogeneous heating of variousmaterials (see microwave continu-ous belt furnaces MDBT-range).

• The 3D-FEM-models provide prac-tically robust results.

• Generally, they offer the possibilityto visualize the development oflocal hot-spots and to optimizethem more by a fast furnace designadaptation.

• The 3D-models of microwaveheating furnaces can be used forcomputer-aided optimization ofindustrial microwave heating fur-naces.

• Ideally, the simulation resultsshould be checked experimentallyin any case.

ConclusionsThis article gives a review of theinnovative microwave heating fur-naces for applications in low- andhigh-temperature ranges for differ-ent industrial sectors which aredeveloped by LINN HIGH THERMGmbH based on long-time experi-ence for laboratory and productionand which are used successfully allover the world. Specific examplesshow the efficiency and the savingspotential of microwave heatingtechnology. The topic of computeraided 3D-FEM-simulation of indus-trial microwave heating furnaces isaddressed as well.

Acknowledgements The authors would like to thank allcustomers and partners of LINNHIGH THERM who contributed suc-cessfully to the development of sev-eral innovative highly efficientmicrowave furnaces with their long-term productive work and experi-ence.

E 48 cfi/Ber. DKG 90 (2013) No. 4

supplying rectangular waveguideleads to the cylindrical multi-modeapplicator. The housing of the appli-cator consists of the metallic cylinder(diameter = 155 mm, length =162 mm) with steel walls which are8 mm thick. The metal grids serve asshields against undesired microwaveleak radiation.The cylindrical filter is manufacturedwith a diameter of 143,76 mm and alength of 152,4 mm and has a massof 1255,5 g. The filter medium iscordierite (Mg2Al4Si5O18).Soot load is approximately 18,4 g.The soot loaded DPF has a mass of1273,9 g. Soot is spread unequallyin the cordierite DPF. From macro-scopic view, the cordierite DPF is aheterogenic medium with six recog-nizable homogeneous areas. Furtherdetails and physical parameterswhich are necessary for the model-ing can be found elsewhere [2, 4].Fig. 13–15 show the experimentalsetup and the results of experimen-tal validation of 3D-modeling of the

Process Engineering

Fig. 14 Results of FEM-simulation: a) electrical field distribution, b) heating source density distribution and c) temperature distribution [4]

Fig. 15 Results ofexperimental simu-lation validation [4]

cfi/Ber. DKG 90 (2013) No. 4 E 49

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