Table Of ContentGREEN TECHNOLOGY
TECHNOLOGY STATUS REPORT
Industrial Microwave Applications
Ministry of the Environment
ISBN0-7778-7466-0
TECHNOLOGY STATUS REPORT
Industrial Microwave Applications
January 1998
Prepared by:
Jack Klieb
EnvironmentalTechnologies Services
Industry Conservation Branch
Ministry ofthe Environment
Copyright:
Queen'sPrinterforOntario, 1998
Thispublicationmaybereproducedfornon-commercialpurposeswith
appropriateattribution.
Cettepublicationtechniquen'estdisponiblequ'enanglais.
PIBS3646E
©
Printedonrecycledpaper
Disclaimer
Thisreportwaspreparedforthe Ontario MinistryoftheEnvironmentaspart oftheMinistry's
informationtransferactivities. Wehope thatthe reportwillprovide perspective andencourage
discussioninarapidlychanging technologicalworld. The views and ideasexpressedinthis
report are based oninterpretations ofvariousreferenced authorsand do notnecessarilyreflectthe
positionorpoUciesofthe MinistryoftheEnvironment, nordoesmentionoftrade namesor
commercialproductsconstituteendorsementorrecommendationforuse.
Anypersonwhowishedtorepublishallorpart ofthisreportshould obtainpermissionto do so
fromthe EnvironmentalTechnologyServices Section, IndustryConservationBranch, Ontario
Ministryofthe Environment, 2 St. ClairAve. W., 14*Roor, Toronto, Ontario, M4V 1L5.
Acknowledgement
The authorwouldlike to thankMr. Terry Strack, PrincipalResearchEngineer, Energy
Technologies, ProcessDevelopmentUnit, Ontario Hydro Technologies; and Dr. RamanSood,
President. Ceramics KingstonCeramiques. fortheirassistance inproviding technicalreviewsof
thisreport. Thisreportwouldnothave beenpossible without theiressentialcontributions.
1
TableofContents
Executive Summary 1
1. Background 1
2. DielectricHeating: 3
2.1 Microwaves '^
2.2 Radio FrequencyWaves 4
3. Microwave Systems 5
3.1 MicrowaveTubes 5
3.2 Applicators 5
3.3 Handling Systems 6
4. TechnologyDevelopment 6
5. Applications ^
6. Benefits ^
7. Economics 10
7.1 Case Study: PastaDrying 1
8. Impediments ^^
8.1 Regulatory 12
8.2 Technical 12
8.3 Economic 12
9. Outlook 12
10. References 1-^
Appendix 1^
ExecutiveSummary
Sometypesofradiantwavessuchasmicrowavescanbe usedto heatcertainnon-metallic
materials. Microwavesheat the bodyofamaterialunlikeconventionalheating whichraisesthe
temperature ofthe surface. Theycanbeused to heatmostmaterialsalthoughwaterpolarsolvents
and productswithahighdielectric-lossfactorcanbeheated fasterthanothers (1).
Electricityisconvertedto microwaveenergyusing inverters, transformers and wave generators
calledmagnetrons. Theelectronic systemsare usuallythe sameregardlessofthe application,
butthe applicatorgenerallymustbe designed to suit theparametersofthe process andthe
equipment (2).
The benefitsofradiantheating compared to conventionalheatingincludelowerenergycosts,
reducedfloorspace fortheheatingequipment, lowerexhaustemissions, lowerlabourcosts,
greaterproductivity, andfasterheating,improvedqualityandreducedprocessingtime ofthe
product (3).
1. Background
Conventionalheating actsonthemolecules atthe surface ofthematerial. Thisheatisthen
transferred throughsuccessivelayersofmoleculestowardsthecentre ofthematerial(4). This
meansthat the surface ofthe materialmaybe overheatedbeforethecentre reachesthe desired
temperature.
Radiantheating useselectromagnetic wavessuchasinfrared, ultraviolet, radio-frequencywaves
andmicrowaves. Radiantheating differsfromconventionalheatinginthatthebodyandnotthe
surface ofthe materialisheated. Radiantenergyalso canbe more preciselyfocused and
modulated whichallowsthe locationand quantityofheatto be bettercontrolled. Thisresultsin
anefficientheating processwhichcanbeusedincertainproductionprocessesto save energy,
time and materials (5).
The type ofelectomagneticwave (Figure 1) musthoweversuitthe applicationbeacause
electromagnetic wavesofdifferentfrequencies (oscillationspersecond) orlength(whichisthe
inverse offirequency) willpenetrate amaterialto differentdepths. Using thecorrectwave
frequencyalso improvestheefficiencyandcontroloftheheatingprocess.
Ultravioletwavesare the highestfrequencyelectromagnetic wavesandtheydo notpenetrate
deeply. Infrared wavesarelowerinfi-equencyand theycanpenetrate slightlymore. These types
ofwave are describedinmoredetailinanappendix.
Figure1-Theelectromagneticspectrumshowingthelocationofindustrialmicrowaveandradio-frequencyheatingregions.
Microwaveshave afrequencyinthe regionof300GHzwithwavelengthsfrom 1 mmto 1 metre.
Theycanpenetratematerialsto adepthofafewcentimetres. Radio-frequencywaveshave
frequenciesbelow 300MHzandwavelengthsgreaterthan 1 metre. Lowfrequencyradiowaves
canpenetrateasdeep asonemetre ormore.
Microwaves and radiowavescanbe usedto heatsome non-metallicmaterials. The temperature
whichthematerialreachesdepends onthe type ofmaterial, itsabilityto absorbenergyandthe
powerlevelofthe generating source (6).
Microwavetechnologiescanincreaseenergyefficiencyandproductivityandimproveproduct
quahtyinmanymanufacturingprocesses. Microwavescanreplace theburning offossilfuelsto
heatprocessequipmentand thusreduce emissions ofgreenhouse gases.
These benefitshavepromptedmicrowavesto be investigated fornon-traditionalapplications
including treating ceramicsandcomposites, drying pharmaceuticalcapsules and thawing logs
(7). Althoughradiantheating hasbeenavailableforover40yearshowever, ithasnotbeen
widelyapplied inCanadianindustry.
2. Dielectric Heating
Microwavesand radio-frequencywavescause the moleculesinthe bodyofsome materials to
rotate rapidlyinalternative directions. Thismovement millions oftimeseachsecond creates
frictionatamolecularscalewhichgenerateswhatisknownasdielectricheat.
Onlymaterialsthat are non-metallic and which are poorinsulatorsorconductorsofelectricity
wUlrespond to dielectric heating. Agoodconductorsuch ascopperoragood insulatorsuchas
airwillnot be heated (8).
Dielectricheating isadirect andefficientwayto heatresponsivematerials. Suchmaterialsoften
also are poorthermalconductors and theymaybebulkyand/orparticulate whichmakesthem
diificultto heat byconventionalmethods. Dielectricheatisgenerated withinthe material which
meansthere is little orno need to transferenergybyconductance, convectionormechanical
work. The heat also canbe uniformlyappliedexactlywhenandwhere itisneeded (8).
Adielectric heating systemhastwo maincomponents, whichwUldifferbetweenmicrowaves
and radio frequencywave systems:
a generatorused to convertelectricityof60Hzfrequencyto therequired
operating wave frequency
an applicatorto applytheenergyto the material.
Radiowavesand microwavesdifferintheirfrequencyandwavelength, buttheyheatdielectric
materials the same way. The particularwavelengthto be used in anapplicationmusthoweverbe
carefullychosen to assure optimumtechnicalandeconomic results. Considerationsinclude:
dielectric lossfactorofthe material
receptivityofcompounding ingredients
shape and dimensionsofthe materialto be heated
required penetrationofthe energyinto thematerial
available space
cost
availabilityofequipment.
There are three waysinaconfined areato applyand distribute the waves:
standing wave mode whichconcentrates theenergyonto asmallarea
multimode in whichthe microwaveshit the materialfromalldirections
travellingwave modewherethemicrowavesare directeddownaconveyor.
2.1 Microwaves
Thepropertiesofthematerialbeing heatedand themicrowavesystemandtherelationship
betweenthemmustbeconsideredwhendeveloping anappropriateproductprocessingsystem.
Thesepropertiesinclude (9):
SystemParameters
microwavepoweroutput/rate ofheating
frequencyofthemicrowave system
use ofauxiliaryheatforms
distributionofthemicrowave field
movementofthe productthroughthe field.
ProductParameters
viscositychangeswithtemperature
balancing specificheatsofmulti-componentsystems
wateractivity
shape, size andgeometryofthe product
formulationchangesandmoisturecontent.
Frequency
Industrialmicrowaveheatingequipmentcould interferewithradio communication,
controlandmeasuring devices. TheFederalDepartmentofTransportationand
Communicationsthusregulatesthefrequenciesthat maybeused andestablishes
tolerancesformicrowave systems. The microwave frequencieswhichmaybeused in
industrialapplicationsare915 MHzand 2450MHz. Everymaterialhasanoptimum
frequencyatwhichheatingbymicrowavesismosteffective. Thecloserofthetwo
permittedfrequenciesto thisoptimumgenerallyisused.
MostcurrentappUcationsofmicrowavesareinthefood, rubberandmetalfabricationindustries.
2.2 RadioFrequencyWaves
Triode oscillatorsnormallyareusedto generateradio fi-equencyenergywavesforindustrialuse.
Applicatorstake theformofparallelplateforthickmaterials, strayfieldforthinmaterials and
staggered strayfieldformaterialsofmediumthickness. TheenergycanbepreciselyappHed to
the materialusingelectroniccontrols.
Radiowavesare more commonlyassociatedwiththe textiles, plastics, wood andpaperproduct
industries.
