Increased interest is being shown in infrared drying today because of the environmental and technological advantages offered by this method. In order to assess the advantages of this drying process, extensive trials have been carried out. The objective of this investigation was to study the drying rate of infrared drying. This was achieved with the use of scanning pyrometer and image analysis.

Haghi, A. K.

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A. K. Haghi

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Acta Polytechnica

Experimental Investigations on Drying of Porous Media Using Infrared Radiation

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionDrying is one of the most energy-intensive processes inmany industrial fields (food industry, paper industry, textileindustry, etc.). Therefore, studies of this type of problem havebecome very important and have for several decade attractedthe attention of researchers. This article discusses an experi-mental study of heat transfer during the drying of porousmedia, using image analysis with the aim of improving pro-cess performance.Textile drying machines are among the most expensiveand most energy-consuming machines used in wet pro-cessing. Most of them consist of hot air dryers from whicha proportion of the circulating air is discharged to waste asexhaust air. The air discharged from the chimney stack,represents a significant energy loss. Infrared dryers havea variety of advantages, in particular when they are usedfor treatment of textile sheet material in wet state. First,they permeate into and heat the textile sheet very quickly.Secondly, they heat only the required sections of the object.There is no heating of unnecessary sections of the object,thereby avoiding wastage of thermal energy. Thirdly, sinceinfrared irradiation causes, an almost simultaneous tempera-ture rise in different sections of an object exposed to it,regional variations in temperature within the object can besignificantly reduced leading to ideal and uniform heatingof the object. Finally, adjustment of the output voltage toinfrared irradiation enables simple, easy and swift control ofheating condition in accordance with the demands in actualtreatment of textile sheet materials.2 ExperimentalThe experiment consisted a simple 20 kW quartz tubelaboratory infrared dryer. The dryer sends the heat to thefabric surface. An infrared camera observes the thermal sceneand produces a real-time, monochrome thermal image. Theanalog video images are recorded on tape for the entireexperiment, with later extraction of the temperature data.The IR camera, which is calibrated at certain temperature,maps the spectral radiance onto a thermal image of the fabricsurface. Following image acquisition, analog video imagesare digitized and treated to image processing routines(algorithms) to improve the signal-to-noise ratio and trans-formation of useful data. This image processing system hasalready been described in detail in previous papers [1–6].Generally, a typical image processing system containsthree fundamental elements:• an image-acquisition element,• an image-processing element, and• an image display element.A continuous scene is converted to a digital imageand stored in compacted memory by the image acquisitionelement. This image is displayed in some form by theimage-display element for human viewing. The image-pro-cessing element is designed for various tasks, which generallyfall into three main groups:• image enhancement,• image analysis and• image coding.The scanning pyrometer rests on the fabric surface, ensur-ing that the temperature measurement does not result indamage to the fabric.Knowing the values of water content of the fabric beforedrying (Hi) and after drying (Hf), one can calculate the globalrate of water evaporation from the fabric in the dryer at agiven total power of the infrared source [7]: EVAP H H SLDi f  (1)WhereS = speed of fabric in the oven (m/s),L = fabric width (m),D = superficial weight of dry fabric (g/m2).The energetic efficiency (EFF) may be calculated from theevaporation rate using [7]:EFFEVAP HP100 (2)WhereH is the latent heat of evaporation of water (J/g).Alternatively, EVAP and P can be expressed in units of(gH2Om–2s–1) and Wm–2 respectively.Equation (2) predicts that EVAP will be directly propor-tional to P provided EFF remains constant. The value ofEFF applies only to this particular dryer and depends on theoperating conditions.The characteristics of the various fabrics in this dryingstudy are given in Table 1.©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 55Acta Polytechnica Vol. 41  No.1/2001Experimental Investigations on Dryingof Porous Media Using InfraredRadiationA. K. HaghiIncreased interest is being shown in infrared drying today because of the environmental and technological advantages offered by thismethod. In order to assess the advantages of this drying process, extensive trials have been carried out. The objective of this investigationwas to study the drying rate of infrared drying. This was achieved with the use of scanning pyrometer and image analysis.Keywords: infrared dryer, porous media, drying rate, image analysisTable 1: Fabrics used in the drying studyFABRIC FIBER WEIGHT, g/m2A COTTON 130B POLYESTER 150In the drying process, water is evaporated from the ex-posed yarn surfaces and replaced by water migration fromwithin the fabric structure. During this process, a steady stateis established and the fabric temperature and evaporationrate remain almost constant. At the of the so-called constantrate drying period, the fabric temperature begins to increaseand the rate of evaporation decreases, because below a certaincritical water content migration of liquid water, and laterwater vapor, to the exposed surfaces becomes more difficultand eventually ceases.3 Results and DiscussionThe drying curves obtained (Fig. 1) show moisture contentversus time. The slope of this curve is the drying rate, the rateat which moisture is being removed. The curve begins with awarm-up period, where the material is heated. The dryingrate can be negative in the warm-up period. As the materialheats up, the rate of drying increases to a peak rate that ismaintained for a period of time known as the constant rateperiod. Eventually, the moisture content of the material dropsto a level known as the critical moisture content, where thehigh rate of evaporation cannot be maintained. This is the be-ginning of the falling rate period.The greatest effects of humidity on the moisture contentversus time curves occur in the warm-up and constant rateperiods. Once the critical moisture content is reached, thecurves are similar. In the second falling rate period, humidityhas little effect on the time required to remove an incrementof moisture.The warm-up period corresponds to the region wheremoisture content is the highest and the drying rate is rapidlychanging. The plots reveal that the warm-up period is rela-tively short and represents a small part of the total dryingtime.An image analysis of the surface is shown in figure 2. Theshape of the wave is influenced by the pore size distribution ofthe fabric. The flat region of the surface depicts the highesttemperature. In phenomenological terms, the fabric reachesa critical temperature when there is an irreversible shift in theheat balance between the heat generation rate and the heatlosses. Therefore, factors such as fabric orientation affect themeasurement of this parameter. As weight increases, the tem-perature of the fabric increases at slower rate. This is asexpected, because increasing fabric thickness increases the56 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No.1/2001Time [s]Moisturecontent[%]Fig. 1: Moisture content versus timeFig. 2: Surface temperature distribution at 90 seconds and surface isothermal distributioninternal resistance to heat transfer within the fabric. Thesurface isotherm distributions are also shown in figure 2.The fabric surface temperature depends on the balance be-tween the rates of heat generation and heat loss. Surfaceradiation is a major part of the total heat losses and dependstrongly on the surface temperature. A small change in sur-face temperature leads to significant changes in surface heatlosses. The surface isotherm vary with the fabric properties,including weight, construction and porosity. The irregularityof the isothermal contour lines is due to surface non-unifor-mities and yarn structure.4 ConclusionsIn this study, the rate of drying was essentially independ-ent of the nature of the fabric type, provided the final watercontent was not much below the critical value correspondingto the end of the constant rate drying period. The drying ratein the constant rate period decreases with increasing hu-midity if the temperature is low enough to prevent thermaldamage. Humidity had little effect in the falling rate period.References[1] Haghi, A. K.: An Experimental Approach for Study of HeatDissipation from the Surface of Small Electric Machines. TheAnnals of the Stefan Cel Mare University, Romania,2000, 7 (13), pp. 66–70[2] Haghi, A. K.: A Thermal Imaging Technique for MeasuringTransient Temperature Field – An Experimental Approach.The Annals of the Stefan Cel Mare University, Romania,1999, 6 (12), pp. 73–76[3] Haghi, A. K. et al.: Determination des Coefficients deTransfert de chaleur lors du Sechage. 2nd Int. Conference onDevelopment & Application Systems Proc., Romania,1994, pp. 189–196[4] Haghi, A. K.: Determination of Heat Transfer Coefficientsduring the Process of Through Drying of Wet Textile Mate-rials with an Optico-Mechanical Scanning Pyrometer & IRThermograph. 3rd Int. Conf. on Development & Applica-tion Systems Proc., Romania, 1996, pp. 25–32[5] Haghi, A. K.: Controle de Materiaux par ThermographieInfrarouge: Modelisation et Experiences, 4th Int. Conf. onDevelopment & Application Systems Proc., Romania,1998, pp. 65–76[6] Haghi, A. K. et al.: Determination des Coefficients de transfertde chaleurs lors du Sechage de Textiles par ThermographieInfrarouge et Microscopie Thermique a Balayage. Poster Pre-sentation, SFT, Paris, France, 1994[7] Broadbent, A. D.: Predrying of Textile Fabrics with InfraredRadiation, Textile Res. Journal, 1994, 64 (3),pp. 123–129Dr A. K. HaghiUniversity of GuilanP.O.Box 3756RashtIRANe-mail: Haghi2002@yahoo.com©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 57Acta Polytechnica Vol. 41  No.1/2001

Experimental Investigations on Drying of Porous Media Using Infrared Radiation

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