Microwave Drying of Expanded Perlite Insulation Board - American

Article pubs.acs.org/IECR

Microwave Drying of Expanded Perlite Insulation Board Blaz Skubic,† Mitja Lakner,‡ and Igor Plazl*,§ †

Trimo d.d., Prijateljeva 12, SI-8210 Trebnje, Slovenia Civil and Geodetic Faculty, University of Ljubljana, Jamova 2, SI-1000 Ljubljana, Slovenia § Department of Chemical Engineering, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, SI-1000 Ljubljana, Slovenia ‡

ABSTRACT: The microwave drying process intensification of new inorganic thermal insulation board, containing expanded perlite and inorganic binder, was investigated in a multimode microwave applicator, with a 1.8 kW, 2.45 GHz microwave source. Ability to generate heat inside moistened material and to avoid the problem of heat conduction to the core of the material is especially important when dealing with efficient thermal insulators. A 3D unsteady mathematical model was developed, considering Lambert’s law of exponential decay of microwave energy and nonuniform power distribution inside the microwave cavity, for temperature and drying rate predictions. The predicted temperature profiles at critical locations of the exposed board and cumulative drying rate were compared and verified with experimental data.

1. INTRODUCTION Inorganic insulation materials represent the majority of insulation products used in Europe, with approximately 60% market share.1 The most extensively used are fibrous materials, mineral and glass wool, but others like foam glass, vermiculite, and expanded perlite also represent an important part, especially in special applications. Besides the construction industry, expanded perlite can also be used in industrial, chemical, horticultural, and petrochemical industries.2 Perlite is obtained from pumice, which is a glassy form of rhyolitic or dacitic magma. It contains from 2 to 6% water, which causes expansion of crude ore during rapid heating to temperatures above 900 °C.3 Water inside the ore produces steam, and this forms bubbles within the softened rock to produce a frothylike structure.4 The process causes a volume expansion from 15 to 20 times of the original ore volume. Reports on a number of different mixtures have been published and patented, containing expanded perlite as a basic material for insulation product production, with multiple different binders, organic and inorganic. Physical and mechanical properties of perlite insulation products can vary in a density range from 110 to 240 kg/m3, with heat conductivity from below 0.06 W/m K to as much as 0.36 W/m K.5−8 The conventional perlite insulation board is made of an aqueous slurry of expanded perlite particles, binder, and additives (fibers, waterproofing agents, fire retardant, etc.), that is formed into a board by a Fourdrinier process and subsequently dried. Another type of board production is suggested, with mixing expanded perlite particles and sodium or sodium−potassium silicate, pressing the mixture in a mold and drying the pressed board. Conventional and microwave techniques are possible for drying such a board. The common limitation for all conventional drying methods is that, regardless of the principal mode of heat transfer from the heating medium to the surface of the board, the transfer from the surface to the inside of the board is principally by conduction. Since such a mixture is an efficient thermal insulator, the conduction of heat into the core of the thick board is very slow, so the process for © 2012 American Chemical Society

very thick boards takes a great amount of time. Microwave drying can, in that case, have an advantage over conventional hot air drying because of the direct effect on water molecules inside the material and consecutively bypassing the surface-tocenter conduction stage. Microwave drying has been extensively studied in various applications, including drying of silica sludge, borax, and cotton, and in particular in food processing.9−13 The study of drying behavior of expanded perlite insulation plates during microwave irradiation presented in this work is related to the industrial project. The technology for the production of a new lightweight building material for sandwich panels includes drying in two stages. In the first stage, the controlled microwave drying allows homogeneous cross-linking of bonding material mixed with perlite and therefore basic mechanical properties of insulation plates. At the end of the process it is necessary to provide water repellency of formed insulation plates, and it is therefore necessary to dry moistened plates again. Especially drying during the process of hydrophobization significantly affects the economics of the industrial technology. The project research from the laboratory to industrial level has confirmed the advantages of microwave heating from a conventional heating due to the optimum time, energy, and economic efficiency. Especially in the last stages of microwave drying of a perlite insulation board, the problem of rapid temperature rise inside the board can occur. This can cause problems in controlling the temperature at the end of the process, so it becomes very important to stop the process exactly at the time all the moisture has evaporated. In order to predict the occurrence of such phenomena, it is necessary to analysis a mathematical model from which insight might be gleaned into an inherently complex physical process.14 Mathematical modeling can predict the necessary residence time for microwave drying process and Received: Revised: Accepted: Published: 3314

August 11, 2011 January 4, 2012 January 31, 2012 January 31, 2012 dx.doi.org/10.1021/ie201790w | Ind. Eng. Chem. Res. 2012, 51, 3314−3321

Industrial & Engineering Chemistry Research

Article

can also present a basis for controlling the process on a large, industrial scale. The objectives of the present work were to develop a mathematical model capable of predicting temperature profiles inside the microwave dried expanded perlite insulation board and to predict the drying rate of the board so treated. Lambert’s law was used to describe the internal heat generation term, and the adopted water load method was developed to determine the volumetric power absorbed on the outside of the tested board. The numerical predictions of temperature profiles and drying rate were compared with experimental data.

keff

∂T = h(T − T∝) ∂x

x = L1

keff

∂T = h(T − T∝) ∂y

y=0

keff

∂T = h(T − T∝) ∂y

y = L2

keff

∂T = h(T − T∝) ∂z

z=0

2. THEORETICAL BACKGROUND 2.1. Microwave Drying. A typical microwave drying process of a porous material can be divided into three stages, namely, initial heating up period, constant drying rate period, and falling drying rate period. In the initial heating up period, microwave energy is required to raise the temperature of the dry material and water to the boiling temperature of water. Afterward, in the constant drying rate and in the falling drying rate periods, the absorbed microwave energy is used to evaporate the water from inside the moistened material. To describe heat transfer in the initial heating up period, energy balance can be used with the internal heat generation part15

keff

∂T = h(T − T∝) ∂z

x = L3

∂T (ρc p)eff = ∇(keff T ) + P ∂t

α=

T = Tint

0 < x < L1

keff

∂T = h(T − T∝) ∂x

P0 =

t>0

(10)

(11)

ε′( (1 + tan 2 δ ) − 1) 2

(12)

ε″ ε′

(13)

m ΔT cp V Δt

(14)

In calculating the volumetric power absorption with eq 14, it is assumed that care is taken to account for the heat capacity of the water holder and the thermometer. Also any cooling effects and magnetron lag-time must be taken into account. Volumetric power absorption at the surface can be measured with the water load method (section 3.3.) and calculated with eq 14 for the entire plate surface. To calculate the contributions from all six power sources normal to volume increment on the surface, eq 15 is written for all the measurement data and the system of equations is solved. Later, eq 15 is used for

(3)

(4)

x=0

(9)

The use of Lambert’s law requires the calculation of P0, obtainable from calorimetric measurement with the water load method.20

0 < y < L2

0 < z < L3

2π λmw

tan δ =

with the initial and boundary conditions t=0

t>0

(8)

The ratio of the loss factor to the dielectric constant is called the loss angle (δ), and it is used to quantify the looseness of a dielectric, as in the following equation.8

(2)

∂T = ∇(keff T ) + fP − I ΔH ∂t

t>0

(7)

where P0 is the power absorbed on the surface, d the distance measured from the surface to the selected position, and α the attenuation factor (function of dielectric constant ε′ and loss factor ε″). The dielectric constant, ε′, is a measure of the ability to store electrical energy in the material, and the loss factor, ε″, is the ability of a the material to convert electromagnetic energy into heat. Materials with a relatively high value of ε″ are microwave-absorbent, whereas materials whose ε″ is close to zero are microwave-transparent.19

The final form of energy balance for our system can then be rewritten as (ρc p)eff

t>0

P = P0e−2αd

(1)

p(m ) p(mo)

t>0

(6)

2.2. Microwave Power Absorption. Microwaves are used in industry primarily as a source of thermal energy.18 High temperature inside the exposed material is rapidly obtained even in materials with low thermal conductivity, due to selective absorption of electromagnetic energy by water molecules. Penetration of microwaves inside the material can be described by Lambert’s law approximation, which considers an exponential decay of microwave absorption within the product, according to the following expression

where P is the volumetric power absorption, specific for different positions inside the material, cpeff is effective heat capacity for the moist material, and keff is effective thermal conductivity for the moist material, calculated from Ochs et al.16 When the moist material reaches the boiling temperature of water, the constant drying rate period starts. The evaporation part (IΔH) must be added to eq 1, where I is the spatially distributed volumetric rate of evaporation which can vary with time.15 During the drying process, the total load inside the microwave cavity is decreasing when the evaporated moisture is being carried out of the cavity with the ventilation system. The total volumetric power absorption by the load is changing with the mass of the load, according to Lin et al.17 The power change factor can then be employed to describe the dependence of the absorbed power at the current load and the absorbed power at the start of the process: f=

t>0

(5) 3315

dx.doi.org/10.1021/ie201790w | Ind. Eng. Chem. Res. 2012, 51, 3314−3321


Article

Figure 1. Schematics for volumetric power absorption calculation for a random point inside the plate.

130 g/L. A hole of 13 × 13 mm was cut in 36 different plates at different positions. By combining six boards with no holes and one with a hole, one could simulate an insulation board with dimensions 200 × 200 × 70 mm, with a 13 × 13 × 10 mm hole at some random position on the board surface. With some transformation of the cut plates, all the surface area of the 200 × 200 × 70 composed plate could be covered with a hole at different positions. Figure 2a shows an example of hole positioning and a combination of cut and uncut plates. The hole was filled with a specially made void container, made from gypsum and coated with epoxy resin, so it would contain water as shown in Figure 2b. For every experiment, six thermostated uncut plates were prepared, with spraying exactly the same amount of distilled water with the same temperature as the plates on both of the largest surfaces, and then left for 10 min, so that moisture homogeneity was obtained. The same procedure was used for the cut plate with the selected opening. All seven plates were then put together and placed into the microwave cavity. The composition of the plates was on precisely the same position inside the cavity for every series of experiment. The gypsum container was filled with approximately 0.9 g of distilled water and then inserted inside the hole of the cut plate. The x, y, and z distances were measured according to Figure 2a. Optical thermosensor Opsens OTG-A (2 m long with −40 to 250 °C temperature range) was used for temperature measurement, attached to an Opsens single channel PicoM data acquisition system, connected to a PC. The system allowed temperature readings to be taken directly during the microwave treatment of the plates, with a 0.2 s sampling interval. 3.4. Dry Insulation Material Power Absorption. For the determination of volumetric power absorption of dry insulation material, several temperature measurements were made at different locations inside the insulation plate. Seven uncut plates were heated at 200 °C for a day and placed at the desiccator until reaching the uniform temperature of 20 °C. The plates were arranged in the same procedure as in section 3.3 and placed at the center of the cavity. The temperature increase rate was measured at the same operating conditions. 3.5. Drying Rate. For the final verification of temperature and mass profiles of the dried plate, 10 thicker (70 mm) plates were prepared and moistened according to the procedure of preparing plates for volumetric power absorption measurement and were dried inside the microwave. Weight measurements were made at different time intervals. The time elapsed

volumetric power absorption calculation through the entire volume. Figure 1 shows the schematics for the use of eq 15. P(x , y , z) = P(0, y , z)e−2αx + P(L1, y , z)e−2α(L1− x) + P(x , 0, z)e−2αy + P(x , L 2 , z)e−2α(L 2 − y) + P(x , y , 0)e−2αz + P(x , y , L3)e−2α(L3 − z) (15)

3. EXPERIMENTAL WORK 3.1. Energy Distribution. First, the energy distribution inside the microwave cavity of a Panasonic NE-1880 microwave oven (maximum power output 1800 W, operating frequency 2.45 GHz) was investigated to determine the optimum position for sample placement. The microwave cavity with dimensions of 535 × 330 × 250 mm was divided into 45 zones (3 levels in height containing 15 positions each). One hundred milliliters of distilled water in a glass beaker was placed on the acrylic support at the center of the zone and heated for 20 s at maximum power. Water temperature was measured before and immediately after the heating, within 8 s of intense mixing by using a type K thermocouple attached to a NATIONAL data acquisition system, connected to a PC. 3.2. Effect of Load on Power Absorption. To determine the effect of loading on microwave power absorption, the microwave power absorbed by distilled water at the center of the cavity was measured. An acrylic container of 200 × 200 × 70 mm with wall thickness of 2 mm and 1000 and 100 mL glass beakers were used for the comparison of rectangular and cylindrical load on microwave power absorption. Water mass inside the container ranged from 200 to 1600 g, from 50 to 1000 g in the 1000 mL beaker, and from 10 to 100 g in the 100 mL beaker. The microwave heating time used for each sample was predetermined according to the size of the load, so that the final water temperature would not exceed room temperature by more than 10 °C.18 Temperature was measured following the same procedure as for determination of energy distribution within the microwave cavity (section 3.1.). 3.3. Sample Surface Power Absorption. To determine the volumetric power absorption on the surface of the exposed insulation plate, a set of experiments was carried out. According to Plazl et al.,8 96 perlite boards were prepared with dimensions of 200 × 200 × 10 mm and density in a range between 128 and 3316



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Figure 2. (a) Composition of six cut and one uncut plate to create one plate with greater height; (b) water container for insertion in the hole of the cut plate.

Figure 3. Hot−cold spot distribution in microwave cavity at three levels for 100 mL water load.

between sample removal from the microwave oven and weight measurement was minimized and recorded.9 Comparison was made between one 70 mm thick sample and a composition of seven plates of 10 mm thickness.

According to the calorimetric method described in section 3.2, the power absorbed by different loads and different geometries was calculated. Although the experimental testing only included rectangularly shaped samples, glass beakers were also used for power absorption determination, because the temperature change for small water loads (below 200 g of distilled water) could not be measured with the acrylic container with the same dimensions as the sample. Figure 4 shows the power absorbed by different geometries and different loads. A nonlinear equation was developed, based on the experimental results, to determine the dependence of volumetric power absorption on water mass. The average volumetric power absorption was calculated for all loads, without considering different geometries, and the following relationship was obtained:

4. RESULTS AND DISCUSSION 4.1. Power Absorption Calculation. Figure 3 shows the absorbed energy inside the microwave cavity with the 100 mL water load, calculated from experiment from section 3.1. The distribution is expectedly nonuniform throughout the whole cavity. It can be seen that the maximum of absorbed energy is at the center of the cavity. It was expected that energy distribution would change with different geometry and weight of the load inside the cavity, but the center of the cavity was still selected for sample positioning to receive the most homogeneous distribution of energy on the sample surface.

P = 2.74*103ln(mH2O) + 2. 1*104 3317

(16)



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Figure 4. Power absorption by different geometries and water loads in the center of the cavity.

Figure 5. (a) Calculated volumetric power absorption on the surface of the tested plate; (b) calculated volumetric power distribution at the center of the plate (x = L1/2; y = L2/2) through the thickness of the plate.

Because the values of volumetric power absorption, calculated for experimental locations (volumes) on the plate surface with eq 14, take into account the contributions from all six directions normal to the selected volume, a corrected power distribution must be calculated to calculate the volumetric power absorption for the entire volume. Writing eq 15 for all calculated data on the plate surface, where P(x, y, z) is the value calculated for specific location with eq 14, a system of 870 equations is obtained with 870 different unknowns, presenting the power sources from all directions, normal to the surface of the selected location. The corrected power distribution is obtained by solving the system of equations with known x, y, and z for every location. The corrected power distribution takes into account the influence of all power sources from all six directions, normal to the surface of the plate, and can then be used for the calculation of the volumetric power absorption for the entire volume with eq 15, which is the input parameter for solving the energy balance for the system. The dielectric properties used for volumetric power absorption calculation

Inserting eq 16 in eq 2 and inserting the initial water mass of 890 g, the following expression is obtained: f = −1.13*ln(mH2O) + 8.79

(17)

Power absorption on the surface of the insulation plate sample was determined calorimetrically from experimental data obtained by the procedure described in section 3.3 on 870 different locations on the plate surface. With the use of eq 14, the volumetric power absorption on this location was calculated, taking into account the mass and heat capacity of the water holder, and is shown in Figure 5. It can be seen that the power absorbed on the surface of the plate is much more homogeneous than expected from the measurement of absorbed power for empty microwave cavity with only 100 mL water load on different positions. Still, some hot and cold spots appear on the surface, such as the one in the top left corner in the direction of the cavity door. 3318



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Figure 6. Predicted and experimental temperature profile for locations with maximum and minimum calculated volumetric power absorption.

where ρp is the dry insulation plate density, ρ(t) is current plate density, and cp,H2O and cp,p are the water and dry perlite insulation plate heat capacity. After all the water has evaporated from the plate, the temperature of the plate starts to increase rapidly due to the microwave energy absorption of the dry expanded perlite. The volumetric power absorption by the dry insulation plate was found to be more homogeneous than that of a moist plate. Fifteen measurements were made at the positions of the predicted hot and cold spots, and the measured value was 68 kW/m3 ± 7%. Figure 7 shows the completely dried plate,

were considered constant, the values taken from Wei et al.21 The power absorbed at the center of the plate is shown in Figure 5b. It can be seen that the absorbed power follows the assumption of decay of microwave absorption through the plate, as described by Lambert’s law. The center of the plate is selected to minimize the contribution of power sources in directions x and y. 4.2. Temperature Distribution and Mass Balance. A finite difference method was used for solving eq 3 with the corresponding initial and boundary conditions. Figure 6 shows the predicted temperature profiles for the locations with the maximum and the minimum calculated volumetric power absorption on the sample plate. It can be seen that even with uneven power distribution at the beginning of the drying process the time needed for different positions to reach the boiling temperature and the period of constant drying rate are very similar, the reason most probably being rapid heat transfer inside the porous structure of the perlite plate due to water convection. The effective thermal conductivity used in solving eq 3, which takes into account the transport of evaporated moisture, air movement, and water redistribution within the pores, was calculated according to Ochs et al.22 for moistened thermal insulation materials. The heat transfer coefficient, which is crucial in controlling the plate temperature in the initial heating up period, was calculated to be 3.61 W/m2 K, the value being in the range of the heat transfer coefficients used for domestic microwave ovens (Geedipalli et al.23). After the start of evaporation, the temperature of the plate is stable, slightly changing around the boiling temperature of water. It can be seen that all the absorbed microwave energy in the constant drying rate period is used for evaporation of the water inside the plate. The heat capacity at any given time was calculated according to eq 18, taking into account the evaporation of moisture from inside the plate: ρp ⎞ ⎛ c p(t ) = ⎜1 − ⎟(c p,H2O − c p,p) + c p,p ρ(t ) ⎠ ⎝

Figure 7. Hot spot inside the perlite insulation plate.

still being subjected to microwave heating and the creation of the hot spot. The temperature of the plate rapidly increases and finally reaches the melting temperature of the plate, melting the surroundings of the plate and creating voids inside it. This phenomenon must be avoided during the plate production process with good temperature control. The relationship between the temperature and the mass of the plate can be seen from Figure 8. All three drying stages can be seen with the corresponding temperature profilesinitial

(18) 3319



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Figure 8. Average density and temperature profiles for the insulation plate.

t time (s) T temperature (°C) V volume (m3)

heating up period with almost no mass change, constant drying rate period with temperature being constant around the boiling temperature of water, and falling drying rate period with the temperature of material rapidly increasing again. As seen in Figure 7 the process must be stopped at this stage to avoid damaging the material.

Greek symbols

α attenuation factor (/) δ loss angle (/) ε′ dielectric constant (/) ε″ loss factor (/) λ wavelength (m) ρ density (kg/m3)

5. CONCLUSIONS A mathematical model was developed to predict the temperature profiles for a perlite insulation plate inside the microwave cavity with Lambert’s law approximation, used for the calculation of energy absorption inside the plate. The plate density was calculated and compared with experimental data. The results show good agreement between the predicted and experimental data and can be used to predict the necessary residence time for a perlite insulation plate to be completely dried during the production process. The process of volumetric power absorption determination can be modified for large scale insulation boards by using glass beakers on the locations where hot or cold spots can be predicted and used to determine the energy distribution inside a larger, industrial cavity.

■

Subscripts

■

eff effective int initial H2O water p dry plate

REFERENCES

(1) Papadopoulos, A. M. State of the art in thermal insulation. Energy Build 2005, 37, 77−86. (2) Singh, M.; Garg, M. Perlite based building materialsA review of current applications. Constr. Build. Mater. 1991, 5 (2), 75−81. (3) Mladenovič, A.; Šuput, J. S.; Ducman, V.; Škapin, A. S. Alkali− silica reactivity of some frequently used lightweight aggregates. Cem. Concr. Res. 2004, 34, 1809−1816. (4) Ilker, B. T.; Burak, I. Effect of expanded perlite aggregate on the properties of lightweight concrete. J. Mater. Process. Technol. 2008, 204, 34−38. (5) Hillmer, R. W.; Morrone, N. F.; Slahetka, C. R. Perlite insulation board and method of making the same. U.S. Patent 3,988,199, October 26, 1976, http://www.freepatentsonline.com/3988199.html. (6) Ruff, D. L.; Nath, N. G. Perlite boards and method for making same. U.S. Patent 4,313,997, February 2, 1982, http://www. freepatentsonline.com/4313997.html. (7) Deporter, C. D.; Dawson, S. D.; Battaglioli, M. V.; Sandoval, C. P. Perlite-based insulation board. U.S. Patent 6,149,831, November 21, 2000, http://www.freepatentsonline.com/6149831.html. (8) Plazl, I.; Kavcic, M.; Franko, U. D.; Bohor, D. Inorganic filling for panel core and method for its manufacturing. Patent Application PCT/SI2007/000019, October 18, 2007, http://www.wipo.int/pctdb/ en/wo.jsp?WO=2007117225.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +386-1-24-19-512. Fax: +386-1-24-19-530.

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NOMENCLATURE cp specific heat (J/kg K) d distance from the surface (m) f factor of power change (/) h convective heat transfer coefficient (W/m2 K) ΔH heat of evaporation (J/kg) I evaporation rate (kg/m3 s) k thermal conductivity (W/m K) m mass (g) P volumetric power absorption (W/m3) P0 surface volumetric power absorption (W/m3) 3320



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(9) Idris, A.; Khalid, K.; Omar, W. Drying of silica sludge using microwave heating. Appl. Therm. Eng. 2004, 24, 905−918. (10) Kocakusuak, S.; Kolroglu., H. J.; Golzmen, T.; Savasu, O. T.; Tolun, R. Drying of wet borax pentahydrate by microwave heating. Ind. Eng. Chem. Res. 1996, 35, 159−163. (11) Lyons, D. W.; Hatcher, J. D.; Sunderland, J. E. Drying of porous medium with internal heat generation. Int. J. Heat Mass Transfer 1972, 15, 897−905. (12) Ali, A. A.; Gaikar, V. G. Microwave assisted process intensification of synthesis of thymol using carbonized sulfonic acid resin (CSA) catalyst. Ind. Eng. Chem. Res. 2011, 50, 6543−6555. (13) Campanone, L. A.; Zaritzky, N. E. Mathematical analysis of microwave heating process. J. Food. Eng. 2005, 69, 359−368. (14) Makinde, O. D. Solving microwave heating model in a slab using Hermite−Padé approximation technique. Appl. Therm. Eng. 2007, 27 (2−3), 599−603. (15) Datta, A. K. Porous media approaches to studying simultaneous heat and mass transfer in food processes. I. Problem formulations. J. Food. Eng. 2007, 80, 80−95. (16) Ochs, F.; Heidemann, W.; Muller-Steinhagen, H. Effective thermal conductivity of moistened insulation material as function of temperature. Int. J. Heat Mass Transfer 2008, 51, 539−552. (17) Lin, Y. E.; Anantheswaran, R. C.; Puri, V. M. Finite element analysis of microwave heating of solid foods. J. Food. Eng. 1995, 25, 85−112. (18) Ayappa, K. G.; Davis, H. T.; Crapiste, G.; Davis, E. A.; Gordon, J. Microwave heating: An evaluation of power formulations. Chem. Eng. Sci. 1991, 46 (4), 1005−1016. (19) Robinson, J. P.; Kingman, S. W.; Barranco, R.; Snape, C. E.; Al-Sayegh, H. Microwave pyrolysis of wood pellets. Ind. Eng. Chem. Res. 2010, 49, 459−463. (20) MacLatchy, C. S.; Clements, R. M. A simple technique for measuring high microwave electric field strengths. J. Microwave Power 1980, 15 (1), 7−14. (21) Wei, C. K.; Davis, H. T.; Davis, E. A.; Gordon, J. Heat and mass transfer in water-laden sandstone: Microwave heating. AIChE J. 1985, 31 (5), 842−848. (22) Ochs, F.; Heidemann, W.; Müller-Steinhagen, H. Effective thermal conductivity of moistened insulation materials as a function of temperature. Int. J. Heat Mass Transfer 2008, 51, 539−552. (23) Geedipalli, S. S. R.; Rakesh, V.; Datta, A. K. Modeling the heating uniformity contributed by a rotating turntable in microwave ovens. J. Food. Eng. 2007, 82, 359−368.

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Microwave Drying of Expanded Perlite Insulation Board - American

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