European Drying Conference - EuroDrying'11 Palma. Balearic Island, Spain, 26-28 October 11 Hot Air-Microwave Combined Drying Characteristics of Gypsum Board Won-Pyo Chun, Sung-Il Kim, and Ki-Woo Lee 1 1 Energy Efficiency Research Department, Korea Institute of Energy Research Address, 152 Gajeong-ro Yuseong-gu, Daejeon, 35-343, Korea Tel.:+82-42-86-3153, E-mail:wpchun@kier.re.kr Abstract: This study deals with experiments of hot air drying, microwave drying, and hot air-microwave combination. The electric capacity of the test apparatus is 3kw at 2,45Mhz microwave oven having inside chamber dimensions 3W 3B 15H mm and 6 orifice nozzles(φmm) for supplying hot air are integrated at the top of the test apparatus. The test materials are gypsum board with initial moisture content 35 4%w.b. and the size is W B T mm. In case of hot air drying, the air velocity varies m/s and 3m/s with hot air temperature 13 and 15 respectively. We studied hot air drying according to the variation of air temperature and air velocity. Also, the drying characteristics according to the variation of the generating power of microwave have been investigated. A combined hot air-microwave drying having the same energy consumption to compare with hot air drying as well as microwave drying has been studied. Keywords: hot air drying, microwave drying, hot air-microwave combined drying, drying time, gypsum board INTRODUCTION In general, the dryer using hot air has potential for wide applications because convective air dryers are more widely used in various industries compared with conduction or radiation type dryers. However, the hot air dryer requires high temperature drying or longer drying time in order to dry materials from high moisture contents to low moisture contents. Accordingly, the efficiency of dryer can be improved by combining different types of heating methods depending on physical properties of materials in drying processes of various dryers. In particular, the combined dryer is a useful means in terms of energy conservation and drying performance, which can operate over two stages by unifying equipment. Although drying kinetics for convective drying and microwave drying are similar except the period of internal pressure generation, several advantages of microwave drying stem from volumetric rather than surface heating and from different temperature profiles due to power penetration (Kudra and Mujumdar, 2). Interest in heating processes using microwave energy in a variety of industries has been growing recently. Microwave is electromagnetic waves in the range of frequencies from 3MHz to 3GHz with wave lengths of 1m to 1mm (Khraisheh, Cooper & Magee, 1997). Most industrial applications of microwave utilised the heating effect of microwave in the industrial processes: like cooking, puffing, drying, moulding etc. Microwave heating is known to be transferred to heat by rotation mechanism of dipole or ionic polarization. Heating principle by dipole rotation leads to rotation or friction of polarity molecular by electric element of micro waves as well as high friction energy by continued collision with neigh boring other molecule. In this process, temperature increases rapidly and this heating is called dielectric heating (Roger, 1988; Osepchuk, 1984; Alami, 1988; Newnham, 1991). Microwave heating creates friction energy inside the material and has faster heating speed than conventional convective heating. Equipment can be simplified or made as compact one. In addition, since microwave energy can heat with uniform heating and low energy, it does not transform or destroy molecular structure of materials, improving the quality. Microwave heating is widely utilized in the whole areas of industry such as foods, rubbers, organic chemicals, paper, timber, textiles, plastics, ceramic, and cosmetics etc. (Annapurna & Sisir, ). Recently, studies have been widely performed on hot air-microwave combined dryers which have advantage of hot air and microwave heating concurrently (Salagnac, Glouannec & Lecharpentier,
Duct Damper Computer Chamber Elec. balance Wave guide Magnetron Directional coupler Control panel Hot air generator Power meter Power supply Fig. 1 Schematic diagram of experimental equipment 4; Andres, Bilbao & Fito, 4; McMiinn, Mcloughlin & Magee, 5). This study performed characteristics test of hot air drying, microwave drying and hot air microwave combined drying using gypsum boards as test materials through basic test apparatus. Through this experiment, this study identified the characteristics of hot air microwave combined drying and obtained basic design data. MATERIALS AND METHODS Experimental Equipments The schematic diagram of experimental equipment used in this experiment shows the drying system using both hot air and microwave as shown in Fig. 1. Drying chamber size of this experimental equipment was 4W 4B 3H mm and manufactured with materials STS-34. Hot air chamber(3w 3B 15H mm) was installed on the top of this cavity. 6 round nozzles (mm diameter and 15mm length) were installed at the bottom of chamber in the triangular array with the interval of mm. Microwave heating system was configured with magnetron, waveguide and power supply. In this system, magnetron generating the microwave uses frequency of 2,455MHz and 3kW, and air-cooling type. WG43 was used as a waveguide that conveys and controls the microwave by connecting the applicator irradiating the microwave and the magnetron generating the microwave. In addition, directional coupler and power meter were installed for power indicator. Hot air generator uses 5kW electric heater. Hot air duct is connected to drying chamber with flange and perforated plate was installed for microwave shielding. Teflon table was manufactured and installed on the bottom of the drying chamber to put experimental materials. Test methods Test material used for this experiment is the gypsum Fig. 2 Drying curve of gypsum board board for test material in the size of mm and thickness of mm. Weight of test material was 35g and initial moisture content of 35 4%w.b. was dried to the final moisture content of 5%w.b. In the experimental equipment of Fig. 1, experiment was conducted with hot air temperature changed between 13 15 and hot air velocity changed between 3m/sec. During the experiment, microwave power by magnetron was changed.3 2kW. Experiment was conducted depending on test conditions for hot air, microwave, hot air microwave combined dryings depending on power conditions of microwave and hot air. In addition, each test material was tested for hot air and microwave drying characteristics for each drying rate period. To measure variation of moisture contents of experimental material, drying chamber was placed on electric precision balance (Mettler Toledo model SR641). Change in weight was measured. Variation of weights of experimental weight was measured in the unit of 3 seconds and entered in the computer to calculate moisture content. For measurement of temperature in drying chamber, fiber optic temperature gauge (FOT-L-CRM) was used and infrared moisture balance (Kett FD7) was used for measurement of moisture contents of final experimental materials. Hot air drying RESULT AND DISCUSSION The test material of this experiment was gypsum boards of WxB mm, thickness of mm and initial moisture contents of 35 4%w.b. Experiment was conducted while adjusting operating conditions and changing hot air temperature of 13 15 and hot air speed of 3m/sec. Fig. 2 shows the drying curve measuring the variation of moisture contents in the moisture balance for the test material of initial moisture contents 44%W.B.. As shown in Fig. 2, general drying curves shows short constant
Fig. 3 Drying curves according to hot air temperature Moisture Conten 45 4 35 3 25 15 5 13 vel. 3m/s 15 vel. 3m/s 5 15 25 3 Fig. 5 Drying curves according to hot air velocity Moisture Conten 45 4 35 3 25 15 5 5 15 25 3 Temp. 13 vel m/s Temp. 13 vel 3m/s Fig. 4 Drying curves according to hot air velocity rate period and relatively long falling rate period. Fig. 3 shows the results of experimentation by changing the hot air temperature to 1 15 while adjusting hot air velocity of hot air generator at the experimental equipment in Fig. 1 to 3m/sec as before. In Fig. 3, it took 52 minutes from the hot air temperature of 1 and 45 minutes from 13 to reach the final water contents of 8%w.b.. It took about 37 minutes from hot air temperature of 15 so that the drying speed increases about 4% compared to hot air temperature of 1. Fig. 4 is the result of experiment using the impinging jet air by raising hot air speed from m/sec to 3m/sec while keeping the hot air temperature to 13 for the test materials with moisture contents of 4%w.b. In Fig. 4, it took about 25 minutes to reach final moisture contents of 5%w.b. from the hot air speed of m/sec and minutes from that of 3m/sec. In the same temperature condition, about % drying speed was found to increase at hot air speed of 3m/sec compared to that of m/sec. Fig. 5 is the result of experiment adjusting the hot air temperature 13 15 while keeping hot air speed for material to 3m/sec. As a result of experimentation at hot air temperature of 13, it took about Fig. 6 Variation of moisture content according to Microwave power minutes to reach final moisture contents of 5%w.b. and 17.5 minutes to reach hot air temperature of 15. It was found out that drying speed increased about 12.5% if hot air temperature increased from 13 to 15 at the same speed conditions. As a result of drying characteristics using hot air, increasing hot air speed and using impinging jet air were found to increase heat transfer coefficient and to favorably improve drying speed favorably under same temperature conditions. Microwave drying This experiment was conducted on the gypsum board with initial moisture contents of 35%w.b. to recognize drying characteristics by microwave heating. Fig. 6 shows the results of experiment by changing the input value of power supply (about 7% of microwave power).5 2kW. To reach the final moisture contents of 5%w.b., it took about 26 minutes for input value of.5kw. It took about minutes for 1kW, so that about drying speed increased about 2.5 times compared to power value of.5kw. The more increase the microwave power, the more increase the drying speed. In addition, since inside and outside the test materials
Moisture Content(% 5 4 3 5 15 25 3 Air 13, m/s M/W 1kW Air 13 and M/W 1kW Fig. 7 Comparison of moisture content of hot air, microwave, and hot air-microwave heating Case 2, microwave heating was given for 6 minutes, followed by hot air heating for 11 minutes. In Case 3, hot air heating was given for 5 minutes, followed by microwave heating for 6 minutes and then finally by hot air heating for 5 minutes. In Fig. 8, drying curves were found to be very similar for initial 5 minutes for hot air heating or microwave heating. This section is considered to be constant rate period including preheating period and has no change in moisture content between hot air heating and microwave heating. In the falling rate period beyond the constant rate period, however, different aspects were found for each operation condition. This section is the falling rate period when the temperature of test material rises continuously while moisture inside the material slowly evaporates. It was found out that the reflected wave rapidly increased during the 2 nd falling rate period and performance declined. As a result of experiment, drying speed improved by about % in case 3 compared to case 1. Accordingly, it was concluded to be more advantageous to use hot air during constant rate period, to irradiate microwave during the 1 st falling rate period and to use hot air during constant rate period according to the drying schedule of experimental materials, in order to improve drying speed. Fig. 8 Drying curves according to operating conditions (Case 1: hot air-microwave, case 2: microwave-hot air, case 3: hot airmicrowave-hot air) are heated at the same time with the microwave heating, uniform heating is achieved and dry feature curve is found to be close to straight compared to hot air drying. Hot air-microwave Combined Drying Fig. 7 shows the experimental results for experiments of hot air drying, microwave drying and combined drying. In Fig. 7, it took about 25 minutes to reach the final moisture contents of 5%w.b. with hot air temperature of 13, and about 25 minutes with microwave drying (1kW). In addition, it took about 7 minutes for combined drying of hot air drying (temperature 13, speed 3m/sec) and Microwave (power 1kW) combined drying. Accordingly, it was found out that drying time decreased about 3.6 times for hot-air-microwave combined drying, compared to conventional hot air drying. Fig. 8 shows the results of experiment when alternating the hot air and microwave heating for each drying section. In Case 1, hot air was given for minutes, followed by microwave heating for 6 minutes. In CONCLUSIONS This study conducted experiment on gypsum boards for construction to examine features of hot air, microwave, hot air microwave combined drying depending on each experiment condition and obtained the following conclusion. 1) In general, if using impinging jet air over hot air speed of m/sec for hot air drying, drying speed can be improved with higher heat transfer coefficient compared to existing conventional hot air drying methods. 2) With regard to the results, drying speed improved substantially with microwave drying compared to hot air drying. As a result of experiment, drying time increased about 1.8 times with microwave drying (1Kw) compared to hot air drying (hot air temperature 13, hot air speed 3m/sec). 3) Drying time was found to decrease about 3.6 times with hot air and microwave combined drying compared to hot air drying (13 ). 4) If combining hot air heating and microwave heating according to drying schedule of test material, it was found to be advantageous to use hot air during initial constant rate period, to use microwave during the 1 st falling rate period and to use hot air during the
2 nd constant rate period, in order to improve drying efficiency. REFERENCES Kudra, T. & Mujumdar, A. S. (2), Advanced drying technologies, Marcel Dekker, Inc. Khraisheh, M. A. M., Cooper, T. J. R. & Magee, T. R. A. (1997) Microwave and air drying I. Fundamental considerations and assumptions for the simplified thermal calculations of volumetric power absorption, Journal of Food Engineering 33, 7219 Roger, M. (1988), Engineers, handbook of industrial microwave heating, The Institution of Electrical Engineers. Osepchuk, J. M. (1984), A history of microwave heating application, IEEE Trans. MTT, vol. 32, No. 9, 1-1224. Alami, R. (1988), The place of microwave and radiofrequency application in industrial process, Mat. Res. Soc. Sympo. Proc., Vol. 124, 311-316. Newnham, R. E., Jang S. J., Xu M., & Jones, F. (1988), Fundamental Interactions mechanisms between microwave and mats, Ceram. Trans., vol. 21, 51-68. Annapurna, D. & Sisir K. D. (), Microwave engineering, McGraw Hill Co. Salagnac, Glouannec & Lecharpentier, (4), The Place of Microwave and Radiofrequency Application in Industrial Process, Mat. Res. Soc. Sympo. Proc., Vol. 124, pp.311-316.