EHD-Enhanced Drying with Multiple-Wire Electrode

The enhancement of drying rate by corona discharge from multiple wire electrodes has been experimentally evaluated in this study. The corona wind produced by an array of three wire electrodes, which resembles a series of slot jets, impinges, and removes moisture from the surface of 6 mm glass beads saturated with water. Corona discharges with positive and negative polarities have been considered. Two sets of experiments have been conducted (one with the cross-flow of air and the other without), at 1 kV increment from the corona threshold voltage to a maximum voltage of 25 kV. To evaluate the drying enhancement, a companion experiment was carried out simultaneously for each case under the same ambient conditions but without the application of electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a single wire electrode to evaluate the effectiveness of a multiple-wire system.     

EHD-Enhanced Drying with Needle Electrode

Abstract

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Drying with Needle Electrode

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Water Evaporation

The enhancement of water evaporation by electrostatic field (corona wind) has been experimentally evaluated in this study. Two sets of experiments were conducted, with and without cross-flow, at an increment of 1 kV from the corona threshold voltage until the occurrence of sparkover. Two types of electrodes (wire and needle) were used. In addition, both positive and negative corona discharges were applied. The weight loss of water due to evaporation as well as the ambient temperature and humidity were measured. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of the evaporation enhancement using electric field. Each experiment lasted for at least 5 h. With the application of electric field alone, the enhancement in the water evaporation rate increases with the applied voltage. With the introduction of cross-flow, the enhancement in the evaporation rate becomes nearly independent of the applied voltage and is close to the result obtained with cross-flow alone.     

EHD-ENHANCED DRYING WITH WIRE ELECTRODE

ABSTRACT

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

EHD-ENHANCED DRYING WITH WIRE ELECTRODE

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

EHD-Enhanced Water Evaporation from Partially Wetted Glass Beads with Auxiliary Heating from Below

Earlier studies have shown that an electric field in the form of corona wind can significantly enhance drying rate. It is particularly effective in the early stage of drying when the material is fully wetted. However, the effectiveness of corona wind in drying a partially wetted material, which is commonly believed decreasing, has not been critically examined. For the present study, enhancement of water evaporation from partially wetted glass beads by corona wind is experimentally evaluated. In addition, auxiliary heating provided by a thermofoil heater attached to the bottom surface of the sample container has been used as a possible means to maintain the effectiveness of corona wind. For the present study, a fine copper wire with a diameter of 0.5 mm is used as the emitting electrode. It is charged by direct current with negative polarity from the corona threshold voltage until the occurrence of sparkover. The water level inside the glass beads is below the external surface and is maintained constant in each set of experiments. For each case, a companion experiment is carried out simultaneously under the same ambient conditions but without the application of electric field or auxiliary heating, the result of which is used as a basis in the evaluation of the evaporation enhancement. The weight loss of water as well as the ambient temperature and humidity are continuously monitored. Each experiment lasted for at least 5 h. The results show that the electric field is effective in the enhancement of water evaporation from partially wetted glass beads, but its effectiveness diminishes when the water level in the glass beads recedes. By applying auxiliary heating, this shortcoming of EHD-enhanced drying can be overcome.     

Drying of Mushroom Slices Using Hot Air Combined with an Electrohydrodynamic (EHD) Drying System

In this study, hot air combined with EHD drying is examined as an improved method for drying mushroom slices. The effects of three levels of voltage (17, 19, and 21 kV) and electrode gap (5, 6, and 7 cm) on the drying kinetics, time, effective water diffusion coefficient, and energy consumption of the EHD, pure hot air (provided within a chamber), and hot air combined with EHD drying systems were investigated. ANOVA showed that there were significant differences between EHD treatments and control (pure hot-air-drying treatment) for all of the investigated responses. Voltage and electrode gap factors had significant effects on all investigated responses. The results confirmed that the combination of EHD and hot air can significantly reduce the drying period, resulting in a greater effective water diffusion coefficient and drying rate and reduced energy consumption. As such, this technique offers a promising solution to the considerable energy consumption of the drying industry.     

Energy Aspects in Electrohydrodynamic Drying

Acknowledging favorable quality of products obtained through electrohydrodynamic (EHD) drying, there is a lack of knowledge about energy aspects of this promising, yet not commercialized, technology. This article is a critical review of EHD research, which may be crucial for future studies and industrial applications. In particular, effects of electrodes configuration and operating parameters on drying kinetics, energy consumption, and energy efficiency of EHD as compared to conventional drying are examined. Some engineering considerations for using EHD in industrial dryers are also discussed.     

US-Assisted Convective Drying of Biological Materials

The aim of this work is to promote ultrasound as an energy source that is suitable for the enhancement of drying processes, and in particular of biological materials. The study aims at a more profound recognition of the interaction between ultrasonic (US) waves and the tissues of fruits and vegetables, which may contribute to an intensification of moisture removal during their drying. Absorption of acoustic energy causes heating and structural changes of the drying material due to a series of rapid material compressions and decompressions. The research hypothesis is based on the expectation that the ultrasound waves may enhance moisture removal from the fruits and vegetables during drying due to a special “heating effect,” “vibration effect,” and “synergistic effect.” Convective ultrasonic-assisted drying tests were conducted experimentally on a new hybrid dryer with ultrasonic equipment, and the effects of ultrasonic enhancement by drying are presented based on a drying model and assessed numerically.     

Characteristic Drying Curves of Cocoa Beans

The characteristic drying curves of cocoa beans are determined by using a tunnel drier where conditioned air passes a single cocoa berm suspended from an electronic balance in the test section. Weight loss, and temperatures of air, testa and nib of the cocoa bean is monitored on personal computers. The nornmalised drying rate versus the normalised moisture content is regressed by least square method to fit a new polynomial model for the penetration falling rate period and a linear model for the regular regime falling rate period. It can be concluded that there are three drying periods for cocoa beans namely the constant drying rate period, the penetration falling rate period and the regular regime falling rate period. The polynomial model estimates the penetration period quite well whereas the linear model estimates the regular regime quite well as well. There is no observable influence of relative humidity and air temperature on the characteristic drying curve of cocoa beans. However, the air velocity seems to have some influence on the curve.     

Characteristic Drying Curves of Cocoa Beans

ABSTRACT

The characteristic drying curves of cocoa beans are determined by using a tunnel drier where conditioned air passes a single cocoa berm suspended from an electronic balance in the test section. Weight loss, and temperatures of air, testa and nib of the cocoa bean is monitored on personal computers. The nornmalised drying rate versus the normalised moisture content is regressed by least square method to fit a new polynomial model for the penetration falling rate period and a linear model for the regular regime falling rate period. It can be concluded that there are three drying periods for cocoa beans namely the constant drying rate period, the penetration falling rate period and the regular regime falling rate period. The polynomial model estimates the penetration period quite well whereas the linear model estimates the regular regime quite well as well. There is no observable influence of relative humidity and air temperature on the characteristic drying curve of cocoa beans. However, the air velocity seems to have some influence on the curve.     

闭路循环干燥装置及其应用

简要介绍了闭路循环干燥装置的原理、特点和工艺流程。文中列举了其工业应用实例 ,并与其他干燥设备进行了对比。     

Microwave-Assisted Thin Layer Drying of Wheat

Drying characteristics of Canada Western Red Spring (CWRS) wheat were studied using a domestic microwave convective oven. The effects of microwave power level, grain bed thickness, and initial grain moisture on the drying kinetics were investigated. Wheat samples with initial moisture levels of 0.18 to 0.29 kg water/kg of dry matter were dried for different drying periods of 180 to 360 s. The moisture loss data were recorded at regular short intervals. Then moisture loss data were fitted to various models (Page equation, modified drying equation, and Midilli equation) to study the drying kinetics of wheat. The results showed that wheat moisture loss increased with increasing microwave power level. A mathematical model was developed by coupling mass and energy balances, resulting in a system of non-linear equations. The predicted moisture loss data from the developed model were compared by fitting to experimental microwave data that were in good agreement.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

Multistage Zeolite Drying for Energy-Efficient Drying

This work discusses the potential of three multistage zeolite drying systems (counter-, co-, and cross-current) with a varying number of stages. The evaluation showed that for 2–4 stages with heat recovery the efficiency of the systems ranges between 80 and 90%. Additionally, by introducing a compressor, the latent heat in the exhaust air from the regenerator is recovered and used to heat the inlet air for an additional drying stage. As a result, for the counter-current drying system and compressor pressure 1.5–2 bar, a maximum energy efficiency of 120% is achieved, which results in halving the energy consumption compared to conventional drying systems.     

Evaluation of Thin-Layer Models for Mushroom (Agaricus bisporus) Drying

In the present research, seven well-known mathematical thin-layer drying models were fitted to mushroom (Agaricus bisporus) drying experimental data, implementing nonlinear regression analysis techniques. The experiments were conducted in two laboratory-scale dryers. A range of temperatures 50–65°C and air velocities 1.0–5.0 m/s were tested. The statistical analysis concluded that the best model in terms of fitting performance was the logarithmic model. Correlations expressing this model parameter dependence with the drying air coefficients are also reported.     

Evaluation of Thin-Layer Models for Mushroom (Agaricus bisporus) Drying

In the present research, seven well-known mathematical thin-layer drying models were fitted to mushroom (Agaricus bisporus) drying experimental data, implementing nonlinear regression analysis techniques. The experiments were conducted in two laboratory-scale dryers. A range of temperatures 50-65°C and air velocities 1.0-5.0 m/s were tested. The statistical analysis concluded that the best model in terms of fitting performance was the logarithmic model. Correlations expressing this model parameter dependence with the drying air coefficients are also reported.     

Drying Western Red Cedar with Superheated Steam

This exploratory study evaluated the possibility of drying 50-mm-thick western red cedar with superheated steam. Since there are no industrial facilities in Canada drying western red cedar with superheated steam, the study was designed to explore the potential of this technology in terms of lumber quality, moisture content distribution, and drying time. The experiments showed that the 50-mm-thick product can be dried in less than three days without jeopardizing lumber quality (in comparison with the two weeks that is currently required in conventional kilns), and the percentage of pieces that remained wet after drying was within the 10% to 15% range that is typically tolerated in industry.