Comparison of energy consumption and specific energy requirements of different methods for drying mushroom slices

Energy consumption for drying of mushroom slices was evaluated using various drying methods including hot air, microwave, vacuum, infrared, microwave-vacuum and hot air-infrared. Results of data analysis showed that the lowest and highest energy consumption levels in drying mushroom slices were associated with microwave and vacuum dryers, respectively. The use of vacuum in conjunction with microwave drying increased energy consumption relative to microwave drying alone. Energy consumption in the hot air dryer showed a downward trend with increasing temperature and an upward trend with increasing air velocity. In drying mushroom using infrared radiation, it was observed that increased air velocity increases drying time and consequently the amount of consumed energy. Using a combination of hot air and infrared drying decreased energy consumption relative to infrared drying alone and increased it relative to hot air drying. In the combined microwave-vacuum dryer, drying time and consequently energy consumption decreased in comparison to the vacuum dryer. Hot air-infrared drying of mushroom slices proved to have the lowest energy consumption.     

Heat and mass transfer in a countercurrent moving bed dryer

The objective of this work is to study the heat and mass transfer between air and soybean seeds in a countercurrent moving bed dryer, based on the application of a two-phase model to the drying process. The numerical solution of the model is obtained by using a computational code based on BDF methods (Backwards Differentials Formulas). The experimental data of air humidity and temperature and of seed moisture content and temperature at the dryer outlet are compared to the simulated values, showing a good agreement.     

Modeling and experimental studies on a domestic solar dryer

A domestic solar dryer with transparent external surfaces was designed, built and tested. Thin-layer drying models that describe the drying phenomena in a unified way, regardless of the controlling mechanism; have been used to estimate the drying period for several products. Temperature of the drying medium is one of the factors that affect the drying rate constant of the exponential model. This fact introduces a problem when used to predict the drying rate under solar drying conditions since the temperature of the drying medium is rarely constant. This paper aims to propose a solar dryer with a uniform temperature profile that meets the requirements of the exponential model over a wide range of cases, thus, providing a simple and accurate design tool. The dryer is characterized by collecting the maximum possible solar energy by having a longer drying period, and allows the fixed dryer to approach with its performance the tracked one with all technical and economical advantages of the tracking system. The performance was tested under different operational conditions and the drying characteristics were experimentally investigated by conducting the experiments on two local herbs, Jew's mallow and mint leaves. The dryer was able to reduce moisture of the tested products to the recommended level (6% wb) in about a 12 h period. The reliability of the exponential model was evaluated by comparing the experimental with the predicted curves. A reasonable agreement was found for the different tests carried out for the entire drying period.     

Performance study of a heat pump dryer system for specialty crops—Part 2: Model verification

The experimental and predicted performance data of a heat pump dryer system is reported. Chopped alfalfa was dried in a cabinet dryer in batches and also by emulating continuous bed drying using two heat pumps operating in parallel. Results showed that alfalfa was dried from an initial moisture content of 70% (wb) to a final moisture content of 10% (wb). The batch drying took about 4.5 h while continuous bed drying took 4 h to dry the same amount of material. The average air velocity inside the dryer was 0.36 m s^?1. Low temperatures (30–45°C) for safe drying of specialty crops were achieved experimentally. The heat pump drying system used in this study was about 50% more efficient in recovering the latent heat from the dryer exhaust compared to the conventional dryers. Specific moisture extraction rate (SMER) was maximum when relative humidity stayed above 40%. The dryer was shown to be capable of SMER of between 0.5 and 1.02 kg kW^?1 h^?1. It was concluded that continuous bed drying is potentially a better option than batch drying because high process air humidity ratios at the entrance of the evaporator and constant moisture extraction rate and specific moisture extraction rate values can be maintained. An uncertainty analysis confirmed the accuracy of the model. Copyright © 2002 John Wiley & Sons, Ltd.     

Performance of indirect solar cabinet dryer

In this paper, the development and testing of a new type of efficient solar dryer, particularly meant for drying vegetables and fruit, is described. The dryer has two compartments: one for collecting solar radiation and producing thermal energy and the other for spreading the product to be dried. This arrangement was made to absorb maximum solar radiation by the absorber plate. In this dryer, the product was loaded beneath the absorber plate, which prevented the problem of discoloration due to irradiation by direct sunlight. Two axial flow fans, provided in the air inlet, can accelerate the drying rate. The dryer had six perforated trays for loading the material. The absorber plate of the dryer attained a temperature of 97.2 °C when it was studied under no load conditions. The maximum air temperature in the dryer, under this condition was 78.1 °C. The dryer was loaded with 4 kg of bitter gourd having an initial moisture content of 95%, and the final desired moisture content of 5% was achieved within 6 h without losing the product colour, while it was 11 h for open sun drying. The collector glazing was inclined at a particular angle, suitable to the location, for absorption of maximum solar radiation. A detailed performance analysis was done by three methods, namely ‘annualized cost method’, ‘present worth of annual savings’ and ‘present worth of cumulative savings’. The drying cost for 1 kg of bitter gourd was calculated as Rs. 17.52, and it was Rs. 41.35, in the case of an electric dryer. The life span of the solar dryer was assumed to be 20 years. The cumulative present worth of annual savings over the life of the solar dryer was calculated for bitter gourd drying, and it turned out be Rs. 31659.26, which was much higher than the capital cost of the dryer (Rs. 6500). The payback period was calculated as 3.26 years, which was also very small considering the life of the system (20 years). So, the dryer would dry products free of cost during almost its entire life span. The quality of the product dried in the solar dryer was competitive with the branded products available in the market.     

Performance studies of solar tunnel dryer for drying aonla (embilica officinalis) pulp

A solar tunnel dyer was constructed and evaluated the performance for drying aonla pulp. The dryer consists of a transparent UV stabilized plastic covered solar collector cum drying unit. Evaluation parameters are air temperature, solar insolation, moisture content, relative humidity and airflow rate. A minimum of 111.18 m² solar collector area is required to dry a batch 1000 kg aonla pulp in 16 hours (two days drying period). The initial and final moisture content considered were 424.93 and 10.08% dry basis, respectively. It was observed that on an average 43 per cent of higher temperature was obtained in solar tunnel dryer over the ambient temperature. The results obtained during the test period denoted that the maximum gained energy occurred at 13 o’clock hour and then gradually declined since the maximum solar radiation occurred at this time.     

Experimental investigation of the performance of the solar tunnel dryer for drying bananas

The multi-purpose solar tunnel dryer was used to dry bananas under the hot and humid weather conditions of Thailand in order to investigate its performance. The dryer comprises a plastic sheet-covered flat plate collector and a drying tunnel. The dryer is arranged to supply hot air directly to the drying tunnel using three fans powered by a 53 W solar cell module. The products to be dried are spread in one layer on a plastic net in the drying tunnel to receive energy from both the hot air supplied by the collector and incident solar radiation. This dryer can be used to dry up to 300 kg of ripe bananas in each drying batch. In investigating the performance of the dryer, seven drying tests were conducted at the Royal Chitralada Projects in Bangkok during March–May 1995. Teh temperature of the drying air from the collector varied between 40 and 65°C during drying and the bananas could be dried within 3–5 days, compared to the 5–7 days needed for natural Sun drying. In addition, the bananas being dried in the solar tunnel dryer were completely protected from rain, insects and dust, and the dried bananas were of high quality in terms of flavour, colour and texture. As the fans are powered by the solar module, the dryer could be used in rural areas where there is no supply of electricity from grid. The pay-back period of the dryer is estimated to be about 3 years when the dryer is locally produced.     

Simultaneous convective heat and mass transfer in impingement ink drying

Effective and economical drying of the ink is essential in the printing, packaging and converting industries. In evaporative drying, high heat and mass transfer rates are commonly achieved by means of high velocity impinging air jets To provide data for dryer designer a programme of research has been implemented to study the heat and mass transfer processes which underlie the drying of thin ink rims The heat transfer situation under impinging air jets is outlined and some experimental results are presented. Optimization of nozzle arrays for impinging air jets is analysed for practical applications. A non-contact infra-red technique for continuously monitoring the ink drying process is described and drying curves for an ink based on a single solvent (4-Methyl-2-pentanol-MIBC) are presented. Heat and mass transfer theory has been used to predict dry ing limes in the constant rate drying period These predictions have been compared with experimentally determined doing times This research has served to confirm the fundamental importance of the dry ing curve as a basis for dryer design.     

Performance assessment of contact heat pump drying

A comparative analysis is presented of a novel plate‐type isothermal heat pump dryer and a conventional adiabatic heat pump dryer. The energy performance and drying capacity of each dryer type is investigated subject to operational constraints on the maximum acceptable relative humidity and temperature of air passing over the product. The analysis demonstrates that for applicable products, a significant opportunity exists for improving the energy efficiency of heat pump drying, by a factor of 2–3 times compared with current adiabatic heat pump dryers. The moisture extraction rate is also increased in the contact heat pump dryer, by a similar factor. However, these improvements are shown to be sensitive both to the impact of product thickness on heat transfer and to the relative humidity constraint. Copyright © 2010 John Wiley & Sons, Ltd.     

Mathematical modelling of thin layer drying process of long green pepper in solar dryer and under open sun

An experimental study was performed to determine the thin layer drying characteristics in a solar dryer with forced convection and under open sun with natural convection of long green pepper. An indirect forced convection solar dryer consisting of a solar air collector and drying cabinet was used in the experiments. Natural sun drying experiments were conducted for comparison at the same time. The constant rate period is absent from the drying curves. The drying process took place in the falling rate period. The drying data were fitted to 13 different mathematical models. Among the models, the logarithmic model for forced solar drying and the Midilli and Kucuk model for natural sun drying were found best to explain the thin layer drying behaviour of long green peppers. The performance of these models was investigated by comparing the coefficient of determination (R), reduced chi-square (χ²) and root mean square error (RMSE) between the observed and predicted moisture ratios.     

Experimental investigations of a chimney-dependent solar crop dryer

An experimental investigation into the performance of a solar crop dryer with solar chimney and no air preheating is described. Tests were first performed on the cabinet dryer, using a normal chimney. The trials were repeated with a solar chimney. Still with the solar chimney, further trials were carried out with the roof of the drying chamber inclined further to form a tent dryer.The described tests include no-load tests for airflow rate measurements and drying tests, with cassava as the crop. Air velocities, temperatures, ambient relative humidity and the drop in crop moisture contents at different stages of the drying process are also presented. The effects of the various configurations described above on the drying process are deduced and discussed while comparing the experimental results with one another. In addition, the performance of the dryer in relation to other natural convection dryers is discussed. The results show that the solar chimney can increase the airflow rate of a direct-mode dryer especially when it is well designed with the appropriate angle of drying-chamber roof. However, the increase in flow rate only increases the drying rate when the relative humidity (RH) of the ambient air is below a certain mark (60% for cassava).     

Solar drying system for energy conservation

A low cost portable farm solar dryer was evaluated for drying goose berry candy in the conditions of Vidarbha region of Maharashtra state. Temperature profile at top, middle and bottom in its seven trays loaded with candy was studied with respect to ambient temperature during the course of drying and maximum solar radiation of 1120 W/m² was observed at 11.30 to 12 h. The solar radiation was varied from 720–500 W/m² at 9.00 h to 16.00 h. The minimum temperature of 27°C was observed at bottom tray of the dryer and maximum of 44°C in top tray at 9.00 h. The maximum temperature of 70°C was attained at 11.30 h. The conventional drying method took 8 days to dry the product. The moisture content was reduced from 36.38 to 8.33 per cent (wb) in three days in solar drying method. The product recovery was 71.55 per cent as compare to 35 per cent in conventional drying method. The drying period was reduced by 62 per cent and product recovery was doubled using portable farm solar dryer. The goose berry candy was also dried with and without shade drying methods. The temperature variation of dryer without shade was found, in the range of 23–36°C, 31–48°C and 38–55°C in bottom, middle and top trays respectively of dryer. The weight loss of 810, 870 and 820 g were observed in three days at bottom, middle and top trays of the dryer respectively. The thermal efficiency of the dryer onepy first day drying was found 15.55 and 15.23 per cent in shade and without shade drying methods respectively. Appearance, taste and flavour of goose berry candy dried in farm solar dryer with shade were superior to conventional drying. The cost of final product was Rs 114/kg. The profit from a single unit of farm solar dryer per year was Rs 57588/-.     

Temperature and pretreatment effects on the drying of Hass avocado seeds

The objective of the present experimental work was to determine the influence of five different drying air temperatures (313, 323, 333, 343, and 353 K) on the drying kinetics and the degree of moisture evaporation from Hass avocado seeds. The drying experiments of the non-pretreated and pretreated (sliced and crushed) Hass avocado seeds were performed in a heating furnace, where the pretreatment process was found to accelerate the drying process. The obtained results suggested that increase in the operating air temperature stimulated the rate of moisture evaporation, but resulted in the charring of the seed surface. The drying air temperature of 313 K was concluded to be suitable for the reasonable drying of Hass avocado seeds. The slicing pretreatment process was found to be better indicative of the total moisture amount present in Hass avocado seeds. The drying process removed a maximum of 58% of the initial water mass of Hass avocado seeds. An additional investigation was performed where the physical appearance of Hass avocado seeds immersed in water at different temperatures (303, 318, 325.5, 333, and 348 K) was examined. The observed study suggested that the surrounding temperature higher than 313 K could damage the physical appearance and reduce the quality of Hass avocado seeds.     

A solar air heater with composite–absorber systems for food dehydration

Development of appropriate technologies for conversion of solar radiation to thermal energy is essential for food preservation. A solar air heater, comprising two absorber systems in a single flat-plate collector, was designed on the principles of psychrometry. The heater was integrated to a drying chamber for food dehydration. This collector design offered flexibility in manual adjustment of the thermal characteristics of the solar dryer. The performance of the dryer was evaluated by drying fresh samples of mango (Mangifera indicus). Both fresh and dried mango samples were analysed for moisture content (MC), pH and ascorbic acid. During the dehydration period, meteorological measurements were made. The air heater converted up to 21.3% of solar radiation to thermal power, and raised the temperature of the drying air from about 31.7 °C to 40.1 °C around noon. The dryer reduced the MC of sliced fresh mangoes from about 85% (w/w) to 13% (w/w) on wet basis, and retained 74% of ascorbic acid. It was found that the dryer was suitable for preservation of mangoes and other fresh foods.     

Thin layer solar drying of rough rice

This paper presents a set of simple empirical equations for natural air flow solar drying of rough rice in mixed-mode type dryer, box-type dryer and open floor drying system. The moisture contents predicted by the equations were in good agreement with the observed values. The effect of drying air temperature on the drying rate constants for these three cases were found to be insignificant. The equilibrium moisture content appeared to be the most important variable controlling the drying rate. The highest drying rate was observed in case of mixed-mode dryer. The drying rate of box dryer was next to that of mixed-mode dryer. This study shows that the introduction of solar dryer for drying of rough rice is highly recommendable in Bangladesh.     

Drying of apricots in a rotary column cylindrical dryer (RCCD) supported with solar energy

Rapid deterioration of fruits and vegetables necessitates their consumption in a limited area. The solutions to this problem may be provided by technological precautions, which not only increase economic efficiency but also motivates agricultural developments. Among the technological operations applied to increase the market life of the foods, drying operation is of prime importance which is applicable to almost all the agricultural products. In this work, drying of apricots in a newly developed rotary column cylindrical dryer (RCCD) equipped with a specially designed air solar collector (ASC) was investigated to find optimum drying air rate and rotation speed of dryer, to maintain uniform and hygienic drying conditions and to reduce drying times. For this purpose, the changes in the moisture contents of the Sugarpiece ( ekerpare) foods were studied. Drying is an operation of prime importance used for almost all the agricultural products. Type apricots dried in the present dryer were compared to those of the same type apricots dried open on the ground. It was shown that co-operation of RCCD and ASC increased drying rate, reduced drying times and rotation of drying chamber provided gains in both time and labour.     

Multi-phase flow and drying characteristics in a semi-circular impinging stream dryer

A physical model was proposed to describe granular material drying in a semi-circular impinging stream dryer, and the multi-phase flow characteristics as well as the heat and mass transfer were numerically investigated. Specially, the influence of various factors (inlet air temperature, mass flow-rate ratio, initial moisture content etc.) on drying process was inspected. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate curvature radius, flow-rate ratio, air velocity and higher inlet air temperature should be chosen for improving the drying performance, and decreasing the energy consumption and operation cost. The numerical predictions were compared with the available experimental results, and they are in quite good agreement with each other.     

Experimental study and analysis of porous thin plate drying in a convection dryer

In this study experiments were conducted to study the porous thin plate drying process. An analysis was also carried out to determine the moisture transfer coefficients that vary with the moisture content of the plate during drying. The study is useful for analyzing the transient drying process of porous thin plates. Furthermore, energy and exergetic analyses based on the measured results were carried out to determine the dryer performance. It is found that, although the drying rate increases, the energy utilization ratio of the drying process decreases with either an increase in the rate of air flow or an increase in the size of the drying air inlet. The results indicate that a significant amount of the energy provided to the system was lost and not utilized for drying, especially for high drying rate dryers. An exergetic analysis showed that by adjusting the configuration of the air inlet and flow rate, the exergetic efficiency of the dryer can be significantly increased (from 0.5% to 10%). An overall transport phenomena analysis and performance evaluation including the transient moisture transfer coefficients, drying rate, energy utilization ratio, and exergetic efficiency in the current study provide comprehensive and important insights to the thermal physics of porous thin plate drying processes and the design of efficient dryer systems.     

Determination of pistachio drying behaviour and conditions in a solar drying system

The main object of this study is to determine the drying behaviour and conditions of shelled and unshelled pistachio samples using both solar assisted and open sun drying. For each drying experiment, 100 g each of unshelled and shelled pistachio were used. The least‐squares method was applied to find the drying curve equation of pistachio. During the experiments, shelf temperatures, weight loss of pistachio, moisture content of air, and distribution of solar radiation were measured; and presented depending on the drying time. Also, the mass shrinkage ratios of shelled and unshelled pistachio samples were determined, and the experimental uncertainty ratio was calculated as 15–16.5 per cent based on the experimental results. It was deduced that the shelled and unshelled pistachio samples in the solar assisted forced convection dryer were perfectly dried at temperatures of 50±10°C in the time period of 6 h. Whereas, the samples in the open sun drying were not sufficiently dried at temperatures of 28±4°C in the same time period. Hence, it is suggested that the pistachio samples with approximately 29.0 per cent of moisture are dried in the solar assisted convection dryer at 50±10°C of temperature in the time period of approximately 6 h in order to protect from the negative climatic and environmental effects. However, it is not desirable to dry the pistachio samples in the open sun because of greater drying time, dirt, dust and harmful insects. Copyright © 2001 John Wiley & Sons, Ltd.