SOLAR GRAIN DRYING IN HUMID AND HOT-TEMPERATE CLIMATES

A natural solar drying system has been tested with wheat. Simple solar collectors give the small temperature rise needed for low temperature drying. The system, with a capacity of 35 tomes, is placed in a region of humid hot-temperate climate. It has been designed to dry all the locally produced grain (wheat, corn, soybean, sunflower, sorghum, etc.), without a supplemental heat source.

Two different types of solar collectors of the same effective area were tested, with one a storage-collector and the other a bare plate collector

Data from four field tests performed between 1981 and 1983 are given and discussed. The mean mass of water removed from the wheat per hour was of 6.54 kg, under an average air flow of 4.3 m3 /min. tonne     

PRACTICAL APPLICATIONS OF USING SOLAR COLLECTORS FOR DRYING PURPOSES IN AGRICULTURE

The main application for solar collectors in agriculture are drying of agricultural crops. Different types of active air type solar collectors have been examlned In small scale experiments and in full scale trials. This report deals with practical applications OF solar collectors for dryers. The investigations have shown that solar collectors can replace conventional heating units based on 011, electricity etc. for the heating. The most economical and practical solution is received lf the collector is integrated in the roof or a wall of a farm building. The report gives recommendations for the construction and For the choice of materials. Design dates and operation schedules are also presented.     

DRYING OF MEDICINAL PLANTS WITH SOLAR ENERGY UTILISATION

ABSTRACT

In the paper, a potential of solar energy for drying of medicinal plants in Polish conditions is estimated and development of solar drying technologies is presented. The results of economic assessment of flat-plate solar collectors applied for drying of medicinal plants on a farm are promising. In some specific conditions, e.g. drying of wild grown medicinal plants in remote areas, even application of photovoltaic modules for driving of a fan of a solar dryer is a profitable option and enables easy control of the drying air temperature.     

GRAIN DRYING IN COUNTERCURRENT AND CONCURRENT GAS FLOW -MODELLING,SIMULATION AND EXPERIMENTAL TESTS

ABSTRACT

Mathematical models and numerical techniques for simulation of parallel flow grain drying has been proposed and tested. Concurrent flow drying has been simulated using a steady state model. In order to overcome numerical difficulties that appears in simulation of countercurrent flow drying under operational conditions that leads to thermodynamics equilibrium in any section of the dryer stage, a robust approach has been developed based on simulation of the operation starting from initial transient conditions. A lab unit for grain drying has been developed, composed of a countercurrent flow stage coupled on a concurrent flow stage. The mathematical models and numerical techniques has been tested through the comparison between calculated results and experimental data measured for com drying.     

DESICCANT GRAIN APPLIED TO THE STORAGE OF SOLAR DRYING POTENTIAL

ABSTRACT

Sorption storage of solar heat using a layer of wheat as the desiccant was analyzed by means of a deep-bed drying model. Intended to be applied to solar-assisted in-storage drying of agricultural bulk materials, the probability of the persistence of unfavorable weather periods was quantified statistically for Potsdam for the month of August, as an example. Simulation results demonstrate that a relative humidity of the drying air of 65 % can be maintained day and night for weeks without combustion of fossil fuels. Using a simple strategy of control, periods with insufficient solar radiation can be bridged over. The desiccant grain is not endangered by mold growth as a matter of principle. Simple solar air heaters can be used to avoid economic losses due to overdrying and to reduce the danger of decay to a minimum even at unfavorable climatic conditions.     

DESICCANT GRAIN APPLIED TO THE STORAGE OF SOLAR DRYING POTENTIAL

Sorption storage of solar heat using a layer of wheat as the desiccant was analyzed by means of a deep-bed drying model. Intended to be applied to solar-assisted in-storage drying of agricultural bulk materials, the probability of the persistence of unfavorable weather periods was quantified statistically for Potsdam for the month of August, as an example. Simulation results demonstrate that a relative humidity of the drying air of 65 % can be maintained day and night for weeks without combustion of fossil fuels. Using a simple strategy of control, periods with insufficient solar radiation can be bridged over. The desiccant grain is not endangered by mold growth as a matter of principle. Simple solar air heaters can be used to avoid economic losses due to overdrying and to reduce the danger of decay to a minimum even at unfavorable climatic conditions.     

AMARANTH GRAIN DRYING IN A FLUIDIZED BED

ABSTRACT

An empirical equation as a function of drying time and temperature was developed to calculate the moisture ratio required Tor balch fluidized bed dryers design for amaranth grain drying

The relative deviations of the moisture ratio values calculated with the proposed equation with regard to the experimental ones ranged between 0.014 and 0.095 for a drying air temperature between 60 and 100 ° C, and a grain initial moisture content between 23.5 and 16.6 % wb.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

GRAIN DRYING USING CONTINUOUS AND PULSED MICROWAVE ENERGY

A laboratory microwave oven was used to dry high-moisture corn samples. The microwave oven was operated in both continuous and pulsed modes at 250 W of magnetron power setting. In the pulsed mode, two magnetron power-on times of 10 s and 15 s were used each with different power-off times in the range of 20 s to 75 s. Drying was more rapid in the continuous mode than in the pulsed mode. But, the continuous mode required much higher total magnetron power-on times. In the pulsed mode, longer power-on times generally resulted in slightly faster drying; and the power-off times did not strongly influence the drying rate. Longer power-on times should be followed by relatively longer power-off times. For a given on-time, increase in power-off time helps to decrease the total power-on time required for drying. When the microwave oven was operated at 10 s of power-on and 75 s of power-off pulsing, it resulted in the lowest total power-on time.     

COMPARISON OF TIMBER DRYING USING SOLAR ENERGY,ELECTRICAL HEATING AND DEHUMIDIFIER.

ABSTRACT

The performance of three different types of dryers for the hot air drying of sawn-limber planks are compared. These were the electric resistance dryer, solar dryer, and the dehumidifier dryer. Whilst the electric and solar dryers depended only upon hot air for drying, the dehumidifier dryer relied on hot dehumidified air. The results of investigations carried out on timber drying employing these three types of dryers in the Engineering Faculty are compiled and compared here in this paper. The results showed that the electric dryer produced the fastest drying lime and lowest moisture content, followed by dehumidifier drying. The solar dryer achieved a lower moisture content and a faster drying rate compared to natural drying, although the difference in drying times was marginal.     

A COMPREHENSIVE MATHEMATICAL AND NUMERICAL MODELING OF DEEP-BED GRAIN DRYING

This paper deals with comprehensive mathematical and numerical modeling of deep-bed grain drying. In order to build the process model, it is necessary to analyze the transport in both grain and gas phases. Experimental works were carried out for a layer of grain bed in order to validate the models. The models consider momentum, energy, and mass conservation within grain and drying air phase. The two-dimensional dynamic equations of energy and mass conservation are solved numerically by finite-difference method (FDM) and utilizing alternating direction implicit algorithm within grain and drying air phase, while momentum conservation are solved by finite difference method by utilizing Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm. Furthermore, the models will be applied in consideration with developing and designing dryer in order to simulate humidity and temperature profiles of the drying gas together with moisture content and temperature of grain across dryer in term of the dryer performance. The simulations show that the models can be used to predict the dynamic drying characteristic profiles as well as the superficial velocity of drying air phase across dryer.     

GRAIN DRYING OPTIMISATION AT LOW TEMPERATURES

ABSTRACT

The optimisation of grain drying systems was studied through the dynamic programming technique focusing on the minimisation of the energy consumption, lakingin account the conservation of the product quality and avoiding excessive humidity gradients in the bed at the end stages of drying. An algorithm was developed, where the input data were the initial conditions of the product and the climate data of the region. The operational conditions considered were: fan on or off, and different degrees of air heating (1 to 5 °C). The result obtained was an optimised management strategy for the operation of the dryer, specified for each stage of the process, in two situations: calculated from the mean climate dam of ten years, and corrected day by day with up to date climate data.     

SOLAR DRYING. FOR REAL APPLICATIONS

The techniques of professional drying can be utilized in solar drying with the view to satisfy the real needs of large agricultural productions and secure product quality and standardization. A solar batch convection dryer designed along this line is presented and its performance for drying fruits and vegetables is discussed together with the techniques used to dry these products.     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

ABSTRACT

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60?°C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.     

SOLAR DRYING. FOR REAL APPLICATIONS

The techniques of professional drying can be utilized in solar drying with the view to satisfy the real needs of large agricultural productions and secure product quality and standardization. A solar batch convection dryer designed along this line is presented and its performance for drying fruits and vegetables is discussed together with the techniques used to dry these products.     

A COMPREHENSIVE MATHEMATICAL AND NUMERICAL MODELING OF DEEP-BED GRAIN DRYING

ABSTRACT

This paper deals with comprehensive mathematical and numerical modeling of deep-bed grain drying. In order to build the process model, it is necessary to analyze the transport in both grain and gas phases. Experimental works were carried out for a layer of grain bed in order to validate the models. The models consider momentum, energy, and mass conservation within grain and drying air phase. The two-dimensional dynamic equations of energy and mass conservation are solved numerically by finite-difference method (FDM) and utilizing alternating direction implicit algorithm within grain and drying air phase, while momentum conservation are solved by finite difference method by utilizing Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm. Furthermore, the models will be applied in consideration with developing and designing dryer in order to simulate humidity and temperature profiles of the drying gas together with moisture content and temperature of grain across dryer in term of the dryer performance. The simulations show that the models can be used to predict the dynamic drying characteristic profiles as well as the superficial velocity of drying air phase across dryer.     

STOCHASTIC MODELING OF GRAIN TEMPERATURE IN NEAR-AMBENT DRYING

Stochastic analytical model was developed to describe the heat transfer in a thin layer of grain ventilated with ambient air. As an excitation to the drying system, a stochastic analytical model of ambient air temperature variations during the period of drying, based on a sine function, was used. Mean, correlation function, variance and the standard deviation were determined for air and grain temperature, both treated as random processes. The stochastic model of air temperature variation was fitted to the observed historical data. The effect of the standard deviation of air temperature of the three different climates (Canadian in Winnipeg, Polish in Poman and English in Weddington) on the standard deviation of grain temperature is shown.     

STACK-WIDE EFFECTS IN THE MODELING OF SOLAR KILNS FOR DRYING TIMBER

This study examines the stack-wide effects due to the humidification and cooling of air as it passes through a 6 m wide stack of Australian ironbark timber for conditions that are representative of those for solar drying (dry and wet-bulb temperatures of 60 and 50°C, respectively). A solar kiln model for a greenhouse-type design has been modified to account for the drying of timber boards and the possibility of stack-wide effects, in terms of moisture-content differences in the streamwise direction of air flow through the stack. The maximum difference between the moisture contents of the leading and trailing boards is predicted to be 0.011 kg kg^-1 for these conditions, compared with timber moisture contents of 0.15-0.35 kg kg^-1. Hence, the stack-wide effect is insignificant for these conditions in this greenhouse kiln design and may be ignored, reducing the simulation time by over 50%. In addition, 14 elements within a finite-difference model for the drying of the timber boards (25 mm thick) gives predictions of the drying time that are acceptably accurate, while minimizing the computational time.     

EFFECT OF TEMPERATURE AND RELATIVE HUMIDITY ON THE DRYING RATES AND DRYING TIMES OF GREEN BELL PEPPERS (CAPSICUM ANNUUM L)

ABSTRACT

Green bell pepper dices were dehydrated at different dry bulb air temperatures (55°, 60°, 65°, 70° and 75°C) and relative humidities (15, 20, 25, 30, 35 and 40%). The effects of temperature and relative humidity (RH) on the drying rates and drying period of diced green bell peppers were determined. Drying rate curves were characterized by a short induction (heating) phase followed by a falling rate period. Near constant rate drying was observed only at 55°C at 15% RH and at 65°C (15% RH). Drying rates generally increased with increasing temperatures and decreasing RH. The effect of temperature on the drying rates became less pronounced with increasing RH Drying rate maxima at 70°C and RH of 15, 20 25 and 40% exceeded those at 75°C, possibly due to case-hardening.     

SOLAR DRYING OF GRAPES

Mass losses and low quality are the most serious disadvantages of traditional grape drying methods. For the production of high quality raisins an increase in the drying rate is required and the grapes should be protected from rain, dust and insects during drying.

Under the terms of a joint German-Greek research program low-cost solar grape dryers were developed in the Institute of Agricultural Engineering of Hohenheim University and were tested in Greece in cooperation with the Crete Agricultural Research Center.

The required data basis for optimizing solar grape dryers was obtained by additional laboratory tests measuring the influence of various drying parameters on drying rate and quality.

Tests with the solar dryers have shown that it is possible to reduce the drying time and improve the quality significantly compared to the traditional drying methods. Also mass losses due to rain can be prevented.