SIMULATION OF THE MIXED-MODE NATURAL-CONVECTION SOLAR DRYING OF MAIZE

A mathematical model for mixed mode natural convection solar drying of maize grain is presented. The drying is described by a deep bed procedure that includes conduction within the grain bed. The conduction is due to radiative energy falling on the upper surface of the bed. The results show that temperatures at the top and bottom of the bed are higher than that in the middle resulting in two drying fronts one at the top and the other at the bottom of the bed and moving in opposite directions. This results in more uniform moisture content distribution than in an indirect dryer. The results are verified against experimental data from a prototype mixed mode natural convection maize solar dryer. The laboratory solar dryer was constructed at Newcastle University, U.K. and the experiments carried out under a solar simulator. The agreement between theory and experiment is very good.     

ANALYSIS OF A GEODESIC DOME SOLAR FRUIT DRYER

This paper analyzes use of geodesic domes as low cost and practical solar energy drying systems, especially for developing countries. A geodesic dome solar fruit dryer was designed, constructed and tested for drying grapes. The structure was constructed using wooden members, covered with transparent tedlar plastic and a black absorber inner shell. Fresh air is heated in the space between the outer shell and the absorber sheet. The hot air then passes through the fruits on the trays and exits from the top. The dryer was tested for drying grapes under two different modes, namely natural convection and forced air flow. In addition, theoretical results were obtained by thermal-electrical simulation and considering the heat and mass transfer phenomena between the air and the grapes.     

ANALYSIS OF A GEODESIC DOME SOLAR FRUIT DRYER

This paper analyzes use of geodesic domes as low cost and practical solar energy drying systems, especially for developing countries. A geodesic dome solar fruit dryer was designed, constructed and tested for drying grapes. The structure was constructed using wooden members, covered with transparent tedlar plastic and a black absorber inner shell. Fresh air is heated in the space between the outer shell and the absorber sheet. The hot air then passes through the fruits on the trays and exits from the top. The dryer was tested for drying grapes under two different modes, namely natural convection and forced air flow. In addition, theoretical results were obtained by thermal-electrical simulation and considering the heat and mass transfer phenomena between the air and the grapes.     

DRYING RATES AND DRYER CAPACITIES OF DIFFERENT SEED GRAINS

The three most commonly occurring grains --maize, rice and wheat --are compared for single kernel and deep bed drying rates. The grains are dried in a concurrent-flow (CCF) dryer; high grain temperatures are avoided to prevent deterioration of seed viability.

Wheat dries the fastest of the three grains as an individual kernel as well as in the CCF grain dryer, maize the slowest; the throughput with wheat is 23% larger than with maize, and 15% larger than with rice.     

DRYING RATES AND DRYER CAPACITIES OF DIFFERENT SEED GRAINS

ABSTRACT

The three most commonly occurring grains --maize, rice and wheat --are compared for single kernel and deep bed drying rates. The grains are dried in a concurrent-flow (CCF) dryer; high grain temperatures are avoided to prevent deterioration of seed viability.

Wheat dries the fastest of the three grains as an individual kernel as well as in the CCF grain dryer, maize the slowest; the throughput with wheat is 23% larger than with maize, and 15% larger than with rice.     

连续性真空低温玉米烘干塔的研发及性能分析

为了解决粮食的晾晒问题,远弘真空干燥对真空干燥设备进行了资源整合,研发了性能更好,更加高效的连续性真空低温玉米烘干塔,并对其系统组成、烘干流程、技术参数和经济性进行了介绍和分析。     

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.     

FUNDAMENTALS OF LIQUID SPRAY TECHNIQUES

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

Optimization of mixed flow dryers to increase energy efficiency

ABSTRACT

Agricultural driers are used for grain or maize drying with a limited annual operating time of about 100–1,000?h. To compensate the increasing costs of energy, permanent optimization of the drying process and the drying apparatus is necessary to increase energy efficiency. To attain higher energy efficiency, the drying potential of air should be fully utilized. The objective of this study is to investigate the medium flow in mixed flow dryers to identify adverse drying conditions. The investigation is based on experimental and numerical modeling and takes into account the bed motion (discrete element method) and the drying air flow (computational fluid dynamics). The results show superposition of a homogeneous air flow distribution with a particle flow profile, resulting in locally inhomogeneous residence time, different drying conditions, and ultimately uneven grain drying. Uneven drying is one of the main reasons for high energy consumption. Considering the results, a new mixed flow dryer geometry was developed which should equalize the drying process and thus be more energy efficient.     

SOLAR DRYING OF FRUITS IN A MEDITERRANEAN CLIMATE

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

Thesis Summary: Study of a New Atmospheric Freeze Drying System Incorporating a Vortex Tube and Multimode Heat Input

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products. An experimental setup was designed and built to permit simultaneous application of convection, conduction and radiation heat input to the drying material above its freezing point to ensure sublimation using a vortex tube to produce low temperature dry air. Comparison with AFD using fixed bed, fluidized bed dryer, traditional vacuum freeze drying and heat pump drying were carried out to investigate the viability of this new system. A two-layer moving boundary model was developed to simulate the drying kinetics and temperature scenario of thin slab product. Fairly good agreement was found between the predicted values and the experimental data. Finally a three-dimensional (3D) CFD simulation for a vortex tube is carried out to capture the highly swirling compressible flow behavior and to gain basic understanding of temperature separation process. An experimental setup was built to validate the simulation results.     

SOLAR ASSISTED DRIER FOR SEEDS

A high performance solar assisted seed dryer has been designed and constructed at a seed processing station in Hungary.

The dryer is arranged in a building specially designed for this purpose, the roof of it serves as solar air collector field. Two drying cells with individual fans of two RPM stages are formed in the building. Drying air preheated by the collector can optionally be distributed between the two drying cells. Material to be dried can be arranged either in a static bed or in containers depending on the requirements. Moist air leaving the bed streams out of the drying cells through openings of the side walls. As auxiliary air heater a natural gas generator is used.     

Experimental Investigation of a Solar Bamboo Dryer

An experimental investigation was conducted on the performance of a solar box dryer for drying bamboo operating under tropical weather conditions. The dryer is a greenhouse-type designed for multi-crop solar drying. Air circulation was by electrically-operated fan. The results showed that the moisture content of the bamboo could be brought dovn to about 19% from an initial value of nearly 90% in 17 days by operating the dryer over 8 hours each day. under natural drying conditions, the final moisture content reached was only 22%. Although solar drying of bamboo was only marginally faster than natural drying. nonetheless. final moisture content was lower.     

SOLAR ASSISTED DRIER FOR SEEDS

A high performance solar assisted seed dryer has been designed and constructed at a seed processing station in Hungary.

The dryer is arranged in a building specially designed for this purpose, the roof of it serves as solar air collector field. Two drying cells with individual fans of two RPM stages are formed in the building. Drying air preheated by the collector can optionally be distributed between the two drying cells. Material to be dried can be arranged either in a static bed or in containers depending on the requirements. Moist air leaving the bed streams out of the drying cells through openings of the side walls. As auxiliary air heater a natural gas generator is used.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

ABSTRACT

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5?rpm drum speed and quadrupled at 7?rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5 rpm drum speed and quadrupled at 7 rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.     

PERFORMANCE STUDY OF A RE-CIRCULATING CABINET DRYER USING A HOUSEHOLD DEHUMIDIFIER

The performance and operating characteristics of a low temperature re-circulating cabinet dryer using a dehumidifier loop were studied using alfalfa. Chopped alfalfa, initially at 70% moisture content, was dried to 10% moisture content in the dryer. Two dryer setups were used. The dryers in each case had a partitioned cabinet with trays of material on one side and a stack of one or two small household dehumidifiers on the other side. Air was re-circulated through the material from bottom to the top and back through the dehumidifiers. Two drying configurations were tested. In one, the material was left on the trays until drying was complete (batch or fixed tray drying). In the other configuration, the trays were moved from top to bottom, introducing a new tray at the top while removing an old tray from bottom. Drying air temperature ranged from 25 to 45°C. The average air velocity through the material was 0.38 m/s. Alfalfa chops dried in 5 h in the fixed tray drying and in 4 h in the moving tray drying. The specific moisture extraction rate ranged from 0.35 to 1.02 kg/kWh for batch drying and stayed at an average value of 0.50 kg/kWh for continuous/moving tray drying.     

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.     

DRYING OF GRANULAR MATERIALS IN AN INCLINED VIBRATED FLUIDIZED BED BY COMBINED RADIATIVE AND ONVECTIVE HEATING

In an inclined-vibrated fluidized bed (IVF-bed), solid particles are fluidized easily at a relatively low air velocity. Good mixing of the fluidized particles in the direction of the air flow and the renewal of the bed surface take place as a result of the vibration effect. Accordingly when the bed surface is heated by thermal radiation from the top surface, radiant heat is absorbed more effectively on the surface than on a stagnant bed surface. This characteristic of an IVF-bed indicates the applicability to a solar dryer and other similar dryers.

The present study reports the result of model experiments carried out to confirm the effectiveness of radiative heating on drying of wet granular materials. Further, for quantitative evaluation of the radiation effect, a theoretical model is offered and its validity is examined.