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.     

热泵干燥箱的干燥过程

通过对热泵干燥箱的热力学分析和对热泵干燥箱干燥过程的试验研究，认为热泵干燥箱干燥物料具有高效节能、干燥温度低和对周围环境无污染等优点。试验结果表明热泵干燥箱在干燥过程中干燥室内的各点温度和相对湿度在干燥０１～２．５ｈ之间相差最大，最大温差为１３．５℃，这时热泵干燥箱的除水量也最大为６．１ｋｇ水／ｈ。     

DESIGN AND APPLICATION OF A DEHUMIDIFIER DRYER FOR DRYING PINE CONES AND POLLEN CATKINS

A critical issue for the plantation forest industry is the preservation and control of genetic material in order to facilitate the fertilisation process. This is best achieved by drying at low temperature and low humidity prior to extraction of seeds and pollen. For example, freshly picked green cones of Pinus radiata are best dried at 25°C, 20% relative humidity to avoid thermal damage and to prevent mould formation. Similar conditions are required for drying pollen catkins. These conditions can be met efficiently and reliably by the use of a dehumidifier dryer. This paper describes the design of such a drying system, including a review of the dehumidifier and the kiln design parameters necessary to achieve the required drying conditions. Performance data and commercial operating experience are also presented and analysed. The system is effective and reliable, and has increased both the speed of drying and the quality of the product.     

DESIGN AND APPLICATION OF A DEHUMIDIFIER DRYER FOR DRYING PINE CONES AND POLLEN CATKINS

ABSTRACT

A critical issue for the plantation forest industry is the preservation and control of genetic material in order to facilitate the fertilisation process. This is best achieved by drying at low temperature and low humidity prior to extraction of seeds and pollen. For example, freshly picked green cones of Pinus radiata are best dried at 25°C, 20% relative humidity to avoid thermal damage and to prevent mould formation. Similar conditions are required for drying pollen catkins. These conditions can be met efficiently and reliably by the use of a dehumidifier dryer. This paper describes the design of such a drying system, including a review of the dehumidifier and the kiln design parameters necessary to achieve the required drying conditions. Performance data and commercial operating experience are also presented and analysed. The system is effective and reliable, and has increased both the speed of drying and the quality of the product.     

A STUDY OT THE DRYING PERFORMANCE OF A VIBRATING TRAY DRYER

This study reports the results of drying tests with a dryer operated both with and without internal air reheating (isothermal operation and noniaothermal operation, respectively), which provide an overview of dryer performance in removing moisture from coffee beans- The results show that “isothermal operation” has an advantage regarding drying speed, whereas the noniaothermal process shows more advantages when the analysis is carried out for energy consumption.     

A STUDY OT THE DRYING PERFORMANCE OF A VIBRATING TRAY DRYER

ABSTRACT

This study reports the results of drying tests with a dryer operated both with and without internal air reheating (isothermal operation and noniaothermal operation, respectively), which provide an overview of dryer performance in removing moisture from coffee beans- The results show that “isothermal operation” has an advantage regarding drying speed, whereas the noniaothermal process shows more advantages when the analysis is carried out for energy consumption.     

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.     

DRYTNG OF PEPPER SEEDS USING A COMBINED MICROWAVE/FLUIDIZED BED DRYER

Abstract

The drying rates curves in terms of moisture content versus elapsed drying time for white pepper seeds were obtained experimentally using a fluidized bed and a combined microwave/fluidized bed. The combined microwave/fluidized bed employs a microwave field to assist convective drying. For both procedures, the drying rates were found to be dependent on the inlet air temperature and velocity. Significantly improved drying rates were achieved utilized a combined microwave/fluidized bed drying compared with a conventional fluidized bed.     

PERFORMANCE EVALUATION OF A CONVENTIONAL NAUTAMIXER APPLIED AS A DIRECT DRYER

This paper describes the experiments which were conducted with the objective to determine the potential of a conventional Nautamixer applied as a direct dryer. This convective Nautamixer-dryer was used for drying the urea nitrate containing around 15% of water, which is converted from a pasteous form into a powder of a moisture lewel below 1%.

The influence of the following variables was studied: increased hot air velocity and temperature, number of revolutions of the agitator, and mass of the drying material. The effect of inlet air distribution to the periphery of the bed of the material was studied with a perforated plate distributor by blanking the holes (17%). which adversely affected the drying rate. The examination of the effect of external heating through the wall indicated that it is not possible to achieve the desired moisture level of the product without the introducing the flowing hot air. As an addition to the convective drying the indirect drying contributed more to the increase of the outlet air temperature than to the increase of the drying rate. As expected, shorter drying time is obtained with higher air flow rates and temperatures as well as with increased number of revolutions of both the orbital arm and the mixing screw.     

DYNAMIC MODELLING OF A DEHUMIDIFIER WOOD DRYING KILN

ABSTRACT

For the optimum design and operating information of heat pump dehu-midifier wood drying kilns, a comprehensive kiln model has been developed, which solves the fundamental balance equations for the whole system. The model is suitable for analysing the influence of design and control variables on the performance of the system as a whole. To illustrate the application of the model, the operation of a dehumidifier wood drying kiln has been analysed under typical operating conditions and kiln controls. The drying performance has been discussed in detail.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.     

A NOVEL RADIO FREQUENCY ASSISTED HEAT PUMP DRYER

ABSTRACT

This paper compares an experimental heat pump batch dryer with the implementation of volumetric Radio Frequency (RF) heating, in the combination drying of crushed brick particulate. Results are presented showing overall improvements in drying

A simplified mathematical drying model including the RF energy source has been developed using mass and energy conservation, confirming the experimental results.     

A DRYING HEAT PUMP USING CARBON DIOXIDE AS WORKING FLUID

The application of carbon dioxide as working fluid in refrigeration and heat pump systems is regaining increasingly importance in view of the chlorofluorocarbon (CFC) substitution problem. It is both under ecological and economical aspects an attractive alternative to the hydrofluorocarbon (HFC) working fluids being in practical use. The thermophysical properties and characteristics of carbon dioxide are quite different from those of refrigerants used in conventional vapour compression cycles. Its application in conventional vapour compression refrigerating systems is limited by its critical parameters (t_c = 31.1°C and p_c = 73.8 bar). The possibility to use carbon dioxide also beyond these limits in high temperature processes, e.g., heat pumps, is given by the application of a trans-critical process. The design and construction of a commercial drying heat pump system (batch type cabinet dryer with 12 kW heating capacity and closed air circuit) using the natural working fluid carbon dioxide is shown and experimental results of investigations carried out are presented. Possible energy savings calculated theoretically are given for comparison.     

A DRYING HEAT PUMP USING CARBON DIOXIDE AS WORKING FLUID

ABSTRACT

The application of carbon dioxide as working fluid in refrigeration and heat pump systems is regaining increasingly importance in view of the chlorofluorocarbon (CFC) substitution problem. It is both under ecological and economical aspects an attractive alternative to the hydrofluorocarbon (HFC) working fluids being in practical use. The thermophysical properties and characteristics of carbon dioxide are quite different from those of refrigerants used in conventional vapour compression cycles. Its application in conventional vapour compression refrigerating systems is limited by its critical parameters (t _c = 31.1°C and p _c = 73.8 bar). The possibility to use carbon dioxide also beyond these limits in high temperature processes, e.g., heat pumps, is given by the application of a trans-critical process. The design and construction of a commercial drying heat pump system (batch type cabinet dryer with 12 kW heating capacity and closed air circuit) using the natural working fluid carbon dioxide is shown and experimental results of investigations carried out are presented. Possible energy savings calculated theoretically are given for comparison.     

微型台式喷雾干燥器的研制

介绍一种目前国内最小、性能优良的喷雾干燥装置 ,该机经鉴定认为填补了国内空白 ,技术水平处于国内领先 ,可以作为各大专院校、科研院所、大中型企业的探测性试验研究或生产前小试装置。     

MATHEMATICAL MODEL DEVELOPMENT AND SIMULATION OF HEAT PUMP FRUIT DRYER

ABSTRACT

A mathematical model of a heat pump fruit dryer was developed to study the performance of heat pump dryers. Using the moisture content of papaya glace' drying, the refrigerant temperature at the evaporator and condenser and the performance, was verified. It was found that the simulated results using closed loop heat pump dryer were close to the experimental results. The criteria for evaluating the performance were specific moisture extraction rate and drying rate. The results showed that ambient conditions affected significantly on the performance of the open loop dryer and the partially closed loop dryer. Also, the fraction of evaporator bypass air affected markedly on the performance of all heat pump dryers. In addition, it was found that specific air flow rate and drying air temperature affected significantly the performance of all heat pump dryers.     

MODELING AND OPTIMIZATION OF A DRYER

This study deals with the modeling and the search for the optimum production capacity of a tunnel-dryer of the Californian type. The model is obtained from the heat and mass transfer balances at two levels, the product (fruits) level and inside the tunnel itself. The optimum condition is given by the maximum production capacity of a plum dryer satisfying the final product quality. This choice is determined by the fact that a large amount of fruit must be dried in a short time to have a minimum of loss. Recirculation of a proportion of the exhaust air improves the dryer efficiently in terms of energy.     

DESIGN AND TESTING OF A NEW SOLAR TRAY DRYER

A novel low cost tray dryer equipped with a solar air collector, a heat storage cabinet and a solar chimney is designed and tested. The design is based on energy balances and on an hourly-averaged radiation data reduction procedure for tilted surfaces. Measurements of total solar radiation on an horizontal plane, ambient temperature and humidity, air speed, temperature and relative humidity inside the dryer as well as solids moisture loss-in-weight data are employed as a means to study the performance of the dryer. First, detailed diagnostic experiments are carried out with no drying material on the trays. Next, a number of experiments is conducted using a controlled reference material whose reproducible dehydration pattern allows comparisons among runs. Drying is also tested during night operation and under adverse weather conditions. For all the employed conditions, the material gets completely dehydrated at a satisfactory rate and with an encouraging system's efficiency.     

PERFORMANCE OF SPRAY DRYER WITH INTEGRATED FILTER AND FLUID BED

Spray dryers featuring a fluid bed integrated into the base of a spray drying chamber have proved one of the most significant developments introduced into industry during the last decade, producing dust-free particulates under low product temperature conditions. The latest design development involves all particulate collection and exhaust air cleaning within the drying chamber with the use of integrated metallic filter elements (with CIP capability). This eliminates handling of fines outside the drying chamber and simplifies the exhaust air system contributing to lower pressure drop losses and lower overall energy consumption. This paper describes the performance of a pilot plant sized spray dryer featuring a drying chamber with both integrated particulate filters and fluid bed. Various products were tested. The results showed that the placing of particulate filters inside the drying chamber does not adversely affect the agglomeration process and that the powder quality compared with that achieved in a standard Fluidized Spray Dryer can be reproduced in this new design concept, with every possibility for improved quality due to no powder handling outside the drying chamber. The work also showed that by containing the powder within the drying chamber, notable operational advantages are apparent and that scale-up of the design concept represents no apparent difficulties.     

OBSERVATIONS OF FLOW PATTERNS IN A SPRAY DRYER

ABSTRACT

Experiments have been performed on a pilot scale, cylinder-on-cone spray dryer fitted with a vaned-wheel atomiser to observe air flow patterns, with and without water spray. A combination of tufts, smoke streams and a laser light sheet, was used to provide information about regions of recirculation, flow stability, spray trajectories and wall deposition. While atomiser-induced swirl dominated the flow patterns under typical operating conditions, some instability was observed, although different in type, for situations with and without atomiser rotation. Clockwise eddies were observed to form and collapse between the wall and the strongly anti-clockwise swirling core created by anti-clockwise atomiser rotation. Without swirl, large portions of the recirculation zones at the walls were observed to have a weak tendency to change randomly between clockwise, anticlockwise and chaotic behaviour.