在惰性气体环境中热泵干燥食品

在传统的食品干燥技术中,干燥介质是空气,干燥能力由变化的温度控制。通常,干燥过程在高温下进行,空气中氧的存在影响食品的营养物质。想要在低温条件下提高干燥能力,可以通过热泵来改善干燥环境。热泵是闭环系统,冷凝器产生的热控制干燥温度,蒸发器除去干燥室的水分,干燥介质在低温低湿下运行。由于整个系统在闭路下操作,为了去除空气中氧对食品的影响,采用氮气、二氧化碳等惰性气体代替空气作为干燥介质。本文主要研究：采用氮气、二氧化碳和空气干燥一些热敏性水果,如番石榴和木瓜,探讨对食品营养物质和颜色的影响,并与冷冻干燥、真空干燥的结果进行对比。     

Saffron Drying with a Heat Pump–Assisted Hybrid Photovoltaic–Thermal Solar Dryer

Saffron is the most expensive spice and Iran is the largest producer of this crop in the world. Saffron quality is profoundly affected by the drying method. Recent research has shown that hybrid photovoltaic–thermal solar power systems are more efficient in comparison with individual photovoltaic and thermal systems. In addition, heat pump dryers are highly energy efficient. Furthermore, they are suitable for heat-sensitive crops such as saffron. Therefore, in the present study, the performance of a hybrid photovoltaic–thermal solar dryer equipped with a heat pump system was considered for saffron drying, in order to obtain a high-quality product and reduce fossil fuel consumption. The effect of air mass flow rate at three levels (0.008, 0.012, and 0.016 kg/s), drying air temperature at three levels (40, 50, and 60°C), and two different dryer modes (with and without the heat pump unit) on the operating parameters of the dryer was investigated. The results of the investigation showed that total drying time and energy consumption decreased as air flow rate and drying air temperature increased. Applying a heat pump with the dryer led to a reduction in the drying time and energy consumption and an increase in electrical efficiency of the solar collector. The average total energy consumption was reduced by 33% when the dryer was equipped with a heat pump. Maximum values for electrical and thermal efficiency of the solar collector were found to be 10.8 and 28%, respectively. A maximum dryer efficiency of 72% and maximum specific moisture extraction rate (SMER) of 1.16 were obtained at an air flow rate of 0.016 kg/s and air temperature of 60°C when using the heat pump.     

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.     

The Application of Heat Pump in Drying of Biomaterials

ABSTRACT

The UN estimates that 6.4 billion people will be living on Earth by the year 2000. As a consequence, dried biomaterials demand will rise and equipments will have to be built or redesigned to accomadate changes. The design and innovations on industrial dryers require fundamental knowledge conceived in laboratory units. The new heat pump dryer operates at a wide relative humidity and the air temperature is adjustable from –20 to 100'C. Above and belw freezing point drying modes are done in the same plant which allows the control of biomaterial properties. The technique consists of two stages, unbound weakly held water removal by freeze drying. and tightly bound moisture desorption by medium temperature drying. The fluidized and shelf heat pump dryers have the required features to dry heat sensitive biomaterials. The dryers were built and extensive research on biomaterials has been conducted at The Norwegian Institute of Technology. Many emeriments were done on effect of heat pump drying conditions on–enrime and biological active bacteria solutions, fruits, cod fish, fish feed and shrimps. At optimum heat drying conditions, the bacteria Rhodococcus reached 100% viability and dried biomolecules attained full biolosical activity. The heat pump dryer allowed adjustment of the bio–mterial properties and quality according to its phase, shape and size. Dried samples showed improvements on survival rate. rehydration, colour, hardness and mono–multimolecular sorption isotherms over poorly dried products. Beneficial results were achieved for nearly all tests. This indicates that the heat pump dryer application has high potential in processing heat sensitive biomaterials.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

Abstract

The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Abstract

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

Calculation Steps for the Design of Different Components of Heat Pump Dryers Under Constant Drying Rate Condition

Heat pump–assisted dryers are an alternative method for drying heat-sensitive food products at low temperature and less relative humidity with lower energy consumption. The mathematical models of a heat pump dryer consist of three submodels; namely, drying models, heat pump models, and performance models. Heat and mass balance of both refrigerant and air circuits in all components of the system are used for development of mathematical models. The models are used for design of different components of heat pump dryers operating under constant drying rate condition. A simple stepwise design procedure for batch-type, closed-loop heat pump dryer is also presented.     

Heat Pump Drying of Green Sweet Pepper

Thin-layer drying experiments under controlled conditions were conducted for green sweet pepper in heat pump dryer at 30, 35, and 40°C and hot air dryer at 45°C with relative humidities ranging from 19 to 55%. The moisture content of sweet pepper slices reduced exponentially with drying time. As the temperature increased, the drying curve exhibited a steeper slope, thus exhibiting an increase in drying rate. Drying of green sweet pepper took place mainly under the falling-rate period. The Page equation was found to be better than the Lewis equation to describe the thin-layer drying of green sweet pepper with higher coefficient of determination and lower root mean square error. Drying in heat pump dryer at 40°C took less time with higher drying rate and specific moisture extraction rate as compared to hot air drying at 45°C due to lower relative humidity of the drying air in a heat pump dryer though the drying air temperature was less. The retention of total chlorophyll content and ascorbic acid content was observed to be more in heat pump–dried samples with higher rehydration ratios and sensory scores. The quality parameters showed a declining trend with increase in drying air temperature from 30 to 45°C. Keeping in view the energy consumption and quality attributes of dehydrated products, it is proposed to dry green sweet pepper at 35°C in heat pump dryer.     

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

ABSTRACT

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.     

Investigation on Drying Performance and Energy Savings of the Batch-Type Heat Pump Dryer

Simulation of the heat pump cycle and the drying process has been carried out to obtain the design parameters of the dryer. The analysis indicates that a specific moisture extraction rate (SMER) greater than 3.4 kg/kWh can be obtained. A box-type heat pump dryer has been developed and investigated for the performance of drying of shredded radish. Heat pump drying took 1.0–1.5 times longer than hot air drying. However, the heat pump dryer showed considerable improvement in energy savings. The SMER of the heat pump dryer was about three times higher than that of the hot air dryer.     

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

ABSTRACT

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.     

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.     

Drying Kinetics of Tomato by Using Electric Resistance and Heat Pump Dryers

Drying kinetics of tomato was studied by using heat pump dryer (HPD) and electric resistance dryers with parallel and crossed airflow. The performance of both systems was evaluated and compared and the influence of temperature, air velocity, and tomato type on the drying kinetics was analyzed. The use of HPD showed to be adequate in the drying process of tomatoes, mainly in relation to the conversion rate of electric energy into thermal energy. The heat pump effective coefficient of performance (COP_HT,EF) was between 2.56 and 2.68, with an energy economy of about 40% when compared to the drying system with electric resistance. The Page model could be used to predict drying time of tomato and statistical analysis showed that the model parameters were mainly affected by drying temperature.     

The Application of Heat Pump in Drying of Biomaterials

The UN estimates that 6.4 billion people will be living on Earth by the year 2000. As a consequence, dried biomaterials demand will rise and equipments will have to be built or redesigned to accomadate changes. The design and innovations on industrial dryers require fundamental knowledge conceived in laboratory units. The new heat pump dryer operates at a wide relative humidity and the air temperature is adjustable from -20 to 100'C. Above and belw freezing point drying modes are done in the same plant which allows the control of biomaterial properties. The technique consists of two stages, unbound weakly held water removal by freeze drying. and tightly bound moisture desorption by medium temperature drying. The fluidized and shelf heat pump dryers have the required features to dry heat sensitive biomaterials. The dryers were built and extensive research on biomaterials has been conducted at The Norwegian Institute of Technology. Many emeriments were done on effect of heat pump drying conditions on-enrime and biological active bacteria solutions, fruits, cod fish, fish feed and shrimps. At optimum heat drying conditions, the bacteria Rhodococcus reached 100% viability and dried biomolecules attained full biolosical activity. The heat pump dryer allowed adjustment of the bio-mterial properties and quality according to its phase, shape and size. Dried samples showed improvements on survival rate. rehydration, colour, hardness and mono-multimolecular sorption isotherms over poorly dried products. Beneficial results were achieved for nearly all tests. This indicates that the heat pump dryer application has high potential in processing heat sensitive biomaterials.     

热泵干燥装置的调控分析

热泵干燥具有节能、低温干燥及环境友好等特点，控制热泵干燥装置运行在较佳的状态，是保证热泵干燥装置效率和可靠性的基础。以带循环空气旁通的封闭式热泵,干燥装置为例，对热泵干燥装置的调控方法进行了分析，给出了装置的被控参数和一组较佳的控制参数，并建立了各控制参数的计算公式。     

HEAT PUMP DRYERS: RESEARCH AND DEVELOPMENT NEEDS AND OPPORTUNITIES

ABSTRACT

As some previous research works on heat pump dryers (HPD) gave contradictory results, there is a need to review and identify R&D needs and opportunities in HPD. It was found that mathematical modeling cannot ignore the interdependence of the heat pump working fluid and the process air of the dryer. The performances of various HPD configurations with respect to all operating varibles need further investigation. A well defined mathematical model of combined dryer-drying material characteristic is required for the system modeling. The relative specific drying cost and relative useful energy were introduced as the dryer selection criteria. The role of the heat pump in the HPD system, CFC alternatives and non-conventional heat pump cycles using air or steam as working fluids deserve further investigation.     

Energy and exergy efficiencies as design criteria for grain dryers

We devised a novel methodology for optimizing cereal grain dryers grounded on the transient spatial–temporal first and second laws of thermodynamics and associated balance equations. Model equations were solved using a special time-adaptive radial basis function. Comprehensive sensitivity tests show the quantitative effects of initial moisture content, air velocity, and drying air temperature on the temporal profiles of outlet air temperature and moisture content and temporally integrated energy and exergy efficiencies. Drying temperature is the most efficacious parameter in the drying range of 50–90°C. Finally, selected examples show optimized dryer operation points under unrestrained and restrained conditions. Second law efficiency is well suited for expressing drying performance, portraying time, noble energy expenditure, and intrinsic sustainability. Three decision tables, based on simulation results, can be used to define dryer design under normal technical choice. First law efficiency and the specific moisture extraction rate are concepts more adequate for designing in-bin low-temperature dryers. Second law efficiency is indicated when exergy recuperation is at stake: higher drying temperatures, shorter drying times, recirculation drying, and other processes.     

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.     

An Evaluation of Factors Influencing the Energy-Efficient Operation of Well-Mixed Fluidized Bed Dryers

This article describes the results of calculations of specific energy consumption, E_s, performed on a well-mixed fluidized bed dryer simulator. Exhaust air temperature-humidity loci required to yield a specified outlet moisture content were also determined. Most of the calculations related to solids whose drying rate was gas-film controlled. Six model drying curves were employed to examine the effects of drying rate and hygroscopicity in addition to the normal operating parameters. The results indicated that E_s was highest for slow-drying hygroscopic solids and lowest for fast-drying, non-hygroscopic solids. Specific energy consumption increased with decreasing bed temperature and outlet moisture content and with increasing heat loss but was independent of solids loading and airflow rate. For both the aforementioned solids and a much slower drying material (wheat), there was close agreement between the zero heat loss data and a single theoretical curve approximating the performance of an ideal adiabatic dryer. Distinct differences between the behavior of well-mixed and plug flow fluidized bed dryers are reported.     

Bibliography of Literature on Fundamentals and Applications of Vibration in Particle Processing

This article describes the results of calculations of specific energy consumption, E _s , performed on a well-mixed fluidized bed dryer simulator. Exhaust air temperature–humidity loci required to yield a specified outlet moisture content were also determined. Most of the calculations related to solids whose drying rate was gas-film controlled. Six model drying curves were employed to examine the effects of drying rate and hygroscopicity in addition to the normal operating parameters. The results indicated that E _s was highest for slow-drying hygroscopic solids and lowest for fast-drying, non-hygroscopic solids. Specific energy consumption increased with decreasing bed temperature and outlet moisture content and with increasing heat loss but was independent of solids loading and airflow rate. For both the aforementioned solids and a much slower drying material (wheat), there was close agreement between the zero heat loss data and a single theoretical curve approximating the performance of an ideal adiabatic dryer. Distinct differences between the behavior of well-mixed and plug flow fluidized bed dryers are reported.