Transcritical CO2 Heat Pump Dryer: Part 2. Validation and Simulation Results

The simulation model of a transcritical CO₂ heat pump dryer presented in Part 1 has been first validated with available experimental data in this part and then used to simulate the heat pump dryer to study the variation of performance parameters such as heating COP, moisture extraction rate, and specific moisture extraction rate. The validation with experimental data shows that the model slightly over predicts the system performance. The possible reasons for the difference between experimental and numerical results are explained. Simulation results show the effect of key operating parameters such as bypass air ratio, re-circulation air ratio, dryer efficiency, ambient condition (temperature and relative humidity), and air mass flow rate. Results show that unlike bypass air ratio and ambient relative humidity, the effect of dryer efficiency, recirculation air ratio, ambient temperature, and air mass flow rate are very significant as far as the system performance is concerned.     

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.     

内加热式热泵干燥装置的设计

内加热式热泵干燥装置通过导热加热方式提供物料中水分气化所需热能,可获得较高的能源效率和除湿能耗比。在介绍内加热式热泵干燥装置基本结构和工作过程的基础上,给出了其物料衡算、能量衡算和主要部件选型参数的计算公式,为内加热式热泵干燥装置的设计提供了较好的参考。     

MVR技术用于空心桨叶干燥机干燥污泥恒速段实验研究

为降低干燥过程的能耗,基于空心桨叶干燥机建立了一套机械蒸汽再压缩式热泵干燥系统,采用罗茨压缩机驱动,对污泥间歇干燥过程的恒速段进行实验研究,实验结果表明:在恒速段,降低干燥压力、适当减小压缩比、选择合适的转轴频率均有利用提高系统的运行效率;在实验条件范围内,MVR热泵干燥系统的COP为3.9~5.0,SMER为5.2~7.7。     

Exergetic performance assessment of a ground‐source heat pump drying system

In evaluating the efficiency of heat pump (HP) systems, the most commonly used measure is the energy (or first law) efficiency, which is modified to a coefficient of performance (COP) for HP systems. However, for indicating the possibilities for thermodynamic improvement, energy analysis is inadequate and exergy analysis is needed. This study presents an exergetic assessment of a ground‐source (or geothermal) HP (GSHP) drying system. This system was designed, constructed and tested in the Solar Energy Institute of Ege University, Izmir, Turkey. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are determined to assess their performances and to elucidate potentials for improvement. COP values for the GSHP unit and overall GSHP drying system are found to range between 1.63–2.88 and 1.45–2.65, respectively, while corresponding exergy efficiency values on a product/fuel basis are found to be 21.1 and 15.5% at a dead state temperature of 27°C, respectively. Specific moisture extraction rate (SMER) on the system basis is obtained to be 0.122 kg kW^?1 h^?1. For drying systems, the so‐called specific moisture exergetic rate (SMExR), which is defined as the ratio of the moisture removed in kg to the exergy input in kW h, is also proposed by the authors. The SMExR of the whole GSHP drying system is found to be 5.11 kg kW^?1 h^?1. Copyright © 2006 John Wiley & Sons, Ltd.     

带辅助冷凝器的封闭式热泵干燥系统理论分析

介绍带辅助冷凝器的封闭式热泵干燥系统,采用控制变量法对系统中的变量（如干燥室进出口空气温度和湿度、冷凝温度、蒸发温度、过热度和过冷度等）进行调节,得到各变量对除湿能耗量（SMER）的影响。计算结果表明,不同的干燥室出口空气湿度均对应着最佳的空气旁通率使得除湿能耗量最大,增大干燥室进出口空气温度和湿度可增大系统的效率,提高过冷度和过热度也可使系统效率增大。     

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.     

COMBINED INNOVATIVE HEAT PUMP DRYING TECHNOLOGIES AND NEW COLD EXTRUSION TECHNIQUES FOR PRODUCTION OF INSTANT FOODS

There is an increased demand for convenient foods including ready-to-eat and instant products. These products are desired with minimum concentration of synthetic chemicals. This creates challenges for the food industry and dryers manufactures to develop new technologies to process difficult-sensitive materials and to supply final products with high quality and improved properties. Fruits and vegetables are mainly composed of water, carbohydrates, proteins and lipids. Due to modification of chemical and physical bonds the compounds the material becomes viscous and sticky during processing. Conventional dryers have limitations in handling such sensitive materials. Heat pump dryers have been applied in the production of a diversity of ready-to-eat foods and dried instant products for the last five years at SINTEF-NTNU. Besides being energetically efficient and environmentally friendly, the heat pump drying technology provides a wide range of drying conditions as required to produce powders with improved characteristics. This work describes the new technologies and processing line for the production of instant foods as well as the measurements on the properties and quality attributes for raw, intermediate and final instant products.     

Re‐circulating heat pump assisted continuous bed drying and energy analysis

Specialty crops such as ginseng, herbs and echinacea need to be dried at low temperatures (30–35°C) for product quality optimization. A drying system that is both energy efficient and preserves product quality is desired. A re‐circulating heat pump continuous bed dryer system was designed, constructed and field‐tested for this purpose. The heat pump dryer system was experimentally evaluated using several potential herbal and medicinal crops such as alfalfa, catnip, wormwood, red clover, portulaca, dandelion and ginseng. These crops were dried either in chopped, sliced or whole form, depending on the part of the plant (aerial or root). The specific moisture extraction rates (SMER) obtained for various crops were in the range of 0.06–0.61 kg kWh^?1. It took 5 days and 190 kWh of energy to reduce the average moisture content of ginseng roots below 10% (wb), while commercial dryers currently in use would take on an average 14 days and 244 kWh of energy at comparable loading rates. The re‐circulating nature of the heat pump dryer made it 22% more energy efficient and resulted in 65% reduced drying time compared to conventional dryers incorporating electric coil heaters. Copyright © 2005 John Wiley & Sons, Ltd.     

COMBINED INNOVATIVE HEAT PUMP DRYING TECHNOLOGIES AND NEW COLD EXTRUSION TECHNIQUES FOR PRODUCTION OF INSTANT FOODS

ABSTRACT

There is an increased demand for convenient foods including ready-to-eat and instant products. These products are desired with minimum concentration of synthetic chemicals. This creates challenges for the food industry and dryers manufactures to develop new technologies to process difficult-sensitive materials and to supply final products with high quality and improved properties. Fruits and vegetables are mainly composed of water, carbohydrates, proteins and lipids. Due to modification of chemical and physical bonds the compounds the material becomes viscous and sticky during processing. Conventional dryers have limitations in handling such sensitive materials. Heat pump dryers have been applied in the production of a diversity of ready-to-eat foods and dried instant products for the last five years at SINTEF-NTNU. Besides being energetically efficient and environmentally friendly, the heat pump drying technology provides a wide range of drying conditions as required to produce powders with improved characteristics. This work describes the new technologies and processing line for the production of instant foods as well as the measurements on the properties and quality attributes for raw, intermediate and final instant products.