辅助冷凝器冷却水量对闭式热泵干燥系统性能的影响

针对目前工业生产中金属件清洗干燥工艺中常用干燥方式存在能耗高且环境不友好的现状,提出并建立了一种采用直接串联式辅助冷凝器的闭式热泵干燥系统。基于所搭建的性能测试台,实验研究了流经辅助冷凝器的冷却水量这一关键参数对系统运行工况参数、制热/制冷量、系统功耗、性能系数（COP）以及单位时间除湿量（MER）和单位能耗除湿量（SMER）等性能参数的影响。结果表明：冷却水量为30.6kg/h时,冷凝器出风温度达72.8℃;随着冷却水量的增大,系统制热/制冷量、功耗、冷凝器出风温度及MER均呈现下降趋势,COP维持在5.6左右;MER最高可达3.80kg/h,SMER最高可达1.44kg/(kW·h),MER和SMER的变化趋势相反,故生产实际中需要综合考虑冷却水量对两者的影响;另外,在实验研究工况下最大冷却水出水温度达65.2℃,可为工业生产提供可应用的热水,使能源得到充分地回收利用。研究结果可为闭式热泵干燥系统在金属件清洗干燥工艺中的应用及其节能降耗提供新的思路和参考。     

Design and Performance Evaluation of a Solar-Assisted Biomass Drying System with Thermal Storage

A solar biomass hybrid air heating system that does not require a conventional auxiliary heater but can still provide a daily load fraction exceeding 90% and supply hot air at a steady temperature and flow rate continuously for 24 h a day has been developed. The system, which combines an unglazed transpired solar collector, rock bed, and a biomass gasifier stove with heat exchanger, was evaluated by drying chilli using air at 60°C and 90 m³/h. The chilli was dried from 76.7% moisture (w.b.) to 8.4% over 32.5 h of continuous drying. The dryer reduced the drying time by 66% compared to open sun drying and provided 91.6% load fraction during the 24-h operation. The temperature of hot air supplied was stable at 60±3°C for about 21 h during the entire drying duration.     

Modeling and Performance Investigation of a Closed-Type Thermoelectric Clothes Dryer

In present work, a closed-type clothes dryer with thermoelectric elements was developed. The looped air circulation was designed to simultaneously recycle waste heat and enhance dryer performance. A mathematical model of heat transfer, based on one-dimensional treatment of thermal and electric power, is conducted. The cooling and heating productions are both correlated in terms of electric resistance, thermal conductivity, and electric current. Experimental investigation on drying of clothes has been attempted, covering the drying air temperature, initial-input electric power, and total weight of wet clothes, with drying rate and specific moisture extraction rate as evaluating indexes. Generally, the drying rate was found to increase first and decrease afterwards as time decayed. Analytical and experimental results demonstrate that optimal performance of the thermoelectric dryer strongly depends on intensities of these operating parameters.     

Drying performance of a tumbler dryer with condenser

The effects of flow rates of hot air and cooling water on drying percentage and energy efficiency of cotton lint in a cylindrical type drum tumbler dryer (0.54m-ID×0.34m-high) have been determined. Drying of the lint is mainly affected by flow rate of hot air, which is a function of the diameter ratio (D_M/D_F) of motor and fan pulley and motor capacity. During the drying process in the tumbler dryer, temperatures and humidity before and after were measured to determine the drying characteristics. The volumetric flow rate of hot air increases with increasing the diameter ratio of the motor and fan pulley, whereas the volumetric flow rate of hot air decreases with an increase in the mass flow rate of the cooling water through the condenser. The energy consumed by the motor relied more on the diameter ratio of the motor and fan pulley as opposed to the mass flow rate of cooling water. Despite the increase in the drying percentage with increasing the diameter ratio of the motor and fan pulley and the mass flow rate of the cooling water, the energy efficiency decreased.     

OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD

ABSTRACT

Food drying process in tunnel dryer was modeled from Keey's drying model and experimental drying curve, and optimized in operating conditions consisting of inlet air temperature, air recycle ratio and air flow rate. Radish was chosen as a typical food material to be dried, because it has a typical drying characteristics of food and quality indexes of ascorbic acid destruction and browning in the drying. Stricter quality retention constraint required higher energy consumption in minimizing the objective function of energy consumption under constraints of dried food quality. Optimization results of cocurrent and counter current tunnel drying showed higher inlet air temperature, lower recycle ratio and higher air flow rate with shorter total drying time. Compared with cocurrent operation counter current drying used lower air temperature, lower recycle ratio and lower air flow rate, and appeared to be more efficient in energy usage. Most of consumed energy was analyzed to be used for air heating and then escape from the dryer in form of exhaust air.     

Improvement in the Efficiency and Capacity of Chopped Spring Onion Drying

ABSTRACT

Appropriate strategy for drying chopped spring onion with a batchwise flat bed was investigated. Both experimental and simulated results such as product quality, drying capacity and energy consumption were taken into consideration. For simulation work, equations of drying parameters such as specific heat, equilibrium moisture content and thin layer drying were first developed from the lab-scale experimental results. Then a mathematical model including shrinkage for a batchwix flat bed drying was developed. The model was lested with the results obtained from a food processing plant with an acceptable accuracy. Appropriate drying strategy war then investigated. The approximate conclusion was that the drying should be devided into 3 stages. In the 1^st stage, drying air temperature was 80°C, specific air flow rate was 33.9 m³/min -kg dry matter and drying time was 0.5 h. In the 2nd stage, drying air temperature and drying time were kept unchanged but specific air flow rate was decreased to 13.5 m³/min - kg dry matter. In the final stage, drying air temperature was decreased to 67°C, specific air flow rate was also decreased to 6.8 m³/min - kg dry matter and drying time was approximately 1.7 h. Following the suggested strategy, specific primary energy cornsumption was 6.2 MJ/kg H₂O, drying time was 2.7 h and product quality was maintained. It was proven that energy consumption was approximalcly 70% of that of the present practice in the plant.     

Drying Characteristics of Barley Grain Dried in a Spouted-Bed and Combined IR-Convection Dryers

A study was performed to determine the drying characteristics and quality of barley grain dried in a laboratory scale spouted-bed dryer at 30, 35, 40, and 45°C and an inlet air velocity of 23 m/s^?1, and in an IR-convection dryer under an infrared radiation intensity of 0.048, 0.061, 0.073, and 0.107 W cm^?2 at an air velocity of 0.5 m/s^?1. The results show that the first, relatively short, phase of a sharp decrease in the drying rate was followed by the phase of a slow decrease. The time of barley drying depended on temperature of inlet air in a spouted-bed dryer and on radiation intensities in an IR-convection dryer. Barley drying at 45°C in a spouted-bed dryer was accompanied by the lowest total energy consumption. The average specific energy consumption was lower and the average efficiency of drying was higher for drying in a spouted-bed dryer. The effective diffusivities were in the range 2.20–4.52 × 10^?11 m² s^?1 and 3.04–4.79 × 10^?11 m²/s^?1 for barley dried in a spouted-bed and in an IR-convection dryer, respectively. There were no significant differences in kernel germination energy and capacity between the two drying methods tested.     

DESIGN AND CONTINUOUS OPERATION OF A SOLAR CONVECTION DRIER WITH AN AUXILIARY HEATING SYSTEM

A solar drier of the through-draft type (4 trays at 0.5 m² surface area) with natural air convection and an auxiliary gas heating system was constructed. General relationships between the climatic conditions of the ambient air, product load and drying characteristics were established. Test runs with the drier as continuous equipment were carried out over a period of 24 h, using carrot dice as experimental material. Air flow rates through the dryer between 100 and 140 kg/h and overall drying rates between 1.5 and 2 kg/h were reached. The overall energy efficiency coefficient for the hybrid heating system amounted to 27 % as compared to 31 to 37 % for the solar energy heating alone and to 22 to 27 % for gas heating energy alone. A combination of continuous drying in the first stage with subsequent batch-wise finish drying in the same or a supplementary drier seems to be advantageous.     

Performance Analysis of an Electric Clothes Dryer

Improving the energy efficiency of clothes dryers has been the subject of numerous investigations. However, to date the clothes drying process has not been thoroughly tested or analyzed in a comprehensive study. Many of the techniques that have been proposed to improve energy efficiency add to dryer cost or are environmentally unacceptable. In the present study, a commercial Frigidaire dryer was instrumented and evaluated for energy performance by changing operating parameters including heater power, fan speed, drum speed, weight, and initial moisture content of the clothes. The total drying time and energy consumption was monitored as well as the effect of air leakage into the drum. The experimental results from 32 runs of the dryer were evaluated. The results from these tests are discussed and techniques are proposed in an attempt to improve the energy efficiency of this unit.     

DESIGN,TESTING, AND OPTIMIZATION OF VIBRO-FLUIDIZED BED PADDY DRYER

The objectives of this research were to design, construct and test a prototype of vibro-fluidized bed paddy dryer with a capacity of 2.5–5.0 t/h and develop a mathematical model that determines optimum operating parameters. Experimental drying conditions were: air flow rate, 1.7 m³/s; bed velocity, 1.4 m/s; average drying air temperature, 125–140°C; residence time of paddy approximately 1 minute; bed height, 11.5 cm; fraction of air recycled, 0.85 and vibration of intensity, 1 (frequency, 7.3 Hz and amplitude, 5 mm). Moisture content of paddy with a feed rate of 4821 kg/h was reduced from 28 to 23% d.b. Specific primary energy consumption (SPEC) was 6.15 MJ/kg-water evaporated. Electrical power of blower motor and vibration motor was 55% as compared to electrical power of blower motor used in fluidized bed drying without vibration. Comparison between the experimental and simulated results showed that the mathematical model could predict fairly well. To find out optimum operating parameters, the grid search method was employed with criteria based on acceptable moisture reduction and quality and minimum energy consumption.

     

Energy and Exergy Analyses of Thin-Layer Drying of Potato Slices in a Semi-Industrial Continuous Band Dryer

This article is concerned with the energy and exergy analyses of the continuous-convection drying of potato slices. The first and second laws of thermodynamics were used to calculate the energy and exergy. A semi-industrial continuous-band dryer has been designed and used for drying experiments. The equipment has a drying chamber of 2 m length and the inlet air used for drying is heated by gas power. The experiments were conducted on potato slices with thickness of 5 mm at three different air temperatures of 50, 60 and 70°C, drying air mass flow rates of 0.61, 1.22, and 1.83 kg/s and feeding rates of 2.31 × 10^?4, 2.78 × 10^?4, and 3.48 × 10^?4 kg/s. The energy utilization and energy utilization ratio were found to vary between 3.75 and 24.04 kJ/s and 0.1513 and 0.3700, respectively. These values show that only a small proportion of the supplied energy by the heater was used for drying. The exergy loss and exergy efficiency were found to be in the range of 0.5987 to 13.71 kJ/s and 0.5713 to 0.9405, respectively, indicating that the drying process was thermodynamically inefficient and much energy was vented in the exhaust air. In addition, the results showed that the feeding rate and the temperature and flow rate of the drying air had an important effect on energy and exergy use. This knowledge will provide insights into the optimization of a continuous dryer and the operating parameters that causes reduction of energy consumption and losses in continuous drying.     

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.     

Unglazed Transpired Solar Dryers for Medicinal Plants

Three different solar drying methods were carried out on four different medicinal plants to investigate the benefits of using an unglazed transpired solar dryer (UTSD) over other traditional methods. Methods involved included drying in an unglazed transpired solar dryer (using suction air flow rate of 0.06 m³s^?1), drying in the open air under direct sun rays and a common traditional drying method in a shaded drying house. The three drying methods were used to dry the following medicinal plants: henna (Lawsonia inermis L.), rosemary (Rosmarinus officinalis L.), marjoram (Majorana hortensis L.), and moghat (Glossostemon bruguieri L.). Drying processes were carried out under the climatic weather conditions of Ismailia, Egypt. Drying rate, drying ratio, and the medicinal plants qualities in terms of oil quantity and sensation tests were considered. The results showed higher oil quantity obtained from rosemary and marjoram dried in the UTSD, compared with those dried in the shaded drying house and in the open air under direct sun.     

Transient analysis of the steam‐water direct contact condensation in the packed column

The transient heat transfer behaviour of direct contact condensation of steam in spray cooling water in a packed column was investigated experimentally and theoretically in this work for the first time. A new analysis method of the state equation was applied to analyze the molar quantity variation of steam in the course of the experiment. The results showed that increasing the cooling water flow rate properly could significantly accelerate the steam condensation rate and this was more obvious at the beginning of the operation. The higher the flow rate of the cooling water was, the shorter a time it took for the steam to be completely condensed. When the cooling water flow rate was 840 L/h, the steam was mainly condensed within 13 s, which indicated that it would rapidly handle steam by regulating cooling water flow rate. However, the effect of changing cooling water temperature on steam condensation rate is slight with the cooling water temperature inceasing from 23 °C to 42 °C. The volumetric heat transfer coefficient in this experiment is in the range of 1.47 kW · m^?3 · K^?1 to 10.93 kW · m^?3 · K^?1 with the cooling water flow rate inceasing from 120 L/h to 840 L/h, and the maximum uncertainty in the results is 2.2 %.

     

OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD

Food drying process in tunnel dryer was modeled from Keey's drying model and experimental drying curve, and optimized in operating conditions consisting of inlet air temperature, air recycle ratio and air flow rate. Radish was chosen as a typical food material to be dried, because it has a typical drying characteristics of food and quality indexes of ascorbic acid destruction and browning in the drying. Stricter quality retention constraint required higher energy consumption in minimizing the objective function of energy consumption under constraints of dried food quality. Optimization results of cocurrent and counter current tunnel drying showed higher inlet air temperature, lower recycle ratio and higher air flow rate with shorter total drying time. Compared with cocurrent operation counter current drying used lower air temperature, lower recycle ratio and lower air flow rate, and appeared to be more efficient in energy usage. Most of consumed energy was analyzed to be used for air heating and then escape from the dryer in form of exhaust air.     

Screw Conveyor Dryer: Process and Equipment Design

The effects of various process variables and equipment components (geometry) on the performance of a screw conveyor dryer (SCD) were studied in terms of the material throughput and its uniformity, dryer load, specific consumption of mechanical energy, and heat transfer rate. The experimental results for drying of fine crystalline solids (50–100 µm particle size and 550 kg/m³ bulk density) in a 3-meter-long uninsulated jacketed screw conveyer dryer with a 0.072-m screw diameter have been used. The hydrodynamic performance of the SCD was also studied using sand particles of 350 µm size and 1500 kg/m³ bulk density (tapped). The maximum specific consumption of mechanical energy for conveying was found to be 1 kJ/kg. Moreover, the flow behavior of the material at the dryer discharge was found to depend strongly on the screw speed and the material feed rate.     

热负荷分配比例对抽凝-背压供热机组能耗影响

抽凝-背压供热模式是实现能量梯级利用、降低火力发电煤耗的有效途径,研究不同室外温度下供热凝汽器与尖峰加热器热负荷分配比例对机组能耗的影响,确定最佳热负荷分配比例,是抽凝-背压供热机组节能降耗的核心问题之一。本文利用热网变工况模型及Ebsilon软件仿真,以某310MW抽凝-背压供热机组为研究对象,分析了供热期不同温度下供热凝汽器与尖峰加热器热负荷分配比例不同时机组的发电功率及煤耗。结果表明：对于抽凝-背压热电联产机组,并非供热凝汽器热负荷比例越高而发电功率越高,供热期不同阶段,机组发电功率随供热凝汽器热负荷变化呈现不同规律;相同室外温度下,供热凝汽器与尖峰加热器热负荷分配比例对机组能耗影响很大,凝汽器热负荷比例不同时,其极差最小值和最大值分别为2.02g/（kW·h）和5.50g/（kW·h）。     

MOBILE FLUIDIZED BED PADDY DRYER

ABSTRACT

The objectives of this research are to design, construct and test a mobile fluidized bed paddy dryer with a drying capacity of 2.5-4.0 t/h. Suitable drying conditions are recommended as follows : drying capacity 3.8 t/h, bed velocity 2.8 m/s, average drying air temperature 144 °C, bed height 13.5 cm, fraction of air recycled 0.8. Residence time of paddy was approximately 1.3 minutes. Test results showed that moisture content of paddy was reduced from 32.6 % dry-basis to 25.8 % dry-basis. Consumption of electrical power and diesel fuel was 12.9 kW and 21.71 1/h respectively. Primary energy consumption was 910.9 MJ/h. The dryer could evaporate water 218.8 kg/h. Specific primary energy consumption was 4.2 MJ/kg-water evaporated. Cost of paddy drying was 1.48 baht/kg-water evaporated of which 0.53 was fixed cost and 0.95 was energy cost (US$1 =34baht).     

The effect of noncondensable gas on heat transfer in the preheater of the sewage sludge drying system

We used a shell-and-tube type preheater to investigate the effect of noncondensable gas on heat transfer. In the preheater of the drying system, heat is exchanged between steam-air mixed gas which is dryer outlet gas and sewage sludge. To evaluate the performances of the preheater, water was first used in the tube-side material instead of sewage sludge and steam-air mixed gas in the shell-side material. The test variables were as follows: mixed gas inlet temperatures range from 95 to 120 °C; inlet air content, m _air /m _steam from 55 to 83%; tube-side water flow rate from 42 to 62 kg/h. The shell-side heat transfer coefficient varied from 150 to 550W/m²K, which corresponds to the amount of noncondensable gas in the steam-air mixed gas and the overall heat transfer coefficient varied from 60 to 210W/m²K. Using sewage sludge as a tube-side material the overall heat transfer coefficient varied from 60 to 130W/m2K and the outlet temperature of sewage sludge was above 90 °C, which is high enough for reducing energy consumption in the dryer by preheating the sewage sludge.     

COFFEE CHERRY DRYING: A TWO-LAYER MODEL

Drying of Coffea arabica cherry was carried out in a thin layer dryer at air temperatures of 40, 50, 60 and 70°C at bed depths of 50, 62.5 and 75mm with air flow rates of 1.5 and 2m³/m²/ min. Since the coffee cherry consis ts of skin and bean, a two layer drying model was developed to characterize the overall drying process for relative humidity range of 46 to 83 per cent. The diffusion coeffcient was determined using drying and equilibrium data and expressed as an Arrhenius- type function. The study revealed that the qualityof the coffee was unaltered over the temperature range studied.