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.     

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.     

HEAT PUMP DRYING OF AGRICULTURAL MATERIALS

Heat pump-assisted drying is an energy-efficient process because the heat is recoverable. However, the economic feasibility of the heat pump dryer (HPD) is debatable because high grade energy (electricity) is used. In this study, sawn rubber wood and bananas were dried in an experimental HPD. The moisture extraction rates (MER) and the specific moisture extraction rates (SMER) of wood drying and banana drying decreased rapidly with the drying time but the compressor power was relatively constant. If the final moisture content of wood is less than 10%, the maximum average MER and SMER were 2.854 kg/h and 0.572 kg/kWh, respectively. Banana drying yielded the highest average MER of 2.710 kg/h when the drying load was highest. The corresponding SMER was 0.540 kg/kWh. Economic analyses among the HPD, an electrically-heated hot air dryer and a direct-fired dryer revealed that the HPD had the lowest operating cost.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

ABSTRACT

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.     

回热式热泵干燥高湿稻谷的试验研究

应用带有嵌入回热器的热泵装置,对华南地区高湿稻谷进行干燥试验,测定其干燥速率、能耗和品质。并与同期作业的燃油低温循环稻谷干燥机进行对照。试验结果表明：回热式热泵干燥机的干燥速率较燃油低温循环干燥机提高20％,干燥周期缩短约12％,而干燥能耗及费用可节省30％以上,干燥品质两者无差异。     

Modeling of heat pump tumble dryer energy consumption and drying time

The use of heat pump tumble dryers is nowadays more common because they offer huge energy savings compared to conventional tumble dryers. Earlier studies made on conventional tumble dryers have shown that parameters such as heater power, fan speed, drum speed, weight and initial moisture content of textiles and air leakage have a huge impact on the energy efficiency and drying time. In the present study, a modified commercial heat pump tumble dryer was evaluated for energy consumption and drying time by changing operating parameters including fan speed, drum speed, and mass load. The total energy consumption and drying time were measured and corrected for the initial and final moisture content in the textiles. The experimental results based on 27 drying tests were evaluated to develop linear regression models for energy consumption and drying time, which show a good agreement with the experimental data. The results show that a large mass load, a high drum speed, and a low fan speed give the highest energy efficiency, i.e. the lowest energy consumption per kg of drying load. Larger loads extend the length of the drying cycle while higher fan and drum speeds result in shorter drying time.     

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.     

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.     

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.     

Statistical analysis of clothing drying in a venting type dryer

An experimental study of the drying process using the prototype of a venting type dryer is presented. The machine has two motors: one for moving the drum and another for moving the turbine; electrical resistance is used as the heat source. The prototype operates under controlled test conditions in accordance with the Department of Energy (DOE) Standard 10 CFR 430. The aim of this study is to statistically determine the impact of the main factors and their interactions involved in the drying of clothing from a complete design of experiments. Statistical analyses for drying time and the energy consumption are determined from the studied factors. The factors studied are turbine motor frequency (to admit and expel air from the dryer); power supplied to the electric coil, amount and moisture of clothing and restriction of exhaust duct the gases. The results show that all of the studied factors and four of the interactions are statistically significant in the drying of clothes. The shortest drying time and the lowest energy consumption were obtained with the higher level of restriction and the lower initial moisture of clothing.     

SIMULATION OF DRYING IN A BATCH LUMBER KILN FROM SINGLE-BOARD TESTS

A deterministic model was developed to perform a board-by-board simulation of a forced convective batch lumber kiln. Individual board properties may be input and dryer operating parameters varied. The drying rates are empirical correlations based on single-board laboratory tests. The model incorporates the thermodynamic properties of the wood and gas, as well as mass and energy balances within the lumber stack. It also accounts for differences in heat and mass transfer resulting from position and changing gas properties throughout the dryer. The rate of drying predicted by the model and the final moisture content distribution were verified by weighing boards in a batch kiln before, during, and after drying. The application of the model is illustrated by simulating four common scenarios.     

Influence of drying conditions on the moisture diffusion and fluidization quality during multi-stage fluidized bed drying of bovine intestine for pet food

In order to establish the influence of the drying air characteristics on the drying performance and fluidization quality of bovine intestine for pet food, several drying tests have been carried out in a laboratory scale heat pump assisted fluid bed dryer. Bovine intestine samples were heat pump fluidized bed dried at atmospheric pressure and at temperatures below and above the materials freezing points, equipped with a continuous monitoring system. The investigation of the drying characteristics have been conducted in the temperature range ?10 to 25 °C and the airflow in the range 1.5–2.5 m/s. Some experiments were conducted as single temperature drying experiments and others as two stage drying experiments employing two temperatures. An Arrhenius-type equation was used to interpret the influence of the drying air temperature on the effective diffusivity, calculated with the method of slopes in terms of energy activation, and this was found to be sensitive to the temperature. The effective diffusion coefficient of moisture transfer was determined by the Fickian method using uni-dimensional moisture movement in both moisture, removal by evaporation and combined sublimation and evaporation. Correlations expressing the effective moisture diffusivity and drying temperature are reported.Bovine particles were characterized according to the Geldart classification and the minimum fluidization velocity was calculated using the Ergun Equation and generalized equation for all drying conditions at the beginning and end of the trials. Walli's model was used to categorize stability of the fluidization at the beginning and end of the drying for each trial. The determined Walli's values were positive at the beginning and end of all trials indicating stable fluidization at the beginning and end for each drying condition.     

农产品干燥加工节能减排工艺及装备研发

农产品干燥加工占干燥行业能耗的12%左右,是一项高能耗的过程。为促进农产品加工业的健康发展,本文介绍了回热热泵、真空带式干燥、旋流床气流膨化、太阳能和热泵组合干燥等四种节能减排的新工艺新装备,同时还探讨了农产品干燥加工节能减排的发展方向。     

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.     

鱿鱼热泵-热风联合干燥节能研究

以热泵干燥阶段的温度、分阶段干燥的水分转换点、后期热风干燥阶段的温度为影响因素,以单位能耗除湿值（SMER）和总挥发性盐基氮值（T—VBN）两者的综合指标Y为最终评价指标进行响应面优化分析,得出了联合干燥最佳工艺参数：热泵干燥温度（X1=54．3℃）、水分转换点（X2=32．3％）和热风干燥温度（X3=43．5℃）。在最优工艺条件下比较HPD、HPD＋AD、AD三种干燥方式下的样品品质和耗能,联合干燥得到的鱿鱼干品质高于热风干燥,而且降低了干燥能耗38．67％。     

Analysis of CO2 Transcritical Cycle Heat Pump Dryers

The heat pump dryer with CO₂ as the working medium is investigated in this study. The drying systems were designed based on the thermophysical properties and characteristics of CO₂ cycle. The two-stage drying of CO₂ transcritical cycle heat pumps are presented and discussed in comparison with the single compression cycle single-stage heat pump dryer, the two-stage drying heat pumps, and electric drying system. The analysis indicates that the specific moisture extraction ratio of the two-stage drying system can be higher than that of the single drying CO₂ system. The efficiency of energy utilization of the CO₂ heat pump dryer was higher than that of an electric dryer.     

Optimization study of soybean intermittent drying in fixed-bed drying technology

Intermittent drying of materials is an alternative operation that aims at reducing energy consumption, improve the preservation of dried products or decrease effective drying time. Intermittent drying supplies the system with time-varying input air properties that are opposite to traditional operations, where air properties are constant at the dryer inlet. The major objective of this study is to establish the most satisfactory patterns of air temperature and velocity modulation at the dryer entrance to reduce energy consumption. This optimization study was based on a heterogeneous model for the drying of grains in fixed bed validated with experimental data. Intermittent and conventional operation experiments were conducted using equal energy consumption, and the influence of air temperature and velocity modulation on the drying rates related to the percentage of evaporated water were assessed. Results indicated that higher drying rates can be achieved under intermittent operation, and the validated model based on these results could reasonably predict temperature and moisture content profiles. Simulations pointed out that the best modulation patterns of air properties is a function of a variety of system conditions such as initial temperature and moisture content of both soybean and drying air. However, a tendency to reduce energy consumption was observed when the system operation is initially at high temperature and constantly at low velocity.     

水循环式热泵干燥装置的性能分析

水循环式热泵干燥装置是指热泵和干燥部分通过水循环耦合而成的热泵干燥系统，是一种中小型热泵干燥装置的结构型式。对水循环式热泵干燥装置的开机过程、调控特性、能源效率进行了分析，相关结论可为推广应用提供参考。     

Constant and Intermittent Drying Characteristics of Olive Cake

The drying kinetics of olive cake, the solid by-product of the olive oil extraction process, has been experimentally investigated in a small-scale tray dryer using both constant and intermittent (on/off) heating schemes. The parameters investigated include inlet air temperature and intermittency of heat input. The drying kinetics was interpreted through two mathematical models, the Page equation and the Lewis equation. The Page equation was most appropriate in describing the drying behavior of olive cake. A diffusion model was used to describe the moisture transfer and the effective diffusion coefficient at each temperature was determined. The dependence of the effective diffusion coefficient on drying temperature can be adequately explained based on an Arrhenius-type relation. The effective diffusion coefficient varied between 7.6 × 10^?8 and 2.5 × 10^?7 m²/min with an activation energy of 38.55 kJ/mol. Comparison of time evolution of material moisture content due to intermittent and constant drying is also made.     

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.