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.     

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。     

Exergoeconomic Analysis of a Heat Pump Tumbler Dryer

In this study, exergy and exergoeconomic analyses of a heat pump tumbler dryer are carried out by using actual thermodynamic and cost data. The wet cotton fabric is used as the test drying material. The results show that the specific moisture extraction rate (SMER) and evaporation rate of dryer are equal to 1.08 kg/kWh and 0.018 kg/s, respectively. Also, the respective exergetic efficiencies of the heat pump and overall system are equal to 0.07 and 0.11. A parametric study is then conducted in order to investigate the system performance and costs of the components, depending on the operating temperature and mass flow rate of air.     

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.     

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.     

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.     

Efficient Energy Recovery with Wood Drying Heat Pumps

This article presents a 13-m³ wood dryer coupled with a 5.6-kW (compressor power input) heat pump. Drying tests with hardwood species such as yellow birch and hard maple were completed in order to determine the system's energy performance. Supplementary heating to compensate for the dryer heat losses was supplied using electrical coils or steam exchangers. The heat pump running profiles and dehumidification performance in terms of volumes removed and water extraction rates, coefficients of performance, and specific moisture extraction rates were determined for two all-electrical and two hybrid drying tests. The hardwood drying curves, share of the final moisture content, and final quality of the dried wood stacks, as well as total drying energy consumption and costs, were determined for each drying run. Finally, the total energy consumption of the drying cycles using a heat pump was compared with that of a conventional drying cycle using natural gas as a single energy source.     

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％。     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract

An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.     

Experimental research on intermittent heat pump drying with constant and time-variant intermittency ratio

The aim of this article was to estimate the effectiveness of intermittent drying with constant and time-variant intermittency ratio, compared with continuous drying. Intermittent drying was accomplished by periodically changing the heat input and air velocity. To achieve accurate control of drying temperature, a new parallel conversion controller was used. Based on the well-controlled heat pump drying system, the drying kinetics of green soybean were analyzed using seven intermittent drying profiles. The increase of intermittency ratio caused decrease of effective drying time and increase of specific moisture extraction rate (SMER). SMERs of intermittent drying were 1.21–4.94 and 0.93–5.40 times more than the continuous drying for different initial moisture content (23 and 28%), respectively. Changes of drying rate (DR) and SMER became relatively moderate when intermittent drying with time-variant intermittency ratio was used. A comprehensive evaluation parameter, based on DR and SMER, was proposed and calculated. Experimental results indicated that intermittent drying (α?=?5/6) and intermittency drying with time-variant ratio gave the best two comprehensive performances.     

Condenser Coil Optimization and Component Matching of Heat Pump Dryer

The heat pump dryer is an energy-efficient piece of drying equipment, but due to its complicated system of the two interactive working fluids (refrigerant and drying air), the optimum design remains the question. A heat exchanger is the major component influencing the heat pump dryer (HPD) performance. This article reports a study to optimize the condenser coil of the HPD and the component matching in order to obtain optimum performance. The study was carried out by a mathematical model for system simulation and followed by experimental verification. Five HPD configurations were studied, including different designs in air and refrigerant flow paths. It was found that the closed-loop HPD with air bypassing over the evaporator is the most appropriate configuration. The proper coil design is 2-row, 2-circuit configuration with optimum refrigerant mass flow rate of 16-20 g/s/circuit. The optimum air flow rate was found to be in the range of 0.6-0.8 kg/s. The corresponding number of coil modules is 8.     

Condenser Coil Optimization and Component Matching of Heat Pump Dryer

The heat pump dryer is an energy-efficient piece of drying equipment, but due to its complicated system of the two interactive working fluids (refrigerant and drying air), the optimum design remains the question. A heat exchanger is the major component influencing the heat pump dryer (HPD) performance. This article reports a study to optimize the condenser coil of the HPD and the component matching in order to obtain optimum performance. The study was carried out by a mathematical model for system simulation and followed by experimental verification. Five HPD configurations were studied, including different designs in air and refrigerant flow paths. It was found that the closed-loop HPD with air bypassing over the evaporator is the most appropriate configuration. The proper coil design is 2-row, 2-circuit configuration with optimum refrigerant mass flow rate of 16–20 g/s/circuit. The optimum air flow rate was found to be in the range of 0.6–0.8 kg/s. The corresponding number of coil modules is 8.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

复叠式热泵高温段实验研究

热泵高温化可以有效拓展其应用范围,复叠式热泵是实现高温热泵的有效方法之一。以R123为高温工质,以喷水模拟干燥脱水过程,实验研究了用于干燥系统的复叠式热泵的高温段。结果表明:热泵的冷凝温度可达到95℃;蒸发温度60℃冷凝温度90℃时,热泵COP达到最大为5.78;在整个蒸发、冷凝温度范围内COP维持在2.2以上,SMER达到4~5 kg/(kW·h),单位干燥介质除水量为0.006~0.012 kg水/kg干空气。     

Drying Characteristics of Thick Lumber in a Laboratory Radio-Frequeocy/Vacuum Dryer

A series of forty two drying runs of two wood species and two cross-sectional dimensions of wood squares were carried out in a laboratory radio-frequency/vacuum (RF/V) dryer. The experimental temperature, pressure and moisture content levels as functions of space and time are presented. The results showed that western red cedar and western hemlock can be dried to a final moisture content of 15% in about 24 and 32 hours, respectively. The quality of the dried specimens was exceptional- Detailed analysis revealed the absence of internal drying stresses, internal and external checking and surface discoloration. Furthermore, evaluation of moisture content distribution in the longitudinal and transverse direction showed minimum variation compared to conventional kiln drying. The experiments also revealed that RF/V drying rates are directly affected by the level of the electrode plate voltage. Drying rates decreased with time when the voltage remained constant throughout the drying cycle thus resulting in long drying times. That was because of the changing dielectric properties of wood due to moisture content reduction during drying. Raising the voltages with time though, resulted in constant drying rates and shorter diying times.     

Mathematical Modeling the Drying of Poplar Wood Particles in a Closed-Loop Triple Pass Rotary Dryer

Closed-loop drying systems are an attractive alternative to conventional drying systems because they provide a wide range of potential advantages. Consequently, type of drying process is attracting increased interest. Rotary drying of wood particles can be assumed as an incorporated process involving fluid–solid interactions and simultaneous heat and mass transfer within and between the particles. Understanding these mechanisms during rotary drying processes may result in determination of the optimum drying parameters and improved dryer design. In this study, due to the complexity and nonlinearity of the momentum, heat, and mass transfer equations, a computerized mathematical model of a closed-loop triple-pass concurrent rotary dryer was developed to simulate the drying behavior of poplar wood particles within the dryer drums. Wood particle moisture content and temperature, drying air temperature, and drying air humidity ratio along the drums lengths can be simulated using this model. The model presented in this work has been shown to successfully predict the steady-state behavior of a concurrent rotary dryer and can be used to analyze the effects of various drying process parameters on the performance of the closed-loop triple-pass rotary dryer to determine the optimum drying parameters. The model was also used to simulate the performance of industrial closed-loop rotary dryers under various operating conditions.     

Design of Solar–Dehumidification Wood Drying Kiln Characterized by High Capacity and Temperature

ABSTRACT

Solar–;dehumidification wood drying kilns attract more and more interest due to their energy saving characteristics. However. such dryers are only available for small effective drying volume capacities. less than 30 m³, and for low drying temperature under 65 °C, for Chinese domestic products. In fact, wood drying kilns over 60 m³and high drying temperatures upto 95 °C or even higher are commonly desired from economic and process technology points o f view. For instance, if the drying temperature is below 60 °C. a long drying period will result in high operational costs and may also cause mould as well colour changes on the lumber surface. It is also known that the anti- decay ability of the seasoned lumber is lhus reduced. As such. the design of the solar–dehumidification wood drying kiln with an effective drying volume capacity of 60 m³ and a drying temperature upto 95 ⁰C was made. Two compression processes for dehumidification and heat pump systems were adopted. The refrigerant of R1⁽²⁾. which has low RODP and RGE, was utilized as the actuating medium. The values of COP. PER and SMER were 4.42. 1.19 and 3.08 kg ( H20) /( kWh) for the dehumidification system. The valucs of COP and PER were 3.05 and 0.824 for the heat pump (heat supply) system. The averaged total COP and PER over the both were 3.74 and 1.01 respectively.     

A rationally based model applicable for heat pump tumble dryer

The present study proposes a rationally based heat pump clothes dryer model which is capable of handling transient behaviors of both air-side and refrigerant side. The model can take into account the geometrical variation of the heat exchangers. Yet the proposed model is free from extra constraints that were normally imposed by previous studies. The simulation shows that there exist three stages in a typical drying process, including a preheat stage, followed by a constant evaporation stage, and finally a falling drying rate period. The calculation indicates that the variation of the clothes temperature and the system suction/discharge pressure show an appreciable rise in the first stage, remain nearly unchanged at the second stage, and increase again at the final stage of drying process. Also, the corresponding maximum specific moisture extraction rate (SMER) peaks at the end of the second stage while the corresponding coefficient of performance (COP) drops continuously. The simulation also indicates that increasing the air volumetric flowrate yields a lower discharge/suction pressure, a higher COP value, and a shorter drying time. The results also suggested that a good strategy to maximize COP is initiated by a low volumetric rate during the preheat stage, increasing it during the second stage and lowering it to an intermediate flowrate at the end of the drying period. Increasing heat exchanger size, either by increasing heat exchanger width or the number of tube row, results in a lower suction/discharge pressure and a shorter drying time. Although both approaches adopt larger surface area to promote overall performance, it is found that increasing the width of heat exchanger yields more effective results.     

Drying Characteristics of Thick Lumber in a Laboratory Radio-Frequeocy/Vacuum Dryer

ABSTRACT

A series of forty two drying runs of two wood species and two cross-sectional dimensions of wood squares were carried out in a laboratory radio-frequency/vacuum (RF/V) dryer. The experimental temperature, pressure and moisture content levels as functions of space and time are presented. The results showed that western red cedar and western hemlock can be dried to a final moisture content of 15% in about 24 and 32 hours, respectively. The quality of the dried specimens was exceptional- Detailed analysis revealed the absence of internal drying stresses, internal and external checking and surface discoloration. Furthermore, evaluation of moisture content distribution in the longitudinal and transverse direction showed minimum variation compared to conventional kiln drying. The experiments also revealed that RF/V drying rates are directly affected by the level of the electrode plate voltage. Drying rates decreased with time when the voltage remained constant throughout the drying cycle thus resulting in long drying times. That was because of the changing dielectric properties of wood due to moisture content reduction during drying. Raising the voltages with time though, resulted in constant drying rates and shorter diying times.