Heat Pump Drying System Research Articles

A novel absorption-based enclosed heat pump dryer with combining liquid desiccant dehumidification and mechanical vapor recompression: Case study and performance evaluation

Drying process is ubiquitous and represents one of the most energy-intensive processes in varieties of agricultural, residential and industrial applications. As an effective heat recovery technology, the heat pump has recently been applied in various drying systems to provide heating and cooling, exhibiting good energy-saving potentials. However, existing condensing-based heat pump drying (HPD) systems require to significantly cool humid air for dehumidification, deteriorating energy efficiency. In this paper, an absorption-based enclosed HPD system is proposed to achieve total waste heat recovery by combining liquid desiccant dehumidification and mechanical vapor recompression. This study focuses on a comprehensive thermodynamic modeling and performance evaluation of proposed absorption-based HPD system. Analysis results indicate that the proposed system can deliver an energy utilization ratio of 4.54 and specific moisture extraction rate of 4.25 kg/kWh under design operation conditions, which is 18.7% and 18.5% higher than the traditional HPD system, respectively. Additionally, an economic analysis is adopted to appraise the economic performance, and obtained results show that the annual investment and maintenance cost rates of proposed system increases by 5.05%, but the annual operation cost rate is reduced by 19.94% compared to the reference system. The payback period is around 18.5monthes with a project life cycle of 20 years, which indicates that the project is profitable.

Study on the effect of circulating air volume on the performance of closed loop heat pump drying system

• 1. A closed heat pump drying system for high temperature industrial drying is proposed. • 2. The effect of circulating air volume on the system drying performance is studied. • 3. The maximum drying temperature of 71.1℃ can be obtained for drying metal parts. • 4. The system has high energy efficiency and excellent dehumidification performance. Considering the fact that high energy consumption and low drying temperature of current drying system was not appropriate for the high temperature industrial drying, a closed loop heat pump drying system with high drying temperature was designed, established and explored. As a key operation parameter, the circulating air volume was closely related to the drying temperature and could affect the system performance. The effect of circulating air volume on the circulating air temperature, coefficient of performance (COP), moisture extraction rate (MER) and specific moisture extraction rate (SMER) were all experimentally investigated in this study. The results showed that with the increase of circulating air volume, the circulating air temperature was gradually decreased, while the system COP was oppositely increased. And both the system MER and SMER firstly increased and then decreased with increasing air volume. The drying temperature could exceed 70 °C when the circulating air volume was below 539.8 m 3 /h, and reach its maximum value 71.11 °C when the circulating air volume was 388.5 m 3 /h, meeting the high drying temperature requirement. At the same time, both the system MER and SMER could reach their maximums (i.e., 3.27 kg/h and 1.40 kg/(kW·h)), showing the excellent dehumidification performance of this system. Besides, when the circulating air volume was 736.1 m 3 /h, the system COP could reach its maximum 6.56, indicating high energy efficiency. These results can provide guidance for the popularization and application of heat pump drying technology in the industrial drying field.

Simulation investigation on a novel open-loop air cycle heat pump drying system

Different from conventional drying technology, the novel technology of an air cycle heat pump drying (ACHPD) system, using air as its working medium, is proposed to solve the problems of low energy efficiency, HCFC refrigerant use, and so on. A mathematical simulation model is constructed to evaluate system performance under various working conditions, and its verification via experimental results from a constructed test bench produced errors of temperature within ±1.6%, of drying rate within ±0.01%, and of system power within ±7%. Compared with a conventional electric heater drying (EHD) system, the energy-saving rate of the ACHPD system reaches about 15–27% when the mass evaporation rate increases from 0.75 kg h−1 to 3.45 kg h−1. Under set working conditions, an increase of inlet air flow rate (150–400 kg h−1) improved the dehumidification energy efficiency ratio (MER) by about 1%. However, the increase of inlet air temperature (10–40 °C) and relative humidity (30–80%) reduced MER by about 7% and 21% respectively. The efficiency improvements of heat exchanger (0.3–0.8) can improve the energy efficiency of system by 20%; however, the efficiency improvements of expander (0.55–0.8) and compressor (0.4–0.9) can deteriorate the energy efficiency of system by 17% and 36% respectively. These results would provide new technical references for the application of heat pump technology in drying field from a different perspective.

Sensitivity analysis of structural parameters of a low-temperature heat pump sludge drying system

Optimizations of the heat exchanger of the heat pump drying system were usually carried out individually, linkage effect of the whole system in practical operating was usually not considered. In addition, for a heat pump sludge drying system, optimizations mainly concentrated on the operation parameters, few theoretical analyses focused on the effect of structural parameters to the system performance. In the present study, based on the laws of energy conservation and mass conservation, a steady-state model for the heat pump sludge drying system, which interconnects the refrigerant circulation and the moist air circulation, was established. Three variables which are coefficient of performance (COP), dehumidification rate (DR), and specific moisture extraction rate (SMER) are simultaneously introduced to evaluate the system’s operating and drying performances. Effects of the number of horizontal tube rows, the number of longitudinal tube rows, and the fin spacing to the above three variables are analyzed. The results show that the COP decreases with increasing the number of horizontal tube rows while increases with increasing the number of longitudinal tube rows and fin spacing. The DR increases with increasing the number of horizontal tube rows and fin spacing and decreases with increasing the number of longitudinal tube rows. The SMER has a similar variation trend to that of the DR due to their directly proportional relationship. Finally, based on the sensitivity analysis, the heat pump sludge drying system achieves better operating and drying performances when the fin spacing is 2.85 mm, the numbers of horizontal tube rows of the evaporator and condenser are 18 and 14, and the numbers of longitudinal tube rows are respectively 12 and 9.

Design of new heat pump dryer system: A case study in drying characteristics of kelp knots

The heat pump dryer system (HPDS) attracts much attention for its high energy saving and excellent environmental performance. In this paper, a new set of double evaporator and normally closed loop HPDS (DENC-HPDS) with dehumidifying capacity of 3500 g/h and condensation temperature of 60 °C was designed. Drying experiments of kelp knots with DENC-HPDS under the conditions of drying temperature of 35, 40, 45 and 50 °C, wind speed of 0.5, 1.0, 1.5 and 2.0 m/s, loading density of 8.29, 12.29, 22.29 and 32.29 kg/m 3 , tray spacing of 50, 100, 150 and 200 mm were carried out, and the drying characteristics and mathematical drying models of the DENC-HPDS were investigated and analyzed. The designed DENC-HPDS solves the problem of heat and moisture imbalance and pollution of traditional HPDS effectively with short start-up time compared with traditional HPDS. The drying experiments shows that drying time generally decreases with the increase of drying temperature, wind speed and tray spacing. The optimal drying conditions are drying temperature of 45 °C, wind speed of 1.5 m/s, loading density of 22.29 kg/m 3 and tray spacing of 150 mm with the highest mean specific moisture extraction rate of 1.63kg/kWh. In ten drying models, the Verma et al. model has the best fitness to the experimental data and is more suitable to describe the heat pump drying process of kelp knots in this paper. Under the experimental conditions, the calculated effective moisture diffusion coefficients are in the range of 3.4687 × 10 −9 –11.3189 × 10 −9 m 2 /s and the average drying activation energy is 21.02 kJ/mol.

MATHEMATICAL MODELLING AND OPTIMIZATION OF MELON SLICE DRYING WITH RESPONSE SURFACE METHODOLOGY IN A HEAT PUMP DRYING SYSTEM

The objective of this study was to optimize the process conditions (in terms of air temperature, air velocity and thickness of the slices) using response surface methodology (RSM) to achieve minimum specific energy consumption and maximum moisture diffusivity during drying of melon slices with a closed loop heat pump drying (HPD) system. An optimum drying temperature of 45 °C, air velocity of 1 m/s and slice thickness of 5.04 mm were recommended with following predicted responses close to experimental values: drying time 216.58 min, total energy consumption 2.94 kWh, coefficient of performance heat pump (COPhp) 3.08, coefficient of performance system (COPws) 2.75, specific moisture extraction rate (SMER) 0.22 kg/kWh, drying rate 2.53, L* value 82.53, a* value -1.83 and b* value 25.82. The most suitable models to represent the drying behavior of optimum melon slices was chosen Wang & Sing. Effective moisture diffusivities (Deff) of the melon slices were ranging from 7.075E-10 - 1.843E-07 m2s-1 and increasing drying air temperature, drying air velocity and slice thickness led to an increment of Deff.

Bioaccessibility of total phenolics and antioxidant activity of melon slices dried in a heat pump drying system

Bioaccessibility of total phenolics and antioxidant activity of melon slices dried in a heat pump drying system

Study on the effects of auxiliary condenser operation parameters on the performance of closed loop heat pump drying system

Considering the problems of high energy consumption, complex system structure and low drying temperature in current industrial drying system of metal parts, a closed loop heat pump drying system with auxiliary condenser was designed and established. The effects of auxiliary condenser operation parameters on the circulating air temperature, coefficient of performance (COP), moisture extraction rate (MER) and specific moisture extraction rate (SMER) were experimentally investigated, where the compressor rotating speed, circulating air volume, cooling water inlet temperature and flow rate respectively were 3900 r•min−1, 539.8 m3•h−1, 5∼30 °C and 30.6∼50.1 kg•h−1. The results showed that the drying temperature gradually increased with increasing cooling water inlet temperature and decreasing cooling water flow rate. When the cooling water inlet temperature and flow rate respectively were 20 °C and 34.3 kg•h−1, the drying temperature was 70.02 °C, meeting the high temperature requirement of industrial drying. Besides, the cooling water flow rate had little influence on the system COP, while the system COP increased at first and then decreased with increasing cooling water inlet temperature, reaching its maximum value 5.64 when the cooling water inlet temperature was 20 °C. Moreover, when the cooling water inlet temperatures respectively were 20 °C and 5 °C with constant cooling water flow rate 34.3 kg•h−1, the system MER and SMER could reach their maximum values of 3.8 kg•h−1 and 1.68 kg•kW−1•h−1, showing excellent dehumidification capacity. These results can provide guidance for the popularization and application of heat pump drying technology in the industrial drying.

Economic Operation Optimization of Solar Assisted Heat Pump Drying System Considering Heat Storage Characteristics of Drying Chamber

Economic Operation Optimization of Solar Assisted Heat Pump Drying System Considering Heat Storage Characteristics of Drying Chamber

Economic Operation Optimization of Solar Assisted Heat Pump Drying System Considering Heat Storage Characteristics of Drying Chamber

Economic Operation Optimization of Solar Assisted Heat Pump Drying System Considering Heat Storage Characteristics of Drying Chamber

Drying process optimization of garlic slices in closed‐loop heat pump drying system by Box‐Behnken design

Drying process optimization of garlic slices in closed‐loop heat pump drying system by Box‐Behnken design

Experimental investigation of a part air dehumidification cascade-like heat pump drying system

• A part air dehumidification heat pump drying system was proposed and investigated. • The thermal characteristics of the PADHPD were tested. • The improvement of the new HPD system was verified. • The maximum exergy loss occurred in the evaporator for each HPU. • Decreasing the airflow resistance is vital to improve performance of PADHPD system. In the cascade-like heat pump drying system, there are contradictions between the low air flow rate for dehumidification and high air flow rate for anti-overpressure. In order to resolve those contradictions, a new PADHPD (part air dehumidification heat pump drying) system was proposed in this work. A PADHPD system used in the industrial production was designed and manufactured. The thermodynamic performance of this was tested and simulated. The air temperature and humidity in the drying system and the thermal parameters of the HPUs (heat pump units) were analyzed. The hawthorn cake was taken as the drying material. According to the estimation from the experimental results, the average COP (coefficient of performance) of the heat pump system is 3.58 with the average SMER (specific moisture extraction rate) of 2.282 kg kW −1 h −1 when the average temperature and RH (relative humidity) of supplied air is 63.1°C and 20.2 %, respectively. EER (energy efficiency ratio) of the system is 2.68 but SMER is 1.792 kg kW −1 h −1 after the electric energy consumed by circulating fans being considered. Exergy analyses shows that it is necessary to reduce the irreversible loss in the evaporator because it represents the largest exergy loss for each HPU.

Simulation Study on Active Air Flow Distribution Characteristics of Closed Heat Pump Drying System with Waste Heat Recovery

A tridimensional turbulent flow model is established for a closed heat pump drying system with waste heat recovery to improve the drying air flow characteristics and reduce the energy consumption of air circulation. The active flow distribution mode is introduced to guide air flowing in the system’s drying cabinet, top air duct, mixing zone, and heat pump. It is found that the wind velocity in the cabinet’s supply channel is greater than that in the return channel, the velocity distribution in the top duct is uneven, and the outlet velocity of the bypass fan is high and a vortex forms. A partition panel added in the top duct and modulating fans in the drying cabinet are proposed to solve these problems. The simulation results show that removing the circulating fan, changing the volume flow of the bypass fan, adopting the alternative operation mode of the bypass fans, reducing the air volume flow of the supply fan, and reducing the total pressure of the fans can improve the dry air velocity uniformity in the drying cabinet and reduce the energy consumption of the fans. This paper analyzes the closed-loop drying air flow characteristics of the active flow distribution system in the heat pump drying system, which lays a foundation for the wind velocity, temperature, and humidity regulating of heat pump drying system with waste heat recovery.

Study on heating performance of solar-assisted heat pump drying system under large temperature difference

In this study, a heat pump drying (HPD) system platform complemented by solar hot water was designed and established. The objective was to address the declines in the heating performance of air source HPD at high altitudes (more than 3000 m), cold conditions (annual average temperature at 5 °C), and with large temperature differences between day and night (more than 20 °C), i.e. under harsh conditions. A theoretical model was derived to determine the system parameters and to support the experimental analysis. Under no-load conditions, four cases with ambient temperature were tested and compared to explore the relationships between the system parameters and performance, as well as the effectiveness of the supplementary solar hot water. The heating performances under HPD and solar-assisted HPD (SHPD) modes were compared and analysed under load conditions (930 kg Mu Xiang). The results indicate that a decrease in the ambient temperature and increase in the temperature in the drying room causes attenuations of the compressor mass flow rate, evaporator cooling power, condenser heating power, and system coefficient of performance (COP). In addition, solar water heating can significantly improve the heating performance of the HPD system. Under no-load conditions, the average heating power of the HPD-mode system is 19.27 kW, and the average COP is 2.76. The average heating power of the SHPD mode system is 23.34 kW, and the average COP of the system reaches 3.24, an increase of 17.4%. Under load conditions, the average heating capacity power of the HPD mode is 9.64 kW; the COP is only 1.34, and fluctuates significantly with the decrease in ambient temperature. The average heating capacity power of the SHPD mode is 17.62 kW and the COP is 2.42, i.e. 1.8 times of that of the HPD mode. By raising the drying room temperature to the same set temperature, the SHPD mode time is shortened by 70%.

Effect of compressor speeds on performance of a closed loop heat pump drying system

• A novel heat pump drying prototype with relatively high air temperature is proposed. • The effects of compressor speed on the performance of the system is studied. • The maximum drying temperature that the system can achieve is greater than 70 °C. • The drying system has high energy efficiency and excellent drying effect. In view of the current situation about environmental pollution and high energy consumption in industrial metal parts drying methods, a closed loop heat pump drying (CLHPD) system with a relatively high circulating air temperature was proposed and established. The compressor speed is a vital parameter that affects the performance of the CLHPD system. The effect of compressor speeds on circulating air temperature, coefficient of performance (COP), moisture extraction rate ( MER · ) and specific moisture extraction rate ( SMER · ) of the system were experimentally studied in this work. The results showed that, with the raise of compressor speed within 3000 to 4200 r/min, the drying air temperature and the MER · of the system increased, the COP and SMER · of the system decreased. The drying air temperature reached a maximum of 75 °C and the MER · obtained a maximum of 3.90 kg/h at the compressor speed of 4200 r/min. The COP reached a maximum of 6.4 and the SMER · obtained a maximum of 1.62 kg/(kW·h) at the compressor speed of 3000 r/min. In an actual industrial production, the compressor speed should be adjusted according to the specific demand to balance the relationship between MER · and SMER · . These can provide references for further research and application of heat pump drying technology in the metal parts drying and even other industrial drying fields.

Numerical Analysis and Optimization of Solar-Assited Heat Pump Drying System with Waste Heat Recovery Based on TRNSYS

In this paper, a new solar-assisted heat pump drying system with waste heat recovery and double water tanks (SCAHP) was established and the system optimized by TRNSYS and variable air volume experiment. The annual cumulative efficiency of the SCAHP (COPac), buffer tank heating efficiency (ηBT) and hot water storage tank heating efficiency (ηST) are all considered as optimization objectives, and this paper discusses the relationship between the three optimization objectives and studies the influence of hot air volume (qDR), area of solar collector (ASC), inclination angle of solar collector (ISC) and volume of heat storage water tank (VST) on system efficiency in the drying process. In order to explore the general rule of system optimization, numerical analysis and optimization were carried out in five typical cities in five climatic regions of China. The results show that the mixed variable air volume mode can increase the drying rate with less energy loss, and shorten the drying period by 28.6–33.3%. When the system surface body ratio (SBR) in Nanjing is between 3.1 and 4.1, the COPac, ηBT, and ηST can reach the maximum value simultaneously. It is estimated that the cost can be recovered in 5 years when the system configuration parameters are optimized.

Theoretical analysis of open - circuit ground source heat pump drying system

In order to apply the ground source heat pump in the drying industry, a set of open-circuit ground source heat pump drying system was designed for theoretical analysis and compared with the closed air source heat pump drying system, The results are shown below: When the drying temperature (air temperature required for drying) is 40°C-60°C, the system performance coefficient( COP) of the drying system is 3.2-5.2. Dehumidification per unit energy consumption( SMER) is affected by the external ambient temperature. The highest and lowest SMER corresponding to the ambient temperature of 20°C are 4.4 and 1.4, and the highest and lowest SMER corresponding to the ambient temperature of -10°C are 2.6 and 1.1. When the ambient temperature is high and the drying temperature of drying materials is low (less than 50°C or so), the open-circuit ground source heat pump drying system has a slight advantage over the closed air source heat pump drying system, and SMER is higher.

Exergy analysis of a thermostatic heat pump drying system with adjustable bypass air ratios

Drying is the best method to preserve products, but it will consume large amounts of energy. In this paper, a new heat pump drying system is designed, the wet cotton cloth is selected as drying material and the exergy analyses for the main components and the whole system are carried out. The results show that the maximum exergy loss always occurs in the compressor. Both the drying air temperature (DAT) and bypass air ratio (BAR) affect the exergy efficiency of the whole system and components. For the whole system and main components, the optimal BAR at 35°C, 40°C, 45°C are always 40%, 0% and 60% respectively. Under a higher DAT, both the dehumidification ability of the evaporator and its exergy efficiency are reduced by the excess air mass flow rate. Through the exergy analysis, some improvement measures for the exergy efficiency are put forward.

Exergy analysis of a thermostatic heat pump drying system with adjustable bypass air ratios

Drying is the best method to preserve products, but it will consume large amounts of energy. In this paper, a new heat pump drying system is designed, the wet cotton cloth is selected as drying material and the exergy analyses for the main components and the whole system are carried out. The results show that the maximum exergy loss always occurs in the compressor. Both the drying air temperature (DAT) and bypass air ratio (BAR) affect the exergy efficiency of the whole system and components. For the whole system and main components, the optimal BAR at 35°C, 40°C, 45°C are always 40%, 0% and 60% respectively. Under a higher DAT, both the dehumidification ability of the evaporator and its exergy efficiency are reduced by the excess air mass flow rate. Through the exergy analysis, some improvement measures for the exergy efficiency are put forward.

Drying Performance of Coffee in a Batch-Type Heat Pump Dryer

<b>Highlights</b> <list list-type=bullet><list-item> The SMER<i>_hp</i> and SMER<i>_ws</i> values for this heat pump drying system were 0.960 and 0.719 kg kWh^-1, respectively. </list-item><list-item> A non-heating period in the heat pump dryer working cycle may affect the asynchronous negative drying rates of coffee in different layers in the drying chamber. </list-item><list-item> Results indicated that ambient air conditions might affect drying behavior in an open-loop batch-type heat pump dryer. </list-item></list> <b>Abstract</b>. Coffee is an important commercial crop in the world due to its popular flavor and aroma. The critical step in coffee processing is drying, which determines the coffee quality and market value. The most popular coffee drying techniques at present, sun drying and mechanical drying, have inherent disadvantages. In contrast, heat pump drying has been observed to offer higher energy efficiency and produce higher-quality output. Therefore, the objective of this research was to investigate the drying characteristics of coffee in a batch-type heat pump dryer. A total of 370 kg (0.72 m3) of fresh coffee at 70.01% ±1.26% (w.b.) moisture content was dried in an open-loop batch-type heat pump dryer. Samples were collected from six different horizontal layers in the drying chamber and two vertical sampling points. The average moisture content of three replicates was recorded. Wet basis and dry basis moisture content, drying rate, moisture ratio, color, temperature, and relative humidity of the ambient air and outflow air were measured and recorded. The SMERhp and SMERws values for this heat pump drying system were 0.960 and 0.719 kg kWh-1, respectively. The ΔE value for coffee at 10.11% ±0.42% (w.b.) moisture content was 31.65. These results indicate that the ambient air conditions may affect drying behavior in an open-loop batch-type heat pump dryer. Further studies are needed to identify the impact on aroma and flavor when processing coffee in a heat pump drying system to rationalize the advantages of using heat pump drying for coffee.