Heat Pump Dryer Research Articles

Experimental study on the transcritical cycle performance of a closed-loop heat pump drying system based on R744/R290/R32 ternary refrigerant

Given the issue of high operating pressure in CO2 transcritical cycles, this paper proposes a novel non-combustible and low-GWP R744/R290/R32 ternary refrigerant used for heat pump drying systems. Six different mass ratios of the R744/R290/R32 are determined based on the limits posed by non-flammability and low GWP, and the transcritical cycle performance of these mixed refrigerants and pure CO2 applied to a closed-loop heat pump drying system (HPDS) at various drying air supply temperatures (DAST) is studied and compared experimentally. The analytic hierarchy process is used to assess the weights of the evaluation indices. Based on the experimental data, comprehensive scores for seven refrigerants are calculated, leading to the determination of the optimal mass ratio. The results show that at the same DAST, compared to pure CO2, the discharge pressure of the HPDS using the six mixed refrigerants decreases, and the COP increases. The optimal R744/R290/R32 mass ratio is 0.741/0.06/0.199. When the HPDS uses this, the compressor discharge pressure, heating capacity, moisture extraction rate (MER), and specific moisture extraction rate (SMER) decrease by an average of 14.8 %, 7.7 %, 17.6 %, and 0.06 %, respectively, while the COP increases by 4.9 % to 12.4 %. These research results can provide a reference for the development of new environmentally friendly refrigerants and their systems in the heat pump drying field.

Performance analysis of heat pump drying system combined with rotor dehumidification for low temperature drying of mango slices

In order to solve the problems of low drying efficiency and poor drying quality existing in the current closed heat pump drying system (CHPD), a closed carbon dioxide heat pump drying system (CHPRD) with the function of combining rotor dehumidification was designed. This study investigates the drying kinetics and quality characteristics of CHPRD in low-temperature drying processes using mango as the drying material through experimental studies and theoretical analysis. The experimental results showed that the drying rate of CHPRD increased by 29% and 17.8%, the moisture removal rate per unit energy (SMER) by 14.2% and 12.5%, and the coefficient of performance (COP) by 17.3% and 12.0%, respectively, compared with that of CHPD in the low-temperature drying process with loading amounts of 400 and 450 g, respectively. In terms of drying quality, the rehydration factor (Rf ) of CHPRD-dried mango slices increased by 6.0% and 5.0%, the ascorbic acid content increased by 15.0% and 14.4%, and the color difference (ΔE) was lower by 11.0% and 5.6%, respectively. These improvements indicate that CHPRD provides better drying kinetics and higher quality of dried material. Based on the experimental data, simulation fitting was conducted, revealing that the two-term model is the best mathematical model to simulate the drying characteristics of CHPRD for mango.

Analysis of the interaction between airflow and high-voltage electric fields on drying characteristics of carrots using heat pump-electrohydrodynamics combined drying

The study investigates the drying characteristics of carrots under different coupling forms of airflow and high-voltage electric fields (parallel flow, PF, and cross flow, CF) using heat pump-electrohydrodynamics (EHD) combined drying. The results show that, compared to single heat pump drying, the combined drying under PF mode reduces carrot drying time by 13.33 %–31.58 %, increases effective moisture diffusivity (Deff) by 17.80 %–32.32 %, increases specific moisture extraction rate (SMER) by 12.25 %–34.26 %. While the combined drying under CF mode reduces carrot drying time by 12.5 %–18.18 %, increases Deff by 7.27 %–13.14 %, increases SMER by 4.64 %–12.58 %. The improvements in parameters under the CF mode are weaker than those under the PF mode. Additionally, the β-carotene content under PF mode is consistently higher than under CF mode. Based on the experimental results, the Modified Page model was improved, yielding an updated model MR = aexp(-(kt)^n), with an R2 value of up to 0.9999–1, which provides theoretical guidance for optimizing the heat pump-EHD combined drying process.

Thermodynamic and economic analysis of a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators

This paper proposes a solar-assisted ejector-enhanced flash tank vapor injection heat pump cycle with dual evaporators (SEFVIC), designed for heat pump drying application. Compared to the standard flash tank vapor injection heat pump cycle (FVIC), the SEFVIC uses an additional high-temperature evaporator to further minimize irreversible losses during the heat transfer process. Moreover, it integrates solar energy to improve the drying quality and heating capacity of the cycle. Meanwhile, the introduction of an ejector improves the system's performance by reducing throttling loss. The advantages of SEFVIC over FVIC are evaluated using theoretical models based on the first and second laws of thermodynamics. Results show that SEFVIC reveals a 34.1 % improvement in the heating coefficient of performance and a 192.8 % enhancement in volumetric heating capacity compared to FVIC under typical operating condition. Notably, SEFVIC maintains superior performance even at lower evaporating temperatures. Moreover, exergy research indicates that 77.1 % of the total exergy destruction in SEFVIC is attributed to the solar collector. Economic analysis indicates that the integration of solar energy into the SEFVIC is economically beneficial. These findings highlight the prospective application of SEFVIC and guide combining the multi-temperature heat pump drying system and auxiliary solar source.

Technical Evaluation of a Stand-Alone Photovoltaic Heat Pump Dryer without Batteries

This paper presents the results of the technical validation of an innovative prototype for drying alfalfa bales. It is based on a 4.1 kW Heat Pump (HP) that uses an advanced technology (optimized for extracting the humidity from the air) and is directly powered by a 6.6 kWp PV generator without grid or batteries support. The main technical challenges of this work were managing solar irradiance fluctuations due to cloud-passing and achieving good drying efficiency. The prototype has been validated for two consecutive drying campaigns in La Rioja (Spain). There were no abrupt stops generated by cloud-passing. The PRPV, which evaluates the performance of the PV system only during the periods when the PV energy can be used by the HP unit, presented values of 0.82 and 0.85, comparable to a well-performing grid-connected PV system. Although the bales’ initial relative humidity (RH) ranged from 18 to 30%, all but one of them presented a final RH below 16%, which is the limit point to avoid fermentation. The drying times ranged from 1 to 5 h, and the specific energy consumption per liter of water extracted, from 0.7 to 1.46 kWh/L. These values are comparable to traditional diesel and grid-powered systems. It is worth noting that the agricultural drying market represented USD 1.7 billion in 2023.

Physicochemical properties and volatile compounds of Rosa roxburghii fruit using different drying methods

To enhance the drying quality of Rosa roxburghii fruit, this study examined the effects of four methods: Natural Drying (ND), Hot Air Drying (AD), Heat Pump Drying (PD), and Vacuum Freezing Drying (FD). Fresh R. roxburghii fruit was used to assess drying characteristics, color, microstructure, active ingredient content, antioxidant activity, and volatile substances. Drying times for ND, AD, PD, and FD were 28, 7, 6, and 21 h, respectively, with the rate order PD > AD > FD > ND. AD samples showed superior water-binding capacity, solubility, and highest levels of flavonoids and polyphenols, enhancing antioxidant activity. FD samples had the best color performance and retained more intact cell structures and higher Vitamin C and polysaccharide levels. Volatile compounds were analyzed, revealing 45, 60, 61, and 50 compounds for ND, AD, PD, and FD, respectively. AD proved to be the most efficient method, providing optimal drying time, color, active ingredient content, antioxidant activity, and volatile components. This finding offers valuable guidance for selecting a drying method that preserves the quality of dried R. roxburghii fruit and suits industrial production.

The Benefits of Incorporating Vegetables and Fruits Flours into Grain-Based Foods

A study of food losses (flours) from vegetables and fruits obtained from production of juices from purple carrot, purple yams and red apple was conducted. The vegetables and fruits are with very good quality and good price. They are dried in a heat pump dryer, at an initial temperature of 45 ° C and circulating air with an initial moisture content of 20% at constant (5.6 m/s) and variable speed, in a thin layer, with transversely oriented air flow relative to the product layer.

An integrated study of quality variations for Eucommia ulmoides Oliver staminate flower processed with different drying treatments by untargeted metabolomics coupled with chemometrics

Eucommia ulmoides Oliver staminate flower (EUF) has been increasingly served as a potential feedstock for nutraceuticals. An integrated approach of untargeted metabolomics based on gas chromatography hyphenated with mass spectrometry (GC-MS) and ultrahigh performance liquid chromatography hyphenated with high resolution mass spectrometry (UHPLC-HRMS) coupled with advanced chemometric method was applied to investigate the metabolic variations of EUF obtained from various drying treatments. The metabolomics analysis was conducted automatically in our newly developed data analysis platform AntDAS. Chemometric methods were further used to provide additional insight into the chemical differences of EUF. 34 volatile and semi-volatile as well as 89 nonvolatile compounds were identified and demonstrated significant differences among diverse drying treatments. Moreover, among which 27 volatile and semi-volatile compounds along with 35 nonvolatile compounds were verified qualitatively by standards. The results revealed that shade drying was more effective for the preservation of volatile and semi-volatile substances, including sugars and vitamins. Freeze drying was conducive for preserving nonvolatile compounds, such as iridoids, flavonoids, and organic acids. The content of alcohols, amino acids, and nucleosides was abundant in heat-pump drying, oven drying, and hot air drying, respectively. In addition, hot air, heat pump, and oven drying methods showed a moderate level of the majority of bioactive components and suitable for industrialization process. This work potentially highlights the possibility of using chemometrics in conjunction with untargeted GC-MS and UHPLC-HRMS analysis for quality assessment of EUF.

Numerical investigation on the effect of area ratio on an adjustable ejector for heat pump dryer based on dynamic mesh

Adjustable ejector is considered as a promising device to recover the expansion work and enhance the performances of refrigeration or heat pump system. To investigate the impacts of area ratio (AR) of mixing chamber throat to nozzle throat, a dynamic mesh based numerical model of an adjustable ejector for the heat pump dryer was established. The area of nozzle throat could be modified by adjusting the position of needle. The movement of needle was simulated by using the dynamic mesh method. The effects of the nozzle throat area and the mixing chamber throat area on the ejector performances were investigated. Further, the optimal values of dimensionless parameter AR under the different structure parameters of ejector were estimated. The numerical model was validated by the experimental results, and the k-ε standard turbulence model was proved to predict the ejector performances well. The numerical results indicated the optimal AR to obtain the maximum entrainment ratio of ejector ranged from 2.65 to 4.87 at the given working condition. Among the structure parameters of ejector, the length of constant-area mixing section has the greatest influence on the optimal AR. These numerical results provided the guidance for structure optimization of adjustable ejectors in heat pump dryer.

Optimal condition design of cannabis drying for vacuum heat pump dryer using computational fluid dynamic method interaction with heat transfer

Utilizing low temperatures and short drying cycles, the vacuum heat pump dryer (VHPD) is ideal for cannabis drying due to the plant's sensitivity to heat and light. To achieve uniform drying and maintain cannabis quality, it is essential to design the dryer with an efficient temperature distribution throughout the drying chamber. CFD models are developed in this study to simulate various scenarios within the VHPD system. The optimization of pressure and temperature is carried out using a systematic approach with the Latin Hypercube Sampling experimental design. This method generates twenty pairs of values for the pressure and temperature variables, confined to ranges of 20–40 kPa and 30–45 °C, respectively. To confirm the simulation results, an experimental study is performed using a VHPD designed from insights gained through a detailed CFD analysis. The conditions of 20.2 kPa at 31.6 °C are considered optimal due to their more uniform temperature distribution and lower static enthalpy relative to other conditions. The temperature in the upper chamber is maintained slightly lower, within the range of 30.5–31.5 °C. Furthermore, the airflow distribution demonstrates uniform movement within the central region of the chamber, with velocities ranging from 2.5 to 3.5 m/s. The simulated temperature results demonstrated a strong correlation with a temperature difference in the range of 0–0.05 °C, evidenced by an RMSE of 0.0191. The VHPD successfully dries 18 kg of fresh cannabis in 180 min, reaching the desired moisture content range of 10–14 % on a dry basis.

Advancement and Innovations in Drying of Biopharmaceuticals, Nutraceuticals, and Functional Foods

AbstractDrying is a crucial unit operation within the functional foods and biopharmaceutical industries, acting as a fundamental preservation technique and a mechanism to maintain these products' bioactive components and nutritional values. The heat-sensitive bioactive components, which carry critical quality attributes, necessitate a meticulous selection of drying methods and conditions backed by robust research. In this review, we investigate challenges associated with drying these heat-sensitive materials and examine the impact of various drying methods. Our thorough research extensively covers ten notable drying methods: heat pump drying, freeze-drying, spray drying, vacuum drying, fluidized bed drying, superheated steam drying, infrared drying, microwave drying, osmotic drying, vacuum drying, and supercritical fluid drying. Each method is tailored to address the requirements of specific functional foods and biopharmaceuticals and provides a comprehensive account of each technique's inherent advantages and potential limitations. Further, the review ventures into the exploration of combined hybrid drying techniques and smart drying technologies with industry 4.0 tools such as automation, AI, machine learning, IoT, and cyber-physical systems. These innovative methods are designed to enhance product performance and elevate the quality of the final product in the drying of functional foods and biopharmaceuticals. Through a thorough survey of the drying landscape, this review illuminates the intricacies of these operations and underscores their pivotal role in functional foods and biopharmaceutical production.

Effect of Drying Methods and Temperature on Physical Properties of Spine Gourd (Momordica dioica Roxb.)

The present investigation entitled “Effect of Drying Method and Temperature on Physical Properties of Spine Gourd (Momordica dioica Roxb.)” was conducted at the Post Harvest Technology Laboratory, Department of Horticulture, Rajasthan College of Agriculture and Department of Processing and Food Engineering, College of Technology and Engineering, MPUAT, Udaipur, Rajasthan during August to December, 2023. The study consisted of 4 different drying methods (sun drying, tray drying, heat pump drying and fluidized bed drying), pretreatment (water blanching) and different temperatures (50°C, 60°C and 70°C). The experimental design used for this study was completely randomized design with three replications. The various physical observations were also recorded after drying. Among different types of drying methods and temperatures, water blanched and dried at 60°C tray dryer was found best with maximum dehydration ratio (0.174) and recovery % (17.43).

Effects of ultrasonic pre-treatment on the porosity, shrinkage behaviors, and heat pump drying kinetics of scallop adductors considering shrinkage correction

Shrinkage has a negative impact on drying efficiency and quality. However, insufficient information about the effect of shrinkage on drying kinetics hampers the comprehension of the underlying mechanism. This study aims to investigate the effects of ultrasonic (US) pre-treatment and drying conditions on volume ratio (VR), apparent density, and porosity, as well as drying kinetics with and without shrinkage correction of scallop adductors during heat pump drying. The VR and apparent density decreased, while porosity increased with higher drying temperatures and longer drying times. US pre-treatment resulted in decreased apparent density, but enhanced VR and porosity, demonstrating the effective role of US in alleviating shrinkage and promoting pore formation during drying. Ignoring shrinkage phenomenon caused a weak underestimation in drying efficiency but a significant overestimation in the effective moisture diffusivity coefficient. Therefore, shrinkage effect must be considered to properly comprehend drying behaviors. The mechanism of shrinkage was closely related to the glass transition theory. The critical moisture ratio at which the samples underwent a glass transition was increased by 15 % with US pre-treatment in comparison to the control. US-pretreated samples could achieve a glassy state more quickly than the control under the same drying conditions, leading to reduced volumetric shrinkage.

Comparison of different drying methods for Wushan Codonopsis pilosula: Changes in color, microstructure, nutrients, bioactive compounds, and volatile

This study evaluated the effects of different drying methods, namely hot air drying (HAD), heat pump drying (HPD), microwave drying (MD), vacuum freeze drying (VFD), and sun drying (SD) on the quality of Wushan Codonopsis pilosula (WCP). Results showed that HAD and HPD were effective in retaining the alcohol -soluble leachate content, with the retention rates of 91.76 % and 91.32 %, respectively. The most attractive appearance was observed in VFD-WCP. There was no significant difference between the polysaccharide content of VFD-WCP and HPD-WCP, which were 202.62 mg/g and 201.71 mg/g, respectively. MD and SD were detrimental to the retention of color, physicochemical indicators, and atractylenolide III. The content of lobetyolin was increased significantly (P < 0.05) after drying, following this trend: HPD > HAD > VFD > MD > SD > Fresh. The aroma characteristics of WCP were altered by the drying process. However, the principal component analysis (PCA) showed that HAD and HPD produce similar aromatic characteristics with traditional SD. Overall, HPD shows promise as a replacement for SD, not only for increasing drying efficiency but also for improving the quality of WCP.

Performance evaluation and optimization of a novel heat pump clothes dryer with ESD-compressor

This study proposes a novel heat pump drying cycle, which uses an enhanced single-stage suction and dual-stage discharge compressor (HPDESD). The system enables the step heating of the cold air at the evaporator exit, reducing the irreversible losses in both compression and heat transfer process. The simulation results agree well with the experimental results. The energy consumption and drying time of the HPDESD system is 1.63kWh and 156mins, respectively, causing an energy-saving rate of 20.3% and saving 4mins compared to the basic heat pump drying cycle. In addition, the SMER and MER are sensitive to the compressor size and air cycle flowrate rather than clothing mass. When compressor size decreases of 20% and the air cycle flowrate increases 20%, the SMER rises by 16.2% and 5.2%, respectively, compared to the original system. Meanwhile, the SMER and MER of the optimized system are 3.64 kg/kWh and 2.21 kg/h, respectively, which are 11% and 7.3% higher than the original design. It is worth noting that the HPDESD system has higher SMER and MER than other types. In future, utilizing the extra energy and adopting the optimization control strategy of compressor can further improve the performance of the heat pump dryers.

Performance Enhancement of Heat Pump Dryer using Heat Recovery

The purpose of this research was to study on performance of a heat pump dryer using R32 refrigerant by recovering waste heat from an external condenser. Drying was carried out with drying temperatures of 45, 50 and 55 °C and water flow rates in the heat exchanger of 2, 3 and 4 L/min. Criteria for evaluating performance of heat pump dryer include: drying rate (DR), specific moisture extraction rate, specific energy consumption (SEC) and coefficient of performance of heat pump (COPh). The result shown that the performance of a heat pump dryer with heat recovery is higher than that of a traditional heat pump dryer. It was also found that increasing in drying temperature and water flow rate in heat exchanger resulted in an increase in the drying rate, power of the heat pump dryer and the specific moisture extraction rate. Whereas the specific energy consumption had decreased.

Influence of drying condition on the drying constants and activation energy for robusta coffee using heat pump drying

Abstract Heat pump drying (HPD) is a method of drying agricultural products which has great potential due to its high efficiency and energy savings. Coffee is a heat sensitive product, HPD was chosen as an alternative to existing drying methods for coffee processing. Understanding of the HPD process is important to know for optimizing the coffee drying process with HPD especially in drying characteristics. The purpose of this research is to determine the number for the drying rate constant and the energy of activation in coffee drying. The material used in this study was wet parchment Robusta coffee beans (Coffea canephora). The research was carried out experimentally for 5 h at 700 lpm air flow with varying drying air conditions by adjusting the drying temperature and specific humidity. The drying temperatures used were 60, 65, 70, 75, and 80 °C. The drying temperature is limited to 80 °C based on SNI 7467-2008. Variations in specific humidity are determined based on temperature settings in the refrigeration system of 10, 15, 20 °C and without a refrigeration system. The maximum drying rate constant was 10.69×10-3 s-1, achieved at a temperature of drying variation of 80 °C while a specific humidity of 6.16 g H2O/kg dry air, while the minimum activation energy value was 23.43 kJ/mol at the same specific humidity. The lowest drying rate constant value was 5.79×10-3 s-1, achieved at a temperature of drying variation of 60 °C and a specific humidity of 17.24 g H2O/kg dry air, while the greatest activation energy level was 26.18 kJ/mol at the same specific humidity.

Effect of Heat Pump Drying on Quality of Fruits and Vegetables: A Review

Effect of Heat Pump Drying on Quality of Fruits and Vegetables: A Review

Modelling and Simulation of Chemical Adsorption Heat Pump for Drying and Heating Applications

The performance of a chemical adsorption heat pump drying system is largely influenced by the selection of working fluid, operating conditions, and system configuration and design. In this study, tomato slices were dried at five different air temperatures (35 °C, 40 °C, 45 °C, 50 °C, and 55 °C) with a tomato slice flow rate of 400 kg/hr and an air flow rate of 200 kg/hr using a modelled and simulated chemical adsorption heat pump drying system. Four adsorbent-adsorbate pairs were used: magnesium chloride-methanol, calcium chloride-ethanol, calcium chloride-water, and aluminium trifluoride-dimethyl ether, at evaporating and condensing temperatures of 10 °C and 15 °C, respectively. The specific moisture extraction rate ranged from 0.116 kg/kWh to 0.149 kg/kWh. The coefficient of performance (COP) of the heat pump drying (HPD) system ranged from 0.960 to 0.987, and the overall exergy efficiency at the highest ambient air temperature (30 °C) ranged from 3.84% to 20.29%. Exergy analysis of the system's components revealed that the condenser and evaporator contributed significantly to exergy destruction, exhibiting low exergy efficiency.

Evaluation of the Performance of Modeled and Simulated Hybrid Heat Pump Drying Systems for Industrial Applications

System configuration and design is one of the major factors that influence the performance of heat pump drying system. This study aimed to evaluate the performance and drying efficiency of two different configuration of hybrid heat pump drying system for heating process and drying of tomatoes slices at condensing temperature 15 ℃ and evaporating temperature 10 ℃ using calcium chloride-ethanol as adsorbate-adsorbent pair. The energetic coefficient of performance, and exergetic coefficient of performance and overall exergy efficiency of the system are 2.03, 15.6% and 17% respectively for simple adsorption-compression hybrid heat pump drying system while its specific moisture extraction rate is 0.234 kg/kWhr and specific energy consumption is 4.27 kg/kWhr. The energetic coefficient of performance, and exergetic coefficient of performance and overall exergy efficiency of the system are 2.18, 22.6% and 16% respectively for cascaded adsorption-compression hybrid heat pump drying system and its specific moisture extraction rate is 0.457 kg/kWhr and specific energy consumption is 3.17 kg/kWhr.