Dehumidification Performance Research Articles

Study on heat and mass transfer characteristics of internally-cooled hollow fiber membrane-based liquid desiccant dehumidifiers

Membrane-based liquid desiccant dehumidifiers avoid the desiccant carryover, which is better than the packed bed. In the process of liquid dehumidification, the dehumidification performance is reduced due to heat release, and internally-cooled liquid dehumidification is a method to improve the dehumidification performance. This study develops an internally-cooled hollow fiber membrane-based liquid desiccant dehumidifier (IHFMLDD). IHFMLDD is composed of two tubes. The inner tube is not permeable to moisture but only permeable to heat, while the outer tube is a hydrophobic semi-permeable membrane, which can transmit moisture and heat simultaneously. The advantage of this technology is that the liquid desiccant does not directly contact the air in the process, preventing the problem of droplet carryover in the air stream. In addition, cold water reduces the temperature of the solution and improves the dehumidification performance. This paper studies the heat and mass transfer characteristics of IHFMLDD, which uses a staggered triangle arrangement and takes LiCl solution as a cycling fluid. Heat and mass transfer capacities are adopted as performance indices. The effects of the inlet parameters, including air and solution flow rates, air inlet temperature and humidity ratio, solution inlet temperature and concentration, water flow rates, on the indices are investigated. Besides, a coupled heat and mass transfer model is developed and solved with the finite difference method. The results show that the air and solution flow rate greatly influence the heat and mass transfer coefficient, revealing the coupling mechanism of heat and mass transfer and the mutual influence law in IHFMLDD. Correlation expressions of heat and mass transfer coefficients are proposed, in good agreement with the experimental data. The dehumidification capacity of the internally-cooled membrane liquid dehumidifier is 75–140% higher than that of the traditional hollow fiber membrane liquid dehumidifier.

Performance of dehumidifier drying machine for soybean seeds drying

The number of soybean consumption in Indonesia has increased every year. However, the amount of domestic production is still insufficient and continues to decline, so imports must be carried out. A strategy used to increase soybean production is by using a good quality of seeds. Drying with low relative humidity can reduce the drying temperature so that the quality of the seeds is maintained. A performance test was conducted by a dehumidifier drying machine with the temperature of 30°C, 40°C, 50°C and oven drying as control at 40°C. Drying was carried out until the moisture content reach 10-11% with every 30 minutes observations. The result showed that the dehumidifier drying machine gave constant air temperature and relative humidity at the drying chamber during the drying period. Duration of drying at the temperature of 30°C, 40°C, 50°C, and control was 7 hours; 6 hours; 3 hours; and 7.5 hours respectively. The application of a dehumidifier drying machine can reduce humidity up to 54% so that soybean drying can run faster than oven drying. A germination test could reach 87.96% in dehumidifier drying at 40°C which is higher than the control. This result showed that dehumidifier drying did not cause significant damage to soybean seeds.

Dynamic performance analyses and optimization studies on air dehumidifiers using multi-stage desiccant plates

• Dynamic air dehumidification processes of a desiccant plate (DP) were studied. • Three causes for the generation of ineffective sections are found. • For the dehumidifier with multi-stage DPs, recommended stage number is 2. • An advanced dehumidifier is proposed, ineffective DPs of which can be replaced. • Effective working period of the advanced dehumidifier is extended by 2.2%∼34.8%. In this study, a novel transient air dehumidifier that can meet the dehumidification demands at places without a constant energy supply and uses multi-stage desiccant plates was proposed. Based on the simulation methods, parametric and optimization studies were carried out to extend the effective working duration. First, the air handling processes, transient temperature and humidity distribution fields of a typical desiccant plate were investigated. The results indicated that there are three reasons for the generation of ineffective dehumidification sections. To reduce the proportion of the ineffective sections and to increase the effective working duration, parametric analyses were carried out on the desiccant plate, considering factors such as the initial water content ( W ini ), inlet air humidity ratio ( ω a,in ), and length of the desiccant plate. The results show that, if the required outlet humidity ratio ( ω ∗ ) is lower than 12–13 g/kg, W ini = 0.1 kg/kg is preferred for ω a,in between 15 and 19 g/kg. Subsequently, a dehumidifier with multi-stage desiccant plates (original dehumidifier) was designed, and the optimized stage number for different total thicknesses of desiccant plates was investigated to improve the dehumidification performance. When ω ∗ is 10 g/kg, the recommended stage number is two. Finally, an advanced dehumidifier is introduced in which an ineffective desiccant plate can be replaced with a new desiccant plate. For the advanced dehumidifier, the proportion of ineffective sections can be effectively reduced by 14.6%–93.2%, and the effective working duration can be extended by 2.2%–34.8%, compared to the original dehumidifier that has the same total thickness as the desiccant plates.

A novel 5-sulfosalicylic acid - Polyvinyl alcohol - Hydroxyethyl cellulose vapor permeation membrane for gas dehumidification

The crystallinity and the hydrophilicity are the two main factors influencing the membrane's vapor permeability for gas dehumidification. Herein, a novel 5-sulfosalicylic acid-polyvinyl alcohol-hydroxyethyl cellulose (SSA-PVA-HEC) vapor permeation membrane composing SSA capturing water layer and PVA-HEC transporting water layer was prepared. To reduce membrane crystallinity and increase vapor permeability, HEC was added to the PVA membrane matrix to form a transporting water PVA-HEC layer. To further improve the PVA-HEC layer's permeability, the surface of the PVA-HEC membrane was modified by the hydrophilic SSA, which acted as a capturing water layer. The results showed that the water vapor permeability of the SSA-PVA-HEC membrane improved from 1.19 × 104 to 1.52 × 105 Barrer. After dealt with SSA-PVA-HEC membrane, the relative humidity (RH%) of the feed (humid nitrogen) decreased from 90% to 23.4%. Overall, the low cost and the excellent dehumidification performance of the SSA-PVA-HEC membrane make it possible for industrialization.

Parametric Analysis of a Universal Isotherm Model to Tailor Characteristics of Solid Desiccants for Dehumidification

Cooling has a significant share in energy consumption, especially in hot tropical regions. The conventional mechanical vapor compression (MVC) cycle, widely used for air-conditioning needs, has high energy consumption as air is cooled down to a dew point to remove the moisture. Decoupling the latent cooling load through dehumidification from the sensible cooling load can significantly improve the energy requirement for air-conditioning applications. Solid desiccants have shown safe and reliable operation against liquid desiccants, and several configurations of solid desiccants dehumidifiers are studied to improve their performance. However, the characteristics of solid desiccants are critical for the performance and overall operation of the dehumidifier. The properties of every desiccant depend upon its porous adsorbing surface characteristics. Hence, it has an optimum performance for certain humid conditions. Therefore, for a better dehumidification performance in a specific tropical region, the solid desiccant must have the best performance, according to the humidity range of that region. In this article, a theoretical methodology has been discussed to help the industry and chemists to understand the porous structural properties of adsorbent surfaces needed to tune the material performance for a particular humidity value before material synthesis.

Development of a new control method for the dynamic liquid desiccant dehumidification process

Liquid desiccant dehumidification technology as an efficient way of handling moisture load has attracted much attention, while the performance of liquid desiccant dehumidifiers has characteristics of uncertainty and time-varying in actual runtime, limiting its application fields. Thus, it is necessary to develop dynamic control methods to achieve stable and efficient operation. As the liquid desiccant dehumidification process is a multi-parameter, time-varying and large delay nonlinear dynamic process, therefore it is difficult to achieve the dynamic control. In this paper, dynamic control methods of the liquid desiccant dehumidification process were studied experimentally for the first time and main difficulties in the dynamic control of the liquid desiccant dehumidification process were presented clearly, helping to learn more knowledge on dynamic control process and facilitate the development of control methods. PID control as a classic control method was applied to control the liquid desiccant dehumidification process firstly, the result showed that it requires long adjusting time to follow the command and is not suitable to control nonlinear dynamic process with large delay. Novel fuzzy logic control (FLC) as an advanced control method was implemented as well, the result demonstrated that it follows command quickly while is unable to maintain the new set point accurately. Then reasons for shortcomings of each control method were analyzed. Along with the above analysis, a new control method, combining the PID control and novel FLC was developed. The command following experiment and disturbance rejection experiment for the newly developed control method were conducted. Control performances are great and validate the feasibility of the new control method. Besides, the newly developed control method also provides a good reference for achieving the dynamic control of a multi-parameter, time-varying, nonlinear dynamic process with large delay.

Charge-assisted coating of silver nanowire transparent conductive layer and application to flexible heater

Transparent conductive layer (TCL) were fabricated from flexible silver nanowires (AgNWs) using charge-assisted coating (CAC) and were subsequently applied in a transparent flexible heater (TFH). The CAC process is a simple and inexpensive process of immersing a substrate in an aqueous solution of charged species and assembling them on the substrate via electrostatic attraction. The AgNWs were synthesized by the polyol method, functionalized with 3-mercaptopropionic acid, and assembled on a polyethylene terephthalate substrate. The optical transmittance of the TCL was controlled by regulating the AgNW solution concentration. The fabricated films were post-processed by calendering. The calendering post-treatment decreased the sheet resistance and increased the structural stability of the AgNW TCL. When the concentration of the AgNW solution was 50 µg/mL, the fabricated AgNW layer exhibited the highest figure of merit possessing a sheet resistance of 8.2 Ω/sq and a transmittance of 82.7%. With a protective layer of polyurethane, AgNW TFH showed a heating temperature of 110°C at bias voltage of 5 V. The dehumidification performance of the TFH was also successfully demonstrated.

Simplified effectiveness and number of transfer unit model for a vacuum membrane dehumidifier applied to air conditioning

• A simplfied prediction model for a vacuum-based membrane dehumidifier is proposed. • Proposed model was validated via numerical simulation and experimental data. • Model dehumidification performance exhibits error bounds below 20%. • Model consumes 42.4% lower vacuum pump energy than existing control method. This paper purposed to develop a simplified model for a vacuum membrane dehumidifier in an air conditioning system. The proposed model is a gray model that uses mathematical equations that are deducted from the effectiveness and number of the transfer unit of the heat exchanger and a regression equation for the ratio of humidity capacities (X factor) analogous to the heat capacities ratio of heat exchangers. The applicability of the simplified effectiveness and number of transfer unit methods to vacuum-based membrane dehumidifiers in air conditioning systems has been validated via numerical simulation and experimental data. The error bound for the validation of the numerical simulation and experimental data for dehumidification performance (i.e., outlet humidity ratio and dehumidification effectiveness) was 20%. In addition, an operation example using the simplified model was performed to control the vacuum pressure of the vacuum-based membrane dehumidifier in the air-conditioning system. The results show that the proposed algorithm using the simplified model exhibits an energy-saving potential of 42.4% compared to the conventional vacuum-based membrane dehumidification operated in a variable air volume system.

Experimental performance of a three-fluid desiccant contactor using a novel ionic liquid

• Novel Ionic Liquid that is not corrosive to aluminum is used as liquid desiccant. • New three-fluid fin-tube desiccant contactor fabricated using aluminum. • Experimental performance of the fin-tube contactor using IL solution is investigated. • The fin–tube contactor showed superior performance than the packed bed contactor. • Lower solution flow rates and pumping power is expected using the fin–tube contactor. The experimental performance of a new three-fluid fin–tube contactor using a novel ionic liquid desiccant solution is investigated and compared with those of an adiabatic packed bed contactor with LiCl solution for liquid desiccant air conditioning system applications. Owing to the compatibility of the ionic liquid solution with aluminum, a fin–tube contactor is manufactured to exploit internal cooling from a third fluid. The results indicate the superior dehumidification performance of the fin–tube contactor, which yielded an outlet air humidity ratio of 10.5 g w ·kg da -1 at a solution mass flux of 3.06 kg·m −2 ·s −1 , as compared to a similar humidity ratio at a solution mass flux of 4.98 kg·m −2 ·s −1 achieved by the packed bed contactor. In terms of cooling performance, the outlet air temperature of the fin–tube contactor is 21.9 °C, which is lower than that of the packed bed contactor even at its highest solution mass flux. This indicates that the cooling performance of the fin–tube contactor has minimal dependence on the solution mass flux in achieving cooling effect, owing to the cooling medium flowing inside the tubes. Relatively lower solution pumping power than that of the packed bed contactor can be expected from the fin–tube contactor, in addition to the ability to achieve comfortable and hygienic indoor air at a cooling medium temperature of 17 °C or higher, the ionic liquid desiccant and three-fluid fin–tube contactor pair for liquid desiccant air conditioning system is elevated a step higher as an alternative to vapor compression air conditioning system.

Experimental investigation on the dehumidification performance of a parabolic trough solar air collector assisted rotary desiccant system

In areas of high humidity, moisture is a major problem for human comfort and air-conditioning systems. As a green solution to this prevalent problem, a solar assisted rotary desiccant dehumidification system has been proposed for air dehumidification. Through an experimental study conducted, performance evaluation of a parabolic trough solar collector assisted rotary desiccant dehumidification system has been performed based on the parameters including air flow rate, regeneration air temperature, inlet-outlet air temperature, and humidity ratios. Findings indicated that the highest regeneration air temperature supplied by the parabolic trough solar air collector to the dehumidifier was 45.8 °C at a flow rate of 60 m³/h. It was also found that the highest dehumidification efficiency was achieved at a regeneration air flow rate of 80 m³/h. The experimental study clearly proves that parabolic trough solar collectors can successfully provide low-medium temperature thermal energy required during the regeneration process in the desiccant wheel. The findings confirm the potential and explain specific conclusions for the practice of solar thermal driven rotary desiccant wheel system.

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.

Effects of fiber bundle nonuniformity on dehumidification performance and energy efficiency of pressure‐driven membrane modules

Effects of fiber bundle nonuniformity on dehumidification performance and energy efficiency of pressure‐driven membrane modules

Effect of operating parameters on the performance of rotary desiccant wheel energized by PV/T collectors

The main energy input of a desiccant air conditioning system is the low-quality thermal ener-gy required for regeneration, which can be obtained from waste heat, geothermal resources or solar energy. Regeneration thermal energy can be produced as well as energizing components such as fans, pumps, auxiliary air heaters, and control elements of the system by using pho-tovoltaic-thermal solar collectors (PV/T). In this study, parametric analyzes were performed to investigate the effect of regeneration temperature and air frontal velocity on the tempera-ture and dehumidification performance of a solid silica-gel desiccant wheel and on the wa-ter-cooled PV/T collectors used to provide the regeneration thermal energy. The regeneration temperature was varied between 50 and 70°C, and air frontal velocity between 1.3 and 4.1 m/s. The analyzes show that the dehumidification efficiency increases from 13.94% to 33.04% as regeneration temperature increased from 50°C to 70°C at 1.3 m/s air frontal velocity at which dehumidification efficiency is maximum. At 4.1 m/s air frontal velocity, the required regener-ation thermal energy is maximum and increases from 49.64 kW to 132.48 kW at the same re-generation temperature change. The low regeneration temperature resulted in desirable latent performance and undesirable sensible heat transfer performance in DEW. Finally, considering the whole system, it was concluded that the optimum regeneration air temperature for the performance parameters is 60°C.

Energy saving potential of passive dehumidification system combined with energy recovery ventilation using renewable energy

Passive dehumidification and solar collection (PDSC) employs fibrous insulation materials with excellent moisture adsorption and desorption characteristics to conduct dehumidification using renewable energy. This study proposes an improved PDSC-integrated energy recovery ventilation (ERV) system (PSE) to dehumidify indoor environments. Energy recovery ventilation (ERV) promotes the exchange of moisture and heat between returned and supplied air to reduce energy loss caused by ventilation. We explained the fundamental moisture movement principle based on thermodynamic energy and designed the air circulation paths of the proposed system for dehumidification and energy recovery. Five house models were simulated and compared: conventional house with no moisture effect, conventional house, conventional house with integrated ERV, the PDSC model, and the PSE model. Simulation results show that the PSE model has the best dehumidification performance, with an approximately 2.9 times latent heat load reduction effect compared with the PDSC model, in hot and humid summer. This study confirms that the proposed system has significant potential for dehumidifying the indoor environment and provides guidance for the future application of the PSE system to dwellings.

Effect of Geometry and Operational Parameters on the Dehumidification Performance of a Desiccant Coated Heat Exchanger

Solid desiccant dehumidification systems are an alternative to vapor-compression-based air dehumidification systems. They use solid desiccant materials to adsorb air moisture in space cooling. DCHE can integrate the cooling component in the heat exchanger to achieve higher dehumidification capacity. Comprehensive numeric modeling would be necessary to assist in the design and operation of the DCHE under development to achieve maximum performance. This paper provides the details of a heat and mass transfer model developed for this purpose. The model aims to cover a gap in existing models by independently modeling heat transfer on the solid desiccant, fin, and tube. The model results were compared with an experimental reference for validation. The air outlet humidity and temperature results for dehumidification and regeneration showed a deviation lower than 15% from the experiment. The validated model was used to perform a parametric study for a series of design and operating conditions. The parametric analysis showed that an increase of 25% in desiccant layer thickness and thermal contact resistance between solid elements reduces moisture removal by 9.1% and 1%, respectively. These results indicate the need to independently model the desiccant layer.

Performance assessment of sustainable energy based novel technique for multistage reciprocating liquid dehumidification system

Liquid desiccant technology is a promising option with a high thermal coefficient of performance (COP), easy storage, and lower regeneration temperature. Single stage static liquid dehumidification approach gave a limited moisture removal rate. Hence, the paper focuses on designing and fabricating a multistage reciprocating desiccant dehumidification system integrated with a conventional vapour compression cycle. The system consists of an absorber unit consisting of calcium chloride liquid desiccant and a duct integrated with Celdek packing for dehumidifying the inlet air. The desiccant concentration is recovered by circulating it to a regeneration unit. Regeneration unit consists of a heating unit, a separator, & a contact device. Experiments are conducted by varying the number of packings, desiccant concentrations, and air Reynolds number. Dehumidification performance parameters such as coefficient of mass transfer (MTC), rate of moisture removal (MRR), sensible heat factor (SHF), dehumidification effectiveness (DE) are evaluated. Energy consumption is determined for varying conditions, and the coefficient of performance of the overall system is determined. Regenerator performance is studied corresponding to change in the desiccant concentration at various stages of regeneration. Experimental results indicated a maximum dehumidification efficiency, moisture removal rate, and the coefficient of performance is equal to 77.6%, 5.2 g/s and 3.5. In the contact device, when packing linear velocity increases from 0.13 m/s o 0.26 m/s, ΔW will be increased by 116%, and ΔT dropped by 17.3% for 40% desiccant solution. Contact device is very effective for regeneration in increasing the desiccant concentration and reducing the temperature.

Flow behavior and mass transfer of humid air across fiber membrane bundles

In the present work, staggered and inline arranged membrane components are modeled to numerically investigate the flow behavior and mass transfer of humid air across the fiber membrane bundles. For the dehumidification purpose, the humid air flows over the outer surface of the composite membrane fibers, and the lumen side of the membrane maintains a negative pressure where the permeated water vapor is removed from the suction port. The study of flow behavior showed that the velocity contours are denser near the membrane outer wall for both configurations, while a fluid stagnation zone is observed in the staggered arrangement. Through the analysis of the flow and concentration fields in the two configurations, it was found that for the fiber membranes with small radius, the inline arrangement possesses more advantages in terms of dehumidification capacity and energy efficiency. Consequently, the effects of the dimension parameters of inline arrangement on the dehumidification performance of membrane bundles were further explored. The results indicated that the increase of tube pitch has a negative effect on the dehumidification rate. In addition, thickening the thickness of the membrane support layer and decreasing the fiber radius are conducive to the dehumidification performance.

Experimental study on performance measurement of planar vacuum membrane dehumidifier with serpentine flow channel designs

In recent years, membrane dehumidification, as a novel dehumidification technology, has received many attentions for energy efficiency in air conditioning systems. Its dehumidification technology is characterized by isothermal dehumidification, which can avoid excess energy consumption. This experimental study focuses on the performance measurement of the planar vacuum membrane dehumidifier. Nafion membrane is used in combination with a planar membrane dehumidifier. A vacuum membrane dehumidification method is also employed to investigate the dehumidification performance. Four serpentine flow channel designs, including three-snake, four-snake, five-snake, and six-snake flow channel designs, are tested at the certain values of temperature and humidity and different inlet air flow rates. Different performance indexes, including water collection, dehumidification rate, pressure loss, and pumping power, are investigated. The results show that the amount of water collected by the cold trap increases with boosting the inlet flow rate in the range of 35 l/min to 55 l/min and it has the largest value when the inlet air flow rate is 55 l/min. However, the amount of water collected by the cold trap decreases with further increase of the inlet flow rate in the range of 55 l/min to 65 l/min.

Mathematical investigation of two-stage dehumidification from single rotary dehumidifier with and without precooling

A one-dimension mathematical model has been developed to investigate the performance of two-stage dehumidification using a single rotary dehumidifier of four sectors. To obtain two-stage dehumidification, the outlet stream of process 1 has been supplied into process 2 and a precooler is also introduced to enhance the performance. The performance of two-stage dehumidification using four sectors with and without precooling has been compared with the single-stage dehumidification using two-sector. The results of this study indicate that the total moisture removal of two-stage-dehumidifier using four sectors with precooling is significantly higher than single-stage-dehumidifier using two sectors, but only a little higher than the Two-stage dehumidifier using four sectors without precooling. The introduction of a precooler not only decreases the exit temperature but also increases the total moisture removal. Hence, this study recommends that a two-stage dehumidifier using four sectors can be used for better performance in place of a single-stage dehumidifier using a two-sector for the same size or space of the wheel in a variety of applications.

Study on the dehumidification performance of vacuum membrane-based dehumidification modules with convex membrane characteristics

Vacuum membrane-based dehumidification technology is one of the key technologies of the next-generation air conditioning system. This work proposes a method for actively constructing convex vacuum membrane-based dehumidification modules by internal fins. A numerical model of internal finned vacuum membrane-based dehumidification is developed and validated experimentally. The dehumidifying and flowing characteristics of two different convex vacuum membrane-based dehumidification modules with internal spiral fins and internal straight fins are comparatively studied. Moreover, the mechanism behind the different dehumidification phenomena is revealed by the concentration and velocity fields. In addition, the effect of the characteristic angle of fins and twist number on the performance of dehumidifying and flowing are analyzed. The results show that the dehumidification performance of the internal spiral finned vacuum membrane-based dehumidification module is weaker than that of the internal straight finned vacuum membrane-based dehumidification module, and the flowing resistance of the internal spiral finned vacuum membrane-based dehumidification module is higher than that of the internal straight finned vacuum membrane-based dehumidification module. Apart from this, the internal spiral finned vacuum membrane-based dehumidification modules cause the direction of moist air to change in some regions, which lead to single-peaked, bimodal, or multi-peaked distribution of humidity ratio under different characteristic angle. Considering the dehumidification effect and flow resistance, the straight finned vacuum membrane-based dehumidification module is more recommended as a vacuum membrane-based dehumidification module with convex membrane characteristics in engineering. This work can provide a reference for the diversified development and some new insights into the mass transfer of vacuum membrane dehumidification modules.