Dehumidification Process Research Articles

An improved indirect evaporative cooler experimental investigation

Air conditioning has enhanced the work efficiency and improved life style by maintaining comfortable environment. The growing demand of air conditioning has negative impact on energy and environment. In 2015, air conditioning consumed 6% of total global electricity produced and it is expected to increase to 20% by 2050. The leveling-off conventional chiller’s efficiency at 0.85 ± 0.03 kW/Rton due to pairing of dehumidification and cooling processes in one machine is not only the major reason of high energy consumption but also the key limitation in efficiency improvement. The de-coupling of dehumidification and cooling processes can be one of the solution to achieve the quantum jump in the performance, 0.6 ± 0.03 kW/Rton, by improving individual processes. We proposed an improved indirect evaporative cooler system for sensible cooling that can be combined with dehumidification processes to achieve sustainable cooling goals. The experimentation on 800 mm long and 280 mm wide generic cell showed that it can produce temperature differential up to 10 °C with small area of heat transfer. It was showed that the proposed vertical heat exchanger configuration with multi point injection of working air is the best configuration of the indirect evaporative cooler, achieving coefficient of performance level of 78 for cooling alone. We expect that overall coefficient of performance level of 7–8 is achievable by incorporating efficient dehumidification processes. We also presented detailed design parameters that can be used as a reference for commercial system design.

Sistema de extraccion y tratamiento de metano

Currently sanitary landfills are considered sources of atmospheric pollutants. Biogas, a product of the biological decomposition of organic waste, incorporates mostly methane (CH4) and carbon dioxide (CO2), which are greenhouse gases. A large number of municipal sanitary landfills do not have control over methane gas emissions to the surface. This project is based on the construction of a portable prototype system for the extraction and treatment of methane gas from landfills. The objective was to manufacture and build a portable prototype system for the extraction and treatment of methane gas from municipal sanitary landfills. That it implements the necessary sections and devices to treat the gas and in this way facilitate its later use as fuel in heating systems, domestic use and / or internal and external combustion thermal machines. Using for this purpose a system of gas extraction, separation of solids, de-humidification and desulphurisation process. In the system variables such as the percentage of methane, temperature and humidity are monitored, in order to give a stability to the gas treatment process.

A Study of Enhanced Heat and Mass Transfer From Variable Height Fin Array Undergoing Natural Convection

Abstract A numerical study is performed on simultaneous heat and mass transfer from a shrouded vertical nonisothermal variable height fin array, representing dehumidification process under natural convection. Fluid properties are treated as uniform, and the fluid is assigned to comply with Boussinesq approximation to include the effect of density variation with temperature and concentration. Semi-implicit method for the pressure linked equations revised (SIMPLER) algorithm is adopted to resolve pressure and velocity coupling. A detailed parametric investigation of fin spacing, variable fin height, and fin tip to shroud clearance for a range of thermal and mass Grashof number is undertaken. Results indicate that in case of smaller fin spacing, involving fin length of 0.3 m, coefficients of sensible and latent heat transfer increase with the decreasing variable height (H1*) of fin and become maximum at H1*=0.5, for all thermal and mass Grashof numbers considered presently. Further, total heat transfer analysis on a particular base length due to sensible heat shows a maximum of 24.4% enhancement, whereas same due to the latent heat shows a maximum of 25.8% enhancement, depending on the values of clearance. Induced velocities also increase with the decreasing variable height of fin (H1*), which influences the heat and mass transport. The output parameters of this analysis, like induced velocities and overall Nusselt numbers due to the sensible and latent heat, are correlated with the governing parameters. The correlation coefficients are found to be in a range from 0.97 to 0.99.

Optimization of fibrous air filter on the basis of particle condensational growth during the air cooling and de-humidification process using mathematical modeling

Indoor ultra-fine particles have significantly adverse effects on human health. Under the condition of high concentration, the widely used HEPA (High-Efficiency Particulate Air) requires expensive operation cost. Previous study indicated that air cooling and dehumidification system could greatly increase particle size (from dozens of nanometers to several micrometers) due to the condensation effect during the process of air conditioning in hot and humid regions. Because of the condensational growth of particles, low-grade air filters can be used to achieve high efficiency and lower cost. This study developed mathematical model to predict particle size variation in cooling and dehumidification process and investigate its effect on fibrous filter performance. The steps are as follows: 1) Based on the state-space model, distributions of supersaturation ratio in condensational dehumidification process were simulated; 2) The modified Lagrangian method and SSR-based model were used to simulate particle trajectory and size increase; 3) Based on the prediction of condensational growth of particles, the air filter performance model were adopted to investigate the condensation effect on fibrous filter performance. The results shows that condensational dehumidification improve the filtration efficiency of middle-efficiency fibrous from 72% to 92%, quality factor from 0.016 to 0.031. Utilization of condensational dehumidification could decrease energy consumption of fibrous filtration system by 53%. This study is of great significance to improve the effectiveness and economy through the coordinated control for indoor pollutants, temperature and humidity control in humid and hot climate region.

Drying rate and efficiency energy analysis of paddy drying using dehumidification with zeolite

Paddy must be dried to prolong the storage life and maintain the quality. There was some researches about paddy drying system: bin dryer. The dehumidification process giving advantages in high efficiency energy, but it is taking the heat for regeneration. Based on the literature study, it is hard to find the literature about drying efficiency analysis during the drying of paddy. This research aims to observe the effect of drying temperatures and addition of zeolite in drying system. Paddy grains were dried from initial moisture content 22% (wet basis) under different temperature 40, 50, and 60°C for 90 minutes drying. Zeolite was used as adsorbent to decrease the relative humidity of air dryer. Results showed that use of zeolite lowering the air humidity and increasing driving force for drying. These made the drying rate increased. The higher drying temperature can also increase drying rate. Energy efficiency decrease during the drying process. Also, addition of zeolite in drying process affected the drying efficiency.

Simulation and experimental studies on the liquid desiccant dehumidification of horizontal tubes having a staggered arrangement

Field synergy theory has been widely used in the field of single-phase heat transfer, which is of importance to heat transfer optimization. However, few scholars apply the field synergy theory to the field of mass transfer, especially to the field of liquid desiccant dehumidification. The innovation of this paper is the application of the technology of the falling-film evaporation of a horizontal tube and the theory of field synergy to liquid desiccant dehumidification. The objective of this paper is to study the falling film liquid dehumidifier and get the dehumidification performance and mass transfer synergy angle under the influence of multiple factors. The heat and mass transfer characteristics of the system are studied in simulation and experimentally. The mass transfer synergy angle comprising the velocity vector and concentration gradient vector is taken as a standard to measure the mass transfer synergy, while the moisture content of the outlet air is taken as a standard to measure the dehumidification performance of the system. Fluent simulation software is used to simulate the dehumidification process of the equipment with staggered horizontal pipes, establish physical and mathematical models and analyze the velocity field and moisture content field in the model. The moisture content of the outlet air and the distribution of the mass transfer angle are obtained. The mass transfer angle of the system is about 96°. The effects of various factors on the air moisture content at the outlet of the equipment are studied experimentally. Results show that the dehumidification amount decreases with an increase in air velocity. Increases in the solution concentration, air inlet moisture content and solution flow rate increase the amount of dehumidification, following the same trends as in simulation. A comparison of the simulation results with the experimental results shows the relative error is within 20%. This study shows that it is feasible to combine the field synergy theory with liquid desiccant dehumidification. On the basis of this study, researchers in related fields can further study the effect of mass transfer synergy angle on mass transfer synergy in liquid desiccant dehumidification system.

Solid desiccant dehumidification-based air-conditioning system for agricultural storage application: Theory and experiments

This study experimentally investigates desiccant dehumidification and indirect evaporative cooling for agricultural products' storage. Thermodynamic advantages of the proposed system are highlighted and compared to vapor compression systems. Significance of proposed system is discussed in relation to agricultural storage application. Factors affecting the postharvest quality of products are discussed, and consequently, the psychrometric zones are established for optimum storage. Hydrophilic polymeric sorbent-based desiccant units are used for the experimental investigation. An open-cycle experimental apparatus is setup by which desiccant dehumidification and regeneration processes are analyzed at various conditions. Thereby, a novel correlation is developed by which desiccant dehumidification process can be simulated precisely. The correlation is successfully validated against the experimental data of various conditions. Desiccant air-conditioning cycle is analyzed for two cases (i.e. case-A: dry-bulb temperature = 31 ℃, humidity-ratio = 6 g/kg-DA; and case-B: dry-bulb temperature = 13 ℃, humidity-ratio = 6 g/kg-DA) to investigate the proposed system's applicability for agricultural storage. The results show that the thermal coefficient of performance is highly influenced by ambient air conditions and decreases with the increase in regeneration temperature. The thermal coefficient of performance for case-A is higher as compared to case-B, and for both cases, it increases with the increase in wet-bulb effectiveness of the evaporative cooling unit.

Drying in a desiccant wheel dehumidification system-assisted hot air dryer: desiccant wheel effectiveness and carrot drying

A desiccant wheel dehumidification system was used in this study to reduce humidity of the air supplied to a hot air dryer. Desiccant wheel effectiveness was determined based on the ideal adiabatic dehumidification process. Best results of adiabatic dehumidification effectiveness and adiabatic enthalpy effectiveness in the range of 0.89 to 0.99 were achieved. Specific adsorption of silica gel in the desiccant wheel was rotational speed dependent. The desiccant wheel system could reduce the relative humidity from 70 to 44%. Drying of cylindrical carrot in the desiccant wheel dehumidification system-assisted hot air dryer was also conducted in this study. The drying temperatures were varied from 80 to 120˚C. The desiccant material regeneration process reused exhaust drying air for heat exchanging with the regenerating air, higher drying temperature caused higher moisture evaporated from the desiccant material. Comparing between dehumidified hot air drying and conventional hot air drying, greater drying rates of dehumidified hot air drying were achieved at the drying temperatures of 100 and 120˚C.

Water productivity enhancement in variable pressure humidification dehumidification (HDH) desalination systems using heat pump

Variable pressure humidification dehumidification (VPHDH), humidification compression (HC), and conventional humidification dehumidification (HDH) processes are coupled to a heat pump (HP) for enhancement of their performances and desalination rates. Process simulation of the modified processes reveals the superiority of the proposed VPHDH-HP process to HC-HP, HDH-HP and the conventional HDH processes. Due to the superiority of VPHDH processes over conventional HDH processes, coupling them to HP units as a novel idea improves the desalination rate and energy utilization. The effects of dehumidifier to humidifier pressure ratio (PD/PH), humidifier pressure, and the inlet water to air mass flow rate ratio (w/a) on desalination rate and performance ratio (PR) are studied for the VPHDH-HP process as the superior process. Increasing PD/PH within 2.0 to 3.0 range, leads to an increase in desalination rate by 215%. Increasing w/a from 1.5 to 2.0 increases the desalination rate by 15%, while the same from 2.0 to 3 leads to a 53% decrease in the desalination rate for humidifier pressure of 0.5 bara. A cost analysis yields a water cost of US $4.68/m3 in VPHDH-HP system, indicating its performance superiority against its counterparts.

Solar-Driven Humidification–Dehumidification Process for Water Desalination Analyzed and Optimized via Equilibrium Theory

This study investigates a desalination system based on a humidification–dehumidification process driven by solar modules and proposes a solution within the framework of equilibrium theory. Although...

Performance analysis and mixing ratio selection of mixed liquid desiccant for a dehumidification system

Liquid-desiccant dehumidification system in building air-conditioning has high energy efficiency in comparison to traditional dehumidification technology that needs to cool air below the dew point temperature. Since the dehumidification process dominates the performance of a liquid-desiccant system, this study aims to develop a deeper understanding in the dehumidification performance of some liquid desiccants. Then, an experiment test was conducted to determine the CaCl2-LiCl mixed liquid-desiccant system in a dehumidification system. The effects of the pure and mixture liquid desiccant solution on the moisture removal rate, refrigerating capacity and dehumidification performance were compared and analysed. Results show that with the increase in the LiCl concentration, the dehumidification capacity and dehumidification coefficient first increased rapidly and then increased slowly after 30% LiCl concentration. According to the economic analysis of mixed salt solution, the mixed LiCl and CaCl2 solution with a mixing ratio of 3:1 would achieve the best optimal dehumidification efficiency and would incur a low system cost under the working conditions of a dry-bulb temperature at 26°C and a relative humidity of 60% of fresh and return air.

Development of liquid-air mass transfer correlations for liquid desiccant dehumidification considering the liquid/air contact and film instability

The focus of this work is on the liquid-air mass transfer mechanism that is critical for liquid desiccant dehumidification and many other absorption processes. Most existing mass transfer correlations heavily rely on specific experiments and show poor universality. Therefore, we proposed a new set of mass transfer correlations theoretically based on the film instability during falling film dehumidification. The flow dynamic, Marangoni effect and liquid/air contact conditions that affecting the interface characteristics and wetting factors are considered. The correlations were verified by comparing with experimental data from several widely-cited literatures. The tests in these literatures were conducted under a wide range of operating conditions and dehumidifier types. The newly-developed correlations provide an acceptable prediction for liquid-air mass transfer, showing close trends to all previous experimental results. The overall error of the new predictions, 20–30%, is close to those of empirical equations built in the specific literature. The factors that affect the interphase mass transfer by changing the film instability and the wetting factor are also analyzed. The increase in liquid Reynolds number shows the most significant effect as it could effectively increase the film instability and liquid-air contact area. The liquid contact angle on solid surfaces, regarding the wettability, also affects the mass transfer considerably. By reducing the contact angle from 90° to 10°, although the increase in Sherwood number is slight due to the suppression of film instability, the wetting factor is almost doubled, resulting in a significant growth in mass transfer performance. This new correlation examines the falling film mass transfer process in more detail, and is based on fewer simplifying assumptions and attempts to take more realistic situations into account. Findings presented herein contribute to a more fully understanding on the falling film behaviors during liquid/gas contact such as liquid desiccant dehumidification.

Numerical investigation of mass transfer characteristics for the desiccant-coated dehumidification wheel in a dehumidification process

Desiccant dehumidification systems provide an environment-friendly and energy-saving option for air humidity regulation. The dehumidification wheel as the key component occurring mass transfer process determines the dehumidification performance of the systems. In this study, a two-dimension numerical model was developed to conduct parameter studies on the mass transfer characteristics of dehumidification process for the desiccant-coated dehumidification wheel. The mass transfer coefficient Kd and Ktotal, the integral-average of mass transfer coefficient K¯d,eff and K¯total,eff determined by the effective adsorption time (teff) and the dimensionless parameter χ(t) were introduced to evaluate the mass transfer performance. Effects of main operational and structural parameters on the mass transfer performance were analyzed and discussed. The results show that these evaluation indices significantly depend on the various parameters, and the preferable dehumidification channel shape is related to cycle time. Research efforts have been made to obtain a correlation for the mass transfer coefficient, which provides an effective tool to predict the dehumidification characteristics of the dehumidification systems.

Heat current method for analysis and optimization of a refrigeration system for aircraft environmental control system

Optimization of refrigeration system for aircraft environmental control system (ECS) benefits the whole aircraft performance significantly. However, due to the complex dehumidification process, the heat transfer and heat-work conversion processes in the system, numerous intermediate variables in the system model aggravate the nonlinear and multivariate properties of the governing equations. Taking the typical two-wheel high-pressure de-water ECS as an example, this contribution introduces the heat current method to model the refrigeration system, which reveals the system topology and heat transfer and heat-work conversion characteristics simultaneously. Analogous to electrical circuit, the governing equations of whole system are obtained according to the circuit principle. Furthermore, a comparison is conducted to clarify the differences between the traditional energy flow method based on Sankey diagram and the heat current method. Next, taking the minimum takeoff gross weight as the objective, using the Lagrange multiplier method directly solves this nonlinear optimization and offers the optimal structural and operating parameters of the system. The optimized results show that the cruise flight hour has little impact on the optimal design variables, while the cabin heat load has an approximately linear relationship with the optimal design variables. Finally, when only the cabin temperature changes, the node temperatures are almost constant, which can be taken as a criterion for the design of each component.

Effect of water wall prototype on air humidity

The presented document focuses on the effect of the water wall on air humidity (humidification and dehumidification of air). For the purpose of impact verification, a water wall prototype that was tested under laboratory conditions (climate chamber) was designed and built. Also a set of measuring sensors has been installed, these sensors are able to continually record air temperature and humidity and water temperature. In the case of air humidification, the water temperature and air temperature were equalized approximately after 2 hours and after 6.5 hours the air relative humidity reached 92%. This represents an increase in the specific humidity of 4.5 g/kg versus the initial state. The article also describes methodology and measurement of condensation ability of the prototype. In case of air dehumidification process, three different relative humidity was tested at the same air temperature. During the measurement periods, there was increasing the amount of condensate several times by increasing the relative humidity.

Chitosan-boehmite desiccant composite as a promising adsorbent towards heavy metal removal

Owing to the widespread occurrence and potential health effects, many treatment strategies have been developed across the world to remove the heavy metal contaminants in water. Developing affordable and sustainable nanoscale materials are the prime factors for the success of such treatment systems in the field. The present study explores the use of desiccant waste, exhausted after several cycles of dehumidification processes. The granulated composite desiccant is composed of boehmite nanoparticles reinforced with chitosan fibrils. The composite was synthesized via a simple and scalable one-pot sol-gel route at atmospheric pressure and room temperature. The desiccant was employed for dehumidification/regeneration cycles. The reuse potential of exhausted desiccant towards enhanced removal of metal ions was analyzed and demonstrated. After adsorption the nanocomposite was characterized to establish its chemical composition and structure. Batch and fixed-bed column adsorption experiments were performed to evaluate the removal efficiency of the nanocomposite and to assess the parameters that influence the adsorption process. The experimental evidences confirm the fast kinetics of adsorption/desorption and effective regeneration of the composite. The enhanced removal capacity, excellent reuse potential, high stable granules, eco-friendly synthesis approach makes the adsorbent an excellent candidate for the removal of wide range of heavy metals in water.

Porous polymer/inorganic composite matrices as efficient desiccants for air dehumidification

A viable option for dehumidification of air stream moisture in closed environments is the use of super adsorbent materials with high capacity for water retention. Selective transport of water molecules across porous materials makes it possible to reduce the humidity of an air stream. The use of such materials has advantage over many commercially available desiccants because they allow an independent control of humidity, being nontoxic materials that adsorb water molecules. It is critical to design new super adsorbent materials for air dehumidification. For instance, a technical solution proposed in this work refers to the development of a specific super adsorbent matrix based on polymer-molecular sieves composites. The polymers are deposited on the rigid structure of different molecular sieves providing a composite structured network with synergistic properties. Theoretical screening indicates that these composites can be used for water retention as efficient desiccants alternative to other available adsorbents and were proved by water adsorption experiments. These polymer-inorganic composites are critical for controlling air dehumidification process.

On the in-depth scaling and dimensional analysis of a cross-flow membrane liquid desiccant dehumidifier

Membrane liquid desiccant dehumidification is deemed as an advanced alternative to air dehumidification, as its operation can be sustained using low-grade waste heat. Many of the existing studies focused on the development and investigation of the dehumidifiers with different flow patterns and geometries, however, the key factors that dominate the dehumidification process are not clear. Therefore, in this paper, an in-depth scaling and dimensional analysis is carried out to investigate the basic physical process in the membrane liquid desiccant dehumidifier. A cross-flow flat-plate membrane liquid desiccant dehumidifier has been developed and tested. Concurrently, a 2-D computational fluid dynamics (CFD) model has been proposed. The model was converted into a dimensionless form so that the governing dimensionless numbers/groups were identified. The relative importance of these dimensionless numbers was determined. A key finding emerged from this study revealed that the relevant dimensionless numbers that dictate performance in the dehumidifier are Rea,Rel,HL,δH,DvmDva andβ. In addition, the heat and mass transfer process is controlled by heat and mass convection, as well as transverse heat conduction and mass diffusion. Finally, a correlation has been judiciously formulated for the dehumidified air temperature and humidity which demonstrated good predictive accuracy within ±3.5% and can potentially evolve to become an invaluable tool to predict and optimize the dehumidifier’s performance.

Hollow fibre membrane-based liquid desiccant humidity control for controlled environment agriculture

Humidity control is an important factor affecting the overall sustainability, productivity, and energy efficiency of controlled environment agriculture. Liquid desiccants offer the potential for pinpoint control of humidity levels in controlled environments. In the present work, a dehumidification processes utilizing liquid desiccants pumped through the lumens of triple-bore PVDF hollow fibre membranes is implemented in a bench scale controlled environment agriculture system. Hydrophobic hollow fibre membranes were combined into an array and placed near the crops. Concentrated magnesium chloride liquid desiccant solution with a low vapour pressure was pumped through the hollow fibre lumens. The dehumidification permeance rate responded dynamically to the changing transpiration rate of the plants, as influenced by changes in environmental factors such as light, temperature, and vapour pressure. The dehumidification permeance rate increased from an average of 0.26–0.31 g m−2 h−1 Pa−1 as the velocity of the liquid desiccant through the hollow fibres increased from 0.023 to 0.081 m s−1. Humidity levels were targeted to be maintained within a range of 70–90% relative humidity at 23 °C. The membrane-based liquid desiccant system was demonstrated to successfully control humidity within a bench-scale controlled environment agricultural setup.

A Full-Solid-State Humidity Pump for Localized Humidity Control

Summary Humidity management is essential and widely needed in various fields. Traditional moisture removal technologies are mostly based on cooling condensation or desiccant dehumidification. These methods come with inherent defects of energy efficiency and are bulky and complex for small-space humidity-control applications. Here, we demonstrate a full-solid-state humidity pump using the advantages of commercial thermoelectric coolers and silica gel materials. Silica gel is coated on the surface of heat sinks to minimize contact thermal resistance between the material and heat source while maintaining enough heat and mass transfer area between the desiccant and the air. The humidity pump device exhibits 0.61 g W−1 h−1 humidity transfer efficiency, corresponding to a humidity transfer rate of 28.38 g h−1. The proposed device neither uses refrigerant nor introduces liquid water. It opens up the possibility of use in localized humidity control applications with higher efficiency and a wider ambient temperature range than traditional technologies.