Air Dehumidification Research Articles

空气除湿处理过程性能改善分析:从理想到实际流程

Dehumidification is an important component of air-conditioning systems. Improving their energy performance would contribute significantly to the overall energy conservation of buildings. In the present study, the efficiency of ideal air dehumidifiers is analyzed. By accessing various processes in ideal air dehumidifiers using either the condensing or the liquid desiccant method, methods to calculate their energy efficiencies are deduced. Performance discrepancies between these ideal dehumidifiers are then investigated and the associated efficiencies of these dehumidifiers are evaluated in typical environments in cities such as Beijing, Shanghai, Wuhan, and Guangzhou, are compared. Better energy performances are obtained if the liquid desiccant method is adopted. That is, dehumidifiers using liquid desiccants offer the potential to improve energy efficiency. Based on the performance of these ideal humidifiers, the characteristics of actual dehumidifiers using liquid desiccants are then explored. In actual processes, the dissipation between moist air and solutions is the result of limited heat and mass transfer capability, unmatched flow rates, and unmatched inlet parameters. The so-called unmatched coefficient ξ m is chosen as the index to evaluate the unmatched dissipation in actual air dehumidifiers using liquid desiccants. As the process between air and solution approaches the iso-concentration line of the liquid desiccant (the air iso-relative humidity line), ξ m approaches unity, indicating that the limited heat and mass transfer ability is given full play. Using air dehumidifiers as a typical example, the energy performance and matching properties of key components of two typical air handling processes are investigated. For these two processes with different regeneration approaches, heating the circulating solution for regeneration exhibits a lower ξ m values than heating the inlet regeneration air. The energy performance discrepancy correlates well with the discrepancy in the unmatched coefficient, which offers a criterion in developing actual air dehumidifiers using liquid desiccants: specifically lowering the unmatched coefficient helps to lower the unmatched dissipation and improve system performance. The present study focuses on air dehumidifiers and the transfer dissipation method is chosen to investigate its characteristics. Based on the transfer dissipation and transfer resistance analysis, the unmatched coefficient ξ m is defined and provides an effective approach for analyzing treatments of air using liquid desiccants.

Synthesis and Characterization of a Chitosan/LiCl Organic–Inorganic Hybrid Membrane for Air Dehumidification

Synthesis and Characterization of a Chitosan/LiCl Organic–Inorganic Hybrid Membrane for Air Dehumidification

Statistical Investigation of Air Dehumidification Performance by Aqueous Lithium Bromide Desiccant in a Packed Column: A Thermodynamic Approach

Lithium bromide solution is used as a desiccant in air dehumidification systems. Liquid desiccant is a solution that facilitates the removal of humidity directly from the air. In this work, effectiveness of a LiBr based air dehumidifier was studied by correlating the vapor–liquid equilibrium data with a proposed thermodynamic model. For this, the nonelectrolyte Wilson nonrandom factor (N-Wilson-NRF) model and the Pitzer–Debye–Huckel formula were used to represent the contribution of the short and the long range ion–ion interactions. In particular, the proposed model assumed that the electrolyte solution is treated as a mixture of undissociated ion pairs and solvent molecules. The proposed equation of this study is valid for the temperature range of 20–35 °C and concentration range of 0.40–0.60 kg/kg. This relation was employed to estimate the equivalent humidity ratio, and then, the humidity ratio from the previous step was used to calculate the effectiveness of a LiBr based dehumidifier. The response surface methodology (RSM) was applied for the design and analysis of the dehumidification experiments. A quadratic model was implemented to predict the dehumidification effectiveness. This model studies the implications of four primary variables on the effectiveness of a dehumidification process. The optimal values to achieve the maximum effectiveness were found to be 32.5 °C for the air temperature, 0.0210 kg/kg for the air humidity ratio, 2.17 for the mass flow rate ratio, and finally, 0.50 kg/kg for the desiccant concentration. These values gave the dehumidification effectiveness of 0.544. The result of the model was in good agreement with the experimental value 0.542, thus verifying the accuracy of the proposed model.

Study of influence of operational parameters on the mass condensate flux in the condenser of seawater greenhouse at Muscat, Oman

The main objective of this theoretical work is to study the influence of the operating parameters such as relative humidity, dry-bulb temperature, seawater temperature, humid air flow rate, and seawater flow rate on fresh water produced by dehumidification of humid air in the condenser of an existing seawater greenhouse that is located in Muscat, Oman. The theoretical results show that the mass condensate rate of the process increases with the augmentation of the relative humidity, the dry-bulb temperature, and the humid air flow rate. However, the seawater temperature and the seawater flow rate have a negative effect. The maximum theoretical mass condensate rate obtained was about 155 kg/h. This result is obtained under the following conditions: for air dry-bulb temperature Tdb = 50°C, relative humidity RH = 97%, inlet seawater temperature Tswin = 28.33°C, humid air flow rate m˙air=23.22kg/s, and seawater flow rate m˙sw=5.87kg/s.

Experimental investigation on compressed air drying performance using pressurized liquid desiccant

ABSTRACTAs a new compressed air-drying method, the compressed air dehumidification using a pressurized liquid desiccant was proposed in our previous study. The pressurized dehumidifier is a complex and core component of the drying system. The mass transfer performance between the compressed air and LiCl aqueous solution is experimentally studied in a counter-flow pressurized dehumidifier filled with structured packing. The humidity ratio of outlet compressed air, vapor removal of processed compressed air, moisture removal rate, and dehumidification efficiency were selected as the performance indices. The results show that the minimum humidity ratio of processed compressed air could reach 0.23 g/kg under 0.71 MPa. Compressed air-drying performance could be remarkably enhanced through increasing the air pressure and liquid desiccant inlet concentration while the influence of liquid desiccant temperature is negative. Furthermore, in order to ensure high compressed air-drying performance, reduce the power consumption of the air compressor and liquid desiccant pump, and the possibility of carryover, the optimum ratio of liquid to compressed air flow rate is recommended to be around 1.5 under pressure around 0.50 MPa. Meanwhile, the energy consumption for per-gram moisture removal of a liquid-desiccant-based compressed air-drying system can reach 1.42 kJ/g lower than cooling dehumidification under 0.3 MPa, which is 16.0% lower than a compressed air-cooling dehumidification system.

Heat and mass transfer of composite desiccants for energy efficient air dehumidification: Modelling and experiment

Desiccant dehumidification technology provides a method of drying air before it enters a conditioned space. When combined with conventional cooling systems, desiccant dehumidification provides an energy-efficient way of supplying thermal comfort air. This paper presents a combined experimental–analytical study on the heat and mass transfer dynamics of composite desiccants during air dehumidification. The composite desiccants are silica gel–calcium chloride, silica gel–lithium chloride, and silica gel–polyvinyl alcohol (PVOH). The derived model is validated against experimental observations of different desiccant types with silica-gel as the host desiccant. Predictions are shown to agree well with extensive experimental measurements conducted using an in-house experimental setup as well as data published in the literature. Experiments were conducted on several promising composite desiccants. The effects of process air velocity, inlet air temperature and humidity on moisture removal capacity, regeneration rates and the associated pressure drops were investigated. Relying on a holistic energy performance index, desiccant coefficient of performance (DCOP), results have indicated that the moisture removal capacity, regeneration rates and the associated pressure drops of composite desiccants outperformed that of pure silica gel by at least 11%.

Adsorption and desorption of silica gel circulating fluidized beds for air conditioning systems

Abstract This article investigates low-energy consumption silica gel circulating fluidized beds for the dehumidification of air conditioning systems. The system consists of an adsorption bed, a desorption bed, and two fans. The fans drive silica gel particles upward to form fluidized beds; as the particles descend, funnels inside the beds allow particles to move between beds through connecting pipes. The particles circulate between absorption and desorption beds to create a continuous operating dehumidification air conditioning system. To achieve a fluidized state, air velocities between 4.0 m/s and 6.0 m/s and regeneration temperatures between 40 °C and 60 °C, which simulate low-temperature waste heat or solar thermal conditions, were chosen for the study. Altering funnel heights and adding oblique baffles to the system allowed us investigate different adsorption/desorption performances. The results show that single-tube fluidized bed can increase adsorption/desorption performance by 20% and lower the pressure drop by about 30%, compared to the packed bed. The circulating fluidized bed system has the highest Energy Factor by 0.554 kg/kW h, and improve the packed bed system by 124% due to the absence of a motor and has the largest total adsorption rate. Increasing air velocities and regeneration temperatures cause adsorption/desorption performance to rise in circulating fluidized bed systems. A regeneration temperature of 60 °C has the highest total adsorption rate of 343 g/h. Compared to the other funnel heights, a 200 mm funnel height with an oblique baffle that increases circulatory effects improves the total adsorption rate by 14% to reach an adsorption rate of 230 g/h.

Experimental investigation of a building integrated photovoltaic/thermal roof collector combined with a liquid desiccant enhanced indirect evaporative cooling system

Large consumption of limited conventional fossil fuel resources, economic and environmental problems associated with the global warming and climate change have emphasized the immediate need to transition to renewable energy resources. Solar thermal applications along with renewable energy based cooling practices have attracted considerable interest towards sustainable solutions promising various technical, economic and environmental benefits. This study introduces a new concept on solar thermal energy driven liquid desiccant based dew point cooling system that integrates several green technologies; including photovoltaic modules, polyethylene heat exchanger loop and a combined liquid desiccant dehumidification-indirect evaporative air conditioning unit. A pilot scale experimental set-up was developed and tested to investigate the performance of the proposed system and influence of the various parameters such as weather condition, air flow and regeneration temperature. A cost effective, easy-to-make polyethylene heat exchanger loop was employed underneath PV panels for heat generation. In addition, a liquid desiccant enhanced dew point cooling unit was utilized to provide air conditioning through dehumidification of humid air and indirect evaporative cooling. The experimental results show that the proposed tri-generation system is capable of providing about 3kW of heating, 5.2kW of cooling power and 10.3MWh/year power generation, respectively. The findings confirm the potential of the examined technology, and elucidate the specific conclusions for the practice of such systems.

Corrosion Failure of Aluminum Heat Exchanger Tubes

This paper investigates the corrosion failure of aluminum tubes on a heat exchanger user to remove heat from the windings of an electric motor. Only the tubes on the perimeter (bottom, top, and sides) at the air inlet location were affected, which is associated with the heat distribution. This investigation concludes that the observed corrosion on the tube is caused by increased chloride concentration from dry out or alternate wetting and drying of the protective aluminum oxide which breaks down, forming a saturated solution of aluminum chloride. The initial aqueous chloride solution is formed by condensation and the high salt content of the inlet air from a marine environment which is then gradually concentrated as it dried out due to heat from the hot nitrogen side of the tube, hence the observed white powder at the tube inlet and vicinity. The concentrated aluminum chloride with an acidic pH of approximately 3.5 becomes very corrosive to the aluminum tubes resulting in the observed pitting and intergranular corrosion. The failure may be seasonal, based on ambient air conditions and the operating temperature of the tube inlet end. Recommendations for prevention are fourfold: change of process/design e.g., dehumidification of the inlet air in order to prevent condensation on the outer tubes; change of material (e.g., use of Incoloy 625 tubes); application of internal coating to prevent condensation (e.g., blast cleaning and coating with a solvent-free epoxy); and replacement of corroded tubes with annual cleaning to remove salts. Selection of the preferred option should involve evaluation of the relative advantages/disadvantages.

Air dehumidification by hollow fibre membrane with chilled water for spacecraft applications

A novel hydrophobic membrane-based dehumidification technology with chilled water is proposed to substitute the traditional dew point method for humidity control in spacecraft, which fulfils the tough tasks of air/water separation and water reuse under low gravity at the same time. A prototype hydrophobic membrane module was designed and fabricated by packing 853 hollow fibres in the shell-and-tube configuration, which could prevent the leakage of the tube-side chilled water to the air side. Theoretical models concerning conjugated heat and mass transfer have been established for the explanation and prediction of the moisture migration across the membranes. Experiments were conducted to validate the theoretical models and the feasibility of water vapour removal under various operating conditions. Although undesirable dewing occurred on the outer membrane surface due to the slow mass transfer relative to heat transfer across the membranes, it was verified by the water increase in graduated cylinder that some water vapour actually transferred through the membrane pores under partial pressure difference. A maximum net dehumidification rate (excluding dewing) of 45 g/h has been achieved by the experiment.

Experimental Investigation of A Liquid Desiccant System for Air Dehumidification Working With Ionic Liquids

Abstract Electrically-driven compression chillers are the commonly used technology for cooling and dehumidifying air. Open sorption systems driven by solar heat are an alternative to conventional air dehumidification technology and may reduce primary energy consumption. For air dehumidification, liquid desiccant systems may exhibit some process engineering and thermodynamic advantages in comparison to solid desiccant systems. The liquid desiccant must exhibit low equilibrium water vapour pressures at the available heat rejection temperature level to achieve low air dew point temperatures and thus a strong air dehumidification with comparably low driving temperatures. Desiccant mass fractions should be as low as possible, but in order to achieve low vapour pressures required desiccant mass fractions may surpass the solubility limit. In the paper at hand, first experimental results of an internally cooled and heated, open liquid desiccant system working with an ionic liquid designed for air dehumidification are presented. It is demonstrated that ionic liquids designed according to the boundary conditions of the respective application may be a promising alternative to commonly used desiccants such as lithium chloride for solar air dehumidification with comparably high heat rejection and low driving temperatures.

Preparation of Natural Zeolite for Air Dehumidification in Food Drying

Drying with air dehumidification with solid adsorbent improves the quality of food product as well as energy efficiency. The natural zeolite is one of adsorbent having potential to adsorb the water. Normally, the material was activated to open the pore, remove the organic impurities, and increase Si/Al rate. Hence, it can enhance the adsorbing capacity. This research studied the activation of natural zeolite mined from Klaten, Indonesia as air dehumidification for food drying. Two different methods were used involving activation by heat and NaOH introduction. As indicators, the porosity and water loaded were evaluated. Results showed both methods improved the adsorbing capacity significantly. With NaOH, the adsorbing capacity was higher. The simple test in onion and corn drying showed the presence of activated natural zeolite can speed up water evaporation positively. This performance was also comparable with Zeolite 3A

Energy and cost associated with ventilating office buildings in a tropical climate.

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This study evaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the following factors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor air temperature and humidity, air conditioning system coefficient of performance (COP), and cost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore’s tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment. Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore’s. This result implies that the benefits of increasing outdoor air ventilation rate up to 25 L/s per person — which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave — can be much larger than the incremental cost of ventilation.

Experimental and modeling analysis of membrane-based air dehumidification

In this work, a membrane cross flow filtration is studied specifically for the purpose of air dehumidification. A flat sheet composite membrane comprising ceramic and polymeric layers with varied water vapor permeability and selectivity has been developed. Effects of the feed air velocity and humidity, the membrane permeability and selectivity, and the permeate pressure on dehumidification performance (i.e. water vapor removal) and resulting COP (Coefficient of Performance) under a cross-flow flat-sheet setting is systematically studied. Results have shown that the membrane dehumidification performance does not depend on the feed air humidity while its COP does not dependent on the membrane’s permeability and the feed air velocity. There exists a tradeoff between the membrane’s ability to dehumidify and its achievable COP when both selectivity and permeate pressure are varied.

Modified PGC model and its validation by experiments for heat and moisture transfer analysis in a vertical fluidized desiccant bed

Air dehumidification in fluidized beds utilizing desiccants is an alternative for the refrigeration methods. A variety of pseudo gas controlled (PGC) model are proposed by assuming constant and varying temperature as well as water content for the solid phase to evaluate the conditions of exit air during adsorption processes. Experimental tests for moisture adsorption in silica gel fluidized bed are carried out. The modified PGC model that assumes uniform water content, varying temperature and linear porosity distributions along the bed estimates the temporal average bed water content to agree very well with the experimental data. The RMSE of the numerical results of the present model ranges from 0.2 to 6% and that obtained from the isothermal model are in the range of 6%–68%.

Energy Efficient Dryer with Rice Husk Fuel for Agriculture Drying

Energy usage is crucial aspect on agriculture drying process. This step spends about 70% of total energy in post harvest treatment. The design of efficient dryer with renewable energy source is urgently required due to the limitation of fossil fuel energy. This work discusses the performance of air dehumidification using rice husk fuel as heat source for onion, and paddy drying. Unlike conventional dryer, the humidity of air during the drying was dehumidified by adsorbent. Hence, the driving force of drying can be kept high. As consequences, the drying time and energy usage can be reduced. Here, the research was conducted in two step: laboratory and pilot scale tests. Results showed that the lowering air humidity with rice husk fuel has improved the energy efficiency. At operational temperature 60oC, the heat efficiency of 75% was achieved.

Sensitivity of the performance of a flat-plate liquid-to-air membrane energy exchanger (LAMEE) to the air and solution channel widths and flow maldistribution

Liquid-to-air membrane energy exchangers (LAMEEs) are novel energy exchangers which use semi-permeable membranes to separate air and desiccant solution streams. The semi-permeable membrane allows simultaneous heat and water vapor transfer between the air and the desiccant solution streams. These exchangers, which include bonded elastic membranes, are prone to significant variations in the flow channel widths, which causes flow maldistribution, and this causes performance degradation. This paper uses a numerical model to show the sensitivity of the steady-state performance of a flat-plate LAMEE to the air and solution channel widths, when operated as a supply air dehumidifier and a diluted desiccant solution regenerator. Simulations show that, without air flow channel width variations, the effectiveness of both the dehumidifier and the regenerator can always be increased toward 1.0 by reducing the nominal air and solution channel widths. To account for variations in the air channel widths, which occur due to construction tolerances and pressure differences across the membrane, laminar fully developed flow equations are used to investigate the influence of random variations in the air channel widths on the performance (i.e. effectiveness and air pressure drop) of the flat-plate LAMEE. The main contribution of this paper is that it shows that there exists an optimal air channel width for flat-plate LAMEEs with flow maldistribution due to random variations in the air channel width. Decreasing the air channel width below this optimal value will result in a decrease in effectiveness. It is found that the optimum nominal air channel width for flat-plate LAMEEs may be 5–6mm and the optimum nominal solution channel width is 1–2mm.

Transient and conjugate heat and mass transfer in hexagonal ducts with adsorbent walls

Honeycomb type adsorbent beds, either in the form of cycling adsorbent beds, or in the form of rotary wheels, are increasingly used in air purification, dehumidification, and energy recovery, etc. The temporal behaviors of operation require a dynamic analysis of the heat and mass transfer in the elementary ducts in the wheels or beds. In this paper, the transient heat and mass transfer in the honeycomb adsorbent ducts of hexagonal cross sections are studied both numerically and experimentally. The transient equations governing the momentum, energy and mass conservation in the air stream and in the porous solid walls are solved simultaneously as a conjugate problem. The velocity, temperature, humidity and water content contours in the ducts are obtained. The effects of operating time and solid wall thickness on the air side convective heat and mass transfer coefficients are analyzed. Different from traditional steady-state heat transfer tubes, the effects of wall thickness are disclosed here. The Nusselt and Sherwood numbers for the fully developed (both axially and temporally) hexagonal ducts with various wall thickness ratios are obtained. They provide guidelines for future component design and performance optimization of adsorption systems.

The effect of vent openings on the microclimate inside multi-span greenhouses during summer and winter seasons

In research literature, little attention has been applied to the effect of vent openings on greenhouse climates during cooling and dehumidification processes with natural ventilation, which would provide guidelines for greenhouse management. To address this problem, a 3D CFD model was successfully designed based on 11-span plastic greenhouses. The model was validated with the measured air temperature and relative humidity, and then used to investigate the effects of vent configuration and opening size on the greenhouse climate. The simulations show that the vent configuration affects greenhouse microclimate patterns and the spatial and temporal variations of the internal climate. Meanwhile, the vent opening size affects greenhouse dehumidification time, air temperature and relative humidity during the dehumidification process. Finally, the assessments of ventilation performance highlight that the roof plus side opening is most suitable for summer cooling, while the roof opening is most suitable for winter dehumidification.

Analytical Study of a Heat Recovery/Desiccant Cooling System under Tunisian Climatic Conditions

 Abstract—The solar potential energy in Tunisia can be used like a principal factor for the operation of new technologies of air conditioning. Studies could be carried out like approaches of simulation, analyzes experimental or both unit. In this paper, we present an analytical solution of heat and mass transfer processes in regenerator and air conditioning system (ACS) cores of a heat recovery /desiccant cooling system (HRDCS) under Tunisia weather conditions. Simulations programs were carried out to test the principal components of the HRDC system. The results show that the coefficient of performance of system improves when we use the renewable energy for heating and cooling. This study is significant and effective to prove the efficiency of such HRDC systems in this country. Energy consumption in buildings has been increased in recent years with the development of the economy worldwide, recent studies have shown that buildings are responsible for the consumption of about 40% of the primary energy and the emission of nearly 33% of the greenhouse gases in the world (1). Therefore, liquid desiccant cooling technologies has been researched and demonstrated in recent years, focusing on applications such as cooling of human houses, commercial buildings, hospitals. It can be an efficient way to provide cooling and to reduce building energy consumption. Approximately 30 years ago, (2) accounted for the operation of a device of air-conditioning on a large scale using water- LiCl, where they presented the direct regeneration in collectors open and storage to the cold in the form of regenerated solution. Some investigators tested the use of the organic absorbents, such as tri ethylene glycol (TEG) to reduce the problem of corrosion involved in inorganic salts (3). Researches (4), (5) have been carried on the liquid desiccant systems for cooling and dehumidification by using solar energy to regenerate the liquid desiccant during the regeneration process and to re-use it for the dehumidification of air. In several cases, the direct regeneration of the liquid desiccant to the sun was considered by using a special type of collector.