Adiabatic Dehumidifier Research Articles

A hybrid technology for enhancing VOC transfer in liquid desiccant dehumidifiers: Combining surfactant and internal cooling

Indoor air pollution control has increasingly focused on addressing the presence of volatile organic compounds (VOCs). In this regard, liquid dehumidification is a potential method for VOC removal. However, most of the recent studies focus on the utilization of conventional liquid desiccants and dehumidifiers without demonstrating their potential in VOC removal. To realize this potential and improve removal efficiency, a proper enhancement strategy is needed. In this paper, a hybrid technology named “surfactant + internal cooling” is proposed to enhance the VOC transfer mechanism. A theoretical model is proposed for predicting VOC transfer employing the hybrid technology presented in this study. The proposed technology is compared against other two conventional technologies in terms of removal performance, parameter distribution, and enhancement mechanism. Additionally, the impacts of some of the key parameters on the VOC removal characteristics when employing the hybrid technology are identified, and their respective distributions are characterized and investigated. The results showed that after the implementation of surfactant technology and internal cooling technology, the improvement rates compared to traditional adiabatic dehumidifiers are 22 % and 42 %, respectively. However, with the hybrid technology combining the surfactant and internal cooling, a maximum improvement rate of 63 % is reported. This enhancement can be attributed to the collaborative reduction of the Henry’s law constant by 62 % at the dual-scale level of micro-solution and macro-structure. Overall, it is demonstrated in this paper that the use of the hybrid technology can simultaneously improve the micro and macro driving forces driving the VOC transfer, enhancing the VOC removal performance. The findings of this study provide new insights to improve the performance of liquid desiccant dehumidifiers in VOC removal applications.

A view on the performance comparison between adiabatic and internally-cooled dehumidification processes using liquid desiccant

Humidity control is emphasized in various fields. However, the effectiveness comparison between adiabatic dehumidifier (AD) and internally-cooled dehumidifier (ICD) was only conducted in limited cases, without comprehensive understanding of different cooling mediums and working conditions. In this study, the component effectiveness of these two dehumidifiers is parametrically compared. When chilled water serves as cooling medium, AD has sensible effectiveness of 0.65 and latent effectiveness of 0.84 under baseline case, while those of ICD are 0.52 and 0.85 respectively. AD performs better under higher sensible loads, and ICD is more capable of dealing with more moisture transfer. Low water temperature, low air-desiccant ratio and adequately low water-desiccant ratio also highlight the superiority of AD. When secondary air serves as cooling medium, AD always has higher effectiveness than ICD in the studied range, and the superiority is more significant at lower temperature and humidity of secondary air. Sensible effectiveness of 0.44 and latent effectiveness of 0.78 for AD are obtained under baseline case, while those of ICD are 0.07 and 0.74 respectively. ICD has better adaptivity than AD at a circumstance with great variation of operation parameters. The results contribute to enlightening insights for further design of moisture capture components.

Performance enhancement analysis of a liquid desiccant dehumidifier with internal cooling for indoor benzene removal

Indoor volatile organic compounds (VOCs) pollution is becoming increasingly serious, posing a huge threat to human health. The removal of VOCs has always been a challenge in air purification processes. In this regard, conventional VOC removal methods are often not ideal. Liquid desiccant (LD) dehumidification is a new removal method proposed recently and becoming increasingly used in many applications. Previous studies dealing with LD mainly focused on adiabatic dehumidifiers, but the results are not very promising. The main reason for this is the significant temperature rise of the LD solution, which seriously affects the mass transfer of the VOC. To enhance mass transfer performance, it is important to suppress temperature rise. In this study, internal cooling (IC) technology is introduced to enhance benzene mass transfer. A mathematical model is established for modeling and assessing the mass transfer process. The benzene transfer characteristics are studied and compared, considering dehumidifiers with and without IC. The effects of the cooling water, air, LD solution, and transfer parameters on the benzene mass transfer performance are evaluated and identified. Additionally, the distribution differences of the key parameters along the dehumidifier are revealed. The results indicated that benzene transfer performance can be significantly enhanced by adopting IC, with a performance improvement of 28.42%. This improvement is attributed to the decrease in the LD solution temperature caused by IC and the corresponding Henry’s constant. The cooling water parameters, airflow rate, LD solution status, and NTUbenzene are the key factors affecting the removal performance. Overall, employing IC can effectively improve the transfer driving force for benzene, resulting in better benzene transfer performance. This study demonstrates the feasibility and promising potential of using IC technology to enhance benzene transfer in LD dehumidifiers.

Parametric investigations on a novel bubble column liquid desiccant dehumidifier with internal cooling

Increasing demand for space cooling and proportionate demand for electrical power call for less power intensive technology with lower carbon footprint. The liquid desiccant (LD) based air conditioning systems can remove latent heat more efficiently, mainly using heat energy coming from waste or renewable energy source. A novel, simple and corrosion-free bubble column liquid desiccant air dehumidifier (BCLD_ADH) with provision for internal cooling of LD has been developed in this work. The advantages of providing internal cooling and effect of LD concentration as well as ambient conditions are evaluated in this experimental work. Two new type of effectiveness, one based on ambient wet bulb temperature and the other on crystallization temperature of LD are proposed here to give due credit to higher potential of moisture removal available in internally cooled dehumidifier. At 40% concentration of LD, the dehumidifier effectiveness at wet bulb temperature was 67% for adiabatic dehumidifier, but could be increased to 95% using internally cooled dehumidifier. Using higher concentration is necessary for adiabatic dehumidifier but less important for internally cooled dehumidifier as lower LD temperature helps retain higher potential for mass transfer. The inlet air humidity ratio helps to increase change in humidity ratio while higher mass flow rate also helps increase moisture removal rate proportionately.

Counter-crossflow indirect evaporative cooling-assisted liquid desiccant dehumidifier: Model development and parameter analysis

Using auxiliary internal cooling is an effective way to improve the dehumidification effectiveness of the liquid desiccant dehumidifier (LDD) compared with the adiabatic dehumidifier. Indirect evaporative cooling (IEC) is a promising solution for internal cooling owing to its high efficiency and environmentally friendly features. A hexagonal plate heat exchanger (PHE) consisting of both counterflow and cross flow was used as a core in an IEC-internally-cooled LDD to ensure efficient heat transfer and facilitate easy installation simultaneously. A numerical heat and mass transfer model was established and validated, and an intensive parameter analysis of the thermal performance of the dehumidifier was conducted. The influential parameters include inlet primary air conditions (temperature, humidity, and velocity), inlet solution conditions (temperature, mass flow rate, and concentration), secondary to primary airflow ratio, the geometry of the heat exchanger, and channel gap. The performance of the newly developed counter-cross flow LDD with IEC-assisted cooling was improved by 16 % and 8.4 %, respectively, compared to the adiabatic and staggered flow type dehumidifiers. Among all the design parameters of the heat exchanger, the channel gap has the most significant effect on the dehumidification efficiency, and the recommended optimum height is 0.004 m. The air velocity is the most influential operating parameter on the dehumidification efficiency. The dehumidification effectiveness is reduced by 23 % as the primary air inlet velocity decreases from 4.5 m/s to 0.5 m/s.

Experimental investigation on structured packed bed liquid desiccant dehumidifier: An optimal mixture design of experiments strategy

Abstract Maintaining human thermal comfort and extensive power consumption of the conventional heating ventilating and air-conditioning systems drive the researchers to look for an alternative dehumidification system. Among the potential alternatives, the liquid desiccant dehumidification system has evolved as a long-term renewable and less power consuming system due to its effective controlling capability of humidity. In the present study, an experimental investigation is carried out to demonstrate the performance of lithium bromide (LiBr) and potassium formate (HCOOK) desiccant solution mixture in a spray type counterflow-packed bed adiabatic dehumidifier. Subsequently, ternary plots and 3D-contour trade-off paradigms are patterned to assess the performance parameters using an optimal mixture design methodology. Additionally, empirical correlations are developed to pattern the performance parameters (moisture removal rate (MRR), enthalpy effectiveness (ξh) and moisture effectiveness (ξm)) as a function of blend combinations. Further, the present study endeavours to explore the application of offline calibration methods based on a balanced optimum mixture design-centred desirability approach for optimizing the mixture proportion to obtain the optimal performance parameters (i.e., MRR, ξh and ξm). The wettability area is explained based on the static contact angle on a plane glass. The results showed that the contact angle was decreased from 47.3° to 44.1° for the optimum mixture of LiBr solution (37.2%-LiBr +17.8%-HCOOK), which further confirmed the higher MRR. Lastly, the tuning effect of the liquid to gas flow rate on the dehumidifier performance and economic viability of LiBr-HCOOK blends are studied.

Numerical investigation of performance trade-off characteristics of a packed bed dehumidifier using aqueous blends of lithium chloride and calcium chloride

Current research in dehumidification systems is mainly focused on thermally driven liquid desiccant systems. However, the selection of proper desiccant solution has a greater significance on the performance of a dehumidifier. In this perspective, current work is presented with governing equations of energy, mass and species balance to study the performance of a dehumidifier. The governing equations are solved using finite difference method coupled with Jacobi’s model for an air–desiccant contact system of a cross-flow packed bed adiabatic dehumidifier operating with different desiccant blends of lithium chloride (LiCl) and calcium chloride (CaCl2) solution. The effect of variation in blend proportion on outlet parameters of a packed bed dehumidifier is patterned under various desiccant to air (L/G) mass flow rate ratios. It is observed that the addition of CaCl2 desiccant solution into LiCl changes the nature of thermo-physical properties of blends‚ which are predicted using non-random two liquid (NRTL) equation. Further, a trade-off analysis among the performance parameters is presented. It demonstrates that dehumidifier performance paradox such as enthalpy effectiveness (ɛh), moisture removal rate (MRR) and moisture effectiveness (ɛm) have a tendency to be optimal for 35% LiCl with 5% CaCl2 blend (BL1) among all blends at L/G ratio of 2. The variation of thermodynamic properties (air temperature, humidity, desiccant temperature and desiccant concentration) inside the dehumidifier module is visualized in terms of contour plots. Furthermore, the irreversibility in heat and mass transfer operation for the optimal blend is investigated in terms of physical and chemical exergy destruction paradigm. The result of such investigation elucidated that the maximum chemical exergy destruction is 0.88 kW, whereas the physical exergy destruction is 0.06 kW.

A review of coupled heat and mass transfer in adiabatic liquid desiccant dehumidification and regeneration systems; advances and opportunities

AbstractIn this paper, a comprehensive technical review of liquid desiccant (LD) dehumidification and regeneration techniques is presented. The operational features, processes and performance indices of various flow configurations of adiabatic dehumidifier and regenerator are extensively covered. The heat and mass transfer assessment is presented in terms of past experimental and modelling evaluations and procedures. The existing adiabatic dehumidifier/regenerator heat and mass transfer models are categorized into finite difference, effectiveness-number of transfer units and simple empirical correlation models. The respective performance prediction models are critically analysed in details and compared in terms of assumptions, iterative procedures, solution methods, accuracy, computation time, output variables and applications. The solar regenerator models are also highlighted with a focus on the collector module. The ideal settings, formulation procedures, current state-of-the-art and opportunities for improvements are outlined. The review provides meaningful insight into the research status and available opportunities in the LD adiabatic dehumidifier and regenerator modelling and optimization as well as conceptualization of the applicable models. Finally, some very impactful suggestions for improvement and further research are outlined.

Falling film liquid desiccant air dehumidification

Falling film liquid desiccant dehumidification technology is attracting more and more attention due to lower energy consumption, less pollution, and more flexible humidity control in recent years. This paper conducts a comprehensive review on falling film liquid desiccant dehumidification systems. Firstly, the working principles and features of the liquid desiccant dehumidification are introduced to describe the dehumidification process. The existing liquid desiccants including organic and inorganic desiccants are reviewed. Then, the structures of falling film dehumidifiers including both adiabatic dehumidifiers and internally-cooled dehumidifiers are described. Besides, the simulation models of falling film dehumidifiers, such as finite difference models, effectiveness NTU (e-NTU) models and simplified simulation models are summarized. Finally, the exiting performance enhancing methods of falling film dehumidifiers are collected, which provide valuable guidance to researchers and engineers to improve the dehumidification performance.

Analytical solution of heat and mass transfer process in internally cooled liquid desiccant dehumidifiers using refrigerant as cooling medium

The internally cooled liquid desiccant dehumidifier is regarded as an efficient heat and mass transfer component for liquid desiccant dehumidification systems. Compared with the typically adopted adiabatic dehumidifier, three fluids are used inside the dehumidifier: air, liquid desiccant, and the internal cooling medium. Herein, a simplified analytical solution for the heat and mass transfer model of an internally cooled dehumidifier is proposed and validated. The enthalpy efficiency and moisture efficiency of the dehumidifier are deduced from the analytical solutions and are adopted to evaluate the dehumidifier performance. The influences of different inlet, operating, and device parameters on the performance of the device are investigated using the analytical solution. We found that the enthalpy and moisture efficiencies are decided primarily by six dimensionless numbers, three indicating the inlet states of the fluids, one indicating the mass flow rate of air to desiccant, and two indicating the heat and mass transfer ability of the device.

Characterisation and evaluation of a new phase change enhanced working solution for liquid desiccant cooling systems

Desiccant solutions play an essential role in desiccant cooling systems to absorb moisture from the process air. This paper presents the characterisation of a new working solution for liquid desiccant cooling systems. The new working solution was prepared through dispersion of micro-encapsulated phase change materials (MPCMs) into lithium chloride (LiCl) desiccant solutions to ensure that the dehumidification process was achieved under a low temperature condition and to improve thermal capacity and moisture removal efficiency of the mixture. The properties of the new solution, including density, enthalpy-temperature relationship, particle size distribution, thermal conductivity, and vapour pressure were characterised through either experimental tests or theoretical analysis. It was shown that the density and thermal conductivity of the new working solution slightly decreased with the increase of the mass fraction of the MPCMs in the mixture. The thermal capacity of the new working solution substantially increased in the melting temperature range of the MPCMs used. The vapour pressure of the new working solution decreased due to the existence of the MPCM particles. It is expected that the dehumidification efficiency of adiabatic dehumidifiers can be potentially improved when using this new working solution due to the decreased vapour pressure and increased thermal capacity of the phase change enhanced desiccant solution.

Experimental analysis of a novel internally-cooled dehumidifier with self-cooled liquid desiccant

With respect to air dehumidification, an internally-cooled dehumidifier is more efficient than an adiabatic dehumidifier. In the present study, a new method to control the increase in the temperature of the liquid dehumidifier solution was presented. A self-cooled liquid desiccant solution was defined and prepared. Anhydrous ethanol was added into the lithium chloride (LiCl) aqueous solution as the vaporizable coolant to control the increase in the temperature of the solution. A working hypothesis for the self-cooled solution dehumidification was established with a main feature wherein the heat exchangers in conventional internally-cooled dehumidifier were replaced by using self-cooled liquid desiccant solution. Its performance was compared and experimentally investigated. An environmental chamber and a visible experimental setup were established. Experimental results under different operating conditions were obtained. The performance indices including moisture removal rate and dehumidification efficiency were analysed. When compared with conventional LiCl liquid desiccant solution, the current analysis indicated that the self-cooled solution is superior with respect to the above performance indices.

Studying the heat and mass transfer process of liquid desiccant for dehumidification and cooling

Desiccant-based dehumidification has gained significant attention in recent years to develop the energy efficient hybrid air-conditioning systems particularly for the tropical climate. Thus far, complex and tedious theoretical models have been developed to analyze the heat and mass transfer phenomena involved in liquid desiccant dehumidification processes. In the present study, a simple theoretical model has been presented to quantify the heat and mass transfer rates of the water-cooled liquid desiccant dehumidification and cooling processes for air-conditioning applications. Seven coupled ordinary differential equations have been developed to describe the complex heat and mass transfer processes. As far as the solution approach and practical application are concerned, the governing equations are solved simultaneously using the numerically stable 4th order Runge-Kutta scheme while preserving its accuracy to 6% in construct to full-scale model simulation. The developed model is rationally modified to study the performance of adiabatic dehumidifiers. The dehumidification and cooling performances of the adiabatic and water-cooled dehumidifiers are analyzed over a wide range of operating conditions. Finally, the energy performance enhancement potentials of the hybrid liquid desiccant air-conditioning system have been demonstrated. By analyzing the dehumidification and cooling performance, an energy saving of up to 25% can be achieved using the hybrid air-conditioning systems.

Mathematical model and performance analysis of a novel outside evaporative cooling liquid desiccant dehumidifier

A numerical model of a novel outside evaporative cooling liquid desiccant dehumidifier (OECD) was developed and the effects of inlet parameters, including the inlet temperature and relative humidity of dehumidified air and evaporative cooling (EC) air, as well as the inlet mass flow rate of solution and so on, on the device performances were investigated in this paper. The results show that as the inlet temperature of solution increased from 31 to 42 °C, the moisture removal rates of OECD were increased by 14.0–18.0% and 31.1–101.5% compared to the non-evaporative cooling dehumidifier (NECD) and the adiabatic dehumidifier respectively, whereas the dehumidification rate was only decreased by about 1.6% with increase in the inlet temperature of LiCl solution from 24 to 44 °C. All these results can provide guidance for the structural design and performance analysis of the dehumidifier in the future.

Coupled Heat and Mass Transfer Analysis of an Adiabatic Dehumidifier – Unique Approach

Abstract Liquid desiccant dehumidifier is one of the most desirable systems for dehumidification of air when compared to conventional refrigeration system and solid desiccant based dehumidifier. In the present study, a finite difference model is developed to simulate the coupled heat and mass transfer processes occur in a counter flow adiabatic dehumidifier employing lithium chloride (LiCl) as a desiccant solution. Mat-lab R13 is used as a simulation package for solving the aforementioned model. This model has taken into the consideration of thermal effectiveness, effective height and moisture effectiveness as variable parameters for obtaining the correlation of the desired operating parameters. The predicted numerical parameters for absorption process have good agreement with the experimental data reported in the literature. This model is unique and can be extended to any type of liquid desiccant solution by changing the solution properties. The performance analysis of adiabatic counter flow dehumidifier is also carried out by varying the operating parameters and the detailed results are presented in the full manuscript.

Study on an internally-cooled liquid desiccant dehumidifier with CFD model

The liquid desiccant air conditioning system is considered as a possible substitute of the traditional air conditioner mainly due to its characteristics of energy saving. The dehumidifier is a key component of the system therefore was chosen as the research object. It was found the present models conducted the simulation with lots of assumptions, especially ignoring the effect of flow behavior. Besides, most studies focused on the inlet and outlet parameter changes rather than the interior condition of the dehumidifier. To fill the research gap, a model for an adiabatic dehumidifier was established with CFD technology in authors’ previous study. On the basis of it, a model was further developed in present work for internally-cooled liquid desiccant dehumidifier. The interior heat and mass transfer processes were then simulated with the model, followed by the detailed performance investigation. Analysis was conducted to investigate the influence of some factors, including inlet desiccant temperature, desiccant flow rate and two types of internally cooling, and variable physical properties. The advantage of present study lied in its more in-depth analysis of interior condition of the dehumidifier. Besides, the study also demonstrated the necessity of considering the variable properties of desiccant solution during the simulation.

A review of the mathematical models for predicting the heat and mass transfer process in the liquid desiccant dehumidifier

Abstract The paper aims to overview various mathematical models for modeling the simultaneous heat and mass transfer process in the liquid desiccant dehumidifier. Firstly, the dehumidification principle is introduced briefly. Then the models are interpreted in terms of two classes of dehumidifiers. For the adiabatic dehumidifier, the models are mainly classified into three types: finite difference model, effectiveness NTU (e–NTU) model, and simplified models. For the internally cooled dehumidifier, there are also three kinds of models: models without considering liquid film thickness, models considering uniform liquid film thickness, and models considering variable liquid film thickness. This review is meaningful for comprehending the development process and research status of the models and choosing suitable models for prediction. In addition, some suggestions are proposed for the model improvement.

Experimental study on partially internally cooled dehumidification in liquid desiccant air conditioning system

The dehumidifier is one of the essential components in air-conditioning systems with liquid desiccant. Compared with adiabatic dehumidifiers, internally cooled dehumidifiers can achieve better dehumidification performance. In this study, dehumidification effectiveness and moisture removal rate were adopted to evaluate and compare the performance of these two kinds of dehumidifiers. The results indicated that internally cooled dehumidifiers can significantly improve performance compared with adiabatic dehumidifiers, especially when the solution is at a high temperature and/or low concentration in the downstream section of the liquid desiccant flow. Performance improvement and energy saving can be taken into consideration simultaneously by only supplying cooling water to the latter part of the dehumidifier. Solution temperatures at 20–25°C for adiabatic dehumidifiers are optimal and those at 25–30°C for internal cooled types. Lower temperatures of the cooling water can result in higher values for dehumidification effectiveness and moisture removal rate in an internally cooled dehumidifier; the limited temperature difference between cooling water and solution might seriously weaken the effect of the cooling water. Dimensionless Sherwood number Sh was calculated, and discrepancies between predicted values and experimental data are almost within ±20%.

Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl2 were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.

Performance analysis on the internally cooled dehumidifier using liquid desiccant

Dehumidifier is one of the most important components in liquid desiccant air-conditioning systems. Previous study shows that the internally cooled dehumidifier may have better mass transfer performance than the adiabatic unit. The effect of flow pattern, especially the flow direction of air to desiccant on the internally cooled dehumidifier performance is numerically analyzed in detail. The result shows that counter-flow configuration of air to desiccant has better dehumidification performance, and parallel-flow configuration performs the poorest with the same conditions, due to more uniform mass transfer driving force expressed in the counter-flow configuration. The decrease of the desiccant concentration is the main factor that influences the internally cooled dehumidifier's performance, while the increase of the desiccant temperature is the main performance restricting factor in adiabatic dehumidifier. Internally cooled dehumidifier has better mass transfer performance compared with adiabatic dehumidifier plus external heat exchanger.