Modelling and performance analysis of a cross-flow type plate heat exchanger for dehumidification/cooling

This paper describes the performance analysis of a cross-flow type plate heat exchanger for use as a liquid desiccant absorber (dehumidifier) and indirect evaporative cooler. The proposed absorber can be described as a direct contact, cross-flow, heat and mass exchanger, with the flow passages separated from each other by thin plastic plates. One air stream (primary air) is sprayed by liquid desiccant solution, while the other stream (secondary air) is evaporatively cooled by a water spray. Each thin plate, besides separating the water/air passage from the solution/air passage, also provides the contact area for heat and mass transfer between the fluids flowing in each passage. A parametric study for the primary air stream at 33°C, 0.0171 kg/kg humidity ratio and secondary air stream at 27°C and 0.010 kg/kg humidity ratio using calcium chloride solution was performed in this study. The results showed a strong dependence on the heat and mass transfer area, solution concentration and ratio of secondary to primary air mass flow rates. However, negligible differences were found between the performance of a counter flow and a parallel flow arrangement. The results demonstrate that the proposed absorber will not offset both the latent and sensible load of the primary air and, therefore, an auxiliary cooler or more dehumidification/indirect evaporative cooling stages will generally be required to meet the sensible and latent load in a typical comfort application.     

Performance evaluation of a solar-powered flat-plate multichannel liquid desiccant air conditioning system

There has been a limited application of liquid desiccant (LD) dehumidification systems in space air conditioning until now. The key elements responsible for this restricted implementation are leakage of desiccant solution, corrosion of components, and solution carryover along with the processed air to the space to be conditioned. To remove these problems, an evacuated tube solar heat collector-driven multichannel liquid desiccant air conditioning system has been proposed and experimentally investigated. In this study, dehumidification and regeneration rate, their effectiveness, cooling effect of the dehumidifier, and indirect evaporative cooling unit have been analyzed. The results obtained indicate that the process air has been dehumidified and cooled by 6.32 g kg⁻¹ and 5.26°C, respectively. The regeneration rate and effectiveness have been obtained to be 0.26 g s⁻¹ and 0.31, respectively. In terms of the cooling effect, the system output of 0.703 and 0.130 kW has been obtained from the dehumidifier and indirect evaporative cooling unit of the system, respectively. The proposed system validates the possibility of the novel solar-powered liquid desiccant air conditioning system concept and provides growth and development of the LD air conditioning technology for space air conditioning.     

Performance of a solar liquid desiccant air conditioner – An experimental and theoretical approach

In this paper the results of testing a solar liquid desiccant air conditioner (LDAC) in the tropical climate of Queensland, Australia have been presented. The system uses polymer plate heat exchanger (PPHE) for dehumidification/indirect evaporative cooling, and a cooling pad as the direct evaporative cooler for the dry air leaving the PPHE. Lithium chloride, which is an effective desiccant in air dehumidification, was used in the experiments and a scavenger air regenerator concentrates the dilute solution from the dehumidifier using hot water from flat plate solar collectors. The data obtained from performance monitoring of the solar LDAC operating on a commercial site in Brisbane was compared with a previously developed model for the PPHE. The comparison reveals that good agreement exists between the experiments and model predictions. The inaccuracies are well within the measuring errors of the temperature, humidity and the air and solution flow rates. The above tests further indicate a satisfactory performance of the unit by independently controlling the air temperature and humidity inside the conditioned space.

In order to prevent carryover of the solution particles into the environment, eliminators are used at outlet of the absorber unit and the regenerator. An alternative method in preventing the carryover is the use of indirect cooling, in which the supply air does not contact the solution. The method can be used to produce potable water from the atmospheric air in remote areas.

The liquid desiccant system can be used in the HVAC industry, either as a packaged roof-top air conditioner, or as an air handler unit for commercial applications. The system could also be used for space heating in winter due to the property of desiccants to provide heat when wetted.     

A simplified model for air dehumidification with liquid desiccant

This paper describes a relatively simple model for the preliminary design of an air dehumidification process occurring in a packed bed using liquid desiccant through dimensionless vapor pressure and temperature difference ratios. An expression is derived using the aforementioned ratios to predict the water condensation rate from the air to the desiccant solution in terms of known operating parameters. The model predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. The effects of the cooling water inlet temperature and the desiccant-to-water heat exchanger effectiveness on the performance of the dehumidifier are also studied and the results are presented in this paper.     

带冷却液体除湿系统性能分析的简化方法

采用无量纲蒸汽压差值比和温度差值比对液体除湿系统进行研究,对带冷却液体除湿系统的简化模型进行了计算,并与一组实验数据做了对比,取得了较为满意的效果。分析结果表明用此方法来计算除湿器中除湿量是可行的,且简便易行。该文同时研究了溶液流量、溶液入口温度、浓度等因素对带冷却液体除湿器性能的影响。     

Analytical solution of combined heat and mass transfer performance in a cross-flow packed bed liquid desiccant air dehumidifier

Heat and mass transfer between air and liquid desiccant in a cross-flow packed bed dehumidifier is investigated. Analytical solutions of air and desiccant parameters as well as enthalpy and moisture efficiencies are given in the present study, based on the analogy between the combined heat and mass transfer process in the cross-flow dehumidifier and the heat transfer process in the cross-flow heat exchanger. The results given by the analytical solution are compared with numerical solutions and experimental findings. Good agreement is shown between the analytical solutions and the numerical or experimental results. The analytical solutions can be used in the optimization of the cross-flow dehumidifier.     

Control performance of a dedicated outdoor air system adopting liquid desiccant dehumidification

Liquid desiccant is energy efficient for dehumidification in air-conditioning systems. In this study, a novel dedicated outdoor air system (DOAS) adopting lithium chloride solution as liquid desiccant is proposed to process supply air. The DOAS mainly consists of a membrane-based total heat exchanger, a liquid dehumidifier, a regenerator and a dry cooling coil. It can realize independent temperature and humidity controls for supply air. Control strategies for the supply air dehumidification and cooling process as well as the desiccant solution regeneration process in the DOAS are developed and verified. The control performances of the proposed dedicated outdoor air system are investigated at different operation conditions by simulation tests. The results show that the DOAS is more suitable for hot and humid climates. The effects of the total heat exchanger on the performance of the DOAS are also evaluated. It can improve the system energy performance by 19.9–34.8%.     

太阳能再生式除湿换热器动态除湿性能研究

提出了一种新型再生式除湿换热器,建立了物理和数学模型。通过实验得到了该除湿换热器的实际动态除湿性能;将除湿器除湿性能的模拟结果与实验结果进行比较,验证了数学模型的可靠性。研究结果表明:该文研制的再生式除湿换热器具有良好的除湿性能,在给定工况(温度为24.7℃,含湿量为12.41g/kg)下除湿率可达到43.8%;还分析了处理风速、再生温度以及除湿换热器厚度对除湿性能和压降的影响,获得了使除湿换热器性能最佳的管排、翅片间距和迎面风速参数。     

Experimental and Numerical Study on Heat and Mass Transfer of Cross-Flow Liquid Desiccant Dehumidifier/Regenerator

Abstract

A liquid desiccant air dehumidification system driven by heat pump was established. The performance of cross-flow dehumidifier/regenerator was experimentally investigated. The empirical correlations of Sherwood number for dehumidification/regeneration were obtained by fitting the experimental data. On the basis of the empirical correlations of Sherwood number and thermodynamics analysis of heat and mass transfer process for dehumidifier/regenerator, a cross-flow heat and mass transfer model was established. The effects of air and solution parameters on the dehumidification/regeneration performance were analyzed. The number of mass transfer units and the height-to-length ratio of the packing module were also studied. The results show that there exist optimal number of mass transfer units and height-to-length ratio in the dehumidifier/regenerator.     

Study of heat and mass transfer in a dehumidifying desiccant bed with macro-encapsulated phase change materials

The present article reports on the feasibility of using encapsulated phase change materials (EPCMs) in the dehumidifying bed of a desiccant cooling system. The mathematical model used to simulate the coupled non-equilibrium heat and moisture transfer processes in the porous composite structure containing the EPCM and desiccant particles is presented. Numerical investigations of heat and mass transfer in a desiccant dehumidifying bed composed of silica gel and EPCM particles have been carried out for different values of process parameters. Careful choices of EPCM volume fraction and thermo physical characteristics have been found to increase the overall effectiveness of the desiccant dehumidifier with negligible loss in the dehumidification efficiency. The air stream exits the desiccant/EPCM bed at relatively lower temperature and slightly higher moisture content than from purely desiccant bed. Desiccant cooling systems with less sensible heating and higher cooling capacity can be obtained by employing EPCM in the dehumidifier.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACT

A two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.     

Research progress in liquid desiccant air-conditioning devices and systems

The developments on liquid desiccant air-conditioning systems were illustrated and summarized in this paper. In order to obtain a better dehumidification (or humidification) performance, liquid desiccant should be cooled (or heated) rather than air. Two fundamental modules were proposed, including basic spray module with extra heat exchanger and total heat recovery device, which could be combined to set up various kinds of liquid desiccant air processors. The operating principle of heat pump-driven outdoor air processor as well as heat-driven outdoor air processor was analyzed. The COP_air of the heat pump (or power)-driven outdoor air processor could be as high as 5.0 both in summer and in winter operating conditions. The COP_air of the hot water-driven processor (65°C–80°C) was 1.19 and 0.93, respectively, using evaporative indoor exhaust air or cooling water to cool the dehumidification process. The liquid desiccant air processor-based temperature and humidity-independent control air-conditioning system could save 20%–30% operating energy compared with the conventional air-conditioning system.     

节能型冷却除湿复合式空调系统

介绍了冷却除湿空调的基本原理,对已有系统提出了改进.利用热回收装置,对除湿过程中的吸附热进行回收,并采用室内回风作为间接蒸发冷却器的二次风,能够更有效的降低处理空气的温度.对两种方案的热利用效率进行了实例计算,改进系统更有效.     

Investigation on a two-stage solar liquid-desiccant (LiBr) dehumidification system assisted by CaCl2 solution

A two-stage solar powered liquid-desiccant dehumidification system, for which two kinds of desiccant solution (lithium chloride and calcium bromide) are fed to the two dehumidification stages separately, has been studied. In the studied system air moisture (latent) load is separately removed by a pre-dehumidifier using cheap calcium chloride (CaCl₂) and a main dehumidifier using stable lithium bromide (LiBr). Side-effect of mixing heat rejected during dehumidification process is considerably alleviated by an indirect evaporative cooling unit added between the two dehumidification stages. The feasibility of high-desiccant concentration difference achieved by reusing desiccant solution to dehumidify air and regenerating desiccant repeatedly is analyzed. By increasing desiccant concentration difference, desiccant storage capacity is effectively explored. It is found that the pre-dehumidification effect of CaCl₂ solution is significant in high ambient humidity condition. Also seen is that the desiccant investment can be decreased by 53%, though the cost of equipments is somewhat increased, and the Tcop and COP of the proposed system can reach 0.97 and 2.13, respectively.     

液体除湿空调系统的数学模型与性能分析

建立了一种液体除湿空调系统，核心部件为液体吸收式除湿器，蒸发冷却器是重要组成部分，两者的主体均采用蜂窝结构。给出了统一的数学模型，对除湿器和冷却器内复杂的传热传质过程进行描述。数值模拟结果与实验数据基本一致。运用上述模型编制程序，对系统性能进行预测，表明液体除湿空调系统方案可行。     

三种太阳能液体除湿空调系统除湿器的比较

太阳能液体除湿空调的除温器是系统的重要组成部分。文章通过对三种典型结构的除湿器的传热传质性能、被处理空气与除湿溶液的质量流量比率（MR）和蓄能能力（SC）等方面的比较，得出绝热型除湿器具有比表面积大，被处理的空气流量大等优点，但加湿器内沿程压降较大；水冷型除湿器蓄能能力强，但结构复杂；而交叉流型板式除湿器由于能充分利用回风，是一种可供选择的节能型除湿器。     

A numerical solution for cooling and dehumidification of air by a falling desiccant film in parallel flow

The heat and mass transfer process between falling liquid desiccant film and air in parallel flow heat exchanger is investigated numerically. The governing equations with appropriate boundary and interfacial conditions describing the physical problem are derived. The control volume approach is used to predict the outlet conditions for both the air and the desiccant solution. The effect of inlet conditions, mass flow rates and channel geometry on the air cooling and dehumidification processes is also predicted. The average Nusselt and Sherwood numbers for air flow are correlated in terms of Prandtl number, Schmidt number and channel geometry. Typical numerical experiments showed good agreement of the present results with the available data in literature. Moreover, a parametric study is conducted to illustrate the general effects of various variables on heat and mass transfer processes in cooling and dehumidification of air.     

Simulation and analysis of a novel liquid desiccant air-conditioning system

In this work a novel energy efficient air-conditioning system utilizing lithium chloride (LiCl) solution as liquid desiccant has been proposed and simulated. The simulation of this system is mainly formulated with two packed columns, one for regenerating the weak desiccant and the other for the dehumidification of ambient air. The air is first dehumidified in the dehumidifier and then sensibly cooled in the indirect and direct evaporative coolers. First and second laws of thermodynamics have been used to analyze the effect of five key variables on the performance of the system. High efficiency could be achieved if proper values of these variables are selected.     

Artificial neural network analysis of liquid desiccant dehumidification system

The dehumidification process involves simultaneous heat and mass transfer and reliable transfer coefficients are required in order to analyze the system. This has been proved to be difficult and many assumptions are made to simplify the analysis. The present research proposes the use of ANN based model in order to simulate the relationship between inlet and outlet parameters of the dehumidifier. For the analysis, randomly packed dehumidifier with lithium chloride as the liquid desiccant is chosen. A multilayer ANN is used to investigate the performance of dehumidifier. For training ANN models, data is obtained from analytical equations. Eight parameters are used as inputs to the ANN, namely: air and desiccant flow rates, air and desiccant inlet temperatures, air inlet humidity, desiccant inlet concentration, dimensionless temperature ratio, and inlet temperature of the cooling water. The outputs of the ANN are the water condensation rate and the outlet desiccant concentration as well as its temperature. ANN predictions for these parameters are validated well with experimental values available in the literature with R² value in the range of 0.9251-0.9660. This study shows that liquid desiccant dehumidification system can be alternatively modeled using ANN with a reasonable degree of accuracy.