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.     

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.     

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.     

Heat and mass transfer in a cross-flow membrane-based enthalpy exchanger under naturally formed boundary conditions

Heat and mass transfer mechanisms in a cross-flow parallel plate membrane-based enthalpy exchanger for heat and moisture recovery from exhaust air streams are investigated. The flow is assumed laminar and hydrodynamically fully developed, but developing in thermal and concentration boundaries. Contrary to the traditional methods to assume a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, in this study, the real boundary conditions on the exchanger surfaces are obtained by the numerical solution of the coupled equations that govern the transfer of momentum, thermal energy, and moisture in the two cross-flow air streams and through the membrane. The naturally formed heat and mass boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the cross-flow passages, in the developing region and thereafter. A comparison was made with those results under uniform temperature (concentration) and uniform heat flux (mass flux) boundary conditions, for rectangular ducts of various aspect ratios. An experiment is done to verify the prediction of outlet moisture content.     

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.     

Conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor for liquid desiccant air dehumidification

The fluid flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor are investigated. The shell-and-tube like contactor is used for liquid desiccant air dehumidification, where numerous fibers are packed into the shell and air flows across the fiber bank. To overcome the difficulties in the direct modeling of the whole contactor, a representative cell, which comprises of a single fiber, a liquid solution inside the fiber, and an air stream across the fiber, is selected as the calculation domain. The air stream in the cell is surrounded by an assumed outer free surface. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams are solved together with the heat and mass diffusion equations in the membrane. The friction factor and the Nusselt and Sherwood numbers on the air and stream sides are then calculated and experimentally validated.     

An experimental study of a cross-flow type plate heat exchanger for dehumidification/cooling

The thermal and dehumidification behaviour of a standard cross-flow type plate heat exchanger, intended for use as a dehumidifier/cooler, has been investigated both experimentally and numerically. Three sets of experiments have been carried out where air is blown into the primary and secondary sides of the exchanger, while water and liquid desiccant were being sprayed in a counter flow arrangement. The first set represents the indirect evaporative cooling of the primary stream by the secondary air stream. The second set is with liquid desiccant only and no indirect evaporative cooling. In the third set of experiments the primary air stream is indirectly evaporatively cooled by the secondary air stream and dehumidified by the liquid desiccant sprayed into the primary side of the exchanger. The above experiments indicate that the heat exchanger performs well when used with liquid desiccant. Furthermore, for an exchanger angle of 45°, there is an optimum value of air mass flow rate at which the effectiveness and dehumidification efficiency of the plate heat exchanger are maxima. To investigate the effect of the ambient air conditions on the PHE performance, further experiments were carried out using a heater element and a humidifier. The results show that under laboratory conditions the exchanger effectiveness and dehumidification efficiency increase with increasing primary air inlet temperature and humidity ratio. The experimental results were used to validate a computer model developed for the cross-flow type plate heat exchanger/dehumidifier. Comparison indicates that the numerical results are in good agreement with the experiments.     

Theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger

This paper presented theoretical and experimental investigations of a liquid desiccant filmed cellulose fibre heat and mass exchanger, a new type of exchanger with the potential to be an alternative to a conventional exchanger. Owing to the complexity of the desiccant assisted heat and mass transfer and difficulty in determining its associated parameters, work started from the simulation of a clear fibre exchanger by developing a dedicated numerical model, and its validation by using the data from the manufacturer of the exchanger. Further to this, laboratory testing was carried out with the same exchanger, but filmed with a liquid desiccant fluid, i.e. LiCl. Comparison between the data of the clear and desiccant filmed exchangers suggested the use of correction factors for heat and mass transfer resistances with desiccant operation. A revised model for the desiccant filmed exchanger was then established taking into account the correction factors. By using the updated model, influence of geometrical sizes and operating conditions of the liquid desiccant filmed exchanger on the exchanger efficiency were studied and the optimal values of these were obtained. The results indicated that the exchanger efficiencies (heat, mass and enthalpy) are largely dependent upon the exchanger channel length, air flow rate and less related to the exchanger channel height, intake air temperature and intake‐to‐outgoing air moisture content difference. It was also suggested that the air speed across the channels should be in the range 0.5–1.5 m s^?1. The height of air channel (passage) should be set at 6.5 mm or below and its length should be 1.0 m or more. A simulation was carried out under UK typical summer operation conditions, i.e. the intake air streams at 30°C db and 70% rh and outgoing air streams at 24°C db and 50% rh, and the results indicated that the exchanger with the above recommended geometrical sizes can achieve an energy efficiency of 87%, which is 30% higher than for non‐desiccant filmed operation. Copyright © 2009 John Wiley & Sons, Ltd.     

An analytical model for heat and mass transfer processes in internally cooled or heated liquid desiccant–air contact units

Internally cooled or heated liquid desiccant–air contact units can be used for effective air dehumidification or desiccant regeneration, respectively. One-dimensional differential equations were utilized in the present study to describe the heat and mass transfer processes with parallel/counterflow configurations. The effects of solution film heat and mass transfer resistances, the variations of solution mass flow rate, non-unity values of Lewis factor and incomplete surface wetting conditions were all considered in the differential model. On considering the relatively narrow ranges of operating conditions in a specified application, the equilibrium humidity ratio of desiccant solution was assumed to be a linear function of its temperature and concentration. Constant approximations of some properties and coefficients were further made to render the coupled equations linear. The differential equations were rearranged and an analytical solution was developed for newly defined parameters. For four possible flow arrangements and three types of commonly used liquid desiccant solutions, results of analytical solutions were compared with those of numerical integrations over a wide range of operating conditions, and the agreement was found to be quite satisfactory. Further, the heat and mass transfer performances were analyzed and some guidance to improve the unit design was provided.     

Effectiveness of heat and mass transfer in packed beds of liquid desiccant system

A liquid desiccant system (using CaCl₂) is presented for air dehumidification using solar energy or any other low grade energy to power the system. The system utilizes two packed beds of counterflow between an air stream and a solution of liquid desiccant for the processes of air dehumidification and solution regeneration. To simplify the prediction of the performance of the system an effectiveness of heat transfer and an effectiveness of mass transfer in the packed beds are defined. A finite difference model is developed to model the heat and mass transfer in packed beds during the air dehumidification mode and the solution regeneration mode. This finite difference model is used to calculate the effectiveness of heat and mass transfer in the packed beds at various bed heights, various air and solution flow rates, various inlet temperatures of air and solution to the bed, and various concentrations of CaCl₂ solution at the bed entrance. Charts of the effectiveness of heat and mass transfer are presented in a convenient form. A designer of a liquid desiccant system may use the charts in predicting the performance of these systems without having to use the finite difference model for this purpose.     

Multi-Objective Design Optimization of Hollow Fiber Membrane-Based Liquid Desiccant Module Using Particle Swarm Optimization Algorithm

ABSTRACT

Hollow fiber membrane-based liquid desiccant air dehumidification is an effective method for air dehumidification. The benefit of this technique is that the carryover problem of air mixing with desiccant solution can be avoided. Optimal design of the hollow fiber membrane-based liquid desiccant module is presented in this paper. Two objective functions are total entropy generation and total annual cost. However, a lower entropy generation often corresponds to a higher cost of the designed hollow fiber membrane module. To balance the total entropy generation and the total annual cost of hollow fiber membrane-based liquid desiccant modules, the multi-objective particle swarm algorithm is developed for the optimal hollow fiber membrane module design problem. The geometric variables like length of fiber, inner diameter of fiber, number of fiber and packing fraction effect on the total entropy generation and total annual cost are studied. The optimal structural parameters of a hollow fiber membrane-based desiccant module are obtained using multi-objective particle swarm algorithm.     

A new method for determining coupled heat and mass transfer coefficients between air and liquid desiccant

This paper presented the characteristic of liquid desiccant dehumidification based on NTU–Le model. The results showed that the Lewis number Le had little effect on air outlet humidity ratio during desiccant solution dehumidification process. A new method called h_D–Le separative evaluation method was developed for determining coupled heat and mass transfer coefficients between air and liquid desiccant, through which the heat and mass transfer coefficients between air and liquid desiccant were calculated to obtain from experimental inlet and outlet parameters of air and desiccant solution. The effects of the air volume flow rate, temperature, humidity ratio and the solution concentration, temperature on the Lewis number, heat and mass transfer coefficient were analyzed according to experimental data and the h_D–Le separative evaluation method. Based on the computation results, it was concluded that the Lewis number greatly depended on the operation parameters and conditions of the air and desiccant. In addition, the correlations of the heat and mass transfer coefficients were developed. The additional 74 groups of experiments validated the developed correlations by comparison of air/solution parameters change with the calculation data.     

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%.     

Conjugate heat and mass transfer in a hollow fiber membrane module for liquid desiccant air dehumidification: A free surface model approach

Conjugate heat and mass transfer in a hollow fiber membrane module used for liquid desiccant air dehumidification is investigated. The module is like a shell-and-tube heat exchanger where the liquid desiccant stream flows in the tube side, while the air stream flows in the shell side in a counter flow arrangement. Due to the numerous fibers in the shell, a direct modeling of the whole module is difficult. This research takes a new approach. A representative cell comprising of a single fiber, the liquid desiccant flowing inside the fiber and the air stream flowing outside the fiber, is considered. The air stream outside the fiber has an outer free surface (Happel’s free surface model). Further, the equations governing the fluid flow and heat and mass transfer in the two streams are combined together with the heat and mass diffusion equations in membranes. The conjugate problem is then solved to obtain the velocity, temperature and concentration distributions in the two fluids and in the membrane. The local and mean Nusselt and Sherwood numbers in the cell are then obtained and experimentally validated.     

Performance characterisation of liquid desiccant columns for a hybrid air-conditioner

Use of liquid desiccant-vapor compression hybrid system is encouraged for low humidity applications. The liquid desiccant is primarily used to further dehumidify the supply air. In the present study, by using psychrometric equations and liquid desiccant property data, heat and mass transfer analysis for the dehumidifier and regenerator columns in counter flow configuration has been carried out. The simulation of the columns corresponds to low solution to air (S/A) flow ratio where precooled air gets dehumidified in the absorber while preheated air is used for regeneration of the solution. A detailed study of the performance characteristics for the absorber and regenerator columns confirms the requirement of the desiccant loop for additional dehumidification of the conditioned air. This need develops the main motive towards the concept of hybrid air conditioning.     

Fluid flow and heat mass transfer in membrane parallel-plates channels used for liquid desiccant air dehumidification

Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.     

Effectiveness–NTU relation for packed bed liquid desiccant–air contact systems with a double film model for heat and mass transfer

One-dimensional models were usually utilized to describe the coupled heat and mass transfer processes in packed bed liquid desiccant–air contact systems. In this paper, a double film model was utilized for both parallel and countercurrent flow configurations. The model considered the effects of non-unity values of Lewis factor, unequal effective heat and mass transfer areas, liquid phase heat and mass transfer resistances, changes in solution mass flow rate and concentration. Within the relatively narrow range of operating conditions usually encountered in a specified application, a linear approximation was made to find out the dependence of equilibrium humidity ratio on solution temperature and concentration. Constant approximations of some properties and coefficients were further made to render the coupled equations linear. The original differential equations were rearranged and an analytical solution was developed for a set of newly defined parameters. Analytical expressions for the tower efficiency and other effectiveness values were further developed based on the analytical solution. Comparisons were made between analytical results and numerical integration of the original differential equations and the agreement was found to be quite satisfactory.     

Coupled heat and mass transfer characteristic in packed bed dehumidifier/regenerator using liquid desiccant

The dehumidifier and regenerator are two key components in liquid desiccant air conditioning systems. The heat transfer driving force and the mass transfer driving force influence each other, the air and desiccant outlet temperatures or humidity ratio may exceed the air and desiccant inlet parameters in the dehumidifier/regenerator. The uncoupled heat and mass transfer driving forces, enthalpy difference and relative humidity difference between the air and desiccant are derived based on the available heat and mass transfer model and validated by the experimental and numerical results. The air outlet parameter reachable region is composed of the air inlet isenthalpic line, the desiccant inlet equivalent relative humidity line and the linkage of the air and desiccant inlet statuses. Except the mass flow rate ratio and the heat and mass transfer coefficients, the air and desiccant inlet statuses and flow pattern have great effects on the dehumidifier/regenerator performance. The counter flow configuration expresses the best mass transfer performance in the dehumidifier and the hot desiccant driven regenerator, while the parallel flow configuration performs best in the hot air driven regenerator.     

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。     

Experimental investigation of the heat and mass transfer between air and liquid desiccant in a cross-flow regenerator

The regenerator is one of the essential components in a liquid desiccant air-conditioning system, whose efficiency directly influences the system performance. A performance test-bed for a cross-flow regenerator was established in the present analysis. Celdek structured packings were used in the regenerator and LiBr aqueous solution was used as the liquid desiccant. Moisture removal rate and regenerator effectiveness are adopted to describe the mass transfer performance of the regenerator. Effects of air and desiccant inlet parameters on the regenerator performance are experimentally investigated, and performance comparisons between present cross-flow regenerator and other counter-flow configurations available in the literature are also carried out. The comparison results show that the impacts of air and desiccant inlet parameters show similar tendency with those of counter-flow regenerators. A dimensionless mass transfer correlation is proposed, which gives results in good agreement with the experimental findings.