Evaluation of rotary dehumidifier performance with and without heated purge

In this paper we evaluate the potential benefits from separating process air stream at the exit of rotary dehumidifier into two streams. One air stream, hot and humid, is called purge air stream and other is remaining process air stream. The remaining process air stream has a lower temperature and humidity ratio as result of separation of initial hot process air stream. It is found that as the purge angle increases the exit humidity ratio of remaining process air stream decreases up to a point where it reaches a minimum. The purge angle for which this occurs is named “effective purge angle”. The effective purge angles for different splits between adsorption and desorption side of the rotary dehumidifier, various regeneration temperatures, non-dimensional lengths and their corresponding optimum non-dimensional times are determined. An existing finite-difference model, developed by the authors of this paper, for simulation of desiccant wheel performance is extended to account for the separation of the process air stream at the exit of rotary dehumidifier and later mixing of purge air stream and outside air to form the regeneration air stream. The performance of desiccant wheel with heated “effective purge angle” is evaluated and compared with performance of the same wheel without purge angle at all. It is found, for all cases considered in this study, that having heated “effective purge angle” has overall positive effect on the performance of the rotary dehumidifier.     

Experimental investigation on the adsorption/desorption processes using solid desiccant in an inclined-fluidized bed

This paper presents an experimental investigation on the adsorption and desorption operations in an inclined-fluidized bed using silica gel as the working desiccant. The experimental system involves a circular glass tube containing the particles of silica gel, which is tested at an inclination angle of 45°. The moisture capacity of the bed is measured using a gravimetric technique. Process air at nearly constant ambient parameters (humidity and temperature) and different values of flow rate are used during adsorption. Moisture concentration in the bed is analyzed through visual observation of the color of silica gel particles. Experimental measurements indicate that the regeneration and adsorption rates are highly dependent on the air stream velocity. A satisfactory regeneration rate is confirmed at regeneration temperature as low as 90 °C when inclined-fluidized bed is applied. The transient-state moisture transfer rates during adsorption and desorption are presented. Finally, observation of the movement and color of the particles in the bed show regular circulation and homogenous distribution of moisture concentration.     

Theoretical model on the heat and mass transfer in silica gel packed beds during the regeneration assisted by high-intensity ultrasound

In the present work, a theoretical model is reported on the heat and mass transfer in silica gel packed bed during the regeneration process by using hot air combined with high-intensity ultrasound. The model consists of two parts: one is about the sound propagation in porous media; the other is about the fundamental heat and mass transfer process in the silica gel (particle) packed bed. The theoretical model is then validated by experiments in terms of the exit air temperature and humidity (kg/(kg dryair)) under different conditions of regeneration. The experimental error due to the measuring instruments is estimated to be within 0.4% and 3.5%, respectively, for the exit air temperature and humidity. The comparison between theoretical and experimental data shows that the mean relative errors (MREs) of the calculated exit air temperature and humidity compared with the experimental ones are mostly within 2.0%, which manifests the model developed in this study has a favorite agreement with the experiments. The theoretical model will help us conduct a further parametric analysis on the regeneration process in the presence of an ultrasonic field, and have a better understanding of the mechanism of enhancement of silica gel regeneration brought by the high-intensity ultrasound.     

Heat and mass transfer in adsorbent bed with consideration of non-equilibrium adsorption

In the solid adsorption refrigeration cycles, the actual adsorption processes are all non-equilibrium. To investigate the heat and mass transfer in adsorbent bed, mathematical model is established and solved by a numerical method. The relations between adsorption temperature, adsorption velocity, adsorption quantity, coefficient of performance (COP), specific cooling power (SCP) and time are discussed during the process of cooling the adsorbent bed. The relations between desorption temperature, desorption velocity, desorption quantity and time are discussed during the process of heating the adsorbent bed. It indicates that there is a peak value for adsorption velocity in the adsorption process and there is also a peak value for desorption velocity in the desorption process. It also shows that the changing rate of the adsorbent temperature tends to let up, and the coefficient of performance value grows nearly linearly in the adsorption process and there is a peak value of SCP in the adsorption process.     

An analytical solution for air dehumidification by liquid desiccant in a packed column

This paper presents a new analytical solution of heat and mass transfer processes in a packed bed liquid desiccant dehumidifier based on the equilibrium humidity on the interface is assumed to be constant. In order to maintain the partial pressure difference on the interface, a high liquid desiccant flow rate is often applied in the practical absorber. Therefore, for a narrow range of operating conditions for practical dehumidification process, we can assume that the equilibrium humidity ratio on the interface is constant. The assumption of constant humidity ratio is applied in this paper for derivation of the analytical solution. The model and the analytical solution predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. According to the Lewis definition in this present study, the Lewis number obtains 0.9. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration have been investigated on the condensation rate. The results present that design variables such as desiccant concentration, desiccant temperature, air flow rate, and air humidity ratio have the greatest impact on the performance of the dehumidifier. The liquid flow rate and the air temperature have not a significant effect. Furthermore, the effects of air and liquid desiccant flow rate have been reported on the humidity effectiveness of the column.     

Adsorption–desorption operations of multilayer desiccant packed bed for dehumidification applications

In this work, the effect of design and operating parameters on the performance of a multilayer desiccant packed bed was theoretically and experimentally studied. In the experimental work, a silica gel packed bed of eight layers has been studied. The transient value of the mass of adsorbed water and desorbed water was measured for different values of the bed length. The theoretical model shows the dependence of the dimensionless value of water content in the bed on the dimensionless time. Also the model shows that the dimensionless temperature depends on the bed characteristics and bed water content. The effect of inlet air humidity and velocity on the adsorption process for each bed layer was studied at different inlet velocities and at different air humidities. The effect of inlet temperature on desorption process for each packed bed layer was also studied at different inlet temperatures. The theoretical model also introduces an equation which can be used to predict the optimum bed length. Also, the optimum length of the bed can be recommended from the experimental results according to the operation time. Good agreement between experimental and theoretical results was found.     

A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier

A one-dimensional coupled heat and mass transfer model, which is expected for use in designing and manufacturing of a honeycombed rotary desiccant wheel, is presented in this paper. The mathematical model has been validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed and verified by experimental data. The numerical results indicate that in the regeneration process a hump curve of air humidity ratio along the channel exists all the time. In the regeneration process the hump of air humidity ratio moves from the duct entrance to the duct exit and increases gradually until the hump reaches the duct exit, where the hump will drop subsequently. The effects of velocity of regeneration air V_reg inlet temperature of regeneration air T_reg and velocity of process air V_ad on the hump moving speed are investigated. To improve the performance of desiccant wheel, it is essential to accelerate the hump moving from the duct entrance to the duct exit as soon as possible.     

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。     

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.     

Dehumidifier batch drying—effect of heat‐losses and air‐leakage

The performance of a dehumidifier dryer is influenced strongly by the operating temperature and humidity. This paper shows how heat conduction and air leakage losses can cause the temperature of a dehumidifier kiln to collapse in a batch drying process, resulting in increased drying time and energy use. By means of a dynamic simulation model it is shown that heat loss due to an uninsulated floor alone may be sufficient to prevent a kiln reaching its normal operating temperature. It is shown that the effect of heat losses is exacerbated when the dehumidifier capacity is modulated for humidity control. Auxiliary heating can prevent temperature collapse in a poorly insulated kiln. This maintains the drying speed but adds considerably to the energy used. To avoid these difficulties the insulation and air seals of a dehumidifier dryer should be appropriate to the power dissipated by the dehumidifier and fans. An example is presented in which sealing and insulating the kiln yields a reduction of 44 per cent in the drying time, a reduction of 32 per cent in energy use, and an increase of 168 per cent in net operating revenue. Copyright © 2000 John Wiley & Sons, Ltd.     

Improved utilization of desiccant material in packed bed dehumidifier using composite particles

Solid desiccant dehumidifiers are widely used in drying processes. In most of these dehumidifiers, the desiccant material is used as packed bed of granule or spherical particles. Investigations of intra-particle heat and mass transfer processes has shown that the entire portion of the particle is not participating effectively during adsorption as well as desorption processes [Pesaran AA, Mills F. Moisture transport in silica gel packed beds-I. Theoretical study. International Journal of Heat and Mass Transfer 1987; 30: 1037–49]. This is because the diffusion rate is very small compared to that of convection. In the present work, a new desiccant composite particle, in which the unutilized portion of the spherical desiccant particle is replaced with an inert particle, is proposed. By replacing the conventional particles with composite particles for the same mass of desiccant material, the available area for heat and mass transfer increases and more amount of desiccant material is effectively utilized. Further, in order to ascertain the improvement in the performance of the desiccant bed using the composite particles, various factors like thermo-physical properties of the inert material, composite particle thickness ratio, bed configuration, bed volume, the pressure drop and the increase in total adsorbed or desorbed mass have to be considered. In view of this, a theoretical investigation of the operation of vertical solid desiccant packed bed dehumidifier, using both conventional silica gel particles as well as the new proposed composite silica gel particles has been reported. A modified solid side resistance (MSSR) model is developed for the prediction of intra-particle temperature and water content profiles. Results of the present theoretical models, when applied to packed bed of conventional silica gel particles, agree well with the experimental results from the literature for both desorption and adsorption processes. From the theoretical results, more utilization for the desiccant material is obtained when ordinary silica gel particles are replaced by composite silica gel particles. For the same amount of desiccant material and same mass flow rate of air, using particles of 0.2 thickness ratio the pressure drop decreases by about 60% for the case investigated. In addition, an increase of about 11.07% and 20.46% in total mass adsorbed and desorbed respectively are obtained. At the time when adsorption process ends, an increase of 15.5% in the bed effectiveness has been obtained. In addition, the expected improvement in total mass adsorbed and desorbed is observed to be dependent on the inert material thermo-physical properties for thickness ratio less than 0.5. An optimization technique relating the composite particle design, resulting savings in pressure drop and bed volume increase is proposed.     

Design of Plate-Fin Tube Dehumidifiers for Humidification–Dehumidification Desalination Systems

A two-dimensional numerical model of a plate-fin tube heat exchanger for use as a dehumidifier in humidification–dehumidification (HDH) desalination systems is developed, because typical heating, ventilating, and air conditioning (HVAC) dehumidifier models and plate-fin tube dehumidifier geometries are not intended for the considerably higher temperature and humidity ratio differences that drive heat and mass transfer in HDH desalination applications. The experimentally validated model is used to investigate the influence of various heat exchanger design parameters. Potential improvements on common plate-fin tube dehumidifier designs are identified by examining various methods of optimizing tube diameter and longitudinal and transverse tube spacing to achieve maximum heat flow for a given quantity of fin material at a typical HDH operating point. Thicker fins are recommended than for HVAC geometries, as the thermal conductive resistance of HVAC fins restricts dehumidifier performance under HDH operating conditions.     

Investigation of heat and mass transfer in a gauze‐type structured packing liquid desiccant dehumidifier

In this experimental investigation, a packed bed column suitable for 5‐ton hybrid cooling system has been designed to study the absorption of water vapour from moist air by contact with aqueous solutions of calcium chloride. The packing material used in the study was two elements of the BXPEP structured packing and the height of the each element was 17 cm. This packed bed dehumidifier handles desiccant flow rates from 10 to 32 l/min. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (calcium chloride) and air in a gauze‐type structured packing dehumidifier. The effects of different independent variables such as air inlet absolute humidity, desiccant inlet temperature, flow rate and its concentration on the performance of the dehumidifier have been investigated. Copyright © 2002 John Wiley & Sons, Ltd.     

Thermoeconomic analysis of an evaporative desiccant air conditioning system

In this paper, a thermoeconomic analysis method based on the first and second law of thermodynamics and applied to an evaporative cooling system coupled to an adsorption dehumidifier, is presented. The main objective is the use of a method called exergetic manufacturing cost (EMC) applied to a system that operates in three different conditions to minimize the operation costs. Basic parameters are the R/P ratio (reactivation air/process air) and the reactivation air temperature. Results of this work show that the minimum reactivation temperature and the minimum R/P ratio corresponds to the smaller EMC. This result can be corroborated through an energetic analysis. It is noted that this case is also the one corresponding to smaller energy loss.     

Experimental Study on Effectiveness of Celdek Packed Liquid Desiccant Cooling System

Dehumidifier and regenerator are the most important components in a liquid desiccant cooling system. Present paper is focused on study the effect of inlet process parameters on the effectiveness of dehumidifier and regenerator of liquid desiccant cooling system. Experimental study is performed with varying inlet process parameters; mass flow rate of air, desiccant solution flow rate, inlet air temperature, inlet solution temperature, inlet specific humidity and concentration of desiccant solution. Celdek structured pads as packing material and calcium chloride as liquid desiccant is investigated first time using counter flow of the desiccant solution and air. It is concluded from the results that the effectiveness of dehumidifier increases with solution flow rate, inlet specific humidity while decreases with increasing mass flow rate of air, inlet temperature of air and desiccant, temperature and concentration of desiccant solution. The effectiveness of regenerator increases with increasing solution flow rate and inlet desiccant concentration and it decreases with increasing inlet air temperature, air flow rate and inlet solution temperature. Present paper adds to effect of inlet specific humidity, inlet temperature of the air and solution on the effectiveness of desiccant cooling system on the past research.     

Simulation studies of a vacuum and temperature swing adsorption process for the removal of VOC from waste air streams

A theoretical study of the cyclic combined vacuum and temperature swing adsorption (VTSA) process for removing of volatile organic compounds (VOC) from waste air streams is done on the basis of computer simulation. The VTSA system consists of two adsorption columns with a fixed bed of activated carbon. The adsorption cycle for each column is operated in four steps: adsorption, indirect adsorbent bed heating, vacuum desorption and cooling. A nonequilibrium, nonisothermal mathematical model of the VTSA process is developed and solved using the numerical method of lines. Exemplary simulation results are presented for the system: 2-propanol–activated carbon Sorbonorit 4. The effect of desorption temperature, pressure and purge gas use on 2-propanol desorption efficiency is investigated.     

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.     

空气源热泵系统无霜化方法概述

从提高空气温度和改变空气湿度两方面详细阐述了空气源热泵系统无霜化处理方法,简单介绍了各方法的原理及相对除霜式空气源热泵所具有的优势.最后对空气源热泵无霜化的研究方向进行展望:可以利用余热或太阳能提高流经室外换热器空气的温度以实现无霜化;进一步研究固体除湿剂在低温环境下的吸附能力和再生能力;重点探究除湿溶液的再生方法以及...     

Desorption characteristics of desiccant bed for solar dehumidification/humidification air conditioning systems

Theoretical and experimental investigation on the desorption characteristics of a packed porous bed is presented in this study. The granules of burned clay are applied as a desiccant carrier. Calcium chloride is used as the working desiccant. The theoretical model defines the transient gradient of air stream parameters (humidity and temperature) as well as desiccant concentration in the bed. In the experimental study, transient concentration gradient in the bed is evaluated by weight method. The bed is divided into seven separate layers. Air stream at low temperature and nearly constant inlet parameters are used for desorption purposes. Concentration gradient in the bed is found highly dependent on the mass transfer rate. For the specified operating conditions and stated assumptions, experimental measurements shows acceptable agreement with the analytical solution.     

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.