Desiccant Flow Rate Research Articles

제습로터의 공기누설모델 개발 및 성능 특성에 관한 실험적 연구

Key Words: Desiccant Rotor(제습로터), Air Sealing(누설방지모), Air Leakage(누설공기), Pressure Leakage(압력누설), Pressure Difference(차압)초록: 제습로터에서발생하는공기누설은로터와브러쉬형태의누설방지모틈새의양단사이차압에의해발생하는압력누설이지배적이며, 실험을통해압력누설특성을파악하였다. 그결과로, 압력누설량을로터와누설방지모의틈새면적과차압의함수로나타내었다. 위의상관식을바탕으로제습로터의공기누설모델을개발하였으며, 실험을통해측정된값과위모델을바탕으로계산된값을비교함으로써공기누설모델을검증하였다. 그리고풍량에따른제습로터의성능실험을수행하였으며, 공기누설모델을적용한성능분석을통하여실질적인제습량과공기누설량을확인하였다. 또한동일한제습로터가적용된제습냉방시스템을고려하여차압변화에따른공기누설의영향을공기누설모델을이용하여고찰하였다.Abstract: This study investigates the pressure leakage characteristics of a desiccant rotor with a brush-type airseal. Through a pressure leakage experiment, a correlation equation for the leakage air flow rate is obtainedas a function of the air seal area and pressure difference. Using this equation, an air leakage model for thedesiccant rotor is developed. By comparing simulation results with the experimental results for the desiccantrotor, the accuracy of the air leakage model is demonstrated. A performance test of a desiccant rotor withvarious air flow rates is carried out. Using the air leakage model, the effective mass flow rate and airleakage rate are found. In addition, the characteristics of the air leakage are analyzed for a desiccant coolingsystem using the developed air leakage model.

An experimental and modeling study of a dehumidification tower

In this paper, the size and performance of a dehumidification tower were studied by simulating varying operating conditions. Based on the experimental results, this study presents the performance of a packed tower absorber for a lithium chloride desiccant dehumidification system. The effects of the main variables - airflow rate, liquid desiccant flow rate, and inlet air temperature on the rate of dehumidification were reported. It was found that the influence of these variables could be assumed to be linear. A finite difference model was developed to determine the packing height of the dehumidification towers. This model was worked out in MATLAB code, which is a suitable model for measuring the optimum height of a tower. The validity of this model was compared with published experimental data and our data. Comparisons between the simulated packing height and the actual packing heights used by experimental studies illustrated that our finite difference model is acceptable. With this model, we predict the packing height for every condition, and then we constructed the dehumidifier based on the results of our finite difference model.

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.

Performance comparison of an adiabatic and an internally cooled structured packed-bed dehumidifier

Abstract Liquid desiccant dehumidifiers are used to improve the indoor air quality and achieve lower humidity levels in conditioned spaces. The performance of these dehumidifiers can be enhanced by providing simultaneous cooling in them. This paper presents the performance comparison of an adiabatic and an internally cooled structured packed-bed dehumidifier. The non-adiabatic structured packed-bed absorber consists of rigid media pads with cooling water flowing through the tubes embedded in the packing. The desiccant (CaCl2 in this study) falls by gravity while air flows in the cross flow configuration. The performance of this absorber has been evaluated at varying desiccant flow rates. The moisture removal from the air, the effectiveness of the dehumidifier and the mass transfer coefficients between air and solution have been compared for the dehumidifier operation with and without internal cooling. It is found that the optimum liquid to gas flow rate ratio for maximum effectiveness is about 1.0.

Simplified analysis of heat and mass transfer model in liquid desiccant regeneration process

Current study reveals an analytical solution of coupled heat and mass transfer processes in a packed bed liquid desiccant regenerator. This solution showed good accuracy when compared with reliable experimental data available in the literature. In order to maintain the mass transfer gradient on the interface, a high liquid desiccant flow rate is often applied in the regenerator. Hence, the solution flow rate and concentration change very little in the whole processes. The experimental results confirmed the validation of this topic. Therefore, for narrow range of operating conditions in practical regeneration process, with acceptable error we can assume that equilibrium humidity ratio is constant on the interface. The assumption of constant humidity ratio was applied in this paper for derivation of analytical solution. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration were investigated on the evaporation rate. Furthermore, a comparison is made between the present study and analytical solution given in the literature. The results showed that the present analytical solution can give better results than other analytical solutions that presented in the literature.

Comparative study on internally heated and adiabatic regenerators in liquid desiccant air conditioning system

Liquid desiccant regeneration has important effect on performance of a liquid desiccant air conditioning system. Compared with conventional packed regenerator, internally heated regenerator is proposed to achieve better regeneration performance. This study emphasized on both regeneration rate and regeneration thermal efficiency to evaluate the performance of both regenerators. A validated heat and mass transfer model was used to analyse and compare the performance of internally heated and adiabatic regenerators. The results indicated that internally heated regenerator not only could increase the regenerate rate, but also could exhibit higher energy utilization efficiency. Different from adiabatic regenerator, internally heated regenerator can provide comparable regeneration efficiency and regeneration rate at low desiccant flow rate, so it should be a good alternative to avoid carryover of desiccant droplets. Higher air flow rate would result in a deduction of regeneration thermal efficiency although achieving higher regeneration rate. Suitable flow rate of the air should be considered carefully in liquid desiccant regeneration. The internally regenerator could have considerable prospect in liquid desiccant air conditioning application.

Performance testing of a counter-cross-flow run-around membrane energy exchanger (RAMEE) system for HVAC applications

A novel run-around membrane energy exchanger (RAMEE) system is designed, built, and tested for heating, ventilating, and air-conditioning applications. The RAMEE system consists of two counter-cross-flow liquid-to-air membrane energy exchangers, one located in the supply and the other in the exhaust air streams of a building. Inside each exchanger, a micro-porous membrane separates the air and desiccant solution streams. This membrane allows heat and water vapor exchange between the two streams. The RAMEE system thus exchanges sensible and latent energy between the supply and exhaust air streams by using a desiccant solution as the energy carrier that is pumped in a loop between the two exchangers. The RAMEE performance is evaluated by testing the system with various air and desiccant solution flow rates during standard summer and winter operating conditions. During summer test conditions, the total system effectiveness increases with increasing desiccant flow rate, but decreases as the air flow rate increases. Under winter test conditions, the total effectiveness changes little with changes in the air and desiccant flow rates. For some test conditions, the maximum total effectiveness of the system is between 50 and 55%. The effectiveness data are compared to available correlations and reasons for discrepancies are discussed.

Heat and Mass Transfer between Air and Liquid Desiccant in Cross‐flow Contact Systems

AbstractIn this paper, an analytical solution of the coupled heat and mass transfer process in a cross‐flow liquid‐desiccant dehumidifier/regenerator is developed. By the use of reliable assumptions, the government equations for the system are solved by an analytical method. The results of the present analytical solution are compared with experimental data from the literature, and they show good agreement. The maximum error produced by the presented analytical solution is less than 12 %. The effect of the operating parameters is considered using the presented analytical solution. The results show that air inlet humidity, air flow rate, desiccant inlet temperature, and desiccant inlet concentration have more influence on the moisture removal rate in the cross‐flow dehumidifier. Also, air inlet humidity, desiccant inlet temperature, desiccant inlet concentration and desiccant flow rate have more effect on the moisture removal rate in the cross‐flow regenerator. The effect of the Lewis number on the performance of the dehumidifier/regenerator is investigated. As the results show, by the use of Le = 1.08 in the dehumidification process and Le = 0.96 in the regeneration process, instead of the value of unity, better results are obtained. The benefit of the present study is a simplified application for the evaluation of the outlet parameters of the cross‐flow air dehumidifier/regenerator.

Investigation of humidity harvest as an alternative water source in the Kingdom of Saudi Arabia

In order to harvest water present in a humid atmosphere, a suitable absorber/desorber was designed. Experiments were conducted in Dhahran to measure the water absorption rate in the nighttime and the water desorption rate in the daytime. It was found that the water absorption rate was 2.11 L/m2/day and the water desorption rate was 1.15 L/m2/day depending on the desiccant flow rate. In order to assess the feasibility of the system, the unit was tested continuously in the nighttime as well as in the daytime and the results are reported in this paper. The chemical characteristics of a set of water samples collected were analysed and the water collected was found to meet most of the World Health Organization drinking water standards. The problem associated with this technology is the mixing of dust and dirt with the desiccant solution, and the fact that a small pump is required to circulate the solution.

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.

Run-Around Energy Recovery System for Air-to-Air Applications Using Cross-Flow Exchangers Coupled with a Porous Solid Desiccant—Part II: Results and Performance Sensitivity

The model for the run-around energy recovery system was developed and verified for sensible and latent energy transfer in Part I (Li et al. 2009). In this paper, the sensible, latent, and overall effectiveness are investigated in each exchanger as well as the run-around exchanger. The overall effectiveness of the run-around energy recovery system is dependent on the airflow rate, the solid desiccant flow rate, the desiccant properties, specific surface area, the size of each exchanger, and the inlet air operating conditions. The run-around system can achieve a high overall effectiveness when the flow rates and exchanger's properties are properly chosen. Comparisons between the solid desiccant and salt solution run-around system effectiveness (Fan et al. 2006) show they are in agreement. In a sensitivity study, the thickness of desiccant on the fiber is investigated in the solid run-around system. It was found that a good performance is obtained with very thin desiccant coatings (1 or 2 μm). During the practical use of this system, a desiccant-coated fiber could be inserted into very porous balls or cages, protecting the desiccant-coated fiber from mechanical wear. The performance sensitivity for this kind of run-around system is demonstrated.

Application of Desiccant Technology for Concentrating Liquid Foods - Experimental Investigation

A single tube, stainless steel, liquid desiccant falling film evaporator was built and tested with the calcium chloride solution as the liquid desiccant to concentrate liquid foods. Experiments were conducted in the above setup with sugar solutions as the liquid food. To minimize water vapor mass transfer resistance, non-condensibles were removed from the system by a vacuum pump. No steam was used to drive this evaporator.The liquid food was heated to about 48ºC when the ambient temperature was 40ºC. The single tube double falling film evaporator was tested to determine the effect of desiccant concentration, temperature, and flow rate on the evaporation rate. The effect of liquid food concentration, temperature, and flow rate on evaporation rate was also studied. An uncertainty and error analysis for the experimental data was performed. It was demonstrated that liquid desiccants could provide enough energy, through heat of absorption, to evaporate a liquid food in a falling film evaporator with vapor transfer lines.

Study of Desiccant Regeneration in a Structured Packed Tower

The performance of a liquid desiccant regenerator, equipped with structured packing made of wood, was investigated for the regeneration of triethylene glycol (TEG). The effects of air and desiccant flow rates, air and desiccant temperature, and desiccant concentration were studied to assess their impact on the regeneration process. The influence of these parameters on the performance of the desiccant regenerator was evaluated in terms of the rate of water evaporation and the effectiveness of the regeneration process. It was found that under the conditions of this study, the desiccant inlet temperature was the most favorable variable, while the inlet desiccant concentration was the least favorable with respect to the regeneration process. The air mass flux increased the rate of water evaporation, but reduced the regenerator effectiveness significantly. The other parameters had a negligible effect. The results of this study were in agreement with the Chung and Luo correlation and published literature.

Analytical solution of adiabatic heat and mass transfer process in packed-type liquid desiccant equipment and its application

This paper presents an integrated analytical solution of adiabatic heat and mass transfer in packed-type liquid desiccant equipment based on proposed mathematical models in both parallel-flow and counter-flow configurations. In the derivation process, the desiccant concentration at the inlet and outlet of the absorber is assumed to be constant. That means this solution is suitable for high desiccant flow rate conditions. This analytical solution has good accuracy when compared with reliable experimental data available in the literature. The moisture removal rate is derived from this solution and every factor that influences performance can be easily analyzed. Another benefit is that it can be used to discover the optimum air-to-solution flow rate ratio with which the driving force may be kept constant throughout the column. The optimum flow rate ratio may be used for optimal system design and to realize a reversible heat and mass transfer process.

Adiabatic water absorption properties of an aqueous absorbent at very low pressures in a spray absorber

In this paper, spray absorber in absorption chillers, a new technique is proposed. This concept of atomization increases the heat and mass transfer rates due to the increases of the expose area of the brine solution compared to falling film technique used in the conventional absorber. Results presented in this paper show the great enhancement of water absorption property by having a pressure atomization. The experimental set up was included a single nozzle adiabatic spray chamber operating at the design pressure, a solution circulating and tempering system, a Laser measurement system to measure the size and the velocity of the droplets, and data acquisition and control system. Based on a first principle “Newman model”, an analytical model was developed to predict the enhance transfer rates. These analytical results were compared with the experimental results that showed good agreement. The experiments were conducted by varying the differential pressure across the nozzle and the liquid desiccant flow rate. The dimensionless viscosity ratio, k, of this disperse/continuous phase flow was around 1300. The nozzles tested were able to produce the expected drop sizes and mean drop velocities. The droplet Reynolds number and Peclet number varied from 10 to 1500 and 0.005 to 0.5, respectively.

Experimental study on mass transfer performances of cross flow dehumidifier using liquid desiccant

The liquid desiccant air conditioning system is drawing more and more attention due to its advantages in energy saving and environmental friendliness. The dehumidifier is one of the essential parts of the system, which affects the whole system performance severely. This paper experimentally studies the performance of the cross flow dehumidifier, which has been less studied than the counter flow dehumidifier, although it is more applicable in practice. Celdek structured packings were used in the dehumidifier, and a LiBr aqueous solution was used as the liquid desiccant. The moisture removal rate and dehumidifier effectiveness were adopted as the dehumidifier performance indices. The effects of the dehumidifier inlet parameters, including air and desiccant flow rates, air inlet temperature and humidity ratio and desiccant inlet temperature and concentration, on the two indices were investigated. Correlations are proposed to predict the cross flow dehumidifier performance, which give results in good agreement with the present experimental findings.

Empirical correlations to predict the performance of the dehumidifier using liquid desiccant in heat and mass transfer

Dehumidifier is one of the essential components in liquid desiccant air-conditioning system, whose hourly performance is required to predict the annual energy consumption of the system. Model complexity and the required amount of computer time usually do not permit the use of models based on a differential element. In order to estimate the hour-by-hour performance of dehumidifier, a simplified approach is proposed in the present study, in which enthalpy and moisture effectiveness are adopted as indexes to describe the heat and mass transfer performances of the dehumidifier. Empirical correlations of enthalpy and moisture effectiveness are expressed by enthalpy difference, moisture difference, air and desiccant flow rate, based on corresponding experimental results. Empirical correlations are given for a cross-flow packed dehumidifier, the average absolute differences between the calculated values and the experimental findings are 6.3% and 6.0% for enthalpy effectiveness and moisture effectiveness, respectively, with discrepancies mainly within ±20%. Good agreements are also shown for counter-flow dehumidifiers available in literatures.

Effect of structured packing density on performance of air dehumidifier

Abstract An experimental study has been conducted to investigate the performance of a liquid desiccant air dehumidifier equipped with a structured packing made of wood for three different densities using triethylene glycol (TEG) as the liquid desiccant. The structured packing densities used were 77, 100 and 200 m2/m3. The performance of the dehumidifier was expressed in terms of the moisture removal rate and the dehumidifier effectiveness under different air and desiccant parameters, i.e. the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and inlet TEG concentration. In general, the trend in the dehumidifier performance was similar to that reported by other investigators using random packing. The effect of packing density on moisture removal rate and dehumidifier effectiveness is assessed. The differences in the effectiveness of different packing densities are attributed to the wetting condition. Lower effectiveness of the column is shown with the packing density of 200 m2/m3 compared to the other two packing densities when the air flow rate, inlet concentration and desiccant flow rate are increased. However, higher effectiveness is shown when either the inlet temperature of the air or desiccant is increased.

Predictions of moisture removal rate and dehumidification effectiveness for structured liquid desiccant air dehumidifier

In hot and humid climates such as in the Sultanate of Oman, the humidity puts extra load on the electric vapor-compression air conditioning (VAC) systems. Liquid and solid desiccants can reduce the moisture content of humid air and thus reduce the latent load imposed on the VAC systems. In the present work, the performance of air dehumidifiers using triethylene glygol (TEG) as desiccant was investigated. Three differently structured packing densities were used (77, 100 and 200 m 2/m 3). The performance of the dehumidifier was evaluated and expressed in terms of the moisture removal rate ( m cond) and the dehumidifier effectiveness ( ε y). The experimental work was undertaken to study the effects of several influencing design factors on this performance. The design factors covered included the air and TEG flow rates, air and TEG inlet temperatures, inlet air humidity and the inlet TEG concentration. The desiccant flow rate investigated was much less than that covered in previous studies and the range of the inlet temperatures of air and desiccant was significantly wider. The objective this study was to use the multiple regression method and the principal component analysis to obtain statistical prediction models for the water condensation rate and the dehumidification effectiveness in terms of these design factors. The results of both techniques agree with each other affirmed that the desiccant flow rate, desiccant inlet concentration and air inlet temperature are the most significant variables in predicting m cond, whereas desiccant flow rate, air inlet temperature and packing density are the most significant variables in predicting ε y.

A Study of the Performance of a Hybrid Liquid Desiccant Cooling System Using Lithium Chloride

This paper presents field test of a hybrid solar liquid desiccant cooling system conducted at a test house at the University of Florida’s Energy Research and Education Park. These tests consisted of operating the air conditioning system at the test house in two configurations: the conventional vapor compression system and the hybrid desiccant system. Experiments were conducted to study the influence of the air mass flow rate, temperature of the inlet air, temperature of the desiccant, and desiccant mass flow rate on the performance of both system configurations. Based on the field test results, it was found that the hybrid desiccant system improves the air conditioning performance in the field test house by decreasing the outlet humidity and temperature of the air.