Desiccant Process Research Articles

Experimental comparison and analysis on silica gel and polymer coated fin-tube heat exchangers

In this paper, two desiccant-coated heat exchangers, which are actually fin-tube heat exchanging devices coated with silica gel and polymer materials respectively, are investigated experimentally. Due to the hygroscopic properties of the desiccant materials, both the sensible heat and the latent heat of the process air can be handled by using this kind of heat exchanger. An experimental setup was designed and built to test the performance of this unit. It is found that this desiccant-coated fin-tube heat exchanger well overcomes the side effect of adsorption heat which occurs in desiccant dehumidification process, and achieves good dehumidification performance under given conditions. The silica gel coated heat exchanger behaves better than the polymer one. The influences of regeneration temperature, inlet air temperature and humidity on the system performance in terms of average moisture removal rate Davg and thermal coefficient of performance COPth were also analyzed.

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.

Developing an Application Analytical Solution of Adiabatic Heat and Mass Transfer Processes in a Liquid Desiccant Dehumidifier/Regenerator

AbstractAn analytical solution of simultaneous heat and mass transfer processes in a packed bed liquid desiccant dehumidifier/regenerator is developed. Various dimensionless parameters and reliable assumptions are used in order to develop this solution. The outlet parameters predicted with the analytical solution show very good agreement with the experimental data available in the literature. The results show that using a Lewis number value of Le = 1.1 instead of Le = 1 gives a better prediction of the performance of the dehumidifier. In addition, the use of Le = 0.9 instead of Le = 1 can give a better prediction of the outlet parameters of the regenerator. The benefits of the present solution are its simplicity and easy application for the simulation of air dehumidification and liquid desiccant regeneration processes.

Development of a novel two-stage liquid desiccant dehumidification system assisted by CaCl 2 solution using exergy analysis method

Air conditioning system based on liquid desiccant has been recognized as an efficient independent air humidity control HVAC system. To improve thermal coefficient of performance, a novel two-stage liquid desiccant dehumidification system assisted by calcium chloride (CaCl 2) solution is developed through exergy analysis based on the second thermodynamic law. Compared with the basic liquid desiccant dehumidification system, the proposed system is improved by two ways, i.e. increasing the concentration variance and the pre-dehumidification of CaCl 2. The exergy loss in the desiccant–desiccant heat recovery process can be significantly reduced by increasing desiccant concentration variance between strong desiccant solution after regeneration and weak desiccant solution after dehumidification. Meanwhile, the pre-dehumidification of CaCl 2 solution can reduce the irreversibility in the regeneration/dehumidification process. Compared to the basic system, the thermal coefficient performance and exergy efficiency of the proposed system are increased from 0.24 to 0.73 and from 6.8% to 23.0%, respectively, under the given conditions. Useful energy storage capacity of CaCl 2 solution and LiCl solution at concentration of 40% reach 237.8 and 395.1 MJ/m 3, respectively. The effects of desiccant regeneration temperature, air mass flux, desiccant mass flux, etc., on the performance of the proposed system are also analyzed.

Introduction of a new definition for effectiveness of desiccant wheels

In common air conditioning methods, both sensible and latent loads are handled by cooling systems. Utilizing desiccant, individual cooling and dehumidification processes are possible. In this paper, desiccant wheel operation has been investigated by experimental study. Experimental conditions include different climates (hot dry and hot humid) at different operating parameters (regeneration temperature and wheel speed). Due to the temperature and humidity measurements of inlet and outlet streams, the desiccant wheel efficiency in each definition was calculated. All experiments show that enthalpy of the outlet process air is notably higher than that of the inlet air. This event leads to a novel efficiency definition which presents the deviation of the outlet process air enthalpy from the inlet air enthalpy. By increase in the dehumidification efficiency, the adiabatic efficiency decreases, whereas it increases by the regeneration efficiency. Hence in some situations the adiabatic efficiency will have an optimum value. According to the adiabatic efficiency concept, it seems to be related to the coefficient of performance of the desiccant cooling systems.

Heat and mass transfer in liquid desiccant air-conditioning process at low flow conditions

This paper investigates the transient heat and mass transfer in liquid desiccant air-conditioning process at low flow conditions. Using local volumetric average approach, one-dimensional non-equilibrium heat and mass transfer models are developed to describe the humid air and liquid desiccant interaction at counter flow configuration. Using triethylene glycol solution as desiccant, some experimental studies are completed. Experimental results are used to justify the numerical models. Numerical results are then obtained to demonstrate process characteristics. The models include a transient desiccant flow model for initial liquid desiccant building-up process, empirical wetted specific surface ratio for mass transfer, and heat and mass transfer coefficients. The objective of this research is to develop a process analytical tool for liquid desiccant air-conditioner design.

Investigation on solar desiccant dehumidification process for energy conservation of central air-conditioning systems

This paper presents the numerical simulation results of an open cycle liquid desiccant dehumidification system, attempts to obtain the best configurations of the solar assisted air-conditioning system and to validate the feasibility of using a liquid desiccant dehumidification system to handle the latent load and improve the energy efficiency of HVAC systems. Firstly, based on the steady state simulation model of a solar collector/regenerator (C/R) system, the energy performance for using exhaust air to regenerate the weak desiccant solution and heating the solution to a temperature higher than the equilibrium value were tested and discussed for different lengths of the collector/regenerator panels. It was found that the thermodynamic performance was significantly improved and it was possible to shorten the length of the solar C/R without degrading the performance greatly. Secondly, the transient performance of the open cycle liquid desiccant dehumidification system was simulated for the weather conditions of Hong Kong. The energy saving, compared with a conventional vapor compression system, is in the range of 25–50%. The higher the portion of latent load in the total ventilation load, the more the energy saving.

Experimental Study of the Heat and Mass Transfer in a Packed Bed Liquid Desiccant Air Dehumidifier

Desiccant cooling systems have the ability to provide efficient humidity and temperature control while reducing the electrical energy requirement for air conditioning as compared to a conventional system. Naturally, the desiccant air dehumidification process greatly influences the overall performance of the desiccant system. Therefore, the effects of variables such as air and desiccant flow rates, air temperature and humidity, desiccant temperature and concentration, and the area available for heat and mass transfer are of great interest. Due to the complexity of the dehumidification process, theoretical modeling relies heavily upon experimental studies. However, a limited number of experimental studies are reported in the literature. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (triethylene glycol) and air in a packed bed absorption tower using high liquid flow rates. A high performance packing that combines good heat and mass transfer characteristics with low pressure drop is used. The rate of dehumidification, as well as the effectiveness of the dehumidification process are assessed based on the variables listed above. Good agreement is shown to exist between the experimental findings and predictions from finite difference modeling. In addition, a comparison between the findings in the present study and findings previously reported in the literature is made. The results obtained from this study make it possible to characterize the important variables which impact the system design.

Parametric study of a packed bed dehumidifier/regenerator using CaCl 2 liquid desiccant

The processes occurring in a packed bed dehumidifier, which is part of a liquid desiccant solar cooling system, are mathematically simulated. The air flows in a counter flow direction to the liquid desiccant (CaCl 2). The effect of varying the air, liquid flow rates and bed geometry are studied in addition to studying the effect due to varying the air and liquid desiccant inlet coditions. The inlet temperature of the liquid desiccant during the air dehumidification process has a strong effect on the other parameters, while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and liquid desiccant enhance the liquid desiccant regeneration processes but by different ratios. The study showed that both the air and liquid desiccant flow rates have negligible effect on the bed exit humidity ratio of air whereas the liquid flow rate has a strong effect on the bed exit moisture content of the liquid. It is also observed that increasing the air flow rate enhances the liquid desiccant regeneration (air humidification) process. Regardless of the inlet moisture content of the liquid desiccant, it is found that as the product ( LA s ) gets larger, the exit air humidity gets less.

The effects of desiccant content and purging process on the performance of a rotary desiccant cooling system

Abstract A computer modeling of a rotary solid desiccant cooling system is presented. The analysis is based on the ARI summer outdoor testing conditions. The desiccant content, f, is set equal to 0.5, 0.75 and 1.0 respectively for the investigation of the effects on the COP and cooling capacity. The effect of a purging flow on the system performance is analyzed by varying the fraction of the purging sector for four different cases, namely, 0, 0.02, 0.06, and 0.1. In the analysis, the size of the wheel and the air mass flowrate in the process stream remain invariant and the value of (mp/Vp ) is kept the same as the value of (ma/Va ).