V.D. Papaefthimiou

A realistic approach to model LiBr-H2O smooth falling film absorption on a vertical tube

Abstract A detailed model is developed for treating the 2-D water vapour absorption into an aqueous solution of LiBr, which is flowing over a vertical tube. Numerical predictions are presented for the spatial variation of solution temperature and concentration in the axial direction and within the LiBr–H2O falling film taking into account the flow and temperature rise of coolant inside the tube. Special care was given to the adequate description of the thermodynamic properties of superheated water vapour, which is treated as a real gas. Emphasis was also given in demonstrating the electrolytic nature of the aqueous solution, introducing thus the effect of variable heat of absorption on the temperature field. A parametric study for examining the effect of absorption pressure, solution mass flow rate and inlet coolant temperature on the absorption process was carried out implementing the model under laminar flow conditions. A good coincidence with experimental observations obtained for solution flows with Re

Critical review of coupled heat and mass transfer models for a liquid desiccant adiabatic dehumidifier and regenerator

Liquid desiccant dehumidification systems have been used for many years in specialized applications. The performance of liquid desiccant systems relies heavily on heat and mass transfer characteristics of two critical components: the dehumidifier and the regenerator. The purpose of this study is to provide a comprehensive review of heat and mass transfer correlations developed to mathematically model the adiabatic absorption and desorption process. There has been an attempt to describe the most important characteristics of each research, such as the method adopted, the assumptions used, validation of the data, as well as the most important results. It was found that most work considers the desiccant solution flowing counter currently with air and the use of structured packing instead of random. Fewer researchers considered co-flow configuration or desiccants other than salts.

Thermodynamic Analysis of an Internally Heated Regenerator Using Three Liquid Desiccant Solutions

Liquid desiccant systems are emerging as promising alternatives to achieve humidity control in a variety of applications with high latent loads and low humidity requirements. Their advantage lies on their ability to handle the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a study of the performance of a counter flow internally heated liquid desiccant regenerator. A tubular heat exchanger is proposed as the internally heated element of the regenerator and water as the heating fluid. The desiccant solution is sprayed into the internally heated regenerator from the top and flows down by gravity. At the same time, ambient air is blown from the bottom, counter-flowing with the desiccant solution. The desiccant is in direct contact with the air, allowing for heat and mass transfer. The water, flowing inside the tubes of the regenerator, provides the necessary heat for regeneration. A heat and mass transfer theoretical model has been developed, based on the RungeKutta fixed step method, to predict the performance of the device under various operating conditions. Experimental data from previous literature have been used to validate the model. Excellent agreement has been found between experimental tests and the theoretical model, with the deviation not exceeding ±6.1%. Following the validation of the mathematical model, the dominating effects on the desorption process have been discussed in detail. The three most commonly used liquid desiccant solutions (LiCl, LiBr, CaCl2) and two different flows (DDU: water downward – desiccant downward – air upward, UDU: water upward – desiccant downward – air upward) have been also evaluated against each other. Considering the flow analysis, the type of flow does not affect the regeneration capacity as much as the type of the desiccant solution. It has been concluded that high regeneration rate can be achieved under DDU flow (water downward – desiccant downward – air upward), low solution concentration, high air inlet temperature, high solution inlet temperature, low air inlet humidity ratio and CaCl2 as the desiccant solution.

Thermodynamic Analysis of a Liquid Desiccant Cooling System Under Mediterranean Climatic Conditions

Liquid desiccant air conditioning systems have recently been attracting attention, owing to their merits in handling the latent heat. Desiccant systems avoid not only the energy penalty caused by overcooling and reheating, but also the bacteria generation caused by condensed water. They can also significantly reduce the electricity peak load caused by conventional compression type air conditioning systems, especially in hot and humid regions. Desiccant systems are thus more energy efficient, healthy and environmentally friendly than conventional mechanical cooling. This paper presents the results from a theoretical study of a liquid desiccant system that provides air conditioning to a typical office building. A coupled heat and mass transfer analytical model was developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under Mediterranean conditions. A parametric analysis was implemented to investigate the effects of ambient temperature and humidity ratio on the dehumidification mass rate, the load coverage and the thermal COP of the system. Simulation results showed that under hot and humid weather, the COP reaches its maximum value, 1.075. However, as the weather becomes more humid, the latent load coverage of the system is decreased and as it becomes hotter, the sensible load coverage of the system is decreased. The maximum latent load coverage, 91.8%, happened at 40°C, 0.011kgw/kgdα. Results can be useful for researchers and engineers.Copyright

Performance Analysis of a Silica Gel-Water Adsorption Cooler: Impact of Nanofluids on Cooler Performance and Size

Thermally driven chillers also known as sorption heat pumps have drawn considerable attention in recent years. They can be divided into two main categories: absorption (liquid-vapor) and adsorption (solid-vapor) systems. Even though adsorption cycles have relatively lower coefficient of performance compared to absorption cycles, however they prevail in terms of heat source, electric consumption for moving parts, crystallization etc. In order to overcome the drawback of low COP and specific cooling capacity, nanofluids, i.e. mixtures of nanometer size particles well-dispersed in a base fluid, can be used as heat transfer fluids as recent experimental and theoretical research has proved that nanofluids can exhibit a significant increase on heat transfer.In this study a two bed, single-stage adsorption chiller which utilizes the silica gel-water pair as adsorbent-refrigerant is simulated. The cooling capacity and the COP of the chiller are calculated for various cycle times. The usage of nanofluids as heat transfer fluids in the chiller evaporator and condenser and their effect on chiller performance and size is investigated. It is proved that the presence of nanofluids at different volume concentrations will enhance the cooling capacity and the COP of the adsorption chiller and therefore will lead to smaller, in terms of size, heat exchangers.Copyright

Preliminary Investigation of a Liquid Desiccant System for Dehumidification and Cooling in Athens

Liquid desiccant air conditioning systems have recently been attracting attention due to their capability of handling the latent load without super-cooling and then reheating, as happens in the conventional compression-type air conditioning systems. In liquid desiccant cooling cycles, a sorbent solution is employed to dehumidify the air, circulating between the two critical components; the dehumidifier and the regenerator. As the strong desiccant solution is sprayed on top of the internally cooled dehumidifier, it flows down by gravity and comes in contact with the process air. The desiccant solution which, by definition, has a strong affinity for water vapor absorbs moisture from the air. The end of the process finds the air cool and dehumidified and the solution diluted. The diluted desiccant solution enters the regenerator in order to retrieve its initial concentration. Hot water derived from a low temperature source supplies the necessary heat to the solution and the excessive water content is evaporated. At the end of the process, the hot humid air is rejected to the ambient and the concentrated solution is driven to the dehumidifier.The complex heat and mass transfer phenomena, occurring both in the dehumidifier and regenerator, has been the subject of earlier work by the authors. Based on the knowledge gained, a liquid desiccant system was installed at the National Technical University of Athens, Laboratory of Applied Thermodynamics, for experimental purposes. The liquid desiccant system was constructed by the German company L-DCS [1].The main components of the system are the dehumidifier, the regenerator and the evaporative cooler. The system uses water as the cooling medium and LiCl solution as the desiccant. It also employs two storage tanks, one for the concentrated solution and one for the diluted. The purpose of this publication is to present the newly installed liquid desiccant system, to predict the performance of the dehumidifier and to carry out preliminary design optimization.© 2013 ASME

Performance Analysis on the Internally Cooled Dehumidifier Using Two Liquid Desiccant Solutions

Liquid desiccant air conditioning systems have recently been attracting attention due to their capability of handling the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a study of the performance of an internally cooled liquid desiccant dehumidifier. A plate heat exchanger is proposed as the internally cooled element of the dehumidifier and water as the cooling fluid. The desiccant solution is sprayed into the internally cooled dehumidifier from the top and flows down by gravity. At the same time, fresh humid air is blown from the bottom or top, counter-flowing or co-flowing with the desiccant solution. The desiccant is in direct contact with the air, allowing for heat and mass transfer. The cooling water, flowing inside the plates of the dehumidifier, carries out the heat of the crossed air and solution. A heat and mass transfer theoretical model has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Experimental data from previous literature have been used to validate the model. Excellent agreement has been found between experimental tests and the theoretical model, with the deviation not exceeding ±4.1% for outlet air temperature and ±4.0% for outlet humidity ratio. Following the validation of the mathematical model, the dominating effects on the absorption process have been discussed in detail. Namely, effects of flow configuration, air inlet temperature, humidity and flow rate, as well as desiccant inlet temperature, concentration and flow rate have been investigated against the dehumidification rate and the cooling efficiency. The two most commonly used liquid desiccant solutions, namely LiCl and LiBr have been also evaluated against each other. The results suggested that high dehumidification mass rate can be achieved under counter flow between air and solution, low air mass flow rates, low cooling water temperature, low desiccant temperature and LiCl as the desiccant solution.Copyright