Dehumidification Efficiency Research Articles

Research on Ratio Selection of Mixed LiBr-CaCl<sub>2</sub> Solution for Liquid Desiccant Cooling System

In recent years, liquid desiccant cooling system has shown its potential research value for its low-grade heat using and energy-saving. Thermal properties of liquid desiccant, especially its vapor pressure greatly affects the dehumidification efficiency of this system. Here comes the problem to find a liquid desiccant with better dehumidification effect and lower cost. To solve this problem, a local composition model: nonrandom two-liquid (NRTL) theory could be used to calculate a mixed solution of LiBr-CaCl2, and calculates the ideal mixing ratio of this solution in a working environment. The best mixing ratio with cost-effectiveness can be worked out by taking into account the initial cost of the solution. Study shows that: compared to single LiBr solution, the dehumidification effect could be raised by more than 8.2% and the costs could be reduced by 20% with mixed LiBr-CaCl2 solution.

Performance comparisons of honeycomb-type adsorbent beds (wheels) for air dehumidification with various desiccant wall materials

This study aims at comparing the performance of honeycomb type adsorbent beds (or desiccant wheels) for air dehumidification with various solid desiccant wall materials, from a viewpoint of system operation. A mathematical model is proposed and validated to predict the cyclic behaviors of the cycling beds or wheels. The influences of regeneration air temperature, process air temperature, and humidity on the coefficient of performance (COP), specific dehumidification power (SDP) and dehumidification efficiency (εd) are predicted with various desiccant wall materials. Totally ten most commonly used desiccant materials are considered, with different adsorption and thermophysical properties. It is found that of the 10 materials, the silica gel 3A and silica gel RD perform better than other desiccants for air dehumidification under typical working conditions and driven by low grade waste heat. The results provide some insights and guidelines for the design and optimization of honeycomb type adsorption beds or desiccant wheels.

Experimental Study on Performance of a Cross-Flow Dehumidifier

Experiment was carried out to test heat and mass transfer performance of cross-flow dehumidifier using metal packing. The effects of air flow rate, solution flow rate and inlet parameters of air and solution on the outlet parameters were also investigated. And a great deal of data obtained from experiment was processed using multiple linear regression method. Empirical formulas were given such as the rates and efficiency of dehumidification evaluating performance of dehumidifier, which provided good reference and necessary information for further theoretical investigation.

A heat pump driven and hollow fiber membrane-based liquid desiccant air dehumidification system: Modeling and experimental validation

A compression heat pump driven and membrane-based liquid desiccant air dehumidification system is presented. The dehumidifier and the regenerator are made of two hollow fiber membrane bundles packed in two shells. Water vapor can permeate through these membranes effectively, while the liquid desiccant droplets are prevented from cross-over. Simultaneous heating and cooling of the salt solution are realized with a heat pump system to improve energy efficiency. In this research, the system is built up and a complete modeling is performed for the system. Heat and mass transfer processes in the membrane modules, as well as in the evaporator, the condenser, and other key components are modeled in detail. The whole model is validated by experiment. The performances of SDP (specific dehumidification power), dehumidification efficiency, EER (energy efficiency ratio) of heat pump, and the COP (coefficient of performance) of the system are investigated numerically and experimentally. The results show that the model can predict the system accurately. The dehumidification capabilities and the energy efficiencies of the system are high. Further, it performs well even under the harsh hot and humid South China weather conditions.

Experimental investigations on desiccant wheels

Experimental investigations on several commercially available and newly fabricated rotors are conducted in two different laboratories to evaluate performance trends. Experimental uncertainties are analysed and the parameters determining the rotor performance are investigated. It is found that the optimal rotation speed is lower for lithium chloride or compound rotors than for silica gel rotors. Higher regeneration air temperatures lead to higher dehumidification potentials at almost equal dehumidification efficiencies, but with increasing regeneration specific heat input and enthalpy changes of the process air. The influence of the regeneration air humidity was also notable and low relative humidities increase the dehumidification potential. Finally, the measurements show that rising water content in the ambient air causes the dehumidification capacity to rise, while the dehumidification efficiency is not much affected and both specific regeneration heat input and latent heat change of the process air decrease. For desiccant cooling applications in humid climates this is a positive trend.

A novel method to evaluate the performance of liquid desiccant air dehumidifier system

The system efficiency of desiccant air dehumidification is usually evaluated by a coefficient of performance (COP) that is defined as the ratio of the cooling capacity and the sum of net electrical power input. The common efficiency evaluation method has many weaknesses especially if applied on liquid type of desiccant, low humidity different between inlet and outlet, and also on the system that has automatic operating control. This paper offers a solution for weaknesses of common method with a proposed novel method to evaluate the performance of liquid desiccant air dehumidification system.

Analysis of Energy Consumption and Mode of Energy Saving of Rotary Wheel

A model of rotary wheel combined with heat pump regeneration was presented and an experimental investigation on it was also carried out. Five normal kinds of dehumidification efficiency evaluation standard were introduced in view of the existing various dehumidifying method while the current energy efficiency evaluation indices were analyzed. SMER which defines the dehumidification quantity of unit energy consumption and dehumidification system energy efficiency were introduced and used to compare energy consumption of rotary wheel with or without heat pump. The comparative analysis showed that the system of rotary wheel combined with heat pump regeneration could significantly improve SMER and dehumidification system energy efficiency of rotary wheel.

Reducing energy consumption in food drying: Opportunities in desiccant adsorption and other dehumidification strategies

This work assesses the energy efficiency of dehumidification drying vis-à-vis conventional convective drying techniques. Mathematical models are developed by means of which the energy efficiencies of different dehumidification dryer types are expressed in terms of that of a conventional convective dryer operating at the same temperature. This permits the isolation of important design and operational parameters specific to each dryer type which when optimized, improve energy efficiency for the same product quality requirement and ensure better product quality for the same efficiency as a conventional dryer. Desiccant dehumidification systems have the advantage of providing further opportunities for beneficial heat integration.

Property Analysis of Solid-Adsorption Desiccant System

Based on the concept of independent dehumidification, a system which can test the dehydration property of both the adsorbent bed and the adsorbent has been designed. As the adsorbent, the activated alumina and the 4A molecular sieve are used to test the dehydration property of the whole adsorbent bed with two kinds of adsorbent. Finally, three new evaluation indexes about the dehumidification efficiency based on the experimental data are proposed in this paper.

Performance analysis of liquid desiccant based air-conditioning system under variable fresh air ratios

In conventional air-conditioning system, fresh air volume is always restricted to save energy, which sacrifices indoor air quality (IAQ) to some extent. However, removing the latent load of air by liquid desiccant rather than by cooling is an alternative way of reducing energy consumption. Therefore, IAQ can be improved by increasing the volume of fresh air introduced into an air-conditioning system. In this paper, a liquid desiccant based air-conditioning system is studied, with the system performance under various fresh air ratios analyzed using simulation tests. In addition, the proposed system and a conventional system are compared. In the proposed system, with the increase in fresh air ratio, the heating load for solution regeneration rises, the dehumidification efficiency increases and the regeneration efficiency drops. The coefficient of performance (COP) of the liquid desiccant based system decreases sharply when the fresh air ratio exceeds 60%. The results also show that the proposed system can save power notably. The maximum power saving ratio is 58.9% when the fresh air ratio is 20%; however, the ratio drops when the fresh air ratio increases. These findings will be beneficial in the selection of fresh air ventilation strategies for liquid desiccant based air-conditioning systems.

Operational experiences with solar air collector driven desiccant cooling systems

Component performance and seasonal operational experiences have been analysed for desiccant cooling systems powered by solar air collectors. Measurements during the commissioning phase in Spain (public library) and in Germany (production hall) showed that the dehumidification efficiency of the sorption rotors was 80% and the humidification efficiency of the contact evaporators was 85–86%. Only in a two-stage desiccant system monitored in China (laboratory building), a dehumidification efficiency of 88% was reached. The rotary heat exchangers only had 62–68% measured heat recovery efficiency, which is lower than specified. Seasonal performance monitoring carried out in the German installation showed that average seasonal COP’s were close to 1.0, when related to all operation hours. COP’s increase if low regeneration temperatures are used with low dehumidification rates, which is often sufficient for moderate German climatic conditions, but much less so in the humid Chinese climate. Electrical COP’s for the German system including air distribution were between 1.7 and 4.6 and reach values of 7.4, when only additional pressure drops of the desiccant unit are considered. It could be shown that conventional control strategies lead to high auxiliary energy consumption, for example if fixed heating setpoint temperatures are used. Furthermore the solar air collector energy yield was very low in the German system, as regeneration was only used when all other options such as humidification at high air volume flows did not reduce the room air temperature enough. The studies showed that the measured auxiliary energy consumption could be reduced to near zero, if regeneration temperature setpoints were not fixed to constant values. The solar air collector efficiency was good at about 50% both for the flat plate collectors used in Spain and Germany and the Chinese vacuum tube solution. A cost analysis demonstrated the viability of the concept, if some funding of the high investment costs is provided.

Dehumidification and regeneration efficiency of the mixed salt solution in the solar dehumidification system

Dehumidification and regeneration efficiency of the mixed salt solution in the solar dehumidification system

Improvements of high-temperature drying heat pumps

Inadequate integration and/or inappropriate operating parameters of heat pump dryers may lead to troubles as too high/low discharge/suction pressures, low dehumidification efficiency and even mechanical damage to the compressor. This paper will help fix a number of common design errors and/or omissions, and suggests original control methods to avoid undesirable operational incidents and improve overall energy performance of high-temperature drying heat pumps. The scope is to provide normal and safe operating conditions, and thus accelerate the implementation of industrial drying heat pumps.

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.

Study of heat and mass transfer in a dehumidifying desiccant bed with macro-encapsulated phase change materials

The present article reports on the feasibility of using encapsulated phase change materials (EPCMs) in the dehumidifying bed of a desiccant cooling system. The mathematical model used to simulate the coupled non-equilibrium heat and moisture transfer processes in the porous composite structure containing the EPCM and desiccant particles is presented. Numerical investigations of heat and mass transfer in a desiccant dehumidifying bed composed of silica gel and EPCM particles have been carried out for different values of process parameters. Careful choices of EPCM volume fraction and thermo physical characteristics have been found to increase the overall effectiveness of the desiccant dehumidifier with negligible loss in the dehumidification efficiency. The air stream exits the desiccant/EPCM bed at relatively lower temperature and slightly higher moisture content than from purely desiccant bed. Desiccant cooling systems with less sensible heating and higher cooling capacity can be obtained by employing EPCM in the dehumidifier.

Experimental Analysis on Chemical Dehumidification of Air in a Packed Column by Hygroscopic Salt Solution: Comparison between Structured and Random Packings

This paper presents the experimental tests on chemical dehumidification of air by liquid desiccant carried out in a packed column with the hygroscopic solution H2O/LiBr. Two types of absorption columns, namely, random packing consisting of 1 in. Pall Ring elements and a structured column Mellapack 250Y, were tested. The experimental tests are presented in terms of humidity reduction, dehumidification efficiency, and air-side pressure drop. The random packing shows the dehumidification rate and efficiency as 20%–30% higher than structured packing, whereas the structured packing shows the air-side pressure drop as 65%–75% lower. The experimental results are also compared with the values predicted by one-dimensional simulation code of a packed column, and a fair agreement is found between experimental and simulated values for both dehumidification rate and air-side pressure drop. The experimental tests show that the chemical dehumidification of air by H2O/LiBr in a packed column with random and structured packings ensures a consistent reduction in the humidity ratio suitable for the application to air-conditioning and drying processes.

Dynamic Performance of a Dehumidification System Using the Absorption-Type Rotary Honeycomb Dehumidifier.

A theoretical and experimental investigation is carried out on the dynamic behavior of a dehumidification system used to dry out the wet floor in a beverage factory. The dehumidification system consists of an absorption-type rotary dehumidifier with lithium chloride salt coated on the honeycomb monolith. Experimental runs were carried out in the factory. By coupling a previous dynamic model of the dehumidifier with an evaporation model of the wet concrete floor, good agreement has been obtained between the simulated and experimental results, thus confirming the applicability of the integrated model. To obtain better performance of the integrated dehumidification system, the model is next used to investigate the effects of various system inputs and parameters on its dynamic preformance. The results show that the paired velocities of dehumidified and regeneration outlet airs, and the pre-heating temperature of the regeneration air have more prominent effect on dehumidification efficiency than the other factors. It is possible to both reduce the room relative humidity below 50% and dry out the water on the floor within 12 hours of dehumidification. The most suitable dehumidification conditions among those investigated are: velocity of room air 0.06 m/s, inlet temperature of regenerated hot air 343 K, velocity of regenerated hot air 0.5 m/s, air velocity in dehumidifying section 0.68 m/s and rotational speed of honeycomb 10 rph.

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.

Thermally Regenerative Monolithic Rotor Dehumidifier for Adsorption Cooling System

A silica-base monolithic adsorbent was prepared by sol-gel synthesis with a corrugated ceramic fiber sheet as a matrix and assembled in an adsorptive dehumidifier of rotary type, which operates with a thermal regeneration cycle. High efficiency of dehumidification was confirmed at a regeneration temperature as low as 80°C when the rotation speed or cycle time was adjusted to the optimal value. A prototype air conditioning unit equipped with the balanced flow dehumidifier was examined in a direct pass ventilation mode. A satisfactory decrease in enthalpy of product air relative to room air was observed. Changes in the state of air during processing are discussed rather rigorously in terms of a humidity chart.

Calorimeter measurements of a heat pump dehumidifier: Influence of evaporator air flow

The performance characteristics of a Rankine cycle heat pump dehumidifier have been measured in a psychrometric calorimeter which provides close control of the air state. The quality of the measurements was monitored by applying energy and mass balance test criteria. The results show that the influence of the evaporator air flowrate on the moisture extraction rate, and on the energy efficiency of the drier, depends on the relative humidity of the evaporator inlet air stream. When the relative humidity is high the dehumidification efficiency is maximized when the evaporator air flow is maximized, within the range of air flowrates investigated. Below 50% relative humidity the specific moisture extraction rate exhibits an increasingly peaked maximum as a function of the evaporator air flowrate. This maximum moves to lower air flows when the relative humidity is lower.