Rotary Desiccant Dehumidifier Research Articles

Comparative analysis of a new desiccant dehumidifier design with a traditional rotary desiccant wheel for air conditioning purpose

Rotary desiccant dehumidifiers for air conditioning purposes are considered one of the most promising air conditioners for regions that suffer from high humidity and temperatures. The heat generated by the adsorption effect represents the most important problem facing the rotary desiccant dehumidifier. So, the present study aims to suggest an innovative design of a desiccant dehumidifier that has a higher dehumidification capacity, in addition, to cooling the process air with cooling rates higher than the heat generated by the adsorption effect. To evaluate the performance of the proposed desiccant dehumidifier, a comparative experimental study was conducted for both the novelty innovative configuration of the desiccant dehumidifier (NICDD) and traditional rotary desiccant dehumidifier (TRDD) under the same weather conditions. The experimental results presented that the utilization of NICDD represents a good choice that can reduce the temperatures of the process air that leaves the desiccant dehumidifier by rates varying between 17.8 and 23.8 °C as compared to TRDD. Also, the use of NICDD causes improved average dehumidification capacity from 5.57 to 9.82 gw/kgda, with an improvement of 70.2%. Moreover; the use of NICDD causes improved the dehumidification coefficient of performance from 0.268 to 1.012, with an improvement of 277.6% compared to TRDD.

Dry gel conversion synthesis and performance of glass-fiber MIL-100(Fe) composite desiccant material for dehumidification

Abstract Metal organic frameworks (MOFs) MIL-100(Fe) is considered to be a promising porous adsorbent for dehumidification due to its good hydrophilicity and hydrothermal stability. However, the powdery MIL-100(Fe) prepared by the conventional hydrothermal method is not suitable for industrial rotary desiccant dehumidifier. In order to produce a high-performance rotor core component, a new dry gel conversion in situ synthesis strategy for glass-fiber MIL-100(Fe) monolithic adsorbent was proposed in this paper. The effects of the reaction sol concentration on the crystalline phase, morphology, pore structure, adsorption/desorption performance and the cycling stability of the monolith were discussed in detail. The results showed that MIL-100(Fe) crystals which evenly dispersed in the surface and voids of glass fiber, could be conveniently synthesized in the reaction sols with molar ratios of 1Fe (III)/0.67BTC/xH2O (x = 93–185). Among them, the MIL-100(Fe) monolith obtained at x = 93 exhibited a high loading (85.7%), large BET specific surface area (1441 m2/g), S-shaped water vapor adsorption isotherm and high saturated adsorption ratio (0.496 g/g) at 298 K and 60% RH, low desorption activation energy (48.28 kJ/mol) as well as a reliable cycling stability. Favorable adsorption/desorption performance of MIL-100(Fe) monolith and convenient dry gel preparation process provide a guidance for the manufacture of high-performance dehumidification rotary wheel.

An overview on use of desiccant dehumidifiers in modern air-conditioning

The solid desiccant based dehumidifier used in conjunction with the conventional HVAC combines the dehumidification of solid desiccant system and with the cooling capacity of the conventional air conditioning system. This hybrid cooling system provides thermal comfort to the occupants of the conditioned space. The hybrid systems main appeal lies in the fact that, it consumes much lesser high grade electrical energy as compared to the dedicated standalone traditional air conditioning systems. The electrical energy usage is possible still lower by use of primary energy sources for to supply the thermal energy needed for the desiccant regeneration. For this purpose freely available renewable solar energy or industrial waste heat can also be used for the regeneration heat source. Sometimes it is also possible to provide condenser waste heat for the part of desiccant reactivation heat supply may increase the overall performance of the system. It was also found that this cooling system with use of air to air waste heat recovery wheel performed better than without it in terms of dehumidification as well as cooling performance. The present study report important literature survey on the dehumidification potentials of desiccant integrated hybrid cooling system operating in hot and humid climates. Keywords: Hybrid air-conditioning; Rotary desiccant dehumidifier; Heat recovery wheel; Regeneration heat; Renewable solar energy; Waste heat

Performance analysis of hybrid solid desiccant–vapor compression air conditioning system in hot and humid weather of India

The performance of hybrid solid desiccant–vapor compression air conditioning system is studied for the typical hot and humid climatic zone of North India (Roorkee). The system consists of a rotary desiccant dehumidifier coupled with heat recovery wheel and a conventional vapor compression air conditioning system. A FORTRAN program was developed to calculate different psychrometric properties like humidity ratio, dry-bulb temperature, wet-bulb temperature, relative humidity, specific heat, etc. at each state point of the system on the basis of ambient and desired inside room conditions, room cooling load, sensible heat factor, and required air flow rate. The overall system performance has been evaluated in terms of the coefficient of performance and the dehumidifier effectiveness. The influence of variation in ambient conditions on regeneration temperature of desiccants has also been discussed. The simulated results are validated using experimental measurements. The present system has ensured a reduction of 79.15% in processing air humidity ratio at an outlet of the desiccant dehumidifier as compared to the outdoor humidity ratio. The results show that the performance of the system is significantly affected by the variations in ambient temperature and humidity ratio. Practical application: The proposed model of solid desiccant–VCR hybrid air conditioning system minimizes the energy usage in comfort cooling applications such as auditoriums, supermarket, hospitals, central library, offices, and lecture halls in hot and humid climate while maintaining an acceptable thermal comfort level and improving the performance. The proposed approach will give valuable insights to the researchers and building engineers to analyze the impacts of ambient conditions on the energy requirements and performance of solid desiccant cooling to ameliorate comfort, energy, and cost savings. Moreover, its potential contribution in environmental protection makes it more attractive at a time where depletion of energy resources and environmental degradation are of major concerns.

Experimental investigation of a desiccant dehumidifier based on evacuated tube solar collector with a PCM storage unit

ABSTRACTA rotary desiccant dehumidifier based on an evacuated tube solar air collector with a phase change material (PCM) storage unit has been experimentally investigated. The PCM (acetamide) simultaneously stores and transfers the heat to the air during day hours. The stored heat of the PCM is utilized to produce hot air during night hours. The performance of the system is analyzed for different air flow rates at different rotational speeds (rph) of desiccant wheel. The dehumidification effectiveness (ϵA), regeneration effectiveness (ϵR), and dehumidification coefficient of performance are 8.17, 10.35, and 0.0327%, respectively, at 16 rph of the desiccant wheel with 127.23 kg/h air flow rate. The system is also used to generate dry air for 14 h a day with PCM, which shows that acetamide used as the PCM has an ability to store an adequate amount of heat to regenerate the desiccant wheel during off-sunshine hours.

Performance prediction of rotary solid desiccant dehumidifier in hybrid air-conditioning system using artificial neural network

Desiccant air conditioning systems are considered as better alternatives to the conventional air conditioning system because of the independent control of temperature and humidity and being environment friendly. An artificial neural network (ANN) model has been developed to predict the performance of a rotary desiccant dehumidifier for different process air inlet conditions. Dry bulb temperature, humidity ratio and flow rate of the process as well as regeneration air streams of dehumidifier and regeneration temperatures are used as inputs to the model. The outputs of the model are outlet dry bulb temperature and humidity ratio of process as well as regeneration air streams of dehumidifier, the moisture removal rate and the effectiveness of the dehumidifier. Moisture removal rate and effectiveness of the dehumidifier are considered as the performance indicators of the system. Experiments are also conducted to investigate the performance of the desiccant wheel and the test results are used as target data to train the ANN model. Performance predictions through ANN are compared with the experimental test results and a close agreement is observed.

Numerical and experimental analysis of a solid desiccant wheel

The rotary desiccant dehumidifier is an important component which can be used in air conditioning systems in order to reduce the electrical energy consumption and introduce renewable energy sources. In this study a one dimensional gas side resistance model is presented for predicting the performance of the desiccant wheel. Measurements from two real sorption wheels are used in order to validate the model. One wheel uses silica gel as desiccant material and the other lithium chloride. The simulation results are in good agreement with the experimental data. The model is used to compare the counter flow with the co-current wheel arrangements and to explain why the counter flow one is more efficient for air dehumidification.

Exergy analysis of solid desiccant-vapour compression hybrid air conditioning system

This paper presents energy and exergy analysis of a solid desiccant vapour compression hybrid air conditioning system. Parametric study has been conducted at varying dead state properties and operating conditions to investigate their effects on the overall system performance. The obtained results show that the exergy efficiency of hybrid air conditioning system is 11.45%. The energy-based performance parameters such as the coefficient of performance (COP) and components effectiveness are also presented. The rotary desiccant dehumidifier and heater are found as major contributors to exergy performance of the system. The analysis provides us beneficial knowledge to determine theoretical upper limit of the system performance.

Formulation and validation of a two-dimensional steady-state model of desiccant wheels

Desiccant wheels are rotary desiccant dehumidifiers used in air-conditioning and drying applications. The modeling of simultaneous heat and mass transfer in these components is crucial for estimating their performances, as well as for simulating and optimizing their implementation in complete systems. A steady-state two-dimensional model is formulated and implemented, aiming to obtain good accuracy and short computational times with the purpose of inclusion in complete system models. The model includes mass and energy balances and correlations for heat and mass transfer based on empirical relations from the scientific literature. Convective heat and mass transfer coefficients are computed locally accounting for the entrance length effects. Mass diffusion inside the desiccant material is neglected. Comparison with experimental data from the literature shows that the model reproduces the physical behavior of desiccant wheels satisfactorily, as the deviation between the computed results and available data is always within 12%. The simulation time is as low as 3 s for a model with 200 control volumes. It is found that for the applied case, the model provides accurate results for the overall flow using an equiangular control volume discretization with 200 control volumes and no axial discretization. More computationally expensive configurations with axial discretization give more accurate results and information on local flow and desiccant conditions inside the wheel.

Hierarchically porous aluminosilicates as the water vapor adsorbents for dehumidification

Hierarchically porous aluminosilicates with large surface area (930–1010m2/g) and ordered hexagonal structureswere synthesized as the water vapor adsorbents for dehumidification in this work. Compared to the commercial microporous zeolite 3A (low Si/Al ratio), and mesoporous SILICA GEL (high Si/Al ratio), the synthesized aluminosilicates showed higher adsorption capacity of water vapor at both high and low humidity ranges. Different from zeolite 3A (type I) and silica gel (type III), the adsorption isotherms of aluminosilicates followed type IV isotherms. The aluminosilicate materials showed low desorption activation energy of water vapor (<20kJ/mol, close to the intermolecular H2O–H2O H-bonding interaction). The aluminosilicates showed a low desorption temperature of 87°C and a relatively fast desorption rate. Moreover, regenerability test in the multiple adsorption–desorption cycles suggested that the synthesized aluminosilicates were regenerable. With the promising dehumidification characteristics, including high adsorption capacity of water vapor, fast adsorption and desorption rates, low regeneration temperature, as well as good regenerability, the synthesized hierarchically porous aluminosilicates can be potential adsorbents for energy-efficient and cost-effective dehumidification process in practical applications, i.e. rotary desiccant dehumidifiers.

Heat and mass transfer modeling in rotary desiccant dehumidifiers

A desiccant wheel model has been developed in the aim to be adapted to building simulation tools. This model fulfils several criteria such as simplicity of parameterization, accuracy, possibility to characterize the equipment under all operation conditions and low computation time. The method of characteristics has been applied to the heat and mass transfer partial differential equations. This transformation provides new equations which are similar to those of a rotary heat exchanger. Then, the model is described by the Effectiveness-NTU method and it is identified from only one nominal rating point. The model has been compared to experimental and manufacturers’ data for a broad range of operating conditions. A good agreement has been found.

Heat-and-mass transfers modelled for rotary desiccant dehumidifiers

A desiccant-wheel behavioural model has been developed. This model fulfils several criteria, such as simplicity of parameterization; accuracy; ability to characterize the equipment behaviour under all operation conditions; and short computation time. The method of characteristics has been applied to the heat-and-mass transfer partial-differential equations. This transformation provides equations which are similar to those of a rotary heat-exchanger. Then, the model is described by the effectiveness-NTU method and identified from only one nominal-rating point. The model predictions have been compared with experimental and manufacturers’ data for a broad range of operating conditions. Good agreement has been 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.

Analysis and Performance of Radial Flow Rotary Desiccant Dehumidifiers

A model is developed to predict the steady periodic performance of a radial flow desiccant wheel. The model is expressed in terms of the same dimensionless parameters that are commonly used in modeling of the conventional axial flow desiccant wheel. In addition a dimensionless geometrical ratio of the volume of the matrix to the volume of the wheel core is found to affect the performance of the wheel. A finite difference technique on staggered grid is used to discretize the governing dimensionless equations. The discretized equations are solved to predict the performance of the desiccant wheel at given values of operation parameters. A sensitivity study is carried out to investigate the effect of changing any of these parameters on the performance of the wheel. The performance of the radial flow desiccant wheel is compared with that of the convential axial flow desiccant wheel having the same values of the operation parameters.

On the Use of the Parabolic Concentration Profile Assumption for a Rotary Desiccant Dehumidifier

The current work describes a model for a desiccant dehumidifier which uses a parabolic concentration profile assumption to model the diffusion resistance inside the desiccant particle. The relative merits of the parabolic concentration profile model compared widely utilized rotary desiccant wheel models are discussed. The periodic steady-state parabolic concentration profile model developed is efficient and can accommodate a variety of materials. These features make it an excellent tool for design studies requiring repetitive desiccant wheel simulations. A quartic concentration profile assumption was also investigated which yielded a 2.8 percent average improvement in prediction error over the parabolic model.

Comparison of the DESSIM Model With a Finite Difference Solution for Rotary Desiccant Dehumidifiers

A heuristic “pseudo-steady-state” model of the heat and mass transfer occurring in desiccant dehumidifiers, embodied in the program DESSIM, has been proposed as a conceptually and numerically simple analysis tool (Barlow, 1982). A comparison is made with a finite difference solution to determine the accuracy and limitations of the pseudo-steady-state model. The comparison indicates that the pseudo-steady-state model can produce accurate results of dehumidifier performance relative to the finite difference solution when used carefully, although at greater computational expense. Substitution of the finite difference solution into the overall DESSIM program results in a potentially accurate and useful analysis tool.