Effect of Desiccant Isotherm on the Performance of a Desiccant Wheel - Engineering Assignment Help

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A one-dimensional mathematical model has been developed to investigate the effect of desiccant isotherm (adsorptive material) on the performance of the desiccant wheel. The model consists of four governing heat and mass transfer equations with some auxiliary conditions which are then solved using FlexPDE Software. The model shows good agreement with the experimental results obtained from the literature as well as with the experiment conducted by the author. In this paper the different adsorption isotherms have been characterized by a single parameter called the separation factor. The separation factors, R = 0.05, 0.1, 0.5, and 1 are analyzed under a different range of operating conditions. Investigation of these parameters is based on the wheel performance, index, that is moisture removal (MR) and dehumidification coefficient of performance (DCOP), as DCOP is more appropriate to reflect the utilization of heat energy in the regeneration process. The result of this study shows that there is a significant effect on the operating parameter on the isotherm shape. Across the entire range of operating condition single isotherm (R = 0.5 or R = 0.1) there is not a better adsorptive material choice but it varies with change in the operating conditions. The results of this study shows that the isotherm shape of R = 0.1 or 0.5 yields better results depending upon whether the given range of operating conditions are higher or lower. 2016 Wiley Periodicals.

In hot and humid climates, the conventional vapor compression system consumes more electricity and also affects the environment by releasing chlorofluoro carbon (CFC) and hydro chlorofluoro carbon (HCFC). Simonson [1] suggested that the ideal cooling of air is from 35 °C and 60% with relative humidity (RH) from to 25 °C and 50%. Relative humidity requires four times as much energy as cooling air from 35 °C to 25 °C with no change in moisture level. An alternative method to provide air conditioning is the desiccant (solid desiccant) based air conditioning system, which is environmentally friendly. A desiccant wheel is the key component of the desiccant based air C?2016 Wiley Periodicals, Inc. 1 conditioning system and it uses a solid desiccant for dehumidification. The rotary desiccant wheel is operated under various operating and design conditions. Zhai and colleagues [2] suggested that the various operating and design parameters of the desiccant wheel include:

• The rotary speed of the wheel.
• Process air temperature, humidity and flow rate. Regeneration air temperature, humidity and flow rate.
• The wheel dimension, such as the wheel length, the wheel diameter and the split between adsorption and desorption sections.
• The channel dimension, such as the channel shape, height and pitch.
• Substrate thickness and thickness of coated desiccant.
• Type and properties of the desiccant.

The desiccant wheel is an air to air heat and mass exchanger and the heat and mass transfer phenomenon in the desiccant wheel is very complicated. Hence, a mathematical model is developed for the discussion and optimization of the above parameters. Person and Mills [3] proposed that for micro porous silica gel surface diffusion is the dominant mechanism of moisture transport, while for macro porous silica gel both Knudsen and surface diffusion are important. Sphaier and Worek [4] compared one-dimensional and two-dimensional mathematical models for both solid side and gas side resistance and found that one-dimensional formulation could be used in desiccant wheel applications, whereas a two-dimensional model was needed for an enthalpy wheel when thermal resistance in desiccant material was high. Gao and colleagues [5] discussed the effect of felt thickness and passage shape on the performance of a desiccant wheel and it was found that as the thickness of sorbent increases, MRC (moisture removal capacity) of the desiccant wheel improves and the sinusoidal air flow passage was the best shape for greater MRC. Jia and colleagues [6] focused on the development of advanced desiccant materials that have improved sorption capacity and better moisture and heat diffusion rates, as well as favorable equilibrium isotherms. Ge and colleagues [7] developed one-dimensional mathematical model to predict the performance of novel Silica gel haloid compound desiccant wheel. Intini and colleagues [8] investigated the performance of an AQSOA Zeolite based desiccant wheel and suggested that this material is most suitable for medium and high temperature applications with regeneration temperature ranging between 80 °C and 100 °C. Goldsworthy and White [9] investigated the influence of the desiccant equilibrium isotherm on the overall wheel performance.

Cui and colleagues [10] proposed that DH-5 and DH-7 have good dehumidifying capabilities and their properties are superior to Silica gel and a 13× Molecular sieve. Also, Silica gel is a desiccant with high performance, but it can be destroyed after rapidly adsorbing a great deal of water and it is not a heat-resistant material. Nobrega and Brum [11] proposed that the different adsorption isotherms are represented by a general equation characterized by a single parameter (the separation factor R), the variation of which allows for the behavior of three different desiccant materials (silica gel, Molecular sieve and 1 M). The results of this study showed that the separation factor R has a great influence over the dehumidification 2 effectiveness, for given regeneration conditions. Dai and colleagues [12] presented that the desiccant isotherm shape is the most important factor in determining the wave front shapes within the desiccant matrix and they also discussed the effect of the separation factor (R = 0.01 to 1) of the isotherm shape on the regeneration temperature of the desiccant wheel. Golubovik and Worek [13] developed a mathematical model for the case (high operating pressure) where condensation is present inside the channels of the desiccant wheel.

They found that depending upon the separation factor and regeneration temperature, the condensation region occuppied 21% to 40% of the regeneration region. In this region, regeneration of the desiccant is not possible and dehumidification of regeneration air usually occurs. They also found that the optimum separation factor of desiccant material increases with operating pressure. Chung and Lee [14] examined the operating/design parameters, the rotation speed and area ratio of regeneration to dehumidification on the basis of MRC for a range of regeneration temperatures from 50 °C to 150 °C. Simulations were also focused on the effect of different isotherms on the optimal condition of these parameters. Yadav and Yadav [15] analyzed the effect of the desiccant isotherm on the performance of the desiccant wheel for various design parameters by solving one-dimensional gas and solid side model with the help of FlexPDE software. Yadav and Yadav [16] mathematically compared the performance of the desiccant wheel for clockwise and anticlockwise rotation of the wheel at different purge angles and it was found that anticlockwise rotation gives better dehumidification; the performance parameter (rotation of wheel, velocity and ambient moisture) showed better results in lower purge sector angles (5° to 10°) as compared to higher purge angles while in the case of regeneration temperature, purge sector angle of 15° gives better performance results as compared to other purge sector angles.

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