Theoretical Study On Heat And Mass Transfer Of Sorptive Material Coated Heat Exchanger And Its Application To The Desiccant System

Dehumidification is an effective way to adjust temperature and humidity of producing and living environments.The dehumidification methods using solid desiccant material,which can be regenerated by low grade heat source and is efficient in dehumidfication,has the advantage of saving energy and environmental benefits.However,the adsorption heat can be released during dehumidifying process and leads sideeffect in terms of improving dehumidifying performance.The sorption material coated heat exchanger called SCHE,which is fabricated by coating solid desiccant material to the surface of the traditional fin-tube heat exchanger,is proposed in this paper.The unique feature of SCHE used in dehumidification process is that the cooling water can be employed to remove adsorption heat from the desiccant material.This is helpful for reducing the irreversible losses during the dehumidification process.In addition,the coated desiccant material can be regenerated by cycled hot water from low grade heat resources during the regeneration process.Compared with other dehumidification methods,SCHE has the significant characteristics of simple structure,low manufacturing cost,flexible operation and small impact on adsorption heat with remaining the advantages of the traditional solid desiccant dehumidification method.In this work,the dehumidification and regeneration performance of the SCHE and its application are investigated systematically.Firstly,the thermodynamic characteristics of SCHE and the dehumidification/ regeneration process based on ventilation cycle are analyzed.Futhermore,a similar thermodynamic analysis with heat recovery is carried on.The performance index of SCHE and the desiccant system are proposed.A novel continually desiccant system with two SCHEs is set up,which can provide continuous dehumidification.The effects of the desiccant system on cycle time,temperature and flow rate of cooling/regeneration hot water,velocity of process air and conditions of ambient air are analyzed.The experimental results show that the average moisture removal and the thermal COP of the desiccant system can approach 5.08 g/kg?DA and 0.34,respectively,under typical summer conditions.The desiccant unit can be integrated with primary return air to further reduce the humidity ratio of the supply air to 9.88g/kg?DA under the high humidity condition.In addistion,the heat and moisture transfer characteristics of a desiccant dehumidification unit using SCHE have been experimentally investigated.The study focuses on the analysis of air side heat and moisture transfer performance in dehumidification mode.The empirical formula on Nusselt and Sherwood numbers are deducted and curve fitted from the test results under various operation conditions(within a given range 200≤Re≤550),using Levenberg-Marquart method.When studying on the influencing factors of moisture and heat transfer properties of SCHE,this work has analyzed the performance impacts of moisture removal and thermal COPby using fin length and process air velocity.Also,the structure of SCHE and the operating condition are optimized according to the analysis conclusions.Based on the typical meteorological data of ARI summer,ARI humid and Shanghai summer,the mean moisture removal of optimized SCHE is increased by 30.3%,47.5%and 73.9%,respectively.And the thermal COP is improved by 58.8%,61.1%and 62.2%,respectively,if it is applied in the desiccant system.When heat recovery device is adopted in the system,the thermal COP is further raised to 1.09,1.12 and 1.18,respectively.Finally,a novel entire fresh air desiccant system using SCHE is introduced in this paper.The higher design requirements in moisture removal and thermal COP are proposed through better irrigations,which can reduce the humidity ratio of supply air below to 8 g/kg?DA and improve the thermal COP of the dehumidification system up to 1.0 by introducing the heat recovery device.The experimental results show that the design of the system has realized the target of the improved designThe feasibilityand advantages of SCHE driven by low grade heat sourceat the temperature in the range of 45-75 ~oC,has proved successfully in the research.It is confirmed that SCHE desiccant dehumidification cycle is an effective method,and is of great potential in the field of demidification and HVAC.