外文原文-空气溶液除湿的除湿速率和除湿效率的预测.pdf

Energy 29 2004 19 34 Predictions of moisture removal rate and dehumidifi cation eff ectiveness for structured liquid desiccant air dehumidifi er S A Abdul Wahab a Y H Zurigata 1 M K Abu Arabib a Department of Mechanical and Industrial Engineering Sultan Qaboos University P O Box 33 Al Khod Muscat 123 Oman b The Middle East Desalination Research Center P O Box 21 Al Kuwair 133 Oman Received 13 January 2003 Abstract In hot and humid climates such as in the Sultanate of Oman the humidity puts extra load on the elec tric vapor compression air conditioning VAC systems Liquid and solid desiccants can reduce the moisture content of humid air and thus reduce the latent load imposed on the VAC systems In the present work the performance of air dehumidifi ers using triethylene glygol TEG as desiccant was inves tigated Three diff erently structured packing densities were used 77 100 and 200 m2 m3 The perform ance of the dehumidifi er was evaluated and expressed in terms of the moisture removal rate mcond and the dehumidifi er eff ectiveness ey The experimental work was undertaken to study the eff ects of several infl uencing design factors on this performance The design factors covered included the air and TEG fl ow rates air and TEG inlet temperatures inlet air humidity and the inlet TEG concentration The desiccant fl ow rate investigated was much less than that covered in previous studies and the range of the inlet tem peratures of air and desiccant was signifi cantly wider The objective this study was to use the multiple re gression method and the principal component analysis to obtain statistical prediction models for the water condensation rate and the dehumidifi cation eff ectiveness in terms of these design factors The results of both techniques agree with each other affi rmed that the desiccant fl ow rate desiccant inlet con centration and air inlet temperature are the most signifi cant variables in predicting mcond whereas desic cant fl ow rate air inlet temperature and packing density are the most signifi cant variables in predicting ey 2003 Elsevier Ltd All rights reserved Corresponding author Tel 968 515 350 fax 968 513 416 E mail address sabah1 squ edu om S A Abdul Wahab 1 On sabbatical leave from University of Jordan Amman Jordan 0360 5442 see front matter 2003 Elsevier Ltd All rights reserved doi 10 1016 j energy 2003 08 001 1 Introduction Desiccants are chemicals with great affi nity to moisture They may be used eff ectively to re move the latent part of the cooling load in vapor compression air conditioners Thus the desic cant dehumidifi cation process can be used as a supplement to conventional vapor compression systems That is the desiccants are effi cient in handling latent loads remove the humidity whereas the evaporator of the vapor compression systems is effi cient in handling the sensible cooling loads lowers the air temperature Hence separating the control of humidity and tem perature by means of desiccants could result in energy savings as well as improved humidity control 1 In addition the desiccant dehumidifi cation process is environmentally safe in comparison to VAC systems VAC systems are responsible for greenhouse gas emissions and they may contrib ute to global warming The refrigerants CFCs and HCFs used in such equipment are them Nomenclature aSurface area to volume ratio of packing m2 m3 A Cross sectional area of the dehumidifi er column m2 G Air mass fl ux kg m2 s L Liquid desiccant mass fl ux kg m2 s mWater condensation rate g s pVapor pressure Pa R2 Coeffi cient of determination TTemperature XDesiccant concentration kg TEG kg solution YAir humidity ratio kg water kg dry zTower height m e Eff ectiveness dimensionless p Dimensionless vapor pressure diff erence Subscripts condCondensation equEquilibrium inInlet outlet Outlet yAir humidity ratio Abbreviations ACAir conditioning PCAPrincipal component analysis TEGTriethylene glycol VACVapor compression air conditioning S A Abdul Wahab et al Energy 29 2004 19 3420 selves greenhouse gases It is possible that some of that refrigerant will escape into the atmos phere during routine installation operation and servicing of the equipment Once released to the atmosphere the refrigerant has the potential to contribute to global warming Moreover most air conditioners are operated by electricity and burning fossil fuels often produces this electricity This burning process leads to the production and release of carbon dioxide that con tributes to global warming Due to the depletion of the ozone layer and global warming poten tial of these refrigerants numerous alternative techniques are explored to conserve high grade energy One of these propositions that are energy effi cient and environmentally benign is the desiccant Several diff erent materials can be employed as the desiccant including both solid and liquid substances Conventional solid desiccants include silica gel activated alumina lithium chloride salt molecular sieves titanium silicate and synthetic polymers Liquid desiccants include lithium chloride lithium bromide calcium chloride and TEG More details about desiccant types properties and the regeneration process are given by Kinsara et al 2 The earliest liquid desiccant system was suggested and experimentally tested by Lo f 3 using triethylene glycol TEG as desiccant Solar heated air was used for regeneration purposes Sheridan 4 investigated a solar operated liquid desiccant dehumidifi cation coolant using lith ium chloride as the working fl uid Reviews of the early work on the development of liquid des iccant systems can be found in Gandhidasan and Gupta 5 Lodwig et al 6 Robison 7 Robison and Houston 8 Factor and Grossman 9 Peng 10 Grossman and Johannsen 11 Johannsen 12 and Howell 13 In addition more than 80 publications regarding liquid desic cant liquid desiccant cooling system and the analysis of important system components were reviewed by O berg and Goswami 14 The performance of a packed bed absorption tower operating as an air dehumidifi er or a des iccant regenerator is infl uenced by many design operating parameters and conditions 1 desic cant fl uid characteristics viscosity density and surface tension packing type shape size and material desiccant distribution over the packing fl ow confi guration counter or co current fl ow tower height fl uid fl ow rates and the inlet conditions of the desiccant temperature and concentration and the air temperature and humidity To improve the eff ectiveness of the des iccant air dehumidifi cation process the impact of these parameters has to be evaluated Good system performance and energy savings can be achieved if the proper values of these variables are selected A number of experimental studies are reported in the literature regarding this mat ter For example Chen et al 15 Patnaik et al 16 McDonald et al 17 Chung et al 18 Potnis and Lenz 19 and Ullah et al 20 carried out experiments on packed bed dehumidifi ers using salt solutions as the desiccants Chung et al 21 Oberg and Goswami 1 Martin and Goswami 22 Park et al 23 Peng and Howell 24 Chebbah 25 Park et al 26 27 Chung and Luo 28 and Chung and Wu 29 reported some experimental fi ndings using triethylene glycol as the desiccant Furthermore Zurigat et al 30 carried out experiments on dehumidi fi ers with structured packing using triethylene glycol as the desiccant They evaluated the per formance of the dehumidifi cation system under hot and humid conditions with low packing density 77 m2 m3 by conducting a series of runs with two structured packing materials wood and aluminum The eff ect of diff erent design variables on the performance of dehumidifi cation process was investigated and expressed in terms of the moisture removal rate and the dehumidi fi er eff ectiveness 21S A Abdul Wahab et al Energy 29 2004 19 34 In general the usefulness of a particular liquid desiccant depends upon the application For example triethylene glycol does have a small vapor pressure that causes some of the glycol to evaporate into the air stream 11 14 The TEG vapor is thus carried over into the conditioned space and may condense on walls windows and equipment forming a viscous fi lm Further more the viscosities of the glycol are much higher than that of the aqueous salt solutions and the pumping cost is also higher 28 On the other hand lithium chloride has good desiccant characteristics and does not vaporize in the air at ambient conditions A disadvantage with lith ium chloride is that it is corrosive Based on the literature review most investigators used random packing Also for the per formance of a packed bed absorption tower only a limited number of correlations were indi cated 20 22 28 31 Furthermore there was a need to broaden the range of validity for these correlations by including additional experimental data In the present work the liquid desiccant TEG was used as the working fl uid Compared to previous studies structured packing with diff erent densities was used to study experimentally the impact of a number of design variables on the performance of the dehumidifi cation process The work concentrated on carrying out experimental work to cover a wider and extended range of conditions reported in the literature The liquid fl ow rate used in this study 9 1 0 192exp 0 615 TGin TLin X 21 498 9 8 3 where X is defi ned as a function of the vapor pressure depression of the desiccant solution to the vapor pressure of pure water Pwater Psoln Pwater a is the surface area to volume ratio of packing in m2 m3 z is the packing height in m TGinand TLindenote the inlet temperatures of the air and the liquid respectively Ginand Lin represent the mass fl ux of air and liquid respect ively Both multiple regression method using the stepwise procedure and the principal component analysis were then used in order to obtain statistical prediction models for the water conden S A Abdul Wahab et al Energy 29 2004 19 3424 sation rate and the dehumidifi cation eff ectiveness with six deign parameters air fl ow rate desic cant inlet concentration desiccant fl ow rate desiccant inlet temperature air inlet temperature and the packing density as predictor variables The study focused specifi cally on identifying those factors that have signifi cant eff ects on the water condensation rate and the dehumidi fi cation eff ectiveness in a structured packing dehumidifi cation system 4 Results and discussion 4 1 Analysis of data In this study the performance of the dehumidifi cation system was evaluated by conducting a series of runs with three packing densities 77 100 and 200 m2 m3 The results of these experi ments are depicted graphically in Figs 2 3 These fi gures show the water condensation rate mcond and the dehumidifi cation eff ectiveness ey for three packing densities 77 100 and 200 m2 m3 as a function of air fl ow rate G desiccant inlet concentration XIN desiccant fl ow rate L desiccant inlet temperature TL IN and air inlet temperature Ta IN Looking at Figs 2 and 3 it can be seen that the trend of the dehumidifi er performance was similar to that reported in the literature using random packing The moisture removal rate increases with increase in air fl ow rate desiccant fl ow rate and desiccant concentration while decreases with increase in air or desiccant inlet temperature for the three packing densities On the other hand the dehumidifi er eff ectiveness increases with increase in desiccant concentration desiccant fl ow rate and desiccant inlet temperature while decreases with increase in air fl ow rate and air inlet temperature for the three packing densities In general the eff ect of packing density varies depending on the design variable investigated The column with packing density of 200 gave lower eff ectiveness than the other two packing densities when air fl ow rate increased This could be due to not having adequate wetting of the packing with higher fl ow rates The moisture removal rate becomes better for the 200 than the 77 and 100 packing den sity when either the inlet air or desiccant temperature is increased The column with packing density of 200 gave lower eff ectiveness than the other two packing densities when air fl ow rate inlet concentration and desiccant fl ow rate increased but gave higher eff ectiveness when the in let temperature of air or desiccant was increased This could be due to inadequate wetting which was improved as the temperature inside the column was increased There are discrep ancies in the literature about the eff ect of process variables on the dehumidifi er performance using TEG as the liquid desiccant The eff ect of air fl ow rate desiccant fl ow rate and inlet des iccant concentration on the dehumidifi er performance were found to increase the rate of moist ure removal for the three packing densities used in this study which is in agreement with some and in disagreement with other reported work The eff ect on the rate of removal due to chang ing inlet temperature of air or desiccant was found to decrease with increasing the temperature which is also in agreement with some and in disagreement with others Also there are discrep ancies with respect to the eff ect of the process variables on the dehumidifi er eff ectiveness More details with experimental fi ndings can be found elsewhere 30 25S A Abdul Wahab et al Energy 29 2004 19 34 4 2 Comparison with Chung and Luo correlation To draw possible relationships between the eff ectiveness calculated from Chung and Luo s correlation 28 based on the value of the parameters used in this study and that found exper Fig 2 The eff ect of various design variables on condensation rate for packing densities 77 100 and 200 m2 m3 S A Abdul Wahab et al Energy 29 2004 19 3426 imentally in the present work the scatter plots were drawn as shown in Fig 4 for three packing densities Looking at Fig 4 it seems that there is a possible positive correlation The plotted points are grouped together or clustered which indicate that the two variables are associated Fig 3 The eff ect of various design variables on dehumidifi cation eff ectiveness for packing densities 77 100 and 200 m2 m3 27S A Abdul Wahab et al Energy 29 2004 19 34 with each other The correlation coeffi cient for the relationships existing between them was in the range 0 71 0 77 This refl ects the fact that the correlation between them was strong positive and quite signifi cant 4 3 Correlation matrix To decide whether a signifi cant relationship exists between the paired data the correlation co effi cient values were developed Table 1 shows the correlation matrix for the various design parameters and for both the water condensation rate and the dehumidifi er eff ectiveness The last two columns in this table indicate the tendency of the water condensation rate and the de humidifi er eff ectiveness to change with the change in various parameters Those parameters where a signifi cant correlation exists have been highlighted in Table 1 As indicated in this table the correlation coeffi cients between the water condensation rate and L XIN and packing den sity were positive The most signifi cant correlations were with L 0 631 and XIN 0 367 denot ing that the increase in their values was associated with an increase in the water condensation rate values However the correlations of water condensation rate values with packing density values were insignifi cant 0 039 On the other hand the coeffi cients of correlations of water condensation rate values with G Ta INand TL IN were negative and they were only signifi cant with Ta IN 0 355 The coeffi cients of correlations of eywith G Ta INand packing density were negative and they were only signifi cant with packing density 0 477 and Ta IN 0 422 The correlation with G was generally poor and not signifi cant In contrast the correlation coef fi cients between ey L and XIN were positive They were signifi cant with L 0 626 and they were less signifi cant with XIN 0 109 4 4 Development of the statistical models This section shows development of statistical models for the estimation of the water conden sation rate and the dehumidifi er eff ectiveness as a function of various input design parameters The models were determined by using a stepwise multiple regression modelling procedure This procedure is a combination of forward selection and backward elimination multiple regression procedures It is one of the most widely used methodologies for expressing the dependence of a response variable on several independent predictor variables It is used automatically to select independent parameters that are of most importance and eliminates those that are of least importance In this work six independent variables were selected as inputs G L XIN Ta IN TL INand a The dependent or response variables are mcondand ey The results of the multiple regression equations that were formulated to predict the moisture removal and the dehumidifi er eff ectiveness are presented in Table 2 The results indicate that the prediction equation of the mcond was more highly correlated with desiccant fl ow rate desiccant inlet concentration and the inlet air temperature Packing density seemed to be unimportant The statistical evaluation of the model using only desiccant fl ow rate desiccant inlet concentration and the inlet air temperature yielded coeffi cient of determination R2 of 0 78 This means that 78 of the varia tions in mcond data can be explained by the variations of the desiccant fl ow rate desiccant inlet concentration and the inlet air temperature data The unexplained variance was of the order 22 Such unexplained variance was attributed to the probable occurrence of other variables S A Abdul Wahab et al Energy 29 2004 19 3428 that had eff ects on mcondand were not considered in the statistical treatment On the other hand the desiccant fl ow rate and inlet air temperature as well as packing densities were found Fig 4 Values of dehumidifi cation eff ectiveness calculated from Chung and Luo correl