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6、Services Abstracts. Cui-i-ent contents arc in ICI Engiiiccring, Coiiiputing accepted August I, 2005 The pressure drop, carryover, leak, and corrosion in traditional liquid desiccant dehumidijca- tion towers are of serious concern in real applications, especially when using tri ethylene glycol (TEG)
7、as a desiccant. In this study the cellulose rigid media pads are used as the packing for its low pressure drop. A special coated material is used to prevent corrosion and leakage prob- lems. The packing arrangement minimized solution carryover. The alternative method that can be used to predict the
8、dehumidifier outlet conditions is the effectiveness method. Theoretical and experimental studies of the simultaneous heat and mass transfer to evaluate the effectiveness of the packed dehumidifier are conducted. The model pre- dictions are compared with the experimental results with very good agreem
9、ent. Through the experimental study of the dehumidijler, important design variables that affect the effectiveness are defined and cornpared with the correlations reported by previous studies. The previous cor- relations are assessed and the errors are reported. INTRODUCTION Higher ventilation rates
10、are dictated both by better comfort requirements and by the most recent standards, such as ASHRAE (62.1-2004). More outdoor air for indoor air quality gave the designers a much bigger load to be removed from the air, especially in humid climates. The out- door air must be conditioned to the desired
11、comfort level of humidity and temperature before being supplied to the occupied spaces. Cooling is usually obtained with refrigeration machinery, and often some post-heating is required to heat the air before it is supplied to the rooms. The other possibility is chemical dehumidification by using de
12、siccants. Solid or liquid desiccants are able to reduce the water vapor content in moist air. Chemical dehumidification had, until now, few applications. The most widespread systems are desiccant wheels, which use mainly solid sorption, and packed towers, which use liquid desiccants. Liquid desiccan
13、t systems essentially consist of an absorber for dehumidifying the air, a regenerator for regenerating the solution, and heat exchangers for precooling and preheating of solution. The amount of dehumidification depends on the concentration, temperature, and characteristics of the hygroscopic solutio
14、n because of the vapor-pressure difference between the air and the liquid desiccant. Several liquid desiccants, including aqueous solutions of organic compound and aqueous solutions of inor- ganic salts, have been employed to remove water vapor from air. Packed tower configurations have received mor
15、e attention. Many researchers worked on packed absorbers and compared the results with theoretical models. Although random packed towers facilitate more mass transfer by providing larger area in a relatively smaller volume, the air pressure drop through the packing is generally high. Structured pack
16、ings have shown excel- lent performance characteristics with a relatively low ratio of pressure drop to heat and mass transfer coefficient per unit volume (Bravo et al. 1985). Several liquid desiccants have been employed for dehumidification of air. Some of the studies related to the use of various
17、liquid desiccant in a packed column are represented in Table 1. Esam Elsarrag is an associate professor at the Technical Studies Institute, Zayed Military City, Abu Dhabi, United Arab Emirates. 3 4 HVACi i II i : ; . HVAC also a sample from the tank was taken for analysis. The dry-bulb and wet-bulb
18、temperatures of the outdoor air were also measured before and during the desiccant experiments. The desiccant and air were allowed to flow through the packed absorber. The measurements were taken after allowing enough time for steady-state readings. The measurements during the experiment are shown i
19、n Figure 2. The specifications for different measuring devices are shown in Table 3. Several sets of experiments were conducted for packing heights of 0.5, 0.45, and 0.4 m. RESULTS AND DISCUSSION Performance Variables The heat and mass transfer found from experimental data, the finite difference mod
20、eling (the- oretical model), and the existing correlations were depicted graphically along with the design variables. The heat and mass transfer effectiveness decreased with the increase of the airflow rate, as shown in Figure 4. The lower effectiveness is due to high outlet humidity ratio when the
21、airflow rate is increased. This is in good agreement with Elsayed et al. (1993), Chung et al. (1994), and Martin and Goswami (2000). However, an increase of the liquid flow rate affected an increase in the effectiveness, as shown in Figure 5. The liquid flow rate increased from 1.7 to 2.2 kg/m2.s wh
22、ile the airflow rate kept constant at 1.75 kg/m2.s. The increase in the effectiveness is mainly due to the good wetting of the packing when high liquid flow rates were employed. But the effectiveness showed a slight dependency on the liquid flow rate when high liq- uid-to-airflow-rate ratios were em
23、ployed, between 1.9 and 2.4, as shown in Figure 6. This is mainly because the sufficient amount of liquid flow rate to achieve maximum wetting of the packing was obtained. This is in agreement with the findings obtained by Oberg and Goswami (1998) and Martin and Goswami (2000). On the other hand, th
24、e desiccant vapor pressure indicates the equilibrium humidity ratio. The vapor pressure is a function of the desiccant temperature and concentration. For desiccant tem- peratures from 3 1.2“C to 33.8“C and a concentration of 92%, the equilibrium humidity changes from 9.7 to 1 1.4 gmwikga. Within thi
25、s range, the effectiveness did not show a significant depen- dency on the equilibrium humidity, as shown in Figure 7. This is mainly because the difference between the inlet and outlet air humidity decreases and the maximum difference also decreases Table 3. Measuring Device Specifications Device Ty
26、pe Accuracy Operative Range Fluid Thermometers Digital RTD 0.1“C 0C-100C Air (DB 03 4 024 I % O8 1 12 14 16 18 2 22 m; (kgims) OI 1.7 1.8 1.9 2 2.1 2.2 2.3 mi imr 1, a o9 O8 0 - 07 o - - - Wnm o1 OC 17 18 19 2 21 22 23 mL (kg/mzs) I . . - _ _ _. . . . Figure 4. Influence of airflow rate on effec- ti
27、veness. effectiveness. Figure 5. Influence of liquid flow rate on when the equilibrium humidity increases. The previous studies used either the desiccant temper- ature or concentration to study their effect on the performance. The comparison between their findings can be done by considering that the
28、 increase of desiccant temperature or the decrease of concentration will affect an increase in the equilibrium humidity. This is in agreement with Oberg and Goswami (1998). A slight increase in the effectiveness was observed with the increase of the inlet air humidity ratio, as shown in Figure 8. Th
29、is is in agreement with the previous studies. By increasing the packing height, the heat and mass transfer effectiveness increased, as shown in Figure 9. This is due to the increase in the area available for heat and mass transfer by increasing the packing height. This is in agreement with Elsayed e
30、t al. (1993) and Martin and Goswami (2000). It can be observed that in all figures, finite difference modeling has shown good agreement with experimental results and gave good predictions. Correlation Assessment Chungs Humidity Effectiveness Correlation. As shown in the figures, the humidity effec-
31、tiveness calculated by the Chung (1 994) correlation, as well as that obtained from the theoretical model, were close to the experimental results. The main advantage of the Chung correlation (Equation 18) is that it included the vapor pressure of the desiccant, the driving force, which is a function
32、 of the desiccant temperature and concentration. VOLUME 12, NUMBER 1, JANUARY 2006 13 0.7 0.6 0.5 0.4 03 0.2 o1 Em _*.- c .- 0: 18 2 2.2 24 mJm. O .” . . ” o,7/ ._._.*,-*- ch 064 O5 I 04; 03 1 02. o1 i O 18 2 22 24 o. o.2 1 I 1 I 0.9 I i 0.4 .I I oJ- 1 9 9.5 10 10.5 11 11.5 Figure 6. Influence of hi
33、gh liquid to air- flow rate ratio on effectiveness. Figure 7. Influence of equilibrium humidity on effectiveness. Martins Humidity and Enthalpy Effectiveness Correlations. The values obtained by Martin and Goswamis (2000) correlations (Equations 19 and 20) had a big discrepancy when the airflow rate
34、 was increased. This is mainly because an increase in the airflow rate is reflected as a decrease in the liquid-to-airflow-rate ratio. In fact, the correlations were developed for high liquid-to-airflow rates, from 3 to 11. Therefore, the lower the airflow rates, the lower the dis- crepancy and good
35、 predictions. This can be observed clearly in Figure 6. Martin and Goswamis correlations (Equations 19 and 20) gave good predictions when high liquid-to-airflow-rate ratios were employed. However, the correlations included effective variables, such as the wettability of the liquid and packing, but t
36、wo terms in their correlations affected the higher discrepancies. The term (rnL/rn,) with a validity range of 3 to 11 is very high for ventilation applications, as explained and discussed by Elsarrag et al. (2005). Also, the term (hu,i/hL,j) in the dehumidifica- tion correlation was used to express
37、the air and desiccant inlet conditions. The desiccant enthalpy is a function of the temperature only, so the concentration was not included in the cor- relation, which is a vital factor. The use of desiccant pressure might be more effective than the use of the enthalpy for humidity effectiveness pre
38、dictions. Abdul-Wahabs Humidity Effectiveness Correlation. It can be observed in all figures that the statistical prediction model developed by Abdul-Wahab et al. (2004) (Equation 2 1) couldnt predict the humidity effectiveness. This is mainly because the variables were not defined well in addition
39、to the limitation of the parameters used. The correlation didnt include the airflow rate, 14 HVAC but for high liquid flow rates, the effectiveness has no dependency on the liquid flow rate, so Em,ha(m J. Therefore, if another range of validity for the iiquid-to-airflow-rate ratio within reasonable
40、limits (e.g., 1:2 to 2:5) is correlated along with the existing set of correla- tions, it will be valuable for packed tower predictions for different applications (e.g., ventila- tion). CONCLUSIONS In this investigation, theoretical and experimental studies of simultaneous heat and mass transfer to
41、evaluate dehumidifier effectiveness have been conducted. The effect of different design parameters on the heat and mass transfer effectiveness of a structured packed column VOLUME 12, NUMBER 1, JANUARY 2006 15 Table 4. Performance Variables and Correlation Discrepancies Correlation Performance m, mL
42、 22 0a.i me Z m, - + f f Cm Model disc. 3 to 8.5% 8 to -1% 8.1 to 4% 9.4 to 8% Present study - + f f Eh 4.7 to 4.3% Model disc. -3 to 8% 13 to 1.8% 6.5 to 1% - + f f Em Chung et al. -1.5 to 4.5% (1994) Discrepancy 8 to -1.5% 8 to -1% 8.5 to 6% - f f f Em -1O*t -35t -6to 4Oto -50% -25% -3%* -32% - f
43、f f Martin and Discrepancy Goswami (2000) Eh -50to -47 to -15 to -58 to -20% 44% -13%* 47yo Discrepancy Em Abdul-Wahab f + c f -120 to -135 to -150 to -120 to -100% -80% -93% -76% et (2004) Discrepancy f 4 to 7.7% f 3.5 to 9% f 3 to 1% f 49 to -57% f -56 to -66% f -124 to -1 15% + 3.1 to 3.8% + 0.6
44、to 2.7% + 8 to 14% i -20 to -13% + -33 to -24% f -100 to -80% * Martins correlations are valid (high liquid flow rates or low airflow rates) + Effectiveness increases with the variable. - Effectiveness decrease with the variable. i Slight or no significant dependency. using TEG has been investigated
45、. The effect of air and liquid flow rates, air humidity, and desic- cant equilibrium humidity have been reported with regard to humidity and enthalpy effective- ness. The effectiveness didnt show any dependency on the liquid flow rate when high liquid-to-airflow-rate ratios were employed. The correl
46、ations found in the literature were assessed and the errors were reported. The correlation developed by Chung (1 994) well pre- dicted the mass transfer effectiveness. However, the correlations developed by Martin and Gos- wami (2000) predicted tbe heat and mass transfer effectiveness only when high
47、 liquid-to-airflow rates were employed, !?b 2.4. The statistical model developed by Abdul-Wahab et al. (2004) could not predict the huridity effectiveness. The heat and mass transfer effectiveness predicted with the finite difference model described in this paper shows good agreement with the experi
48、- mental findings. It is found that if another range of the liquid-to-airflow-rate ratio within reasonable limits is correlated along with the existing set of correlations, it will be valuable for packed tower predic- tions for different applications. NOMENCLATURE a = area of heat and mass transfer,
49、 m2/m3 Cp = specific heat, kJ/kg.K a, = specific interfacial area of packing, D, = diffusion coefficient, m2/s m .- m2/m3 16 deq = equivalent diameter for structured pack- ing, m Fc = gas phase mass transfer coefficient, kmoiim2.s F, = liquid phase mass transfer coefficient, kmoi/m2.s hc = heat transfer coefficient, kW/m2.K K = mass transfer coefficient, kmoi/m2.s k = thermal conductivity, W/m.K Le = Lewisnumber m = flow rate, kgls or kglh Greek y = latent heat of condensationhaporization, kJk Subscripts a = ai
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