EXPERIMENTAL INVESTIGATION OF THE AIR CLEANING EFFECT OF A DESICCANT DEHUMIDIFIER ON PERCEIVED AIR QUALITY

EXPERIMENTAL INVESTIGATION OF THE AIR CLEANING EFFECT OF A DESICCANT DEHUMIDIFIER ON PERCEIVED AIR QUALITY L Fang 1,2,, G Zhang 1,2 and PO Fanger 1,2 1 International Centre for Indoor Environment and Energy, (www.ie.dtu.dk), Department of Mechanical Engineering, Technical University of Denmark, DK-2800 Denmark 2 School of Environmental Science and Technology, Tianjin University, Tianjin 300072, China ABSTRACT A laboratory study was conducted to investigate the co-sorption effect of a desiccant rotor on improving perceived air quality. A rotary desiccant dehumidifier was used for the investigation. Carpet and linoleum emissions as well as human bio-effluents were used as air pollution sources. Up to thirty subjects served on a sensory panel to test and judge the effectiveness of pollutant removal by a desiccant rotor. The results of the experiment showed that a desiccant rotor has great potential for air cleaning. The effectiveness of air pollutant removal in terms of sensory pollution load was better than 80% for the three types of air pollutant. The air-cleaning effect was found to be independent of air humidity and the processed airflow rate of the dehumidifier. The effect of the reactivation air temperature on the effectiveness of pollutant removal by the dehumidifier remains to be further investigated. The observed air-cleaning effect of a desiccant rotor can be used to improve indoor air quality and decrease energy consumption of HVAC systems that use desiccant cooling. INDEX TERMS Desiccant Rotor, Air Cleaning, Indoor Air Quality, Sensory Pollution Load. INTRODUCTION Poor indoor air quality is a recognized worldwide problem having adverse effects on people s health, comfort, productivity and quality of life. For most nonindustrial indoor environments the concentration of chemical pollutants in the air is normally at a relatively low level (e.g. the level of ppb) but covers a broad spectrum. It is usually this spectrum of chemicals at low concentrations that causes people to feel uncomfortable or sick. Great efforts have been made to remove indoor air pollutants by means of air-cleaning devices, but major difficulties are encountered because of the wide range and low concentration. Currently available air cleaners use various technologies, including sorption filtration, photocatalytic oxidation, ozone oxidation and air ionization etc. Recent studies have found that among the different technologies, sorption filtration is more effective than other approaches (Chen et al. 2005). However, an ideal air cleaner that can remove the major air pollutants effectively has still to be developed. Sorption filtration technique utilizes an adsorbent to adsorb the chemicals in the air. The adsorbent consists usually of porous materials such as active carbon, silica gel, zeolite. To adsorb indoor air pollutants having a wide range of chemicals (e.g. different sizes of molecules), the adsorbent materials must contain a mass of pores of different sizes. Some adsorbent materials, such as silica gel, fulfill this requirement. Silica gel is usually used as a desiccant for removing moisture from air, a typical application being the rotary desiccant dehumidifier. The principle of this dehumidifier is shown in Figure 1. A rotary desiccant dehumidifier contains a reactivation section that removes the moisture adsorbed by the desiccant material and restores the adsorbability of the rotor. The same principle can be used for an air cleaner. The adsorption performance of a desiccant material, e.g. silica gel, on selected chemical samples, e.g. 1, 1, 1-trichloroethane, toluene, carbon dioxide, formaldehyde and radon, was studied by Hines et al. (1993) and Hines and Ghosh (1993). The study shows that silica gel has adsorbability for both water and the chemical vapors tested. Their study also indicated that the adsorbability of silica gel on water and chemical vapors compete with each other for adsorption of the silica gel. Since concentrations of VOCs in nonindustrial indoor air are usually at very low levels, the competition between VOCs and water vapor may not influence significantly the co-sorption performance of a desiccant rotor on vapor from VOCs. The desiccant rotor may still have sufficient capacity to adsorb VOCs in the air and remove them by reactivation even at moderate levels of indoor air humidity. Corresponding author email: fl@mek.dtu.dk. 2976

Furthermore, the results of Hines et al. were obtained from a laboratory experiment with small samples of a few selected desiccant materials. The adsorption performance of a wider range of chemicals (especially VOCs in indoor air) was not studied. This paper presents a study that investigated the practical application of a commercially available desiccant rotor and its ability to improve perceived indoor air quality in a real indoor environment, and discusses the practical application of the desiccant adsorption technique for indoor air cleaning. Humidifier Outdoor Air Partition Wall Recirculation Air Cooling coil Chilled water Reactivation Air Damper 3 Test room Fan Process Air Materials Exhaust Air Damper 1 Damper 2 Figure 1. Principle of rotary desiccant dehumidifier Figure 2. Experimental set-up METHODS The experiment was carried out in two adjacent office rooms of 14 m 2 each. One room was used as a test room to simulate an air-conditioned and ventilated space, and the other was used for the technical installation (see Figure 2). A rotary desiccant dehumidifier with silica gel rotor was installed to process the air that was recirculated from the test room. Since the air was warmed up after it passed through the dehumidifier, a cooling coil was installed downstream of the dehumidifier to cool the air before it was supplied to the test room. The air temperature in the test room was maintained at 23 C by this cooling coil. Ultrasonic humidifiers were installed in the test room to control the humidity in the room. The outdoor air supply rate to the test room was controlled by a fan and a damper installed on the window to supply a constant airflow rate of 13 L/s. Flooring materials and human subjects were used as pollution sources to replicate actual indoor air pollution sources that provide a real spectrum and concentration of chemicals. The flooring materials used were carpet and linoleum, both commonly used in offices. The carpet and linoleum were cut into small pieces of 0.2 1.7 m 2 and stapled back to back. They were hung on a stainless steel rack in order to outgas constant chemicals. Twenty five pieces of carpet were used in one experiment, and fourteen pieces of linoleum were used in another. Human bioeffluents were obtained from three volunteers (one male and two female subjects) who worked in the test room at a moderate metabolic rate. Sensory assessments were used to evaluate the air quality in the test room. The sensory assessments were made by a group of 30 untrained subjects. The assessments were always made after a few hours ventilation when steady-state was established for each test condition. The subjects used acceptability and odor intensity scales to judge the air quality. The acceptability scales had been used in many of the previous studies and can be used to calculate the percentage of persons dissatisfied (PD) with the air quality (Fang et al. 1998). The mean acceptability and odor intensity assessments of the 30 subjects were used to evaluate the quality of the indoor air. Nonparametric Wilcoxon matched pairs test was used for analyzing the results of the assessments. Table 1. Experimental design Pollutant Dehumidifier bypassed (RH=40%) (ACR=5h -1 ) RH (40%) RH (15%) RT (high) RT (low) RT (high) RT (low) ACR 5h -1 ACR ACR ACR ACR ACR ACR ACR 2.5 h -1 5 h -1 2.5 h -1 5 h -1 2.5 h -1 5 h -1 2.5 h -1 Carpet Linoleum Human no Note: RH Relative Humidity; RT Reactivation Temperature; ACR Air Change Rate; Conditions tested. 2977

The reactivation temperature was controlled at two levels. At high level, the process air temperature downstream of the dehumidifier was controlled in the range 35-40 C. At low level, the corresponding temperature was controlled in the range 45-50 C. The two levels of air change rate in the tablewere calculated by the airflow rate of recirculation, corresponding to 54 L/s and 27 L/s. The outdoor air supply rate was always controlled at 13 L/s for all conditions. The experimental design is shown in Table 1. This design enables the effect of air cleaning by a desiccant rotor to be observed and shows how it can effect an improvement on the indoor air quality. Since the effect of some factors such as indoor air humidity, reactivation temperature and the process airflow rate may influence the pollutant removal efficiency of the desiccant rotor, the effect of these factors were all included in the experimental design. Although it was not a complete design for all the factors that were investigated, the possible effect of these factors can still be observed with a view to the design of further experiments. RESULTS Figures 3 and 4 show the mean perceived air quality assessments (expressed by PD) and odor intensity of the air in the test room when the air was polluted by the three different pollution sources with and without the desiccant dehumidifier connected into the recirculation airflow. The results showed that a relatively higher level of indoor air pollution (above 70% PD) was established when the dehumidifier was bypassed from the recirculation system. When the dehumidifier was used to clean the recirculation air, the perceived air quality was improved dramatically, to the level of around 20% PD. This result was observed for all the three conditions where the air was polluted by three different pollution sources. Meanwhile, odor intensity of the air in the test room was reduced significantly from moderate odor to slight odor for the same three conditions when the recirculated air was cleaned by the dehumidifier. PD (%) 100 80 60 40 20 0 P<0.0108 P<0.0219 Odor intensity Overpowering Very strong Strong Moderate Slight No P<0.0002 P<0.0019 P<0.0214 Figure 3. Perceived air quality (PD) in the test room at 23 C, 40%RH, and 5h-1ACR, and using high RT when the dehumidifier was in operation. Figure 4. Odor intensity in the test room at 23 C, 40%RH, and 5h -1 ACR, and using high RT when the dehumidifier was in operation. PD (%) 100 80 60 40 20 0 RH 40% RH 15% P<0.1398 P<0.0366 P<0.7960 Sensory pollution load (olf) 20.00 15.00 10.00 5.00 0.00 Figure 5. Perceived air quality in the test room at 23 C and 5h -1 ACR, with dehumidifier operated at high RT. Figure 6. Sensory pollution loads in the test room at 23 C, 40%RH and 5h -1 ACR, and using high RT when dehumidifier was in operation. 2978

Figure 5 shows that decreased humidity in the test room improved the perceived air quality. However, a significant improvement was observed only for the condition when the air was polluted by linoleum. Since air humidity can have a direct impact on the perception of air quality (Fang et al. 1998), its effect on the effectiveness of pollutant removal of the dehumidifier could be confounded with the direct effect of humidity on perception of air quality. Excluding the direct impact of humidity on the perception of air quality, the effect of humidity on the effectiveness of pollutant removal of the dehumidifier may be marginal. The impact of reactivation temperature on the performance of pollutants removal of the dehumidifier was tested only when the air was polluted by carpet. The results show that the effectiveness of pollutant removal was slightly better at lower levels of reactivation temperature. However, the effect was not significant. Figure 6 shows the sensory pollution load in the test room, calculated from the perceived air quality and the outdoor air supply rate, for three conditions of air pollution, with and without dehumidifier being used to clean the air. The results show a substantial reduction of the sensory pollution load in the test room. The effectiveness of pollutant removal for all three conditions was better than 80%. This result implies that 80% of the outdoor air supply rate can be saved when a desiccant rotor air cleaner is used for indoor air cleaning. DISCUSSION This study shows the great practical value of using a desiccant rotor as an air-cleaning device. The air-cleaning performance of a desiccant rotor seems to be better than that of any existing air cleaner. Since the desiccant rotor used in this study was primarily designed for dehumidification, the air cleaning performance may not be optimized. But after the design of a desiccant rotor has been optimized for indoor air cleaning, further improvement may be expected. It is also expected that the effectiveness of pollutant removal should increase with increasing reactivation air temperature. However, this was not observed in the experiment. The reason could be that the temperature in the reactivation section of the rotor did not follow linearly the temperature of the reactivation air since the temperature of the reactivation section of the rotor was also influenced by the dehumidification capacity. By increasing the temperature of the reactivation air, the dehumidification capacity also increased. This may counteract the increase of the rotor temperature in the reactivation section due to increased moisture evaporation in that part of the rotor. The rotor temperature should be a key factor that determines the effectiveness of pollutant removal. However, it was not measurable during the experiment. Therefore, it should not be concluded that the effectiveness of pollutant removal by means of the desiccant rotor was independent of the reactivation air temperature. Further studies are needed to investigate the effect of this factor. The marginal effect of humidity on pollutant removal of the desiccant dehumidifier observed in the experiment suggests that humidity has little influence on its air-cleaning performance. The reason could be that the concentration of chemicals in the air was so low that the rotor had sufficient capacity to remove these chemicals even if part of the capacity was used by water vapor. Furthermore, a desiccant dehumidifier would always decrease indoor air humidity to a relatively low level. The rotor always had capacity for removing chemicals in the air. Therefore, the competition of adsorption between water vapor and chemicals on the rotor is not a problem for pollutant removal by the desiccant rotor. Besides improving the air-cleaning performance of the desiccant rotor, the energy consumption for reactivating the rotor is of major concern for further optimization of the design. Most of the energy consumed by the desiccant rotor is used for heating the reactivation airflow. This energy is partly let out together with the reactivation airflow and partly added into the process airflow that is warmed up. Since reactivation heat energy consumption is essential for the application value of this technique, separate studies were made to investigate the possibility of utilizing this heat energy. Two solutions are suggested for systems that require heating or cooling. In buildings where heating is needed, the desiccant air cleaning technique can be combined directly with heating. Since most of the reactivation heat can be recovered, the energy consumption for air cleaning is not significant. The major problem is possibly that the indoor air becomes too dry after use of the desiccant rotor. The instrument should, therefore, be optimized so as to be less effective for moisture. In buildings where cooling is needed, the desiccant air-cleaning can be combined with a desiccant cooling technique. In this case, the desiccant rotor is used for removing latent cooling load. In both cases, the significant effect of air cleaning using a desiccant rotor can reduce substantially the outdoor air requirement. A detailed analysis of the energy consumption of a desiccant rotor for both heating and cooling will be presented in a separate paper. 2979

This paper presents the air-cleaning effect of a desiccant rotor in terms of perceived air quality. The results have been confirmed by chemical measurements. A special measuring technique known as proton-transfer-reaction mass-spectrometry was used for real-time monitoring of concentrations of different indoor air chemicals. The results of the chemical measurements are still being under analyzed and will be published elsewhere. CONCLUSIONS The effectiveness of air pollutant removal by the silica gel rotor in terms of sensory pollution load was better than 80%. The air-cleaning effect of the silica gel rotor was found to be independent of air humidity and the processed airflow rate for a humidity level at or below 40%RH. The effect of the reactivation air temperature of the silica gel rotor on the effectiveness of pollutant removal remains to be further investigated. ACKNOWLEDGEMENTS This study was supported financially by the Danish Technical Research Council s funding of the research programme for the International Centre for Indoor Environment and Energy. REFERENCES Chen W., Zhang JS. and Zhang Z.2005, Performance of Air Cleaners for Removing Multiple Volatile Organic Compounds in Indoor Air, ASHRAE Transactions, Vol.111, Part 1, pp. 1101-1114. Fang L., Clausen G. and Fanger PO. 1998. Impact of temperature and humidity on the perception of indoor air quality, Indoor Air, Vol. 8, (2), pp 80-90. Hines AL., Ghosh TK., Loyalka SK. and Warder RC. 1993, Investigation of Co-Sorption of Gases and Vapors as a Means to Enhance Indoor Air Quality Phase 2: A summary of Pollutant Removal Capabilities of Solid and Liquid Desiccants from Indoor Air, Report of Gas Research Institute: GRI-92/0157.1, Chicago. Hines AL. and Ghosh TK. 1993, Investigation of Co-Sorption of Gases and Vapors as a Means to Enhance Indoor Air Quality Phase 2: Water Vapor Uptake and Removal of Chemical by Solid Adsorbents, Report of Gas Research Institute: GRI-92/0157.2, Chicago. 2980