Healthy Buildings 2017 Europe July 2-5, 2017, Lublin, Poland. Experimental study of a novel dew point indirect evaporative cooler

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1 Healthy Buildings 2017 Europe July 2-5, 2017, Lublin, Poland Paper ID 0232 ISBN: Experimental study of a novel dew point indirect evaporative cooler Wenhua Chen, Zhiwei Zhang, Jiayu Li, Junjie Liu * Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, China * Corresponding jjliu@tju.edu.cn SUMMARY Designed a new type of composite separator material that had good water absorption and was inexpensive for dew point indirect evaporative coolers. Effects of some factors on the performances of two coolers were experimentally studied, including the secondary / primary air flow ratio, the supply air volume of primary air, the dry bulb temperature and moisture content of inlet air. And the running conditions of two coolers were optimized. The results showed that the secondary / primary air flow ratio should choose 0.9 to 1.1. In order to reduce the volume of equipments, the primary air flow rate should be as high under the premise to meet the supply air temperature. The results showed that the supply air temperature reached the lowest after closing the pump and in the case of composite separator material, intermittent operation of pump was recommended, because it both saved energy and improved cooling effect. KEYWORDS Indirect evaporative cooling, dew point temperature, separator material, influence factors, running conditions. 1 INTRODUCTION Dew point indirect evaporative cooling technology can achieve the dry bulb temperature of supply air to be lower than the wet bulb temperature and close to the dew point temperature of outdoor air and it has high research significances and broad application prospects (Chen, 2008). Practical engineering tests on dew point indirect evaporative coolers have been done by Chen et al. (2008). The results showed that effects of the wet bulb temperature and the dry bulb temperature of inlet air on the dry bulb temperature of supply air were large and the optimal secondary/primary air volume ratio was 1.48: 1. Cui et al. (2012) found that in order to maintain a higher cooling efficiency and lower supply air temperature, it was appropriate to take the air flow rate as 3 m/s and the primary/secondary air flow rate ratio as 1.5: 1; the dew point efficiency reached the highest when the draining water content was 3 dm 3 /min while the temperature of circulating water had less effect on the performances of dew point indirect evaporative coolers. Zhou et al. (2013) compared the structures of several dew point indirect evaporative coolers that emerged at home and abroad in recent years, and summarized the design elements of dew point indirect evaporative coolers including the inlet air conditions,

2 the supply air volume, the secondary / primary air ratio, the length or height of channels, the materials of core and so on. Effects of the thermal conductivity, the porosity and other parameters of the separator material on the cooling efficiency have been theoretically analyzed by Zhao et al. (2008). And the study showed that the porosity of materials had the largest impact on the cooling effect and the impact of thermal conductivity was small. Studies on dew point indirect evaporative coolers are mostly in the experimental stage currently and the dew point efficiency is still difficult to achieve 100% when coolers are used alone (Zhao, 2008, Riangvilaikul B and Kumar S, 2010, Lee J and Lee D Y, 2013). And dew point indirect evaporative coolers are mainly used in public buildings and the size of equipments is large (Riangvilaikul B and Kumar S, 2010). To further improve the performances of coolers and reduce the size of coolers under the premise of meeting the requirement of cooling capacity and save energy. Measurements were made to determine the separator materials and the operating parameters in two coolers, a new type of composite separator material that had good water absorption and an aluminum foil separator cooler. The performances of the two coolers have been analyzed and compared quantitatively and the running conditions of two coolers were optimized. 2 MATERIALS/METHODS The experimental platform of dew point indirect evaporative coolers included the following components: (1) air pretreatment system, (2) boxes of cooler prototypes, (3) air system, (4) water system, (5) data measuring devices. To compare with traditional aluminum foil material, this paper set up two cooler prototypes of aluminum foil separator and composite separator and the structure schematic diagram of the experimental table has been shown in Figure 1. Cooler cores Figure 1. Structure schematic diagram of the experiment platform. Table 1. Performance parameters of separator materials Thermal Material Separator Material Separator material conductivity thickness thickness composition (W/m K) (mm) (mm) Unit absorption (g/m 2 ) Hydrophilic aluminum foil PS plastic Composite Nylon villi 0.5

3 Table 1 shows the performance parameters of two kinds of separator materials, Table 2 shows the structural parameters of two kinds of cores, Figure 2 shows the physical graphs of two kinds of separator materials. Table 2. Structure parameters of cores Separator material Length Width Height (mm) Channel height (mm) Resistance (Pa) Hydrophilic aluminum foil Composite Figure 2. Physical diagrams of separator material, (a) Aluminum foil separator, (b) Composite separator. Experimental methods and processes The experiment has set three states of inlet air and the state parameters of inlet air were shown in Table 3. On this basis, adjust the fans and test the performances of two coolers in different secondary / primary air flow ratios and supply air volumes. The air operating parameters were shown in Table 4. Dry bulb temperature ( C) Table 3. State parameters of inlet air Moisture content (g/kg) Wet bulb temperature ( C) Table 4. Operation parameters of air Dew point temperature ( C) Parameter Setting value Secondary / primary air flow ratio 0.68/0.94/1.04/1.15 (Aluminum foil separator cooler) 0.52/0.68/0.81/0.94 (Composite separator cooler) Supply air volume (m 3 /h) 35/55/85 3 RESULTS The experiment has tested the performances of two kinds of coolers with the secondary / primary air flow ratio changing under different inlet air conditions and the supply air volume of primary air Qs = 55 m 3 / h, the results were shown in Figure 3a to 3c. The experiment has tested the performance of two kinds of coolers with the supply air volume of primary air changing under different inlet air conditions and the secondary / primary air flow ratio r = 0.94, the results were shown in Figure 3d to 3f.

4 The experiment has tested the peformance of two kinds of coolers with the dry bulb temperature of inlet air changing when the secondary / primary air flow ratio r = 0.68, the supply air volume of the primary air Qs = 85 m 3 / h and the moisture content of inlet air d=16.0g / kg, the results were shown in Figure 4a. The experiment has tested the peformances of two kinds of coolers with the moisture content of inlet air changing when the secondary / primary air flow ratio r = 0.68, the supply air volume of the primary air Qs = 85 m 3 / h and the dry bulb temperature of inlet air t = 32.5 C, the results were shown in Figure 4b. Figure 3. Effects of the secondary / primary air flow ratio on the performances of two coolers, (a) Supply air temperature, (b) Wet bulb efficiency, (c) Dew point efficiency and effects of the supply air volume of primary air on the performances of two coolers, (d) Supply air temperature, (e) Wet bulb efficiency, (f) Dew point efficiency.

5 Figure 4. Effects of the of inlet air on the temperature of supply air and efficiency, (a) Dry bulb temperature, (b) Moisture content. 4 DISCUSSION Figure 5. Variation curves of the two coolers supply air temperatures with time. Figure 5 shows the variation curves of the two coolers dry bulb temperatures of supply air in the same operating conditions (t = 32.5 C, d = 16.0g / kg, Gs = 55m 3 / h, r = 0.68) and the pump was opened at 00:00 and closed at 00:15. As shown in Figure 5, when the pump was turned on, the supply air temperature of the aluminum foil separator cooler droped, but the cooling extent was not obvious. After the pump was turned off, the supply air temperature had a larger cooling extent in a short time and the cooling effect reached the highest. Then the supply air temperature gradually increased and reached higher than the temperature before turning off the pump. In the composite separator cooler, the supply air temperature was stable at the beginning and with the opening time of the pump increased, the supply air temperature increased on the contrary. After the pump was turned off, the supply air temperature gradually decreased and the cooling effect gradually increased. The supply air temperature of the two coolers reached the minimum after the pump was turned off. 5 CONCLUSIONS

6 The performances of an aluminum foil separator cooler and a composite separator cooler are experimentally studied and the running conditions of two kinds of coolers were optimized and the main conclusions were as follows: (1) with the increase of the secondary / primary air flow ratio, the performances of two kinds of coolers improve but it was not economical for the overall operation of the coolers to excessively increase the air flow, so the more reasonable secondary / primary air flow ratio was 0.9 to 1.1. With the increase of the supply air volume of primary air, the performances of two kinds of coolers dropped. In order to reduce the size of equipments, the air flow rate in the dry air flow channels should be as high on the premise of meeting the supply air temperature. (2) with the increase of inlet air temperature, the cooling capacities of two kinds of coolers increased. But with the increase of inlet air moisture content, the cooling capacities of two kinds of coolers decreased. And with the increase of inlet air temperature and moisture content, the wet bulb efficiency and the dew point efficiency increased slightly. (3) the performance of the composite foil separator cooler was better than that of the aluminum foil cooler. And the supply air temperature of the composite separator cooler can achieve lower than the wet bulb temperature. (4) the hydrophilic property of the separator material had significant impacts on the performances of coolers. In the case of composite separator material, intermittent operation of the pump was recommended which both saved energy and improved the cooling effect. 6 ACKNOWLEDGEMENT This work was supported by the indirect evaporative cooling air-conditioning in kitchen program supported by Jiyuan City, the Lan Man Energy Saving Technology Co., Ltd. 7 REFERENCES Chen J. P, Huang X, and Xuan Y. M Application analysis of the dew point indirect evaporative cooler. Refrigeration and Air-conditioning, 8(5), Cui J. J. and Liu Z. B Design and experimental study of a counter-flow dew point indirect evaporative cooler. Household Appliance Technology, (3), Duan Z, Zhan C, and Zhang X Indirect evaporative cooling: Past, present and future potentials. Renewable and Sustainable Energy Reviews, 16(9), Lee J. and Lee D. Y Experimental study of a counter flow regenerative evaporative cooler with finned channels. International Journal of Heat and Mass Transfer, 65, Riangvilaikul B. and Kumar S An experimental study of a novel dew point evaporative cooling system. Energy and Buildings, 42(5), Riangvilaikul B. and Kumar S Numerical study of a novel dew point evaporative cooling system. Energy and Buildings, 42(11), Zhao X, Li J. M, and Riffat S. B Numerical study of a novel counter-flow heat and mass exchanger for dew point evaporative cooling. Applied Thermal Engineering, 28(14), Zhao X, Liu S, and Riffat S. B Comparative study of heat and mass exchanging materials for indirect evaporative cooling systems. Building and Environment, 43(11), Zhou H. D, Huang X, and Fan K Comparative analysis of configuration on dew point evaporative cooler. Fluid Machinery, 41(2),