Performance evaluation of two stage indirect/direct evaporative cooler with alternative shapes and cooling media in direct stage Kulkarni.R.K 1, Rajput S.P.S 2 1 SVPM College of Engineering, Malegaon(bk), Tal. Baramati, Dist. Pune (M.S.) India 2 Mechanical Engineering Department, Maulana Azad National Institute of Technology, Bhopal (M.P.) rkk288in@gmail.com ABSTRACT Two stage indirect/direct evaporative cooler with wet surface plate heat exchanger type indirect stage and different shapes and cooling media in direct stage is fabricated and tested. Rectangular, semi cylindrical and semi hexagonal shaped cooling pads made up of wood wool, rigid cellulose and aspen fiber are used as cooling media in direct stage. The performance was tested in direct cooling mode and combined cooling mode for constant secondary air flow rate of about 1 kg/s and primary air flow rate varying between 0.078 and to 1.011 kg/s. Average inlet dry bulb temperature was varying between 39 0 C and 43 0 C and relative humidity between 37 % and 46 %.The results show that saturation efficiency of direct evaporative cooler varies in the range of 98.3 % to 71.9 %. Overall efficiency of the unit varies in the range of 119.5 % to 74.3 % and outlet temperature of air between 27.3 0 C and 32.4 0 C. The cooling capacities in direct cooling mode range between 3240 and 45427 kj/h and that for combined mode range between 4679 and 43771 kj/h for different combinations. Such a cooler would be beneficial than stand alone direct or indirect systems. Keywords: Indirect Direct Evaporative Cooler, Cellulose, Aspen, Woodwool Media, Alternative Shapes, Saturation Efficiency. Nomenclature A dp Primary air outlet duct area (m 2 ) A ds Secondary air outlet duct area (m 2 ) C pa Specific heat of primary air (J/kgK) M p Mass flow rate of primary air (kg/s) M s Mass flow rate of secondary air (kg/s) Primary fan power (W) P p P s Q ic Q dc Q dcc Q o t p t s t 1 t 2 t 3 Secondary fan power (W) Cooling capacity of IEC (kj/h) Cooling capacity of DEC (kj/h) Cooling capacity of DEC in combined mode (kj/h) Overall cooling capacity of two stage cooler (kj/h) Primary fan energy meter pulse time (sec) Secondary fan energy meter pulse time (sec) Dry bulb temperature of primary air at inlet of IEC (combined mode) OR at inlet of DEC (only direct mode) ( 0 C) Dry bulb temperature of primary air at outlet of IEC (combined mode) OR at outlet of DEC (only direct mode) ( 0 C) Dry bulb temperature of primary air at the outlet of DEC 800
t w1 t w2 V ap V as V fp V fs ( 0 C) Wet bulb temperature of primary air at inlet of IEC(combined mode) ( 0 C) OR at inlet of DEC (only direct mode) ( 0 C) Wet bulb temperature of primary air at outlet of IEC(combined mode) ( 0 C) OR at outlet of DEC (only direct mode) ( 0 C) Average velocity of primary air (m/s) Average velocity of secondary air (m/s) Volume flow rate of primary air (m 3 /s) Volume flow rate of secondary air (m 3 /s) Greek letters Ρ Density of air at outlet temperature (kg/m 3 ) η dc Saturation efficiency of DEC (%) η dcc Saturation efficiency of DEC in combined mode (%) η ic Saturation efficiency of IEC (%) η o Overall saturation efficiency of the unit (%) Abbreviations AS CL DBT DEC IEC WBT WW Aspen Cellulose Dry bulb temperature Direct evaporative cooler Indirect evaporative cooler Wet bulb temperature Wood wool 1. Introduction Evaporative cooling is environment friendly and more efficient air cooling method. The efficiency of evaporative cooling systems increases with an increase in temperature and decrease in humidity. Therefore in hot and dry climates, evaporative cooling can save a large amount of energy used for conventional air conditioning systems. Direct evaporative cooler (DEC) uses a wetted pad with large air water contact surface area through which air is passed at uniform rate to make it saturated. However this process is accompanied by an increase in humidity which is sometimes not desirable. In indirect evaporative cooler (IEC) the air to be cooled (called as primary air) does not come into direct contact with water. Instead, it is in contact with a surface that is maintained at lower temperature by flow of water and air (secondary air) on the other side of surface. In such a process primary air will be sensibly cooled and its absolute humidity is not affected. In both the above cases minimum temperature to which air can be cooled theoretically is the wet bulb temperature (WBT) of the incoming air. Therefore a combination of above two systems called indirect direct two stage evaporative cooling is sometimes used to improve the performance of the whole system. Such a system reduces the dry bulb temperature (DBT) of incoming air in a heat exchanger before it passes to the direct stage. Such a system can ultimately reduce the temperature of the incoming air below its WBT. Different researchers 801
have made the efforts to improve the performance of these systems by changes in design, process and materials. G. Heidarinejad, et al. 2009 experimentally investigated the cooling performance of twostage indirect/direct evaporative cooling system in various simulated climatic conditions in Iran. They used plastic wet surface heat exchanger as an IEC unit and 15 cm thick cellulose pad for a DEC unit. They obtained an effectiveness of IEC unit in the range of 55 % 61 % and effectiveness of IEC/DEC unit in the range of 108 % 111 %. They also reported 55 % more water consumption than DEC unit and power consumption as 33 % of mechanical cooling systems. Jain 2007 developed and tested a two stage evaporative cooler to improve the efficiency by using wooden shave as the packing material. Room return air was evaporatively cooled and used in a heat exchanger to cool the incoming dry air. The effectiveness ranged from 1.1 to 1.2 and this cooler could achieve favorable temperature and relative humidity conditions for storage of tomatoes for 14 days. Camargo et al. 2005 developed a mathematical model for a DEC and presented their experimental results of the tests using rigid cellulose media with wetted surface area of 370 m 2 /m 3. The effectiveness relation derived in terms of heat transfer coefficient, mass flow rate of air, wetted surface area and humid specific heat is useful in predicting the performance of different pad materials. They concluded that the effectiveness is higher at higher dry bulb temperature and lower air speeds. El Dessouky et al. 2004 carried out theoretical and experimental study on small scale evaporative cooling unit using structured packing material of high density polythene with wetted surface area of 420 m 2 /m 3. They used a combination of DEC and IEC and concluded that the efficiency of IEC unit is less than DEC but a combination of both can reduce the temperature of incoming air below its WBT. Maheshwari et al. 2001 compared the power requirements of an IEC unit with conventional packaged air conditioner. They concluded that the best performance of IEC unit coincides with the hour of maximum cooling capacity and peak power demand of conventional unit and it offers maximum reduction in cooling capacity and peak power demand. El Dessouky et al. 1996 used the concept of pre cooling the air before DEC without using cooling tower. Structured, sheathy leaf and natural fiber packing was used as cooling media with varying thickness and flow of water. They observed that effectiveness increased with packing thickness and flow rate of water to IEC. Structured packing showed higher effectiveness than sheathy leaf and natural fiber. Peterson 1993 theoretically investigated complex heat and mass transfer process occurring within IEC and developed a simple but powerful model for calculating the theoretical IEC performance. Their model represented cooling effectiveness in a single algebraic equation that can be solved manually in three to four iterations. Chen et al. 1991 developed a heat and mass transfer model for thermal and hydraulic calculations of IEC performance. They performed thermal calculations for tube and plate 802
type IEC using room or outside air as secondary air. Cooling capacities and coefficient of performance are found to be much higher when room air is used as secondary air. Dowdy and Karbash 1987 tested rigid impregnated cellulose media experimentally to determine heat and mass transfer coefficients for evaporative cooling process. They used the sample of rigid cellulose media having wetted surface area of 350 m 2 /m 3 with thickness 305 mm and obtained saturation efficiency between 90 to 94 %. This paper attempts to evaluate the performance of two stage indirect direct evaporative cooler with three different cooling media for direct stage. Wood wool (WW) is locally available in the market while Aspen fiber (AS) and Cellulose (CL) media are ordered. An attempt is made to use alternative shaped cooling pads to allow for larger volume flow rates of air at moderate velocities. 2. Experimental Set Up Figure 1: Experimental Set up The experimental set up shown in figure 1 consists of two parts: plate heat exchanger type IEC and DEC. Two views of set up are shown in figures 2 and 3. 803
1. IEC consists of alternate channels for primary and secondary air formed with the aluminum plates. A total of 33 Aluminum plates of grade 1100, 1.5 mm thick were used to form 34 channels of 15 mm spacing in each channel. Secondary air is sucked from the bottom duct and delivered outside through the upper duct on the same side by two fans. Secondary air channels are made wet by flow of water from the upper side. Water is circulated from the sump tank to the upper side of the channels by a pump and again flows along the plates to the tank. Figure 4 shows the flow arrangement inside IEC. 2. DEC consists of a cooling pad arranged inside a removable cage and water distribution arrangement from the upper side of the pad. Water is circulated from the sump tank to the upper side of the pad by a pump and again flows through the pad to the tank. Three different pad materials WW, CL and AS used as cooling media can be accommodated in three different shaped cages. Rectangular shaped pad has the size 600 mm by 600 mm and thickness 150 mm. The dimensions of other pad shapes are shown in figures 5 and 6. Primary air is sucked by a fan mounted on opposite side of cooling pad. In order to simulate hot and dry atmospheric conditions, four finned heaters of 500 W capacity are mounted on the inlet side of primary and secondary ducts to artificially heat the incoming air. Figure 2: View from inlet side Figure 3: View from outlet side 3. Instrumentation The control panel has two energy meters, digital temperature indicator, fan regulators and necessary switches. Eight thermocouples are used to measure the DBT and WBT of inlet primary and secondary air, DBT and WBT of primary air after IEC and DEC. Ambient DBT and WBT are measured by separate temperature indicator kept in the test room. The velocity of primary and secondary air is measured with the help of vane anemometer in the outlet ducts. The primary and secondary fan power is measured by energy meters connected to the respective fans. 804
Figure 4: Flow arrangement inside indirect cooler. Figure 5: Semi Cylindrical shape of pad 805
4. Process Figure 6: Semi hexagonal shape of the pad. In IEC, hot and dry secondary air enters the channels from the bottom duct. Water flowing along the plates in downward direction gets evaporated due to heat of air and takes heat from air as well as plates. As a result the temperature of plates is reduced. Warm and humid secondary air is discharged through the upper duct on the same side with the help of two secondary fans. The primary air enters the IEC from the middle duct due to suction of primary fan mounted on the opposite side. The cooling effect produced in secondary channels is transferred to the primary air flowing through the alternate channels and thus primary air gets cooled. The primary air travels in downward direction through the IEC and enters the DEC on the opposite side. In DEC, primary air comes in contact with wetted pad surface. Water on the pad surface gets evaporated by taking heat from incoming air. Thus the temperature of primary air is reduced in two stages in IEC and in DEC. The cooled primary air is discharged by primary fan on the opposite side. 4.1 Performance Tests Performance tests in direct and combined mode are conducted for different pad materials and shapes. Combined mode includes the tests on IEC. Hence IEC performance can be evaluated on the basis of combined mode observations. In each mode, primary air flow rate is varied in five steps by varying the fan regulator switch. The secondary air flow rate through the IEC is kept constant throughout the experimentation. 806
4.2 Equations Used V fp = V ap A dp m 3 /s (1) V fs = V as A ds m 3 /s (2) M p = V fp ρ kg/s (3) M s = V fs ρ kg/s (4) 1 P p = 1000 t p 3200 3600 1 P s = 1000 t s 3200 3600 W (5) W (6) t 1 t 2 η ic = η dc = 100 % (Watt and Brown 1997) (7) t t 1 w 1 Q ic = Q dc = M p C pa (t 1 t 2 ) 3.6 kj/h (Maheshwari et al. 2001) (8) t 2 t 3 η dcc = 100 % (9) t t 2 w 2 Q dcc = M p C pa (t 2 t 3 ) 3.6 kj/h (10) t 1 t 3 η o = 100 % (11) t t 1 w 1 Q o = M p C pa (t 1 t 3 ) 3.6 kj/h (12) 5. Results and Discussions The equations shown in section 6 are used and the performance parameters of the cooler in terms of primary air mass flow rate, saturation efficiency and cooling capacity are calculated. The outlet temperature of primary air is directly obtained from observation tables. 5.1 Primary air mass flow rate Minimum and maximum values of primary air mass flow rate for different combinations are shown in table 1. When material or pad shape is changed, primary air mass flow rate is affected. Semi cylindrical and semi hexagonal shapes are having slightly more surface area and are able to maintain slightly higher mass flow rates of air through them. As CL material offers lesser resistance for the air flow, the higher mass flow rates are obtained with CL material with same fan speed than WW and AS material. AS material offers more resistance to air flow and hence lower mass flow rates are obtained with it. Table 1: Primary air mass flow rate for different combinations (kg/s) 807
Geometry Rectangular Semi Cylindrical Semi Hexagonal Mode Material Min Max Min Max Min Max DEC Only WW 0.1 0.794 0.128 0.891 0.089 0.864 CL 0.175 1.000 0.145 1.001 0.171 1.011 AS 0.106 0.920 0.095 0.891 0.131 0.869 Combined WW 0.083 0.949 0.128 0.958 0.078 0.809 mode/iec CL 0.139 0.994 0.131 0.998 0.126 0.981 AS 0.103 0.947 0.1 0.908 0.114 0.844 Table 2: Primary air outlet temperature for different combinations ( 0 C) Mode IEC Only DEC Only Combined Rectangular Semi Cylindrical Semi Hexagonal Material Min Max Min Max Min Max WW 34.1 35.9 34 35.6 34 36.8 CL 32.5 35.3 35.5 36 33.8 36 AS 31.7 34.6 36.3 37.2 34 36 WW 24.2 26.6 30 32.6 28.7 29.5 CL 25.1 27.9 28.3 29.5 27.1 28.6 AS 27.3 28.8 27.5 31 29.1 31.1 WW 30.1 31.7 29 30.8 27.3 30.2 CL 27.5 32.4 28.2 31.3 27.7 31.2 AS 28.2 30 29.7 30.6 29 30.9 5.2 Outlet temperature of air Primary air outlet temperature is recorded in each test and its maximum and minimum values are shown in table 2. It varies between 31.7 0 C and 37.2 0 C for IEC, between 24.2 0 C and 31.1 0 C for DEC and between 27.3 0 C and 31.7 0 C for combined mode. It is thus possible to reduce the temperature of incoming air below its WBT by the use of two stage cooler. Such operation will be beneficial in extreme climates. 5.3 Saturation efficiency Saturation efficiency values for different combinations are shown in table 3. The general trend is that saturation efficiency decreases with increase in primary air mass flow rate. The maximum values are obtained for minimum mass flow rates and vise versa. At higher velocities, air spends lesser time in IEC thereby getting lesser cooling effect. Hence IEC efficiency decreases at higher mass flow rates and efficiency values are lower because of indirect cooling process. IEC efficiency values range between 83.3 % and 37.2 %. The reported effectiveness values in literature (Chen et al. 1991) lie between 0.9 and 0.39 for similar arrangement and constant secondary air flow of 0.38 kg/s and varying primary air flow between 0.2 kg/s to 2.3 kg/s. DEC efficiency values for CL material and rectangular pad are obtained between 92.2 % and 83.2 %. Values obtained by in literature (Camarago et al. 2005) are between 82% and 78% by 808
using 0.15 m thick pad and a higher efficiency of 94% to 90% by using 0.3 m thick pad of cellulose material. (Dowdy and Karabash 1987) Table 3: Saturation efficiency for different combinations (%) Mode IEC Only DEC Only DEC with IEC Overall Efficiency Rectangular Semi Cylindrical Semi Hexagonal Material Max Min Max Min Max Min WW 65 58.8 75.6 43 71.7 37.2 CL 74.5 46.7 55.1 38.7 69.8 42.7 AS 83.3 41.9 69.2 40 75.4 41.7 WW 98.3 90.5 92.1 73.1 96.8 90.6 CL 92.2 83.2 97.4 88.2 94.9 91.1 AS 82.5 71.9 98.2 85 98.3 80.8 WW 81.7 66 85.7 64.9 85.9 81.3 CL 85.1 63 88.6 65.6 81.3 71.9 AS 71.8 66 90.5 80.6 80.6 52.6 WW 101.9 87.7 118.3 103.8 113.8 94.4 CL 110.9 74.3 108.8 76.6 110.7 85.4 AS 115.7 81 119.5 95.2 116.4 81.9 Efficiency in only DEC mode is higher than DEC with IEC mode because higher temperature and lower RH at the inlet. Hot and dry conditions are more favorable for the operation of DEC. In IEC, temperature of air gets reduced and RH increases. Inlet conditions become less favorable for the operation of DEC. Hence saturation efficiency of DEC in combined mode decreases. The overall saturation efficiency values are obtained between 74.3 % and 119.5 %. These are higher than that of only DEC or DEC with IEC in majority of the combinations. This is because the overall temperature drop is higher than individual modes. The overall efficiency of the two stage cooler exceeds 100 % in some cases because the DBT of incoming air is reduced below its WBT. In the experiments referred in literature (Heidarinejad et al. 2009) these values are obtained between 108 % and 111 % with 0.15 m thick cellulose pad. A two stage cooler tested with high density polythene packing material of thickness 0.1 to 0.4 m had the overall efficiency in the range of 90 % to 120 %.( El Dessouky et al. 2004) The overall efficiency values are higher with AS material followed by CL and WW material because of better wetting characteristics. At low air flow rates, efficiency of rectangular shape is better. Semi cylindrical and semi hexagonal shapes are able to maintain high efficiencies at maximum air flow rates. Velocity of air with different shaped pads is approximately same at maximum mass flow rates but slightly higher mass flow rates are obtained with semicylindrical and semi hexagonal shaped pads. Hence these shapes are suitable for higher mass flow rates of air. The efficiency with semi cylindrical and semi hexagonal shape is better at lower range of mass flow rates also. 809
5.4 Cooling Capacity Cooling capacity for different combinations is shown in table 4. Its value is obtained between 3111 kj/h and 27829 kj/h for IEC, 3240 kj/h and 45427 kj/h for DEC and 4679 kj/h and 43771 kj/h for combined mode. The maximum cooling capacity occurs at certain combination of air mass flow rate and temperature drop. It may occur at maximum mass flow rate of air at which efficiency may be minimum. The general trend is that cooling capacity increases with air mass flow rate and then decreases. This decreasing trend at higher mass flow rate may be attributed to the decreasing saturation efficiency. Cooling capacity of only IEC is lower due to lower efficiency. Also the cooling capacity of DEC with IEC is lesser than that of only DEC because of lower saturation efficiency. Cooling capacity values in combined mode are lower than DEC mode but the cooling efficiencies are higher. Overall cooling capacity of semi cylindrical and semi hexagonal shapes is higher because of higher mass flow rates of air. 5.5 Power Consumption The major power consumption of the unit is for fans and pumps. The power for secondary fans remains almost constant between 165 W and 175 W because the secondary fan speed is kept constant throughout the tests. Power consumed by primary fan is affected by primary air mass flow rate at different fan speeds. It varies from 56 W to 163 W for DEC mode and from 46 W to 163 W for combined mode. Power consumed for semi cylindrical and semihexagonal shape is slightly higher than rectangular shape. As CL material allows greater mass flow rates, it consumes more power than other two materials in some cases. Based on the pump rating of 40 W for both IEC and DEC pump, the maximum power required for the unit is estimated to be 203 W for DEC mode and 418 W for combined mode. 5.6 Water Consumption The actual flow rates of water for DEC and IEC pump are 16 lpm and 20 lpm respectively. The water was re circulated through both the units and level of water before and after each test was noted. The DEC mode took 3 hours of operation for five sets of readings and combined mode took 5 hours of operation. The combined mode with AS material consumed about 30 liters (6 lph) and CL or WW material consumed 25 liters (5 lph) of water irrespective of the pad shape. DEC mode with AS material consumed about 12 liters (4 lph) and CL or WW material consumed 10 liters (3.3 lph). Hence a water flow of about 18 lph is sufficient run the unit with maximum water consumption of 6 lph. 5.7 Conclusions The performance of experimental indirect direct two stage evaporative cooler is evaluated with different materials and shapes of cooling media in direct stage. Average inlet dry bulb temperature was varying between 39 0 C and 43 0 C and relative humidity between 37 % and 46 %.Saturation efficiency of direct cooling mode ranges between 98.3 % and 71.9 % and cooling capacity ranges between 3240 kj/h and 45427 kj/h for different material and pad shapes. The combined mode gives the overall efficiency of 119.5 % to 74.3 % and cooling capacity of 4679 kj/h to 43771 kj/h for different pad shape and material combinations. The 810
outlet temperature of air ranges between 27.3 0 C and 32.4 0 C in two stage operation. The results show that the cooling efficiency can be improved by adding an indirect stage before the direct stage and the dry bulb temperature of incoming air can be reduced below its wet bulb temperature. Efficiency values are higher for the material with high wetted surface area, low mass flow rates of air and low velocity. Rectangular shapes are having high efficiency with low mass flow rates of air and low velocity. At maximum mass flow rates the velocity of air remains approximately same with different shaped pads but slightly higher mass flow rates are obtained with semi cylindrical and semi hexagonal pads. Hence these shapes are suitable for higher mass flow rates of air. The efficiency with semi cylindrical and semihexagonal shape is better at lower range of mass flow rates also. Water consumption of the cooler varies between 3 and 6 lph depending on the mode of operation and the material used. The maximum power consumption in direct mode is around 203 W while that in combined mode is 418 W. This type of two stage cooler will be suitable for tested range of climatic conditions of 39 46 0 C DBT and 37 46 % RH. Acknowledgement Authors wish to acknowledge the financial support by BCUD, Pune University under research grant to construct the experimental set up and SVPM College of Engineering, Malegaon (bk) Tal. Baramati,Dist. Pune to run the test set up. 6. References 1. Camrago J. R., Ebinuma C. D., Silveria J. L. (2005). Experimental performance of a direct evaporative cooler operating during summer in Brazilian city, International Journal of Refrigeration, 28(7), pp 1124 1132. 2. Chen P. L., Qin H. M., Huang Y. J., Wu H. F. (1991). A heat and mass transfer model for thermal and hydraulic calculations of indirect evaporative cooler performance, ASHRAE Transactions, 97, Part 2, pp 852 865. 3. Dowdy J. A. and Karbash N. S. (1987). Experimental determination of heat and mass transfer coefficients in rigid impregnated cellulose evaporative media, ASHRAE Transactions, 93, Part 2, pp 382 395. 4. El Dessouky H., Ettouey H., Al Zeefari A. (2004). Performance analysis of two stage evaporative coolers, Chemical Engineering Journal 102(3), pp 255 266. 5. El Dessouky H. T. A., Al Haddad A. A., Juwayhel F. I. (1996). Thermal and hydraulic performance of a modified two stage evaporative cooler, Renewable Energy, 7(2), pp 165 176. 6. Heidarinejad G., Bozorgmehr M., Delfani S., Esmaeelian J. (2009). Experimental investigation of two stage indirect/direct evaporative cooling system in various climatic conditions. Building and Environment, 44(10), pp 2073 2079. 811
7. Jain D., (2007). Development and testing of two stage evaporative cooler, Building and Environment, 42(7), pp 2549 2554. 8. J.R. Watt and W. K. Brown. (1997). Evaporative Air conditioning Handbook, 3 rd. Ed. The Fairmont Press, Inc. Lilburn. 9. Maheshwari G. P., Al Ragom F., Suri R. K. (2001). Energy saving potential of an indirect evaporative cooler, Applied Energy, 69(1), pp 69 76. 10. Peterson J. L. (1993). An effectiveness model for indirect evaporative coolers, ASHRAE Transactions, 99, Part 1, pp 392 399. 812