Development and Performance Evaluation of Indirect-Direct Evaporative Cooling System

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1 Development and Performance Evaluation of Indirect-Direct Evaporative Cooling System #1 Mr. Amit T. Sadgir, #2 Prof.N.C.Ghuge #1 PG. Scholar, Matoshri college of Engineering and Research centre, Eklahare, Nasik, Pune university. #2 Project Guide, Matoshri College of Engineering and research Centre, Eklahare, Nasik, Pune university Abstract Air condition system is responsible for increasing human comfort and efficiency mainly in the warm periods of the year. Problem concerned with air condition system high power consumption and high green house gases emission which affect environment as well as human health and comfort. A two stage indirect-direct evaporative cooling system is designed and developed. The indirect section has a plate type heat exchanger and filled with cooling pads to increase the water holding capacity of indirect section. Numbers of tests were conducted at various operating conditions. From the tests data, optimum parameter for high effectiveness is determined. optimum velocity of secondary air 1.5 m/sec at which indirect evaporative cooling works at maximum cooling efficiency for secondary % and primary can be cooled using it with efficiency of heat exchanger 50%. Direct evaporative cooling section offered a saturation efficiency of % with a temperature drop of 10 0 C to C. Overall air temperature is dropped up to C in the developed two stage indirect-direct evaporative cooler with the max effectiveness of the order of 92.5%. Keywords: Evaporative Cooling, Saturation Efficiency, DBT, WB, RH, Humidity ratio and Effectiveness. I. INTRODUCTION Evaporative air cooling is an environmentally friendly and energy efficient method for cooling application. Basically it consists of two methods one is indirect evaporative cooling and second is direct evaporative cooling. Evaporative cooling is one of the best option in hot and dry climate. Evaporative cooling effectively implemented when there is high wet bulb depression. In case of direct evaporative cooler, the air stream to be cooled is in direct contact with liquid water film and cooling is accomplished by the adiabatic heat exchanger between the air stream and liquid water film. The evaporation of water in the air stream leads to reduction of dry bulb temperature and increase in absolute humidity of supply air. Generally maximum possible reduction in dry bulb temperature depends on the DBT and WBT temperature of air stream. The direct type evaporative cooler is simple but introduces moisture into the air stream while cooling it adiabatically. In many applications however the increase of humidity in supply air stream is not desirable. A promising way to increase the thermal performance of evaporative cooling system is to decrease the DBT of the air at constant absolute humidity. Such process achieve by combining indirect type and direct type of evaporative cooler. In indirect and direct evaporative cooling system ultimately reduce the temp of the incoming air near about the its WBT. II. LITERATURE REVIEW Huang et al. (2009), worked on heat and mass transfer between air and water film in the evaporative cooler is theoretically analyzed and built up simplified cooling efficiency correlation based on energy balance analysis of air. Results indicate that the present simplified correlation of cooling efficiency can be used to predict the performance of the direct evaporative cooler. Camargo and Ebinuma. [2003], developed a mathematical model of direct evaporative cooler and presented experimental results of the tests with rigid cellulose media having wetted surface area of 370 m 2 / Concluded that the efficiency is more at higher heat transfer area and lower air speeds. Fouda and Melikyan [2011], a simplified Model for Analysis of Heat and Mass Transfer in a Direct Evaporative Cooler. Show graphical result for cooling efficiency related with pad thickness, frontal velocity of air at constant DBT and WBT. Gonzalez et al. [2013], showed that influence of constructive parameters on the performance of two indirect evaporative cooler prototypes. Concluded that these simple devices achieve energy savings of around 50% by covering from 45% to 80% of the ventilation cooling demand. Hasan et al. [2012], introduced new and modified concept which states that by using only IEC unit temperature can be lowered than dew point temperature of ambient air. Results indicated that for same mass flow rate of primary and secondary fluid obtains maximum effectiveness value up to Dessouky et al. [2004], developed experimental test rig of two stage evaporative cooling unit. Showed that efficiency of IEC/DEC unit varies over a range of %. Results interpreted that heat transfer coefficient for air stream varies over a range of w/m 2 K. 2015, IERJ All Rights Reserved Page 1

2 Heidarinelad et al. [2009], experimental investigation of two stage indirect direct evaporative cooling system in various climatic conditions. Concluded that despite wide variety of climatic conditions, IEC effectiveness varies between 55 and 61% and IEC/DEC effectiveness over a range of %. 2.1 Objective To obtain human comfort condition with minimum energy consumption and emission in dry and warm climate of the year. To develop indirect direct evaporative cooling system for room conditioning. To optimize plate thickness & spacing between two plate for maximum heat transfer performance. To optimize mass flow rate of water and optimize air velocity of both primary and secondary for Maximum performance indirect direct evaporative cooling system. To check suitability of cooling performance of indirect/direct evaporative cooling system across different region of India. To evaluate COP of system. 2.2 Methodology System performance depends on various operating parameters; water flow rates, primary and secondary air flow rates, heat transfer areas, pad material and ambient conditions. Heat and mass transfer takes place simultaneously. In indirect stage three fluids are flowing as primary air, secondary air and water stream. Secondary air is first cooled in wet passage by evaporation and then cooled air is taking out heat from primary air. Accordingly, secondary air and water flow rates are to be optimized. Therefore, an experimental model will be prepared for all above parameter at various operating conditions to realize the performance optimization and identification of the critical operating parameters. III. DESIGN CALCULATION There are two methods available for evaporative cooler selection, sensible heat removal method and air change method. For the chosen environmental conditions and outside air cooling load, air change method is used for design purpose. Test facility is designed to provide the cooling in a room. 3.1 Assumption The following assumption are made in IEC analysis Surface wetting in IEC is complete Miscellaneous loads such as make up water addition,pump heat gain from ambient environment are neglected. Heat and mass transfer coefficient are constant. Temperature and enthalpy at air water interface in IEC is represented by a constant average temperature over the entire surface. 3.2 Ambient Condition Table. 1 show two group of ambient condition based on weather data of Nasik (Maharashtra), India from Dec-15 to April 16 in. The most frequently occurring condition is B and its average is C DBT, 24 0 C WBT and 48 % R.H. This is used as inlet condition for IEC in combined mode and at inlet of DEC for direct mode operation. The properties of air based on this condition. Fig.3.1 flow arrangement inside iec 2015, IERJ All Rights Reserved Page 2

3 Table1. Averaged weather data at Nasik (Maharashtra) Ambien t Conditi on DBT Range 0 C A 28 DBT <32 Reading period Dec-15 Jan-16 Feb-16 Average Max DBT 0 C Ave rage RH % W BT 0 C B 32 D BT<4 0 March- 16 April Saturation specific heat (1) The iterative process for calculation of outlet temp of primary air is started with initial valve of t wo as t wi = +1 0 C. 3.3 Cooling effectiveness The performance of IEC in transferring heat from primary to secondary air is expressed as cooling cooling effectiveness, given by (2) However this form of cooling effectiveness requires the temp t2 to be know. Temp of primary air leaving the IEC is not know. Which is find out by following equation (3) and It is possible that to operate IEC in such manner that secondary air leaving IEC is saturated with water vapor.this implies that the secondary effectiveness is unity.it is also found that in saturating secondary air primary air being cooled at WBT of leaving secondary air, implying primary effectiveness also to be unity.therefore by substituting the relationship in equation (3), cooling effectiveness became. (4) C min = M x C p and C max = m x C wb Table 2. Iterative process step for M P = kg/s Inlet T1 = oc RH = 48 % h1 = 72.5 kj/kg t1w = 24oC Cmin = M = kg/s T2w h2w Cwb Cmax Cmin / Cmax Єc T o , IERJ All Rights Reserved Page 3

4 The equation (4) is used in present analysis because it represent operation of IEC to achieve theoretical maximum cooling effectiveness at any specified capacity ratio. However, Achieving primary and secondary effectiveness of unity may required large heat exchanger area and smaller flow rate which is not practical possible. If primary and secondary effectiveness is less than unity, Eq (3) can be used to find theoretical maximum performance of IEC. The values in table 2 are calculated with 100% saturation efficiency for secondary air which is not practical. Further above relation is valid when saturation efficiencies of primary air side is equal to secondary air side. A moderate value of saturation efficiency of adiabatic direct evaporative cooling is 85 to 95% effective [ASHRAE HVAC Applications, 2011 ] and when sized for 2.5 m/s face velocity with equal mass flows on both sides, reduced to around 60 to 70%. Hence taking Ԑ P = 0.65 Ԑ S = 0.9 and C wb = [table 2] using these values in Eq. (3), Ԑ C = Employing Eq. (2), cooling effectiveness is calculated as T 2 = T 1 - Є c (T 1 t 1w ) Tabulated values are to be verified by the Ԑ-NTU method developed by A Hasan [A. Hasan, 2012]. C c *= m ( kg/sec.) C h *= M* = M x C p /a (5) Where C h *= M* = M x C p /a. C h * ( kg/sec) C min = C h * (kg/sec) C max = C c * (kg/sec) C r = C min / C max Overall modified heat transfer coefficient is given as, (6) Mass transfer coefficient β has to be considered. Thickness of aluminum plate δ =0.5 mm and k=237 W/m.K [ASHRAE,2000] Modified NTU* is given as NTU* = U* X A / C min (7) As we know that, ϵ* = f (NTU*, C r ) (8) Maximum heat transfer in indirect evaporative heat exchanger is given by q max = C min (h s (T 1 ) h 1 ).(9) 2015, IERJ All Rights Reserved Page 4

5 heat transfer from primary air to water surface at temperature t f is given by q = U A (T- t f ) (10) where Ԑ* = q/ q max (11) Solve Eq. (8) and Eq (11) by iterative method for surface area. Values are tabulated below Table 3. Designed values of surface area Iteration No. Assumed area A (m 2 ) exp( A) Ԑ* Calculated area A (m 2 ) The design parameters are obtained as Area required for the heat transfer is m 2 For this selected plate size = * = m 2 No. of plates required = = 14 No. of channels for secondary air flow = 14/2 = 7 No. of channels for primary air flow = 7+1 = To find out spacing between two plates, boundary layer thickness is calculate by (12) where.(13) Properties of air at 33 0 C [26]. Pr = Re = 38.4 X 10 4 Flow is laminar δ = m Thermal boundary layer is given by [D.P Dewitt, 1996] δ t = δ Pr -1/3. (14) δ t = m 2015, IERJ All Rights Reserved Page 5

6 Spacing between two plates = 2 δ t = m o Number of plates = 14 o Plate spacing taking as 2 cm o Dimensions of IEC unit are 0.35 x 0.35 x 0.32 m Direct evaporative cooler Fig.3.2 Indirect evaporative heat exchanger A correlation to find the convective heat transfer coefficients in a rigid cellulose evaporative media Dowdy and Karabash given the equation. (15) Where le is the characteristic length and l is the pad thickness.. (16) The following air properties are used: k = W/m o C; Pr = ; Cpu = 1033 J/kg o C [D.P Dewitt, 1996]. Table 4. Design values of efficiency for different pad thicknes. Sr. No. Pad thickness (m) Convective heat transfer coefficient h (w/m 2 k) Efficiency (%) Convective heat transfer coefficient for several pad thicknesses is calculated at constant velocity. For different heat transfer coefficient, efficiency also calculated by using (3.19) Pad thickness selected as 15 cm. 2015, IERJ All Rights Reserved Page 6

7 3.5 Experimentation The set of experiments were conducted for various combinations of cooling stages, tests were conducted for indirect evaporative cooler unit, direct evaporative cooling and two stage indirect direct evaporative cooling. Performance is measured for different operating parameters like air flow rate, water flow rate and addition of cooling pads in secondary air flow channel of IEC and in direct cooler. Step by step course of action was as follows Beginning of experimentation was done by measuring outdoor air condition with help of sling psychrometer. All devices are properly connected to electrical power supply through watt meter and water sump is filled with water. Speed of secondary fan was regulated with the help of anemometer. Allow to run the water over aluminium plates and evaporative pads. In the same fashion as done before we regulate the primary air velocity. A span of time min. was provided for stabilizing the outlet air temperature. The readings were noted down at various points such as after indirect cooler and increasing pad thickness or after pad thickness 50 mm, 100 mm and 150 mm. Temperature of secondary air and recirculation water were also noted. Based on the recorded data efficiency of system is calculated for each case and compared. System performance is plotted against the operating parameters to find out optimum working conditions. Fig. 3.3 Experimental setup. Fig. 3.4 Schematic diagram of set up 2015, IERJ All Rights Reserved Page 7

8 Fig. 3.4 Measuring Instrument and Experimental Fabrication IV. RESULT AND DISCUSSION A test facility is developed to study the performance of the two stage indirect direct evaporative cooling system under various operating conditions. System performance depends on various operating parameters; water flow rates, primary and secondary air flow rates, heat transfer areas, pad material and ambient conditions. Heat and mass transfer takes place simultaneously. In indirect stage three fluids are flowing as primary air, secondary air and water stream. Secondary air is first cooled in wet passage by evaporation and then cooled air is taking out heat from primary air. Accordingly, secondary air and water flow rates are to be optimized. Experiments were conducted at various operating conditions to realize the performance optimization and identification of the critical operating parameters. Fig. 4.1 Effect of air velocity and water flow rate with additional pads Outlet temperature of secondary air is plotted with secondary air velocity as shown in Fig 4.1. The process depends on air velocity and water flow rate. Secondary air velocity is varied from 0.5 m/s to 3 m/s at two water flow rates 6 l/min and 10 l/min. For with and without cooling pad in indirect section. At lower velocity (0.5 m/s ) outlet temperature is more and as it increases outlet temperature decreases from 33 0 C to 30 0 C and again increases up to the 32.2, For 10 lpm water flow rate. Adiabatic cooling process is depending on time of contact of water and air. For low air velocity time of contact is more but diffusion rate is low. For high air velocity high diffusion rate but time of contact is less. There is an optimum velocity of secondary air velocity 1.5 m/s and 10 lpm water flow rate where maximum cooling is derived. At increased water flow rate the cooling effect increases with the same variation trend with an optimum value of secondary air velocity 1.5 m/s. It is observed that at high air velocities, 2015, IERJ All Rights Reserved Page 8

9 2.0 m/s and above, water drops are carried away along with air. To overcome this problem secondary air channels are fill up with additional aspen wood pad, which improve major heat gain from primary air. Secondary air velocity m/s Fig. 4.2 Variation of secondary air section efficiency with sec air velocity. Saturation efficiency of secondary air section increases with additional pads. As shown in Fig. 4.2 Velocity between 1 to 2.0 m/s gives maximum efficiency of secondary side air for 10 lpm water flow rate. The saturation efficiency increases with increasing cooling surface area. As the secondary velocity increases saturation efficiency increases up to 41.25% for 1.5 m/s velocity, Which improve net sensible heat gain from primary air. It start decreasing because it attain Maximum saturation capacity. For saturation efficiency same pattern is observed for the 6 lpm but we get less saturation efficiency compare to 10 lpm water flow rate. Fig. 4.3 Cooling of primary air process on Psychometric chart at 1.5 m/s and 2.8 m/s air velocity It is clearly seen from fig 4.3 effective heat gain by secondary air from primary air is near about 60 %. Notice that in process the relative humidity of air increases during a sensible cooling process even if specific humidity remains constant. This is because the relative humidity is the ratio of the moisture content to the moisture capacity of air at the same temperature and moisture capacity is decreases with temperature. As the moisture capacity decreases we get maximum effectiveness keeping lower humidity ratio. 2015, IERJ All Rights Reserved Page 9

10 Fig. 4.4 Cooling process in direct evaporative section on Psychometric. Psychometric process for direct evaporative cooling plotted in Fig 4.4, Operating parameters for direct evaporative section are cooling pad area, mass flow rate of air and water flow rate. Water flow rate is kept constant to get wet ability factor is unity. As shown in Fig. 4.4 the cooling with humidification process (1-2), DBT decreases from to 30 0 C TO 24 0 C reduces sensible load up to KW Increased specific humidity of air from kg moisture per kg of dry air to 0.O11 kg moisture per kg of dry air, and latent heat load increases kw which is reduce one so net cooling load is reduced Fig.4.5 Cooling process in indirect and direct section on Psychometric The overall process of the indirect direct evaporative cooler is shown on psychometric chart in Fig The sensible cooling process (1-2), DBT decreases from C to C reduces sensible load up to kw. The cooling with humidification process (2-3), DBT decreases from 30 0 C to 24 0 C reduces, latent load increases up to 0.5 kw. Increased specific humidity of air from kg moisture per kg of dry air to kg moisture per kg of dry air. V. CONCLUSION An in-house test facility is developed to carry out the experimentation on indirect direct evaporator cooler. It facilitates the indirect cooling to primary air through secondary air followed by direct cooling. Corrugated cellulose pads are used which provide wet area where heat and mass transfer takes place simultaneously in the direct evaporative cooler section. The test facility facilitates the variation of operating parameters such as water flow rate, primary and secondary air flow rate etc. 2015, IERJ All Rights Reserved Page 10

11 It is observed that there exists an optimum velocity 1.5 m/s of secondary air offers maximum cooling in indirect section. Efficiency of the indirect section is increased from % to 32 % with additional aspen wood pads. Sensible cooling of primary air gives maximum temperature drop up to C for DBT 1 0 C for WBT drop indirect evaporative cooler give maximum cooling at velocity velocity 1.5 m/s of secondary air.thicker pads perform better temp drop obtained from direct evaporative cooler is C to C,saturation efficiency obtained by direct evaporative cooler is up to % at air velocity 2.8 m/s. The primary air temp is drop to C by employing indirect direct evaporative cooler with max effectiveness of the order of 92.5%. Detail study done for application of indirect- direct system across Indian different climatic condition. and it is conclude that system is give best performance for hot and dry climate region in India. REFERENCES 1. J.D.Palmer (2002), Evaporative Cooling Design Guidelines Manual, Energy Conservation and Management Division, New Mexico, pp S.S.Jadhav, Padale N. D, Shinde N. N. (2012), Performance Evaluation of a Novel Multipurpose Evaporative Cooler, pp J. M. Wu, X. Huang, H. Zhang, (2009) Theoretical Analysis on Heat and Mass Transfer in a Direct Evaporative Cooler, Applied Thermal Engineering 29 pp J.R Camargo, C. D Ebinuma, (2003) Mathematical Model for Direct Evaporative Cooling Air Conditioning System, pp C. Sheng, A.G Agwu. (2012), Empirical Correlation of Cooling Efficiency and Transport Phenomena of Direct Evaporative Cooler, Applied Thermal Engineering, 40, pp J. M. Wu, X Huang, H Zhang,(2009). Numerical Investigation on the Heat and Mass Transfer in a Direct Evaporative Cooler, Applied Thermal Engineering 29, pp R. M. Lazzarin, (2007) Introduction of a Simple Diagram-Based Method for Analyzing Evaporative Cooling, Applied Thermal Engineering,27,pp A. Fouda, Melikyan (2011) A Simplified Model for Analysis of Heat and Mass Transfer in a Direct Evaporative Cooler, Applied Thermal Engineering, 31, pp W. Zalewski, P.A. Gryglaszewski (1996), Mathematical Model of Heat and Mass Transfer Processes in Evaporative Fluid Coolers, Chemical Engineering And Processing,36,pp N. J. Stoitchkov, G.I Dimitrov (1998), Effectiveness of Crossflow Plate Heat Exchanger for Indirect Evaporative Cooling, International Journal of Refrigeration, 21, A. T. Gonzalez A, M.A Chicote, E.V. Gomez, F. J. Rey F. J, (2013) Influence of Constructive Parameters on the Performance of Two Indirect Evaporative Cooler Prototypes, Applied Thermal Engineering,51, pp C.Ren, H Yang, (2006) An Analytical Model for the Heat and Mass Transfer Processes in Indirect Evaporative Cooling with Parallel/Counter Flow Configurations, International Journal of Heat and Mass Transfer,49, pp O. P. Arsenyeva, L.L Tovazhnyansky, P.O. Kapustenko, G.L. Khavin, (2011) Optimal Design of Plate and Frame Heat Exchangers for Efficient Heat Recovery in Process Industries, Energy,36, pp A. Hasan (2012), Going Below the Wet-Bulb Temperature by Indirect Evaporative Cooling: Analysis Using a Modified ϵ- NTU Method, Applied Energy,89, pp Bruno F, (2011) On-Site Experimental Testing of a Novel Dew Point Evaporative Cooler, Energy and Buildings A. Tsymerman, M. Reytblat, (2010) Thermodynamic Fundamentals of Indirect Evaporative Air Cooling and Specific Application Examples, Thermo-Physical Lab, Intech-Mark, pp H. Dessouky, H. Ettouney, A. A. Zeef, (2004) Performance Analysis of Two-Stage Evaporative Coolers, Chemical Engineering Journal, 102, pp G. Heidarinejad, M. Bozorgmehr, S. Delfani, J. Esmaeelian. (2009), Experimental Investigation of Two-Stage Indirect/Direct Evaporative Cooling System in Various Climatic Conditions, Building and Environment, 44, pp R.K Kulkarni, S.P. Rajput (2011), Theoretical Performance Analysis of Indirect-Direct Evaporative Cooler in Hot and Dry Climates, International Journal of Engineering Science and Technology, Vol G. Tale, J. V. Thue, A. Gustavsen, Measurements of the Convective Mass Transfer Coefficient between the Water Surfaces and Still Air. 22. X. C Guo, T. Zhao (1998), A Parametric Study Indirect Evaporative Air Cooler, Int. Comm. Heat Mass Transfer, Vol. 25, pp ASHRAE Handbook, HVAC Applications, (2011) pp F.P. Incropera, D. P. Dewitt (1996), Introduction to Heat Transfer, Third Edition, John Wiley and Sons, ANSI/ASHRAE Standard 143, (2000) Method of Test for Rating Indirect Evaporative Coolers, ASHRAE Handbook, HVAC Fundamentals,(2001), pp ASHRAE Handbook, (2008) HVAC Systems and Equipment, pp S. K. Wang, Air Conditioning and Refrigeration, Mechanical Engineering Handbook, pp Wang S. K. (2000), Handbook of Air Conditioning and Refrigeration, pp , IERJ All Rights Reserved Page 11