CHAPTER 2 LITERATURE REVIEW

Transcription

1 11 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION For better human comfort, cooling of living or work environment is vital in tropical climates. Researches carried out till date in evaporative air cooling process focus mainly on reducing the dry bulb temperature of the incoming air. Theoretical efficiency of 100% can be realized when dry bulb temperature of the room is equal to wet bulb temperature of the outside ambient air; Evaporative cooling efficiency is defined as the ratio between drop in dry bulb temperature across the cooler and the difference between inlet DBT and inlet WBT. Many researchers have worked on improving evaporative cooling efficiency to the maximum possible extent. The present chapter reviews various studies carried out related to the evaporative air cooling recently and the past. The cooling efficiency of the evaporative cooling system is found to be increased by imparting the following three methodologies in the system, viz., (i) Introduction of suitable components for effective evaporation; (ii) Combining direct and indirect cooling systems; and (iii) Utilization of different pad materials with different thicknesses. Section 2.2 deals with the studies related to the improvement in evaporative cooling efficiency by the appropriate system modification in the conventional evaporative air cooling system. Studies on indirect / direct (two stage) evaporative coolers are presented in Section 2.3. Effects of various pad materials are discussed in Section 2.4. Studies conducted to evaluate the

2 12 influence of various process parameters on the cooling efficiency are presented in the section 2.5. The studies related to the development of mathematical models and its validation for analyzing the performance of the cooler are presented in Section 2.6. The chapter ends with foremost conclusions arrived out of this review and objectives of the present research work. 2.2 EVAPORATION RATE ENHANCEMENT STUDIES Giabaklou and Ballinger (1996) have attempted to study the effectiveness of a passive evaporative cooling system employing natural ventilation. The front faces of a building are provided with water guide filaments, where in water flows from the top to bottom by gravity. The incoming air gets cooled and goes inside the building. Such a system is found to reduce the temperature of incoming air by 9.9 ºC, averaged over a day. Giabaklou (2003) has extended the study the using Fanger PMV (Predicted Mean Vote) methodology. It shows that such a system can improve PMV significantly when the number of air changes per hour is higher. Kant and Mullick (2003) have studied on thermal comfort in a room with exposed roof using evaporative cooling system. Hourly values of temperature and humidity are computed and compared with the values that are obtained during unexposed condition. The levels of thermal sensation, which could be obtained with a direct evaporative cooler, are computed. Kittas et al (2003) have investigated the temperature and humidity gradients during summer in a greenhouse equipped with a ventilated cooling-pad system and half shade plastic roof. The cooling performance up to 80 % is reported. The temperature of the greenhouse is lowered by 10 ºC than the outside air.

3 13 Taufiq et al (2007) have conducted an energy analysis of evaporative cooling for reducing energy use in a building. A correlation has been developed between relative humidity and energy efficiency, and between ambient temperature and energy efficiency. The results of the study revealed that when relative humidity is increased, energy use also increases. The study concluded that the evaporative cooling is a feasible technology that can reduce mechanical cooling and energy requirement. Shukla et al (2008) have done an experimental study in a cascade green house with inner thermal curtain. About 5-8 ºC reduction in the temperature of greenhouse is reported during hot summer in Delhi. Energy saving aspects are attempted by Al-Azzawi and Almuhtadi (2009) using of newly designed automated solar powered evaporative cooler. Naticchia et al (2010) have used a new novel technique, water-evaporative wall, which reduces the conduction heat gain across the walls. The experiments have shown to reduce the summer overall heat load considerably. He and Hoyano (2010) have studied the cooling effects of passive evaporative cooling wall constructed of porous ceramic materials. Experimental results showed that the cooling efficiency reached a maximum of 0.7 during sunny daytime periods. A higher cooling efficiency is obtained under windy conditions where wind at a speed of 1-3 m/s is continuously blowing. Heidarinejad et al (2010) have investigated a hybrid system of nocturnal radiative cooling and direct evaporative cooling and up to 13.5 ºC reduction in indoor temperature is reported. 2.3 STUDIES ON INDIRECT / DIRECT EVAPORATIVE COOLING SYSTEMS Datta et al (1987) have experimentally studied an 8.5 ton indirect-direct evaporative cooling system and reported that such a system

4 14 provides a relief cooling rather than comfort cooling. The room could be maintained at 4-5 ºC above the inlet wet bulb temperature using such a cooler. A facility of using indirect-direct evaporative cooling for residential use in arid regions of Israel is attempted by Navon and Arkin (1994). Such a system is shown to provide higher level of thermal comfort where external humidity is around 80 %. Singh et al (1991) have attempted to save the electricity used in the circulation pump. The existing evaporative cooler is retrofitted with an over head tank and a limit switch, which switches on the circulation pump when tank water level is low and stops when tank water level is full. By such process 75 % savings in electricity are reported for the same system performance levels. Sodha et al (1995) have evolved a rule thumb for direct evaporative cooler for various floor areas to be cooled for hot, dry and composite climates. El-Dessouky et al (2000) have developed a membrane air dryer and coupled with conventional direct / indirect evaporative cooler. As the membrane drier, removes the moisture from the incoming air, the air can be cooled to lower temperature by the subsequent evaporative cooler. Using such a system, reasonable cooling has been obtained. When such system is combined with Mechanical Vapour Compression system to achieve to perfect thermal conditions, about 50 % savings in electricity are obtained. Gomez et al (2005) have developed a ceramic evaporative cooling system which acts as a semi-indirect cooler. The water cooled in a cooling tower is passed through the annulus passage of the ceramic tube. The out side air is passed through the central region. Chilled water evaporates by seeping through pores. Such a system permits the recirculation of indoor air, which is not possible in the conventional evaporative cooling system. Use of such

5 15 system is experimentally demonstrated and 5-12 ºC drop in temperatures is obtained under various conditions. Jain (2007) has developed and tested a two-stage evaporative cooler. Such a cooler could provide necessary comfort even though outside humidity is higher. The two-stage cooler is found to provide 20 % better cooling when compared to single stage cooler. A novel dew point evaporative cooling system for sensible cooling of ventilation air has been developed by Riangvilaikul and Kumar (2009) and tested experimentally. Wet bulb effectiveness of % and the dew point effectiveness % are reported. Heidarinejad et al (2010) have tested a ground assisted hybrid evaporative cooling system in Tehran. The ground coupled circuit provides necessary pre-cooling effects. Simulation studies have shown that such a hybrid system can provide cooling effectiveness of %. 2.4 STUDIES ON THE EFFECT OF PAD MATERIAL In case of cabinet type evaporative coolers, the type of pad material used plays a major role. Natural and man-made materials are tested by various investigators. Table 4.2 present the details. With the materials with more wick-like characteristics, the water spreads easily which offers more surface area; hence, better evaporation rate is possible. With commercial pads, about 50 % evaporation efficiencies are reported. Almost all natural materials (except Date - palm fiber) offer equivalent or better performances. PVC sponge and CELdek offer higher evaporative efficiencies of around 80 %.

6 16 Table 2.1 Studies related to the effect of cooling pad material S.No. Author(s) Pad materials tested 1 Taha et al (1994) Charcoal granules C 2 3 Liao and Chiu (2002) Al-Sulaiman (2002) Coarse fabric PVC sponge Fine fabric PVC sponge Date palm fiber 39 % Temperature drop or Evaporative cooling efficiency reported 82 to 85 % 77 to 92 % Jute 62 %, Luffa 55 % Commercial pad 50 % 4 Anyanwu (2004) Porous C Gunhan et al (2007) Darwesh et al (2009) Ahmed et al (2010) Fine pumice stones 93 % CELdek 82 % Volcanic tuff 81 % Coarse pumice stones 76 % Greenhouse shading net 51 % Rice straw C, 77 % Palm leaf fibers C, 88 % CELdek C, 82 % Straw C, 79 % Sliced wood 9.53 C, 86 % 2.5 PERFORMANCE STUDIES ON EVAPORATIVE AIR COOLERS Performance of evaporative air coolers depends upon the outside air DBT and WBT, and the internal arrangement of the cooler which decides the rate of evaporation of water. The evaporation rate depends on air velocity

7 17 across the pad, frontal area of the pad, thickness of pad, water circulation rate and the flow arrangements Studies Related to Direct Evaporative Coolers (DEC) Dai and Sumathy (2002) have investigated a cross-flow direct evaporative cooler, in which the wet durable honeycomb paper is used as the packing material. A mathematical model is developed that include the governing equations of liquid film and gas phases as well as the interface conditions. The interface temperature of falling film is also predicted. The variation of temperature, humidity ratio in the flowing channel, effect of system size and effect of operation parameters as inlet temperature, humidity of inlet air and temperature of feed water are studied. The liquid gas interface temperature is predicted and the evaporative cooling process of falling film is analyzed quantitatively. The optimum length of air channel is also found. Elfatih et al (2003) have investigated the performance of porous ceramic evaporators for building cooling application. Low, medium, high porosity ceramics are tested with direct evaporative cooling system. High porosity ceramic accounted better performance, even when supply water flow rate is increased. There is a drop in dry bulb temperature and increase in relative humidity which are found to be 6-8 K and 30 % respectively. Maximum cooling recorded is about 224 W/m 2. Qureshi and Zubair (2005) have studied the impact of fouling on performance of evaporative coolers and condensers. The result revealed that the fouling of tubes reduces the performance. The maximum decrease in effectiveness due to fouling is found to be 55 % and 78 % for the evaporative coolers and condensers respectively. Dagtekin et al (2009) have studied the performance characteristics of pad evaporative cooling system in a broiler house in a Mediterranean

8 18 climate. There are 5 pads each m in size which are used for experimentation. The cooling system has six exhaust fans with maximum of 42,000 m 3 /h. The average evaporative cooling efficiency and temperature drop are found to be 70 % and 7.3 C. Hasan et al (2009) have studied the efficiency of fan-pad cooling system in green house by building up of internal greenhouse temperature map. The results reveal that the drops in temperatures are found to be C. Considerable differences are recorded between the temperatures in center of the pad and in front of the fans (13 C at right in front, 8 C at middle of the pad and 7 C in front of the fan). Workneh et al (2009) have investigated the effectiveness of forced ventilation evaporative cooling during storage of tomatoes and storability of pre harvest treated tomatoes using evaporative cooling methods. The average dry bulb temperature of ambient air and inside the evaporative cooling are observed as 32 C and 20.5 C respectively. An average temperature drop is found to be 11.5 C. The average relative humidity of ambient air and inside the evaporative cooling are recorded as 40 % and 83.9 % respectively. The average difference between the inside and outside relative humidity during the test is found to be 43.9 %. Tilahun (2010) has investigated the feasibility and economic evaluation of low-cost evaporative cooling system in a fruit and vegetables storage. The evaporative air cooler is able to decrease the air temperature from 36 C to 16.4 C and increase the relative humidity increased from 25.4 % to 91.1 %. 10 cm thickness cooling pad is used and the cooling efficiency obtained ranged between 55 % and 84 %. The power consumption recorded is 1.13 kwh for a fixed air flow rate of 4.3 kg/s. The energy efficiency ratio is found to be 26.3 for mentioned air flow rate. Wong and Chong (2010) have investigated the performance of misting fan in hot and humid climate. Experiments are conducted to evaluate

9 19 the thermal conditions and thermal sensations resulted from the misting fans. The results from the experimental procedure showed that the misting fan is able to effectively reduce the dry-bulb temperature by approximately C. The reduction in temperature comes at the expense of higher relative humidity which results in consistently greater biological (bacterial and fungal) pollutants; being enumerated from samples collected under the misting fan system. In some samples, the bacteria count was very much greater than the samples collected under the non-misting fan, illustrating the potential for a substantial increase in biological pollutants due to the generation of mists. Khandelwal et al (2010) have studied on the energy saving in a building using regenerative evaporative cooling. The result revealed that regenerative evaporative cooling system indicated significant potential for energy savings up to %, where as simple evaporative cooling system provided %. The indoor temperature obtained are between 22 C and 26 C. El-Awad (2010) has studied the feasibility of solar assisted winter air conditioning system using evaporative coolers. The evaporative air cooler obtains the heat through from solar energy for preheating the water supply. A theoretical model is developed for a room of m 3 volume. Typical power consumption is found to be 0.1 kw. It is estimated that, for air conditioning a 500 cfm air flow rate for a minimum of four hours operation, a 150 LPD solar heater is needed and for that of eight hours 250 LPD solar heater is needed. Studies have been carried out to study the influence of various geometric and operational parameters on the performance of cabinet type Direct Evaporative Cooler (DEC). Table 2.2 provides the details of studies on DEC and the related outcomes.

10 20 Table 2.2 Studies on effect of various parameters on DEC performance S. No Author(s) 1 Thepa et al (1999) 2 Kant et al (2001) 4 Lertsatitthanakorn et al (2006) Parameter studied Parameter Pad thickness Air velocity Range cm 1-5 m/s ACH 1-40 By pass of supplied air Mass flow rate of air % kg/s Outcome EC efficiency increased from 65 % to 85 % EC efficiency decreased from 95 % to 80 % When the pad thickness 50 cm. With increase in ACH, room condition moves up from Extended Comfort Zone. With increase in ACH, room condition moves down from Extended Comfort Zone. Efficiency increased from 66 % to 80 % Studies Related to Indirect Evaporative Coolers (IEC) Maheshwari et al (2001) have evaluated the energy saving potential of an indirect evaporative air cooler. Estimated cooling capacity of IEC is found to be 3.4 and 2.4 TR for interior and coastal areas respectively. Power requirements to achieve the above mentioned cooling capacity with air-conditioner for interior and coastal area are 4.93 and 3.85 kw, whereas 1.11 kw is needed for IEC. Chaktranonda and Doungsong (2010) have evaluated the energy savings in a split-type air conditioner with evaporative cooling system experimentally. The results revealed that ambient temperature of air had been much influenced by power consumption of compressor and COP R. When the temperature is raised by 1 C, electrical power consumption is increased by

11 21 around 4 %. Due to high contact surface between water and air-stream, the evaporative cooling system can decrease the power consumption by around 15 %, and can increase COP R up to 45 %. Table 2.3 provides the details of studies on effect of various parameters on the performance of IEC and the related outcomes. Table 2.3 Studies on effect of various parameters on IEC perfromance S. No Author(s) Parameter studied Parameter Range Outcome Velocity of primary air m/s Evaporative cooling effectiveness decreased from 0.93 to Guo and Zhao (1998) Velocity of secondary air Channel width m/s m Evaporative cooling effectiveness increased from 0.7 to 0.98 Evaporative cooling effectiveness decreased from 0.99 to 0.6 Plate wettability % Evaporative cooling effectiveness increased from 0.3 to Zhao et al (2008) Channel height 2.0 mm to 10 mm Dew point and wet bulb effectiveness decreased on increasing the channel height. At the channel height of 10 mm, the dew point and wetbulb effectiveness are 0.2 and 0.85 respectively. 3 Johnson et al (2003) Number of fiber arrays 20 to 160 Approximately 58 fiber arrays are needed to achieve 80 % efficiency Studies Related to Two Stage Evaporative Cooler El-Dessouky et al (2004) have carried out the performance analysis of two stage evaporative coolers. The system is operated as a function of the

12 22 packaging thickness and water flow rate of the DEC unit. The efficiency of IEC and DEC units when operated individually where found to be % and 63-93% respectively, whereas the efficiency of two stage IEC/DEC varied over a range of %. Heidarinejad et al (2009) have investigated the cooling performance of two stage indirect (IEC) / direct evaporative cooling (DEC) experimentally under various simulated climatic conditions. The cooling effectiveness of IEC in the range of % and that of IEC /DEC of % are reported under varying climatic conditions. Such system is found to be better for the hot and humid Iranian climates. El-Dessouky et al (1996) have studied the effect of various parameters on the performance of two stage evaporative cooler. The details are presented in Table 2.4. Table 2.4 Details of the study of El-Dessouky et al (1996) S. No Parameter studied Parameter Range 1 Packing type Packing thickness Mass flux of water Mass flow rate of water Structured Sheathy leaf Natural fiber mm kg/m 2 s kg/s Evaporative Cooling Effectiveness Evaporative cooling effectiveness 0.9 when the water flow rate is 0.03 kg/s Evaporative cooling effectiveness 0.8 when the water flow rate is 0.03 kg/s Evaporative cooling effectiveness 0.75 when the water flow rate is 0.03 kg/s Evaporative cooling effectiveness increased from 0.72 to 0.9, with structured packing Evaporative cooling effectiveness decreased from 0.85 to 0.74, with structured packing Evaporative cooling effectiveness increased from 0.85 to 0.9, with structured packing

13 STUDIES ON MODELING OF EVAPORATIVE COOLING SYSTEMS A mathematical model is the mathematical representation of a process, device or concept; it uses a number of variables to represent inputs and internal states, and sets of equations and inequalities to describe their interaction. Mathematical models developed for evaporative cooling system are used to predict the system behaviors like cooling efficiency, temperature drop, optimum air velocity for various inlet conditions. Landsberg et al (1979) developed a mathematical model to estimate the average air temperature inside the glasshouse, subjected to any specified radiant energy load, outside temperature and humidity. Kimmel et al (1991) developed a theoretical model of an evaporative cooling in a wetted hide; simulations have showed that the evaporative effectiveness and effective wettedness depend on the parameters like temperature, humidity, and air velocity and on intrinsic local properties such as water content and distribution within the hide. Thomson et al (1994) have developed a mathematical model using logarithmic approximation for estimating the performance of natural draft evaporative coolers. Abdel-Wahab (1994) have developed a mathematical model to estimate the water evaporation rate, air flow rate and cooling effect in an evaporative cooling system for a farm structure. Singh et al (1997) have developed a mathematical model to evaluate the relative thermal performance of a building coupled with an indirect and direct evaporative cooler. Zalewski and Gryglaszewski (1997) have developed a mathematical model of heat and mass transfer processes in evaporative fluid coolers. The model consisted of four ordinary differential equations with necessary boundary conditions and some associated algebraic equations. Alonso et al (1998) developed a mathematical model to predict the heat and mass transfer of an indirect evaporative cooler, the model is also be

14 24 used for energy analysis and system optimization. Halasz (1998) developed a general non dimensional mathematical model used for all types of evaporative cooling devices, for each device a unique rating procedure is established, irrespective of flow direction of the fluid involved. Zalewski et al (2000) have developed an algorithm for the optimization of evaporative fluid coolers incorporating the heat and mass transfer, operating costs and production costs of heat exchanger. Sweetland and Lienhard (2000) have developed a mathematical model using a Karman-Pohlhausen treatment of the velocity and thermal boundary layers accounting the evaporation of an entrained water spray and investigated the effect of the water sprays commonly used to cool freshly drawn glass fibers. In general, a reasonably good matching between the model and experimental data are reported in these studies. Al-Nimr et al (2002) have proposed a novel summer airconditioning system incorporating adiabatic dehumification using moisture absorbent and developed a mathematical model to predict the improvement in the performance. Dai et al (2002) investigated a cross-flow direct evaporative cooler, in which the wet durable honeycomb paper constitutes as the packing material; a mathematical model is developed that include the governing equations of liquid film and gas phases as well as the interface conditions. Shammiri (2002) have developed a correlation for measuring the rate of evaporation of water as the function of salinity of water. Kittas et al (2003) have predicted the temperature gradient in a partially shaded large green house equipped with evaporative cooling pads, a simple climate model is developed incorporating the effect of ventilation rate, roof shading and crop transpiration. Fisenko et al (2004) have developed a new mathematical model to predict the performance of mechanical draft cooling tower. Riffat and Zhu (2004) have developed a mathematical model of indirect evaporative air cooler which utilizes a porous ceramic and heat pipe; the mathematical model analyzed the effect of humidity and air velocity of passages. Fuchs et al

15 25 (2006) have developed a procedure to evaluate latent heat cooling by means of crop transpiration and free water evaporation in a wet pad and fan system. Beshkani and Hosseini (2006) have developed a mathematical model of a rigid media evaporative air cooler, equipped with corrugated papers as a wetted medium; saturation efficiency and pressure drop were evolved as a function of air velocity and depth of the media. Hettiarachchi et al (2007) investigated the longitudinal heat conduction in the exchanger wall of a compact plate cross flow indirect evaporative air cooler using NTU (Number of transfer units) method and block iterative numerical method. Wang et al (2008) have developed a fuzzy mathematical method and evaluated the suitability of an evaporative pad cooling system for poultry houses in China. Wu et al (2009a) conducted a theoretical analysis to evaluate the heat and mass transfer between air and water film in a direct evaporative cooler with wet durable honeycomb papers constituting the pad modules. Wu et al (2009b) investigated the heat and mass transfer in a direct evaporative cooler numerically; a simplified mathematical model is developed to describe the heat and moisture transfer between water and air in a direct evaporative cooler. In these studies, the predictions are reported to be in good agreement with the experimental data. 2.7 CONCLUSIONS FROM THE LITERATURE REVIEW major conclusions: From the review of literature presented above, the following are the i. With the ongoing energy crisis and pollutant emission constrains, use of evaporative air coolers are much advantages. ii. The difference between the outside air dry bulb temperature and the wet bulb temperature is the key factor which decides

16 26 the use of evaporative coolers. Larger the difference, usefulness of evaporative coolers is better. iii. iv. Evaporative cooling can reduce the incoming ambient air temperature to the room considerably. It is inexpensive, energy efficient and potentially attractive. Pre cooling of water could enhance a direct evaporative cooler that cools the air even below its inlet WBT. v. The reduction in temperature comes at the expense of higher relative humidity. It may be disadvantages in certain applications. vi. vii. Cabinet type evaporative cooler are most popular. The pad material, pad thickness, air velocity, water circulation rate, and flow arrangements are found to affect the performance of such evaporative coolers. Various attempts have been made to study the effect of various parameters on the evaporative cooler performance. The pad material, and pad thickness are found have the major role for a given air flow rate. viii. For the given conditions, there exists an optimum pad thickness balancing the evaporation rate and pressure drop across the pads. ix. Studies have shown that excessive water circulation does not contribute in improving the performance. Considerable energy savings are possible by optimizing the pump operations. x. Cabinet type coolers are sufficient for cooling of small air volumes. For large spaces, more cabinet coolers are employed.

17 27 xi. xii. For hot and humid climates, Indirect / Direct coolers are used. Such a coolers could provide better comfort with % of energy savings when compared to conventional Mechanical Vapour Compression Systems. Various other methods, like thermal curtain, providing water filament guides over external facet of the building, evaporative walls, are being attempted. xiii. Efforts have been made to model the performance of evaporative cooling systems and reasonable agreements between the predictions and experiments are reported. 2.8 CLOSING REMARKS The efficiency of evaporative air cooler mainly depends on outside dry bulb temperature, wet bulb temperature, inlet air velocity and the rate of water evaporation. The dry bulb and wet bulb temperatures of ambient are uncontrollable and hence for increasing the performance of the system the inlet air velocity and the water evaporation rates are to be enhanced by suitable methods. In general, water evaporates at its surface there by reducing the temperature of air consequently increasing the moisture. There are two methods possible to increase the surface area of water. (i) By installing more number of evaporative cooling pad, thereby increasing the wetted surface area (ii) By splitting of water into fine particle like a mist. The first concept is better when small air volumes are to be handled. It is not suitable for large air volumes, since it increases the cost and energy consumptions. In the present work, a centrifugal type atomizer is conceived and developed. Such an atomizer uses centrifugal force to split a

18 28 stream of water into fine mist, thereby increase the surface area of water many fold. The increased surface area facilitates faster evaporation. Thus, large air volumes can be evaporatively cooled. Thus, the proposed centrifugal type atomizer can evaporatively cool large air volumes, which can be used to cool large industrial sheds in tropical climates like India. This has provided the objectives for the present research work, mentioned in Section 1.4. The following chapters present the details and outcome of this research work.