International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
Experimental Study to Evaluate Mist System Performance Mahmoud Sh. Mahmoud Mechanical Eng. Department Al Nahrain University-
[email protected] ABSTRACT - An experimental investigation is presented to evaluate mist system performance in space which includes study of parameters (Water flow rate, elevation of mist system h n and water inlet temperature) affected on system performance. In the last 20 years, evaporative and desiccant cooling technology for air conditioning systems has increased as an alternative to the conventional vapor compression systems due to electricity cost and environmental protection. The measurement results showed that better performance of the mist system at nozzle elevation is (2.25m) and water flow rate is (1.2L/min). The maximum temperature difference in space is (9.4˚C) and effectiveness of mist system is more than (62.5). Inlet water temperature variations has a slight effecton obtained results which was found to be (15.9%) for air temperature difference. Key words - Evaporative cooling, effectiveness, mist system, dry bulb temperature, wet bulb temperature Nomenclature: h n : Elevation of nozzle (m) mw =water flow rate (L/min). T db : Dry bulb temperature of air (˚C) T wb : Wet bulb temperature of air (˚C) T w : Temperature of water (˚C) Va : Average air velocity (m/s) θ: Nozzle inclination angle (˚) : Effectiveness. DBT: Dry bulb temperature of air (˚C) Max: Maximum RH: Relative humidity (%). WBT: Wet bulb temperature of air (˚C) I. INTRODUCTION: Water will evaporate if air with a lower dew point passes by. This process will result in conversion of sensible heat into latent heat at a constant wet bulb temperature (WBT), therefore the air not only gets cooler but also gets more humid. This phenomenon is called Evaporative Cooling it was used about a thousand years ago in vernacular architecture in the Middle East and North Africa during the era of the Islamic Empire. Evaporative cooling technologies such as mist spraying, sprinkling of roofs, and water spraying onto air-conditioner outdoor units were taken up as environment measures for an investigative study. The only maintenance required for a properly designed Mist system is to change the oil in the pump and cleaning the nozzles at the start of the season as required. The water filter cartridges should be replaced yearly at a minimum with visual check of all fittings and hoses / tubing. Drain the system of water at the end of the season and flush all lines without nozzles with fresh water at the start of the season. Mist cooling systems can be used effectively in most geographical locations because when temperatures reach their peak during the day, humidity is normally at its lowest point. Evaporative cooling is based on a physical phenomenon of converting the sensible heat of air to latent heat which leads to decrease the dry bulb temperature of air and increase the relative humidity to a certain extent. This process occurs by direct contact between water and air. Many systems works using this principle and Mist cooling System is most common. Mist system could be defined as high pressure water that runs through a special Mist nozzle which produces fine droplets that float and evaporates in air. Mist systems have been used as an economical and efficient means of environmental control through outdoor space. Mist system applications include outdoor, livestock cooling, humidification, and dust control. The tiny water droplets quickly absorb the heat present in the environment and evaporate, becoming water vapor. The heat used to change the water to a vapor is eliminated from the environment; hence the air is cooled. Relative humidity is a crucial factor in determining the cooling potential. With lower relative humidity, more water can be vaporized and more sensible heat can be removed. _____________________________________________________________________________________________________ © 2014-15, IJIRAE- All Rights Reserved Page -41
International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
II. LITERATURE SURVEY The developments and other investigations in evaporative cooling was reported by different researchers. O. Amer, et al (2015) [1] They aimed to review the recent developments concerning evaporative cooling technologies that could potentially provide sufficient cooling comfort, reduce environmental impact and lower energy consumption in buildings. Air-conditioning plays an essential role in ensuring occupants thermal comfort. However, building’s electricity bills have become unaffordable. Yet the commercially dominant cooling systems are intensively power-consuming ones, i.e. vapor compression systems. An extensive literature review has been conducted and mapped out the state-of-the-art evaporative cooling systems. The review covers direct evaporative cooling, indirect evaporative cooling and combined direct-indirect cooling systems. The indirect evaporative coolers include both wet-bulb temperature evaporative coolers and dew point evaporative coolers have been of particular interest because of high thermal performance. The dew point evaporative coolers have shown great potential of development and research opportunity for their improved efficiency and low energy use. Mu’azu Musa. (2008) [2], A compromise using horizontal arrangement was considered. Use of pump for supplying water required to moisten the evaporative cooling surface was eliminated. The system was constructed and tested under varying temperature, relative humidity and air flow rates. Results showed significant temperature reduction accompanied with acceptable increase in relative humidity. Temperature drop of (6-10) oC between the inlet and outlet temperatures of the product or supply air was recorded. Increase in relative humidity of the supply air was (6 – 10) % less than the working air. J. R. Camargo et.al. (2005) [3], aims to analyze the influence of some operation parameters, such as the reactivation temperature of the adsorbent, and the relationship between the reactivation air flow and the process air flow (R/P) on the performance of the system. In addition, this paper presents an application of a proposed system in different climate characteristics of several tropical and equatorial cities. Samaan, et.al. (1980) [4], carried out experimental device to study thermal performance of the evaporative coolers, their studies concentrate on changing pack thickness. The increase in pack thickness from (5.5cm) to (13cm) leads to effectiveness more than (51%). Also they studied possibility to develop water distribution system and water pump power. The results indicate that the use of these modifications together leads to effectiveness of more than (90 %). Roy, (1989) [5]. Drip evaporative cooling method was constructed with simple materials and used for the preservation of fruits and vegetables. It consists of a simple low cost cavity wall evaporative cooler constructed from bricks and termed as “Improved Zero-Energy Cool Chamber” in India. Bucklin et al, (2008) [6], Drip or misting systems are one of the older evaporative cooling methods around. In this type a low pressure mist or drip is sprayed directly on a body. As moisture evaporates from the surface of the body, heat is absorbed and therefore it becomes cool. This method was commonly used on animals. As water drips sprayed on the animal skin evaporate, heat is absorbed. The animal therefore enjoys cooling. The method often features use of cooling pads and water drips distribution system. Water drips along the distribution pipe and drains down into the pad material. Sump can be provided at the bottom and should be large enough to hold all run-offs when the pump is turned off. El-Dessouky, et al, (2000) [7]. In this research direct evaporative cooling is used and relatively dry outside air is blown through a water-saturated medium (cellulose) or porous ceramics where it is cooled by evaporation. As stated in the principle of evaporative cooling, the contact between air and water causes small amount of water to evaporate. Subsequently the latent heat of vaporization causes simultaneous reduction in the air and water temperatures. Also a smaller amount of sensible heat is exchanged between the two streams. However, the latent heat transfer amounts to 80-95% of the total heat exchanged. J. R. Camargo, C. D. Ebinuma and S. Cardoso,(2003) [8], presented the basic principles of the evaporative cooling process for human thermal comfort, the principles of operation for the direct evaporative cooling system and the mathematical development of the equations of thermal exchanges, allowing the determination of the effectiveness of saturation and compared them with the experimental results. III. EXPERIMENTAL WORK The experimental work was conducted in Al-Nahrain University / Iraq – Baghdad using mist system to predict the effects of above mentioned parameters. The experimental rig was built using mist nozzle which is solid cone, concentrates distribution of atomized water droplets towards the center of the spray cone pattern, made of brass material, with spray angle pattern (45˚). See Figure (1). Water pump operating with (40bar, suction 40 L/min and 1200R.P.M.). The control on water flow rate from nozzle is through pressure valve. Nozzle tube base made of square frame to ensure stability at high velocities, with 2.25m height and 0.5m width see Figure (2). A meter model (AR847), RH & DBT sensor type k, temperature range (-20˚C to 50˚C) and relative humidity range (5%RH to 98%RH) was used to measurement temperature and humidity. and the air velocity in open space was measured using a digital anemometer model 8901. _____________________________________________________________________________________________________ © 2014-15, IJIRAE- All Rights Reserved Page -42
International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
Figure (1) Mist Nozzles
Figure (2) Nozzle tube base and experimental rig at work. IV. RESULTS AND DISCUSSION The results of the main parameters influencing the mist system performance are presented and discussed below. The parameters are water flow rate, nozzles elevation and inlet water temperature which effect on mist cooling system performance in the open space. The results for maximum dry bulb temperature (DBT) and relative humidity (RH %) are calculated to the average height of human and the important region for cooling. Also the effectiveness was found with respect to the surrounding wet bulb temperature according to equation bellow by using the temperature values measured in the inlet and outlet air flow.
Tdb, in Tdb, new Tdb, in Twb
Figures (5 and 6) shows the variation in measured dry bulb temperature (DBT) and effectiveness with horizontal distance from nozzle respectively. Air condition before cooling process is (DBT=35˚C) and (RH=21%). Water entering at temperature (31˚C) and changing flow rate is (0.42, 0.84 and 1.2L/min). The results obviously reveal that temperature in space decreases with increasing water flow rate because the increase in heat and mass transfer between the mist and surrounding air. Maximum difference in dry bulb temperature (∆DBT) is (9.4˚C) at water flow rate of (1.2 L/min). While effectiveness variation with the horizontal distance is found to have the max. Value of (59.1) at the 2m distance and 1.2 L/min flow rate with respect to 19.1oC wet bulb temperature due to the variance in DBT and flow rate [see Figure (6)]. Relative humidity variation is shown in Figure (7), it could be noticed that the logical variance for RH% with water flow rate along the covered distance. The increase in water flow rate from 0.42 L/min to 0.84 L/min leads to an increase in RH% from (21%) as the initial state to (57%), (45%) and (30%) for the 0.42 L/min, 0.84 L/min and 1.2 L/min water flow respectively at the 2 m horizontal distance and that is due to the increase in evaporative water amount in space.
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International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
40 35
Air DBT (oC)
30 25 20 0.42 L/min 15
0.84 L/min
10
1.2 L/min
5 0 1
1.5
2
2.5
3
3.5
Horizental Distance X (m)
Figure (3) Effect of Water Flow Rate on (DBT). 70
0.42 L/min 0.84 L/min
60
1.2 L/min
Effectivness
50 40 30 20 10 0 1
1.5
2 2.5 Horizental Distance X (m)
3
3.5
Figure (4) Effectiveness variation with horizontal distance.
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International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
60 50
RH %
40 30 20 0.42 L/min 0.84 L/min
10
1.2 L/min 0 1
1.5
2
2.5
3
3.5
Horizental Distance X (m)
Figure (5) Effect of Water Flow Rate on (RH %). Figures (8 and 9) Show the variation in measured dry bulb temperature (DBT) and effectiveness with horizontal distance from nozzle respectively. Inlet air condition is (DBT=35˚C), (RH=21%), and inlet water temperature is (32.5 ˚C). The nozzle elevation is changed as (2.25, 2.5 and 2.75m) from datum. it can be seen that the change in (DBT) tends to the decrease with increasing nozzle elevation. At nozzle elevation of (2.25 m), the maximum change in (DBT) is (11˚C), while its value is (10.2˚C) at nozzle elevation of (2.5m) and at (2.75 m) the maximum change in (DBT) is (6.6˚C). This behavior is due to cooler region became higher than the previous measured region. While the effectiveness variation with the horizontal distance is found to have its max. value of (59.1) at the 2m distance and 1.2 L/min flow rate with respect to 19.1 oC (WBT) wet bulb temperature due to the variance in DBT and flow rate [see Figure (9)]. Figure (10) shows the variation of nozzle elevation with RH%, the results shows that average relative humidity tends to decrease with the increase of nozzle elevation, at (2.25m) elevation the average RH% is found to be (49.11%) while its average value is (44.33%) at (2.5m) and finally its average value at (2.75m) elevation is (28.03%), this behavior is due to cooler region becomes higher than previous measured region. 40
Air Dry Bulb Temprature oC
35 30 25 20 15
2.25 m
10
2.5 m
5
2.75 m
0 1
1.5
2
2.5
3
3.5
Horizental distance X (m)
Figure (6) Effect of Water Flow Rate on (DBT). _____________________________________________________________________________________________________ © 2014-15, IJIRAE- All Rights Reserved Page -45
International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
70 60
Effectivness
50 40 30 2.25 m
20
2.5 m 10
2.75 m
0 1
1.5
2
2.5
3
3.5
Horizental distance X (m)
Figure (7) Effectiveness variation with horizontal distance. 70 60
RH %
50 40 30 2.25 m
20
2.5 m 10
2.75 m
0 1
1.5
2
2.5
3
3.5
Horizental distance X (m)
Figure (8) Effect of Water Flow Rate on (RH %). Inlet air condition is (DBT=45˚C), (RH=17%) and (Va= 1.1 m/s). Water entering at (mw=0.82 L/min) and changed water temperature at (34.8, 39.7 and 44˚C). The nozzle elevation is (2.25m) from datum. Figure (11) shows the effect of water temperature on (DBT). It is clear that, water temperature has a slight effect on (DBT) with a (6.7%) difference between (34.8oC) and (39.7oC) water inlet temperature and a percent of (9.9%) between (39.7 oC) and (44 oC) water inlet temperature with an overall percent of (15.9%) between (34.8 oC) and (44oC) water inlet temperature. This is confirmed by evaporative cooling literatures and I some cases is neglected. The water temperature variation from (0˚C to 100˚C) at standard atmosphere pressure, the slope angle of line evaporative cooling operation changes (7˚) of protractor [8].
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International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
In Figure (12) the effectiveness variation with the horizontal distance for selected water inlet temperature is found to have the max. Value of (58.4) at the 2.5m distance and (39.7 oC) water inlet temperature with respect to (21 oC) (WBT) wet bulb temperature due to the variance in DBT and water inlet temperature.Also the variation of RH % with designated water flow temperature is shown by Figure (13), it could be noticed that the RH % decrease with the increase of water inlet temperature. 45 40
Air Dry Bub Temprature oC
35 30 25 20 15
Twater=34.8 C Twater=39.7 C
10
Twater=44.0 C 5 0 1
1.5
2
2.5
3
3.5
Horizental distance X (m) Figure (9) Effect of Water Flow Rate on (DBT). 70 60
Effectivness
50 40 30
Twater=34.8 C Twater=39.7 C
20
Twater=44.0 C 10 0 1
1.5
2
2.5
3
3.5
Horizental distance X (m) Figure (10) Effectiveness variation with horizontal distance.
_____________________________________________________________________________________________________ © 2014-15, IJIRAE- All Rights Reserved Page -47
International Journal of Innovative Research in Advanced Engineering (IJIRAE) Issue 9, Volume 2 (September 2015)
ISSN: 2349-2163 www.ijirae.com
70 60
RH %
50 40 30
Twater=34.8 C Twater=39.7 C
20
Twater=44.0 C 10 0 1
1.5
2
2.5
3
3.5
Horizental distance X (m) Figure (11) Effect of Water Flow Rate on (RH %). REFERENCES [1] O. Amer, et al “A Review of Evaporative Cooling Technologies”, International Journal of Environmental Science and Development, Vol. 6, No. 2, February 2015. pp.111-117. [2] MU’AZU MUSA. “NOVEL EVAPORATIVE COOLING SYSTEMS FOR BUILDING APPLICATIONS”, PhD. Thesis, University of Nottingham, UK, May 2008. [3] J. R. Camargo, et al “An Evaporative and Desiccant Cooling System for Air Conditioning in Humid Climates”, J. of the Braz. Soc. of Mech. Sci. & Eng., July-September 2005, Vol. XXVII, No. 3, pp. 243-247. [4] Samaan, J.M., et al “Experimental Study of Thermal Performance of Direct Evaporative Air Coolers”, Al-Muhandis, N.77, pp.18, 1980. [5] Bucklin R. A., J. D. Leary, D. B. McConnell, E. G. Wilkerson, “Fan and Pad Greenhouse Evaporative Cooling Systems”. http://edis.ifas.ufl.edu. [6] Roy S.K., “Postharvest technology of vegetable crops in India”. Indian Horticulture. (Jan-June, 1989, p.7678). [7] El-Dessouky H.T., H.M. Ettouney and W. Bouhamra. “A Novel air conditioning system –Membrane air drying and evaporative cooling” (Trans IChem E, Vol.78, Part A, October, 2000). [8] Camargo J. R., Ebinuma C. D. and Cardoso S.,(A Mathematical Model for Direct Evaporative Cooling Air Conditioning System), Engenharia Térmica, no.4, pp. 30-40,2003.
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