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Performance Evaluation of Misting Fans in Hot and Humid Climate

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Performance Evaluation of Misting Fans in Hot and Humid Climate Building and Environment 45 (2010) 2666e2678 Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv Performance evaluation of misting fans in hot and humid climate N.H. Wong*, Adrian Z.M. Chong Department of Building, School of Design and Environment, National University of Singapore, 117566 Republic of Singapore article info abstract Article history: Singapore experiences a hot and humid climate throughout the year. This in turn results in heavy reli- Received 3 March 2010 ance on mechanical systems especially air-conditioning to achieve thermal comfort. An alternative would Received in revised form be the use of evaporative cooling which is less energy intensive. Objective and subjective measurements 14 May 2010 were conducted at an experimental setup at the National University of Singapore (NUS) to evaluate the Accepted 27 May 2010 thermal conditions and thermal sensations brought about by misting fans. Field measurements were also conducted at food centres in Singapore to determine if they are coherent with the objective and Keywords: subjective measurements conducted. Analysis of objective and subjective data showed that the misting Misting fan fi Evaporative cooling fan was able to signi cantly reduce the dry-bulb temperature and thermal sensation votes. This is fi Singapore consistent with eld measurements taken, where regression analysis showed that with the misting fan, Hot thermal neutrality can be obtained at a higher outdoor effective temperature (ET*). However, the Humid 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 is very much greater than samples collected under the non-misting fan, illustrating the potential for a substantial increase in biological pollutants due to the generation of mists. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The use of evaporative-cooling systems are however dependent on climatic conditions and hence the successful use of an evapo- Singapore being situated 1210N of the equator is characterised rative cooler achieved at one particular climatic condition cannot by uniformly high temperatures, high humidity and abundant be guaranteed at another location with a different climatic condi- rainfall throughout the year. This in turn results in heavy reliance tion. As a result, the feasibility of evaporative cooling should be on mechanical systems especially air-conditioning to achieve determined specifically for each individual location and climatic thermal comfort. An alternative would be the use of evaporative condition. Performance of direct evaporative cooling in hot arid cooling. The advantage of evaporative cooling comes from its climates have however been widely studied, particularly for relatively low energy consumption and absence of environmentally glasshouses. A study which applies water droplets through spray aggressive refrigerants. In addition, the use of evaporative coolers mists to the roofs of a glasshouse have shown that roof temperature will certainly result in lower operating and initial costs as of glasshouses with plants growing inside was reduced by nearly compared to a comparable mechanical system [1]. 8 C as a result of direct evaporative cooling [2]. This study goes on The main objectives of this study are as follows: to conclude that much larger reductions in roof temperature were obtained when water vapour deficit in the ambient air was high as 1. Determine the thermal comfort level for semi-outdoor dining compared to under low water deficit conditions where evaporative areas with misting fans cooling reduced air temperature by less than 1 C. Similarly, theo- 2. Determine the possible factors contributing to its effectiveness retical analysis into the effectiveness of direct evaporative cooling or ineffectiveness in a hot and humid climate of glasshouses at areas characterised by high solar radiation 3. Determine if the generation of mist increases the potential for intensities and low humidity by Landsberg et al. [3] indicates that the harbouring of bacteria and yeast in the ambient air single stage direct evaporative cooling can reduce air temperature in the glasshouse by 8e12 C. Apart from studies on glasshouses, an experiment by Aimiuwu [4] recorded that the evaporation of water * Corresponding author. placed in hot arid regions was able to achieve a drop of 10.4 C E-mail address: [email protected] (N.H. Wong). below ambient temperature in still air, while the temperature fell 0360-1323/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.05.026 N.H. Wong, A.Z.M. Chong / Building and Environment 45 (2010) 2666e2678 2667 by 15.0 C under forced convection. One type of evaporative-cool- misting fan setup. During the allocation of participants, it was ing system that has been increasingly used in dinning locations ensured that there would be an equal proportion of males and around Singapore is the misting fan system which utilises atom- females under each setup. During each day, each participant isation nozzles to produce a cloud of very fine water droplets. The would be limited to sedentary activities under their allocated misting fan system is essentially a direct evaporative system. setup during the time period of 9.30e11am (morning), 12.30e2pm ASHRAE [5] defines direct evaporative equipment as one which (afternoon) and 3.30e5pm (late afternoon). For each time period, cools air by direct contact with water, either by an extended each participant would be required to complete a thermal comfort wetted-surface material or with a series of sprays. These droplets questionnaire every subsequent half-hour period after the come into direct contact with the airstream, removing its latent participants have settled in for at least 30 min. Throughout the heat of vaporisation and thus cooling it in the process. experiment, a Babuc A data logger would be used to record each of Under the misting fan system, a cloud of very fine water droplets the four environmental parameters at 15 min intervals from 9am is produced using atomization nozzles, inducing mixing between to 5pm. This is to allow correlation to be established between the the airstream and water, thus allowing the ambient air, to cool from comfort levels and the corresponding ambient conditions that has its dry-bulb temperature to its wet bulb temperature if the droplets been recorded. are fully vaporised [6]. Objective measurements would be taken at a height of 1.0 m Although evaporative cooling is expected to be more effective in above the floor level which represents the respondent’s head level a hot and dry climate, a simulation carried out by Yu and Chan [7] when seated. concluded that evaporative pre-coolers can function properly even under hot and humid climate. Although the use of evaporative cooling is able to reap benefits in terms of energy savings due to its 2.3. Field measurements lower energy consumption and absence of environmentally aggressive refrigerants as compared to mechanical system, there is The 2 semi-outdoor food centres, food centre A and food centre a lack of research into the effectiveness of evaporative coolers B were selected because they have similar orientation, with food particularly in hot and humid climates [1]. centre A using the misting fan system (Fig. 2) and food centre B In addition, the use of misting fans is limited by the plausible using non-misting fans. Data for food centre A was collected on increase in biological pollutants due to belief that mist generated 22nd August 2009 and 20th September 2009, and data for food would provide a more conducive environment for the harbouring centre B was collected on 29th August 2009 and 27th September of bacteria and yeast [8]. High moisture and relative humidity has 2009. The semi-outdoor coffee outlet was selected because it long been hypothesised to be required for optimal growth [9].In utilises the misting line system (Fig. 3) in which mist is generated at addition, of greater relevance was a study on cold-mist vaporizers a lower velocity only at the perimeter of the outlet. For the coffee in which the level of fungal particles increased sharply during the outlet, measurements were taken at different time periods when use of the vaporizers [8]. However, there is a lack of research into the misting line system was operational and non-operational since the bacterial and fungal aerosols arising from misting fans. In view permission was granted to turn the misting system off. In this case, of the potential benefits as well as the increase in use of misting comparison can be made between the data collected during the fans in Singapore there is a need to look into the viability of different time periods (operational and non-operational) to deter- evaporative-cooling systems in hot and humid climate. mine the effectiveness of the misting line system. Data for the coffee outlet (without mist line) was collected on 30th August 2009 2. Methodology and 5th September 2009, and data for coffee outlet (with mist line) was collected on 23rd August 2009 and 13th September 2009. 2.1. Overview Both objective and subjective measurements were carried out concurrently in this study and in the occupied zones of the dining To study and determine the effectiveness of misting fans, both area. In addition, respondents were chosen
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