Design And Testing Of An Evaporative Cooling System


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DESIGN AND TESTING OF AN EVAPORATIVE COOLING SYSTEM USING AN ULTRASONIC HUMIDIFIER
KAEL EGGIE
ADVISOR: DR. G.S.V. RAGHAVAN
Kael Eggie Student Branch Faculty
Advisor: Dr. G.S.V. Raghavan
Department of Bioresource Engineering Room MS1-027, Macdonald-Stewart Building, 21111 Lakeshore Road
Ste. Anne de Bellevue, Quebec H9X 3V9 Tel.: 514-398-7773 | Fax: 514-398-8387
Date of signing: May 12, 2008

STATEMENT OF PURPOSE This paper topic was chosen at the behest of the course coordinator, Dr. Raghavan, who wanted work to be performed with a client’s (Plastique Frapa Inc.) humidifying technology in relation to part of his research interest area: postharvest storage. This paper was part of a senior year design project and reports the results of the experiment work that was performed in analyzing the effectiveness of the designed setup. Aside from the topic definition, the project was self-directed in terms of methodology and analysis. Although it should be noted that research associates of the department were instrumental in solving problems that occurred throughout the experimentation. A co-author for this paper must be recognized for his contributions to the formulation of this paper. Michael Schwalb, worked very closely to assist in setup, conceptualization and other technical aspects.
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EXECUTIVE SUMMARY
Postharvest losses throughout the world account for significant reductions in food supply which negatively impacts incomes of farmers, prices of food, and food availability. Many of these losses may be mitigated by providing reduced temperature and increased humidity. This report contains the details of the design of an evaporative cooling system which is intended to perform these functions. The system’s design was based upon the humidifying capacity of an ultrasonic humidifier supplied by a Québec company, Plastique Frapa Inc. The evaporative cooling system initially took shape as an external input unit to feed cool, moist air into a control volume such as a cold-storage room. After trials which yielded a very limited temperature reduction (max 2°C) but an acceptable relative humidity increase (to 80%). A second setup with the unit directly inside the control volume yielded slightly greater temperature reduction (max 4°C) and significant relative humidity increase to the point of saturation (100%). This second setup’s results were marred by significant condensation within the control volume. A third setup was established with the unit outside of the control volume, however with direct humidity input to the control volume. A similar temperature reduction and relative humidity were obtained as in the second setup however, condensation was limited because the control unit for relative humidity was adjusted to a lower level. The unit was also tested for its effect on produce weight loss. The setup for this test was consistent with the third setup for cooling. The results were compared with a similar experiment conducted by Dadhich et al. (2008), and the values for weight loss from this experiment were found to be considerably higher.
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ACKNOWLEDGEMENTS Throughout the duration of our project, we had quite a bit of help along the way. We relied on the experience and knowledge of a number of people who were able to point us in the right direction regarding a starting point or how to navigate around the many road blocks we encountered. Our first thanks go to M. Yvan Gariepy who gave much of his time. His expertise in problem solving, instrumentation instruction and general testing design was instrumental in the completion of this project. We would also like to thank Dr. Sam Sotocinal who lended his opinion to the setup design, guiding around the machine shop and helping troubleshoot when problems arose. Dr. Raghavan warrants our thanks for providing the project as well as being a guide for project definition.
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TABLE OF CONTENTS
STATEMENT OF PURPOSE ...................................................................................................................... 2 ACKNOWLEDGEMENTS .......................................................................................................................... 4 TABLE OF CONTENTS.............................................................................................................................. 5 LIST OF FIGURES ...................................................................................................................................... 6 INTRODUCTION ........................................................................................................................................ 7 METHOD AND MATERIALS .................................................................................................................. 10 THE ULTRASONIC HUMIDIFYING UNIT ............................................................................................ 10 DESIGN OF A MIXING CHAMBER ....................................................................................................... 10
Design Concerns and Considerations for a Mixing Chamber................................................................. 11 Testing Setup .......................................................................................................................................... 12 EVAPORATIVE COOLING SYSTEM DESIGN ..................................................................................... 16 Design Parameters .................................................................................................................................. 16 Evaporative Cooling Design Set-Up:...................................................................................................... 16 Testing Procedure of Evaporative Cooling Unit..................................................................................... 17 EVAPORATIVE COOLING EFFECTS ON PRODUCE.......................................................................... 25 LIMITATIONS AND SETBACKS............................................................................................................ 30 CONCLUSIONS......................................................................................................................................... 30 REFERENCES ........................................................................................................................................... 31 APPENDIX A – Performance Analysis of Mixing Chamber at Varying Maximum Air Velocities.......... 32 APPENDIX B – Solid Works Representational Model.............................................................................. 35 APPENDIX C – Diagram of MDFD-1 Ultrasonic Humidifier by Plastique Frapa Inc. ............................. 36 APPENDIX D – Thermal Load Analysis ................................................................................................... 37
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APPENDIX E – Calculation of Relative Humidity from Tdry and Twet....................................................... 39 APPENDIX F – Daily Weight Loss of Produce ......................................................................................... 42 APPENDIX G – Psychrometric Chart ........................................................................................................ 44 APPENDIX H - Ideal Mixing Chamber Velocity....................................................................................... 45
LIST OF FIGURES
Figure 1 - Images of parallel and series configurations................................................................ 12 Figure 2 - Initial Test Results: RH and Tdry vs. time .................................................................... 13 Figure 3 - Initial Test Results: Tdry and Twet vs. time.................................................................... 14 Figure 4 - Performance curve relating Tdry and RH to air velocity............................................. 15 Figure 5 - First Setup: Humidifying unit outside control volume ................................................ 17 Figure 6 - Relative Humidity, Tdry,CV vs. time with unit outside control volume..................... 19 Figure 7 - Tdry,ambient vs. time for unit outside control volume................................................ 19 Figure 8 – Tdry,ambient – Tdry,CV vs. time for unit outside control volume............................. 20 Figure 9 - Second Setup: Humidifying unit inside control volume .............................................. 22 Figure 10 – Tdry,CV vs. time with unit inside control volume.................................................... 23 Figure 11 – Tdry,ambient vs. time for unit inside control volume* ............................................. 24 Figure 12 - Tdry,ambient – Tdry,CV vs. time with unit inside control volume*......................... 24 Figure 13 - RH and Tdry inside control volume vs. time ............................................................... 26 Figure 14 - RH and Tdry vs. time outside control volume............................................................. 26 Figure 15 - Carrot weight loss vs. time......................................................................................... 27 Figure 16 - Radish weight loss vs. time........................................................................................ 28 Figure 17 - Spinach weight loss vs. time ...................................................................................... 28
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INTRODUCTION
The global population in the year 2008 has hit an unprecedented level of 6.5 billion. It continues to rise drastically, and is predicted to hit 8 billion by the year 2025 (United Nations, 2008). In addition, with the economic rise of highly populous countries such as China and India, there is also an unprecedented rise in the overall global standard of living. From an agriculture perspective, these facts translate into an immense increase in the demand for food. Naturally, this entails a subsequent increase in the price of food and agricultural products worldwide. It should also be noted that industries such as the biofuel industry constitute market forces that contribute to the rise in the demand and price of agricultural products. Agricultural engineers are faced with the task of not only meeting the food requirements of the ever increasing global population, but to also maintain relatively low costs for food products. From basic economics principles, stabilizing the cost of a product undergoing an increase in its demand requires either an increase in its supply or a decrease in its cost of production. However, with agricultural products, such a task is not so simple: the amount of land that can be cultivated has reached its practical limit. Furthermore, soaring energy prices continue to push the costs of production from the growing stage to the transportation stage of production higher and higher. The only possible way left to stabilize the cost of food is to establish and implement new and efficient methods for crop production, post-harvest drying and storage, as well as distribution and transportation. It should be noted, that such Although all of these aspects are important when trying to tackle the challenge of meeting stabilizing the cost of food, this design project only explores the post harvest storage aspect of food production. In southeast Asia, postharvest losses range from 10%37% for rice (International Rice Commission, 2002). Furthermore, in India, post harvest losses are in the range of 25-50%. These losses translate into a significant loss in the overall supply of agricultural products.
To realize the relevance of this project it is important to recognize the reasons for postharvest losses in relation to temperature and humidity. Losses related to temperature and humidity can include spoilage due to disease, over-ripening, negative physiological and compositional effects, loss of mass (produce water mass), and aesthetic appeal. The goals of postharvest cooling are to counteract these effects by the following mechanisms:
-slow and inhibit water loss
-suppress enzymatic degradation and respiration
-slow and inhibit the growth rate and activity of pathogens
-reduce the production of ethylene or minimize a commodity’s reaction to ethylene
(Narayanasamy, 2006)
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Some background on each aspect is necessary in understanding why it is important to analyze and correct this problem. The first problem of water loss is related to food structure, texture and appearance changes. Water loss after harvest is mainly dependent on two things: 1. the water vapour pressure deficit that exists between produce and its immediate environment, and 2. the surface transfer resistance to water vapour movement (determined by shape and internal resistances as well as surface resistances) which occurs now that the plant is no longer transpiring through its stomata. Transpiration resistance can be increased as a boundary layer of water vapour-saturated air is fashioned because of zero air movement, but the interrelation of factors can be seen in this case because to remove respiratory heat there must be air convection, so a balance must be found. Water loss affects commodities’ quality and taste in that water increases the turgor pressure of fruits and vegetables which lends itself to a perceived “freshness” and “crispness” upon consumption. (Wills et al., 2007)
Very important to the economic side of production is that that fruits and vegetables are largely sold on the basis of weight, loss of water is indicative of a loss in revenue which farmers can ill afford.
Suppressing respiration is integral in food preservation; respiration refers to the breakdown of sugars and other compounds within the cell which releases carbon dioxide, water and heat to the surroundings (linking respiration to water loss). It is important to note that the rate of metabolic activity in cells increases with temperature; thus the importance of minimizing produce temperature (and environment temperature) is clear. Produce is typically harvested at ideal times (i.e. ripeness) depending on whether or not it is considered climacteric or non-climacteric, but remains a living entity; thus, cells perform the normal functions of aging (DeEll et al., 2003). Eventually a stage of senescence is reached whereby the cell continues respiring, but loses the ability to divide and becomes a dying entity. During the stage of senescence, cell-wall structure and composition changes and subsequently produce may become more palatable to a point, but these changes make the produce more susceptible to damage from external forces.
Also involved in ripening and eventual produce edibility and/or degradation is the chemical ethylene which is a hormone that may be either produced naturally by the commodity, or applied as a catalyst to hasten the ripening process. There is a large variability between commodities in terms of the actual production rate of ethylene and the commodities’ response to ethylene. The final effect of pre-cooling treatments that are implemented is to improve the limitation of pathogens and disease found in vegetables. Postharvest diseases causing spoilage in perishable commodities can be significant as shown by Table 1.1 in Narayanasamy (2006). Pathogens and disease tend to flourish in high temperature and high humidity environments and produce may become affected at any point in the postharvest pathway to the consumer; in terms of precooling, removing initial field heat is an important first step to limiting the influence of diseases.
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Moreover, there is a tradeoff to be made with respect to humidity control and it is certainly beneficial to have a high humidity environment for fruits and vegetables with regard to water loss, but leaves the produce more susceptible to disease; another important management decision for producers.
Evaporative cooling is of specific interest to engineers concerned with the efficiency and energy demands of post-harvest storage. While providing a humid environment required for storage, humidifiers also offer the potential to provide some cooling. However, this is only the case with mechanical humidifiers and not with traditional humidifiers that use a heating element to evaporate water. The theory behind cooling effect from evaporation is simple: as water evaporates it absorbs latent heat from the surroundings (notably air) and as a result the ambient temperature is reduced. This phenomenon is precisely why a human being feels cooler when sweating.
In the context of food storage, much research has been done to explore the effects that evaporative cooling may have for extended preservation by counteracting the previously discussed activities of stored produce. In a study which examined the benefits of evaporative cooling for tomato storage by Mordi and Olurunda (2003), an average drop of 8.2°C in comparison to ambient temperature was observed. Also, an increase in relative humidity of 36% was experienced. These changes in air quality had a significant impact on the storage life of the tomatoes as within the evaporative cooling unit, the tomatoes were stored for 11 days in comparison to 4 days of ambient conditions storage. Another study by Dadhich et al. (2008) also demonstrated significantly improved storage life for a number of different commodities due to a simple evaporative cooling technology. In their comparative study between an evaporative cooling environment (0.7 m3 brick structure) and ambient conditions, % weight loss and visual quality were the factors of concern. After 7 days, produce within the evaporative cooler retained much more of its moisture as compared to the ambient conditions. For example, carrots lost 5% and 50% of their mass between the respective conditions and coriander leaves lost 15% and 76% of their mass. In a study by Thiagu et al. (1990) it was demonstrated that moisture loss in tomatoes is 6.5 times as great in ambient conditions (28°C – 33°C, 45%-65% RH) as in evaporative cooling conditions (20°C – 25°C, 92%-95%). They also demonstrated that lycopene development is significantly higher (double in this case) in evaporative cooling storage conditions than ambient conditions.
Plastique Frapa Inc., a small humidifier manufacturer has been exploring the benefits of an ultrasonic mechanical humidifier. The unit itself produces droplets in the range of 1-5 microns compared to specialized fine nozzle diffusers that produce droplets in the range of 70-80 microns. The advantage of the smaller water droplets are less condensation and greater evaporation rate. The goal of this design project is to explore and test an ultrasonic unit in order to identify its potential for use in a storage environment. As such, an evaporative cooling system will be design and built the in order to determine the cooling effect, if any, of the unit. Again,
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this cooling effect has the potential to significantly reduce the energy requirements of a storage system. After such tests, the unit will also be tested for its ability to disperse humidity and droplets into a storage volume. A complete storage environment with a controlled atmosphere will, however not be created and such design details are beyond the scope of this project. It should be noted that the unit itself will only be tested for cooling effect, and the ability to provide humidity without making condensation. The hypothesis is that the humidifier will provide significant cooling and little condensation will occur when used.
METHOD AND MATERIALS
THE ULTRASONIC HUMIDIFYING UNIT
The ultrasonic humidifier, as already mentioned, is manufactured by a small humidifier company called Plastique Frapa. The unit comprises of a water reservoir, a fan, and a small mechanical vibrating transducer. The reservoir is attached to the transducer by a small hose and provides water, through a slight pressure head difference, at such a rate that there is always a thin layer of water sitting on top of the transducer. When the unit is operating, the transducer vibrates ultrasonically with a frequency of roughly 1.65MHz. These mechanical oscillations break the thin layer of water into extremely fine water droplets. The water in the reservoir completes a circuit with the transducer and the fan and is controlled through a simple pinch float mechanism. When the water level in the reservoir is too low, the circuit is broken and the unit is no longer in operation. The unit also has very low power consumption of 48 watts and more specifically operates at a voltage of only 48 volts and a current of only one amp.
DESIGN OF A MIXING CHAMBER
There are two reasons why a mixing chamber is a key component to a storage system design. The first is to continuously provide moisture and air into the control volume and the second is to properly disperse it. Properly dispersing the water droplets outside the control volume is hypothesised to reduce the amount of condensation occurring within the control volume. Dispersing the air and water droplets outside the control volume also has the advantage of keeping the heat loss from the mixing fan outside the control volume. Nevertheless, the hypotheses and assumptions about the benefits of using a mixing chamber were verified by designing and building an evaporative cooling system without a mixing chamber. The amount of cooling and condensation that occurred with and without a mixing chamber will be compared and evaluated. The results of these tests will be presented and discussed in the following section of the paper.
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Design And Testing Of An Evaporative Cooling System