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A mathematical model for direct evaporative cooling air conditioning system

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ABSTRACT

Air conditioning systems are responsible for increasing men's work efficiency as well for his comfort, mainly in the warm periods of the year. Currently, the most used system is the mechanical vapor compression system. However, in many cases, evaporative cooling system can be an economical alternative to replace the conventional system, under several conditions, or as a pre-cooler in the conventional systems. It leads to a reduction in the operational cost, comparing with systems using only mechanical refrigeration. Evaporative cooling operates using induced processes of heat and mass transfer, where water and air are the working fluids. It consists in water evaporation, induced by the passage of an air flow, thus decreasing the air temperature. This paper presents 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. It also presents some results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer coefficient and to compare with the mathematical model.

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    • "They found that there exists an optimum length of the air channel, which results in the lowest temperature, where the system performance can be further improved by optimizing the operation parameters. Camargo et al. [4] presented basic principles of the evaporative cooling processes for human thermal comfort and developed mathematical equations for thermal exchanges. Wu et al. [5] theoretically studied the Influences of air frontal velocity and thickness of pad module on the cooling efficiency of a DEC. "
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    ABSTRACT: Performance of a direct evaporative cooler (DEC) was numerically studied at various outdoor and indoor air conditions, with geometric and physical characteristics of it being extracted based on thermal comfort criteria. For this purpose, a mathematical model was utilized based on the equations of mass, momentum, and energy conservation to determine heat and mass transfer characteristics of the system. It is found that the DEC can provide thermal comfort conditions when the outdoor air temperature and relative humidity (RH) are in the range of 27–41 oC and 10–60%, respectively. The findings also revealed that by raising the RH of ambient air, the system will reach the maximum allowed RH faster and hence a smaller heat exchanger can be used when the ambient air has higher RH. Finally, performance of the DEC in a central province of Iran was investigated, and a design guideline was proposed to determine size of the required plate heat exchangers at various operating conditions.
    Full-text · Article · Aug 2014 · Journal of Thermal Science and Engineering Applications
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    • "They found that there exists an optimum length of the air channel, which results in the lowest temperature, where the system performance can be further improved by optimizing the operation parameters. Camargo et al. [4] presented basic principles of the evaporative cooling processes for human thermal comfort and developed mathematical equations for thermal exchanges. Wu et al. [5] theoretically studied the Influences of air frontal velocity and thickness of pad module on the cooling efficiency of a DEC. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Performance of a direct evaporative cooler (DEC) was numerically studied at various outdoor and indoor air conditions, with geometric and physical characteristics of it being extracted based on thermal comfort criteria. For this purpose, a mathematical model was utilized based on the equations of mass, momentum, and energy conservation to deter-mine heat and mass transfer characteristics of the system. It is found that the DEC can provide thermal comfort conditions when the outdoor air temperature and relative hu-midity (RH) are in the range of 27–41 C and 10–60%, respectively. The findings also revealed that by raising the RH of ambient air, the system will reach the maximum allowed RH faster and hence a smaller heat exchanger can be used when the ambient air has higher RH. Finally, performance of the DEC in a central province of Iran was inves-tigated, and a design guideline was proposed to determine size of the required plate heat exchangers at various operating conditions. [DOI: 10.1115/1.4028179]
    Full-text · Article · Aug 2014 · Journal of Thermal Science and Engineering Applications
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    • "Several authors dedicated their researches to the development of direct and indirect evaporative cooling systems. Watt [1] developed the first serious analyses of direct and indirect evaporative systems; Leung [2] presents an experimental research of the forced convection between an air flow and an inner surface of a horizontal isosceles triangular duct; Halasz [3] presented a general dimensionless mathematical model to describe all evaporative cooling devices used today; Camargo, Cardoso and Travelho [4] developed a research, where a thermal balance study for direct and indirect cooling systems was developed; Camargo and Ebinuma [5] presented the principles of operation for direct and indirect evaporative cooling systems and the mathematical development of the equations of thermal exchanges, allowing for the determination of heat transfer convection co-efficients for primary and secondary air flow; Dai and Sumathy [6] investigated a cross-flow direct evaporative cooler, in which the wet honeycomb paper constitutes the packing material and the results indicate that there exists an optimum length of the air channel and the performance can be improved by optimizing some operation parameters; Liao and Chiu [7] developed a compact wind tunnel to simulate evaporative cooling pad-fan systems and tested two alternative materials; Al- Sulaiman [8] evaluated the performance of three natural fibers (palm fiber, jute and luffa) to be used as wetted pads in evaporative cooling; Camargo, Ebinuma and Silveira [9] presents a thermoeconomic analysis method based on the first and second law of thermodynamics and applied to an evaporative cooling system coupled to an adsorption dehumidifier; Hasan and Sirén [10] investigated the performance of two evaporatively heat exchangers operating under similar conditions of air flow and inlet water temperatures; Camargo, Ebinuma and Cardoso [11] presents the basic principles of the evaporative cooling processes for human thermal comfort and presents the mathematical development of the thermal exchanges equations, allowing the determination of the effectiveness of saturation. This paper develops a mathematical model for direct evaporative cooling system and presents the experimental results of the tests performed in a direct evaporative cooler that took place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, located in the city of Taubaté, State of São Paulo, Brazil. "
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents 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. It also presents the results of experimental tests in a direct evaporative cooler that take place in the Air Conditioning Laboratory at the University of Taubaté Mechanical Engineering Department, and the experimental results are used to determinate the convective heat transfer co-efficient and to compare with the mathematical model.
    Full-text · Article · Nov 2005 · International Journal of Refrigeration
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