An Introduction to Solid Desiccant Cooling Technology
Air-conditioning industry is facing important environmental and regulatory challenges, such as the phaseout of CFCs refrigerants, the requirement of higher ventilation rates and improved air quality by new design standards. The result is the development o
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1 Introduction Although a thermal comfort zone has been defined by standards in a psychrometric chart, the actual sensation of thermal comfort is an individual response, which requires input from physical, physiological, and even psychological nature [47]. Among other factors, skin evaporation has a major impact over the skin temperature. It has been estimated that evaporation accounts for as much as 20 % of the energy balance in a human body [7]. Moreover, it is well known that allergic and respiratory diseases are closely related to inadequate humidity levels. The most common air-conditioning system, the vapor compression system, operates running air through a cooling coil which operates below air dew point temperature, thereby promoting air cooling and dehumidification in a single process. In a later section, it will be shown that this technique might result in an ineffective and costly humidity control method. In addition, the increasing concern with environmental impacts and indoor air quality over the last three decades has challenged HVAC designers to develop new technologies to assist or even replace the conventionally used vapor compression systems. Although the Montreal Protocol has recommended some traditionally employed refrigerants to be phased out, potential replacement substances still feature issues related to leak detection, flammability and global warming potential (GWP) [53]. In face of the economic, regulatory, and environmental challenges that have been imposed to the air-conditioning industry, evaporative cooling systems represent a promising solution regarding the ozone layer depletion and the
C. E. L. Nóbrega (&) Centro Federal de Educação Tecnológica, Celso Suckow da Fonseca, CEFET-RJ, Rio de Janeiro, Brazil e-mail: [email protected] N. C. L. Brum Universidade Federal do Rio de Janeiro, COPPE, Rio de Janeiro, Brazil
C. E. L. Nóbrega and N. C. L. Brum (eds.), Desiccant-Assisted Cooling, DOI: 10.1007/978-1-4471-5565-2_1, Springer-Verlag London 2014
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C. E. L. Nóbrega and N. C. L. Brum
indoor air quality standards. It is estimated that there are 30 million of evaporative coolers currently working around the world [10]. Evaporative coolers rely on the water evaporation into an air stream to lower the air temperature. The sensible cooling corresponds to the water enthalpy difference in the vapor and liquid states. Although it offers an alternative from the economical and environmental viewpoints, the use of evaporative coolers is, however, limited to places with low local average humidity levels. Since the temperature drop of the airstream is usually low, evaporative systems typically have to handle high airflow rates, so as to meet a given thermal load. Accordingly, the air movement in the conditioned space is often higher than if vapor compression system is used. This usually results in an increased convective coefficient, improving the thermal comfort sensation. In any case, the lower the absolute humidity of the airstream, the higher will the cooling capacity evaporative unit. If
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