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Article Excerpt
INTRODUCTION
In general the air temperature levels in the South of Portugal (the Algarve region) are not very low during winter, are very high during summer, and are moderate during spring and autumn (there are also moderate winter and summer days), the time of year that represents the wider period of school activities. This research analyzed and developed a forced ventilation system, to be used in spring and autumn (and also on moderate winter and summer days), being of easy implementation in all school building offices, with low initial investment and low consumption level. This ventilation system, which worked in general in spaces not subjected to direct solar radiation or subjected to low direct solar radiation, with acceptable thermal comfort levels, presents acceptable local thermal discomfort and air quality levels.
In order to obtain good thermal comfort and air quality in indoor environments used as educational spaces for young people, the inlet and outlet air, through strategically placed points, are necessary. When entering the spaces, the clean air should create a pleasant micro-climate around the occupants, assure good air quality in the breathing area, and extract contaminants released by the occupants.
In order to evaluate the indoor air and thermal quality, micro-models and macro-models can be used (see, for example, Heinsohn [1989]). In the micro-models, using space discretizing grids, environmental variables are calculated in all grid nodes using computational fluid dynamics (CFD), while in macro-models these variables are calculated inside all spaces and bodies.
In order to climatize the indoor space, the heating, ventilation, air-conditioning, and radiation systems are usually considered. These systems can, in general, operate in accordance with basic piston flow, perfect mixing, short circuiting, hybrid, displacement, natural, or cross-flow ventilation topologies or in combinations of these. To study these systems with these topologies, experimental chambers or adapted full-scale chambers can be used. Koskela et al. (1998), for example, presented an experimental study in a test room to evaluate the displacement airflow efficiency, and Hashimoto et al. (2000) measured airflow profiles to evaluate the energy consumption and thermal comfort for a new heating, ventilation, and air-conditioning system using low air temperature. In other studies, when the full-scale compartment is not available, the scale model is also used (see, for example, Sandberg and Elvsen [2004]).
A combination of CFD numerical models and full or other scale models is frequently shown. Teodosiu et al. (2000) and Gobeau and Saunders (2002), for example, showed applications of this technique.
The characterization of the expected thermal comfort conditions for a given environment is frequently made through the evaluation of the human thermal sensation using the predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) indices described in Fanger (1970). This kind of evaluation is used, for example, in CR 1752, Ventilation for Buildings: Design Criteria for the Indoor Environment (CEN 1998) and international standards like ISO 7730, Moderate Thermal Environments--Determination of the PMV and PPD Indices and Specification of the Conditions for Thermal Comfort (ISO 1993). The above-mentioned CR 1752 (CEN 1998) defines three comfort categories (A, B, and C), establishing limits for PMV and PPD indexes. These indexes consider the human body as a whole and are able to make a good analysis of the thermal sensation of an occupant.
A person can express thermal comfort sensations and present local thermal discomfort problems. Local thermal discomfort sensations in localized regions of the body may occur, produced namely by incident airflows from ventilators, due to their intrinsic characteristics such as exit air velocity and airflow symmetry as well as their location in relation to the occupants. According to Fanger et al. (1988), local thermal discomfort depends on the local air temperature and velocity and turbulence intensity. This draft risk index is also presented in ISO 7730 (ISO 1993) and CR 1752 (CEN 1998), defined in the latter standard as three local thermal discomfort categories (A, B, and C). According to Fanger and Pedersen (1977), local discomfort sensations associated with air velocity frequency fluctuations, are verified in frequencies between 0.3 and 0.5 Hz and according to Zhou and Melikov (2002) and Zhou et al. (2002) are verified to equivalent frequencies between 0.2 and 0.6 Hz.
The airflow rate inside an occupied compartment can be calculated using different recommendations and methodologies presented in national and international standards (see Olesen [1997]). According to ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality (ASHRAE 2004), for offices in buildings the recommended values are 2.5 L/s (5.3 [ft.sup.3]/min) per person (for outdoor air rate) and 8.5 L/s (18 [ft.sup.3]/min) per person (for combined outdoor air rate). The Portuguese normalization presented in D.-L. n[degrees] 79.2006 of April 4th, Regulamento dos Sistemas Energetico de Climatizacao em Edificios (DR 2006) defines an airflow rate to each occupant and kind of space of 35 [m.sup.3]/h (9.7 L/s [20.6 [ft.sup.3]/min]) per occupant. CR 1752 (CEN 1998), where the airflow rate is based on the occupants' comfort level, was also analyzed. In this methodology, which defines three quality levels (Category A, with 10% of dissatisfied persons; Category B, with 20% of dissatisfied persons; and Category C, with 30% of dissatisfied persons), occupants and materials existing inside the compartment are considered pollution sources. This perspective, based in olf and decipol units, is presented in detail in Fanger (1988). Categories A, B, and C are recommended, respectively, for 16 L/s (33.9 [ft.sup.3]/min), 7 L/s (14.8 [ft.sup.3]/min), and 4 L/s (8.5 [ft.sup.3]/min) per olf. In all categories it is also necessary to consider the...
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