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134 2 Mass transfer Moist air, a mixture of dry air and water vapour (suffix a), has as ideal gas equation: PaV ˆ maRaT Concentration thus becomes: ρa ˆ Pa=…RaT† In both relations, Pa is the total air pressure in Pa, equal to the sum of the partial dry air and partial vapour pressure (Pa ˆ pda ‡ pv). Compared with dry air, the presence of water vapour modifies the air mass ma, the gas constant Ra and the concentration. However, for temperatures below 50 °C the effect is so small that the following holds: Ra Rda; Pa pda; ρa ρda or: Pa=…RaT† pda=…RdaT† Air transfer across assemblies occurs when wind, stack or fans create air pressure differentials between rooms and with the outdoors. For this to happen, the enclosure and building fabric must be air-permeable, which presumes the presence of airpermeable materials, cracks, overlaps, intended leaks, and so on. 2.2.2 Air pressure differentials 2.2.2.1 Wind Wind pressures are given by: Pw ˆ Cp ρav2 2 0:6 Cpv2 (2.11) an equation that follows from Bernoulli’s law applied to a horizontal wind whose velocity drops from a value v to 0 when running into an infinite large obstacle, multiplied by a pressure factor Cp that accounts for the difference between bumping against an infinite as opposed to a finite obstacle. Finiteness, in fact, deflects the air flow at the upper and side edges, where it forms vortexes, while a lee zone develops in front of and behind the obstacle. The pressure factor couples the real pressure at a point on a surface to a reference wind velocity, in principle measured in an open field at a height of 10 m. Of course, other references can be used depending on the situation. For buildings, pressures on the façades often correlate to the velocity above the ridge. A change in reference alters the pressure factors. Also, wind direction, building location, building geometry and the façade spot considered will all affect the pressure factor. A positive Cp means overpressure, while negative Cp is underpressure. Overpressure is found at the wind side, and underpressure at the lee side and along faces more or less parallel to the wind (Figure 2.2). Wind pressure differences are extremely variable and may change sign regularly.


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