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136 2 Mass transfer The resulting stack between the two points becomes: pT;12 ˆ Pstack2 Pstack1 ˆ g Pa;hoh 1 …RaT† m2 1 …RaT† m1 If the temperatures T1 and T2 are different but constant between height zero and height h, while the composition of the air remains the same, stack simplifies to: g Pa;… Ra1T1 Ra2T2 †ho h pT; 12 Ra1T1Ra2T2 ρa0 g Ra1θ1 Ra2θ2 … †h Ram12Tm12 ρagβ θ1 θ2 … †h (2.14) with β(= 1/Tm12) the compressibility of air. If, in contrast, the temperature is constant but the composition, in this case being the vapour concentration, differs, stack changes to: pT;12 ρaRagβz 1 Ra2 1 Ra1 (2.15) In case leaks connect air volumes, a neutral plane, stack zero, appears somewhere between the lowest and highest leak. In the absence of other pressure differentials, the air above the neutral plane moves from warm to cold or higher to lower vapour concentration, while the air below goes the other way. Where that neutral plane sits will depend on the leak distribution and size. Except for high-rises, thermal stack air pressure differences are small. Vapour concentration related stack is even negligible: 20 °C indoors and0°Coutdoors inwinter gives as thermal stack over a 2.5m high room (ρa0=1.2 kg/m3, g=9.81 m/s2, β =1/273.15K1, θm12θ0=20 °C, z=2.5m) pT ˆ 1:2 9:81 1=273:15 20 2:5 ˆ 2:15 Pa. Instead, for a 250m high-rise, thermal stack between the lowest and highest floors will touch 215 Pa, a value as large as the pressure difference between wind- and leeside for a 62 km per hour wind. Thermal stack, however, ismore stable over time than the wind pressure differences are. 2.2.2.3 Fans Fans are part of any air heating, air conditioning or forced ventilation system. The pressure differentials they create are usually quite stable over time. 2.2.3 Air permeances A distinction must be made between open porous materials, air-open layers, unintended and intended leaks. Open porous materials include no-fines concrete, mineral fibre, wood-wool cement and others. Air-open layers include sidings and finishes made of scaly or plate elements such as tiled, slated or corrugated sheet roof covers, lathed ceilings and layers composed of board or strip-like materials, such as plywood, OSB, insulation layers and underlays (Figure 2.3). Unintended leaks include construction parts, boards and other elements with joints in between, cracks formed when tensile strength is exceeded, microcracks between


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