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2.2 Air 147 With thermal stack the flow in both directions equals: Ga1 ˆ Ga2 ˆ C´ f B 3 s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ρagPaH3 Ra 1 T1 1 T2 …kg=s† (2.33) The air permance thus is: ΔpT;max ˆ gPaH=Ra T1 e T1 2 T1 e ‡ T1 1 3450H T1 1 T1 2 : Ka1 ˆ Ka2 ˆ C´ f BH p ffiffiffiffiffi ρa 3 0:5 3450H 1 T1 1 T2 (2.34) with Ra the gas constant of air and C´ f the flow factor for this two-way move. The equation gives a parabolic air velocity along the opening’s height, a consequence of Bernouilli’s law, which for a flow path at height z above the floor states: ρa1gz ˆ ρa2v2a ;z 2 ‡ ρa2gz giving: va;z ˆ s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ρa1 2gz ρa2 ρa2 The air permeance equation follows from integrating the air velocity over half the height, then using the ideal gas law to convert density into temperature and multiplying the result by the flow factor. 2.2.6.4 The conservation law applied Per node in a building (see Figure 2.11), the algebraic sum of the n inflows and the m outflows must be zero: Xm‡n iˆ1 Ga;i ˆ 0 (2.35) The living room, zone 2 in the figure, has two windows, while doors separate it from the kitchen, zone 1, and the hall, zone 3. All partitions and opaque outer walls are assumed air-tight, which may differ from reality. In a first step the building has no fan-driven ventilation and the indoor and outdoor temperatures are equal. So, only wind matters. Air comes in and goes out of the living room across the operable window sashes (Ga,2-e1, Ga,2-e2) and the door bays with the kitchen (Ga,2-1) and hall (Ga,2-3). Wind pressures outdoors are supposed to be known and all four flows are assumed entering the living room: Ga;ˆ 2-e1 Ka; 2-e1 Pa;e1 Pa;ˆ 2 Ga;2-e2 Ka;2-e2 Pa;e2 Pa;2 Ga;2-1 ˆ Ka;2-1 Pa;1 Pa;2 Ga;2-3 ˆ Ka;2-3 Pa;3 Pa;2


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