Page 22

FPREF 13..14

146 2 Mass transfer with Ti the temperature in the space, Te the temperature outdoors, both in K, and z the height. Between equally warm spaces at different heights coupled by a flow path, the pressure difference with the outdoors is: 1 Δ12…Pa ρgz† ˆ Pa1 Pa2 3460 … z1 z2 † Te 1 Ti where z1 and z2 are the vertical distances from the zone points to a lower horizontal reference plane. If the spaces are at different temperatures (Ti,1, Ti,2), the equation changes to: Δ12 … Pa ρgz† ˆ Pa1 1 Pa2 3460 … zo z1 † Te 1 Ti;1 ‡ z2 zo … † 1 Te 1 Ti;2 with zo the height of the horizontal plane where the temperature changes. 2.2.6.3 Large openings Open doors and windows act as large openings, of which the air permeance (Ka) changes with the driving force. While wind and fans give uniform flows, thermal stack activates a double flow, from warm to cold above and from cold to warm below the central horizontal, with their sum being zero (Figure 2.10). In other words, the same amount of air moves in both directions. The question is how to calculate the air permeance? For wind and fans, the airflow is: p Ga ˆ CfBH ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ρaΔPa …kg=s† (2.31) giving as air permeance: Ka ˆ Cf BH ffiffiffiffiffiffiffi 2ρa p ΔP0:5 a (2.32) with Cf a flow factor with value 0.33 to 0.7, B the width and H the height of the opening. h/2 1 2 z Fig. 2.10 Large opening, uniform flow by wind and fans (left), and stack-induced flow (right)


FPREF 13..14
To see the actual publication please follow the link above