Book - School of Science and Technology

354 VentilationTable 13.6 Air volumes for natural ventilation due to stack effectEffectiveheight(m)Air volume (litre/s per m 2 ) free opening for temperature difference (K)2 4 6 8 10 15 20 25 302 240 340 420 480 540 660 760 850 9304 340 480 590 680 760 930 1070 1200 13106 420 590 720 830 930 1140 1310 1470 16108 480 680 830 960 1070 1310 1520 1700 186010 540 760 930 1070 1200 1470 1700 1900 208015 660 930 1140 1310 1470 1800 2080 2320 255020 760 1070 1310 1520 1700 2080 2400 2680 294025 850 1200 1470 1700 1900 2320 2680 3000 329030 930 1310 1610 1860 2080 2550 2940 3290 3600Notes1. Areas of openings are assumed to be equal, i.e. upper ˆ lower.2. Openings are assumed to be 45% effective.Wind effectsNatural ventilation resulting from wind is unpredictable and any attempt to quantify theeffect should be based upon data which give some indication of the frequency of occurrence.Statistics which are reproduced in the Guide Section A2 relate to the meteorologicalwind speed, i.e. as measured in open country at a height of 10 m. For use in practice,factors are published which enable this speed to be adjusted to suit other heights andalternative topographies. In addition, isopleths (contours of hourly mean wind speeds) areavailable together with multipliers which allow frequency of occurrence to be estimated.In the context of ventilation, the design criterion must be aimed at the lowend of thescale and, for a speed which will be exceeded for 80% of the time, a figure of about 3 m/swould appear to be a reasonable choice. Table 13.7 has been prepared, from the simpleTable 13.7 Air volumes for natural ventilation due to windAir volume (litre/s per m 2 ) free opening with notional wind speed of 3 m/sOpenings normal to windOpenings at angle to windEffective heightabove ground(m)OpenCountry Urban City Country Urban CityWith windbreaksOpenWith windbreaks2 1260 990 690 440 690 540 380 2404 1420 1130 820 550 720 620 450 3006 1520 1230 900 630 830 670 490 3408 1610 1300 970 690 880 710 530 37010 1660 1360 1040 740 910 740 570 40015 1780 1470 1140 850 970 800 620 46020 1870 1560 1220 930 1020 850 670 51025 1940 1630 1290 1000 1060 890 700 55030 2000 1690 1350 1060 1090 920 740 580Notes1. Areas of openings are assumed to be equal, i.e. inlet ˆ outlet.2. Wind speed is 3 m/s (exceeded for 80% of time).3. Wind speed is at notional height of 10 m in open country: other speeds are derived from this datum.