Book - School of Science and Technology

Methods of ventilation 355relationship Q ˆ A V, using this wind speed for the columns headed `open country' andadjusted values for other locations. A resistance factor, as noted in the following paragraph,has been applied to produce the volumes listed.Resistance factorsTheoretical air flowquantities due to temperature and wind effects are not achieved inpractice due to flowresistances and the physical arrangement of inlet and outlet openings.For the flowdue to temperature effects, a correction factor of 0.45 was used in preparingTable 13.6 and, for Table 13.7, correction factors of 0.55 and 0.3 were used for the firstand second sets of columns, respectively. These factors are probably appropriate tobuildings where simple louvred ventilators are used but should not be confused with thecomprehensive performance data published by specialist manufacturers of ventilators forthe industrial category.Other considerationsThe combined results of temperature difference and wind effects may be estimated bytheir addition, with respect to sign: that is to say, with the assurance that both are actingto produce air flow in the same direction (which is by no means always the case!).Similarly, the matter of a positive wind effect upon an exposed face of a building and anegative effect upon the lee side must be considered, particularly in circumstances whereopenings may not be of the same size. The entire subject cannot be dealt with in theabstract since the variables are complex and, to some extent, depend upon the shape of thebuilding. The essential factors are given in the Guide Section A4.Natural inlet and mechanical extractA mechanical extract system will function irrespective of wind and temperature differenceand will be positive in action. Since the air to be extracted from the space must be replacedand the means provided for admission of outside air will present some resistance to flow,leakage inward from surrounding spaces is more likely than leakage outward. In consequence,escape of steam, fumes and noxious vapours generated within the ventilated space is less likelythan would be the case if reliance were placed upon natural extract alone.ExtractgrilleExtractduct tofanCorridorOutsideairinletGilled tube incasing accessiblefor cleaningFigure 13.7 Natural inlet and mechanical extract