. Transactions of the Institution of Mining Engineers . snot at all affected by the quantity of air delivered by the fan. For thisreason the static water-gauge in the fan-drift of such a fan is greatestwhen the fan-drift is completely shut off. Fans with blades curved back-wards or forwards behave differently, as the theoretical depression theyare capable of producing is not independent of the quantity of air pass-ing through the fan. The effect of increase of quantity in a fan withblades trending backwards is to reduce the theoretical depression, andin the case of a fan with forward-curved bl
. Transactions of the Institution of Mining Engineers . snot at all affected by the quantity of air delivered by the fan. For thisreason the static water-gauge in the fan-drift of such a fan is greatestwhen the fan-drift is completely shut off. Fans with blades curved back-wards or forwards behave differently, as the theoretical depression theyare capable of producing is not independent of the quantity of air pass-ing through the fan. The effect of increase of quantity in a fan withblades trending backwards is to reduce the theoretical depression, andin the case of a fan with forward-curved blades the opposite effect isproduced. The static depression in the fan-drift of the latter type offan will not be a maximum when the fan-drift is entirely closed. Asthe fan-drift is gradually opened, the static water-gauge will riseslightly, due to the added forward velocity of the air as it leaves the fan-tips. The greater the quantity of air delivered by the fan, the greater willbe the theoretical water-gauge produced by the fan. As against this, the. lao 1^5 ISO 135 REVOLUTIONS PER MINUTE. Fig. 10.—Distribution of Quantity of Am between Two Fans RunningTogether in Parallel at Wellesley Colliery. greater the volume of air, the greater will be the proportion of thewater-gauge lost in the fan itself. Eventually the resistance of the fanwill wipe out the influence of the forward blades, and the static water-gauge in the fan-drift will commence to decrease. On the other hand, the water-gauge produced by a fan with backwardcurved blades will not rise at all as the fan-drift is opened, but willcontinue to fall away gradually. The fan with radial blades will occupy a position somewhere inbetween the other two types of fans. The fan with blades curved forwards may be likened to a direct-current dynamo with compound winding, the fan with backward bladesto a shunt-wound dynan^o, and the radially-bladed fan to a dynamoseparately excited with constant current. With regard to the points
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