Surface Environment, Mine Ventilation and Underground Hazards · Mine Ventilation · 2 marks · Multiple choice
An exhausting fan on a 0.60 m diameter duct carries 4.0 m³/s. Static pressure behind it is −97.5 mm water gauge. An outlet evasee has inlet-to-outlet area ratio 1:4 and efficiency 60%. Choose the resulting fan static pressure in mm water gauge.
The inlet speed is 4/(π×0.60²/4)=14.15 m/s; the outlet speed is one quarter of this. The recoverable velocity pressure is proportional to v₁²−v₂² = v₁²(1−1/16). At 60% efficiency this is about 6.8 mm water gauge under standard-air assumptions. On the exhaust side that recovered pressure permits an additional negative suction pressure: approximately −97.5−6.8≈−104.3 mm, matching A. Adding the recovery algebraically towards zero would reverse the exhaust-pressure sign convention.
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