end-of-duct airflow

End-of-duct airflow is the airflow that actually reaches the working face in a ducted auxiliary ventilation system. In underground mining, this is the number that matters for dust control, gas dilution, and heat/humidity management—yet it is often confused with free-air airflow or fan outlet airflow. A fan can be “rated high” and still deliver poor end-of-duct airflow if the duct system demands more static pressure (Ps) than the fan can provide at the required airflow (Q).

The main reasons end-of-duct airflow drops underground are predictable:

• Duct extension: as headings advance, longer ducts increase friction loss and raise Ps demand.
• Additional bends and fittings: each bend, reducer, and connector adds local pressure loss.
• Duct leakage: poor joints, tears, and loose couplings can steal a large portion of airflow before the face.

That is why selection must target the end-of-duct duty point (Q@Ps at the face), not a free-air number near 0 Pa. A practical approach is to define the required face airflow, estimate total duct resistance under expected conditions, and choose equipment whose curve meets the duty point with usable margin for planned duct extension and realistic leakage deterioration.

Operations should treat duct condition as part of the ventilation “equipment.” Simple actions—sealing joints, replacing damaged sections, reducing sharp bends, and using better duct quality—can move the system resistance curve downward and increase end-of-duct airflow without changing the fan.

Finally, verify performance with measurement rather than assumptions. Establish where and how airflow is measured (at duct end, near face, or at a standardized location), and track changes as the heading advances. A VFD can help maintain stable delivery by adjusting speed as resistance grows, avoiding the cycle of under-ventilation followed by oversized, noisy operation.