system resistance curve

The system resistance curve is one of the most useful concepts in mining ventilation because it explains why a fan that “worked yesterday” can underperform today. In simple terms, the system curve describes how much static pressure (Ps) the ventilation system requires at different airflow (Q) levels. The actual operating point is where the fan curve intersects the system curve—this is your real Q@Ps.

In mines, the system curve is not fixed. It moves as the operation changes:

• Duct extension increases friction loss and raises pressure demand at the same airflow.
• Bends and fittings add localized losses that can be significant in ducted headings.
• Leakage effectively wastes airflow and can make the “required” pressure higher to achieve the same end-of-duct airflow.
• Regulators and doors change network resistance and redistribute airflow across circuits.

When resistance increases, the system curve shifts upward. The intersection with the fan curve moves, typically toward lower airflow and higher pressure. That is why relying on a free-air airflow number (near 0 Pa) is risky: underground systems rarely operate near zero resistance.

For auxiliary duct ventilation, the system curve is heavily influenced by duct length, diameter, condition, and leakage. This is also why duct maintenance is often the cheapest “performance upgrade” available—sealing joints and reducing sharp bends can move the system curve downward, increasing delivered airflow without changing the fan.

For main and booster applications, network changes and operational controls can shift the curve and affect distribution. Booster fans must be evaluated carefully because changes in one district can influence neighboring circuits and create recirculation risk if operating points are not managed.

A VFD provides a practical way to respond to curve movement by adjusting fan speed to maintain required airflow or pressure within a safe operating region. Understanding the system resistance curve turns ventilation troubleshooting from guesswork into predictable cause-and-effect.