The BHP curve of a fan is the part of the fan performance chart that shows how brake horsepower (BHP) varies with airflow at a given speed. It is plotted alongside the pressure and efficiency curves and is essential for selecting the correct motor size and predicting energy consumption in industrial and mining ventilation systems.
Brake horsepower (BHP) is the mechanical power delivered to the fan shaft, after motor and drive losses. It represents the power the fan actually requires to move air at a particular flow and pressure. When manufacturers test a fan in a laboratory, they measure airflow, pressure and shaft power at many operating points. From these data, they construct three key curves: pressure vs flow, efficiency vs flow and BHP vs flow at each speed.
On a typical performance chart, the BHP curve is plotted with airflow on the horizontal axis and BHP on the vertical axis. At low flows, the fan may require relatively low power because either the pressure developed is low or the flow is throttled. As airflow increases towards the fan’s design point, BHP usually rises because more air is being moved against system resistance. Near the upper end of the curve, BHP may increase steeply, especially for certain blade shapes, indicating that operating the fan in this region would require a much larger motor and may be inefficient.
For industrial and mining engineers, the BHP curve is used together with the system curve. The system curve describes how much pressure is needed at different airflows in the ductwork, filters, kilns or mine airways. The intersection of the fan pressure curve and the system curve defines the operating point. By projecting vertically from this airflow to the BHP curve, you can read off how much brake horsepower the fan will require at that duty.
This information is critical when choosing a motor size. The selected motor must have a rated horsepower at least as high as the fan BHP at the design duty, with additional margin for tolerances, wear and possible operating changes. If the motor is too small, it will overload and trip; if it is far too large, capital and running costs will be higher than necessary.
In summary, the BHP curve of a fan is the graph showing how the mechanical power required at the shaft changes with airflow. It is a key tool for correct motor selection, assessment of energy use and safe application of axial and centrifugal fans in industrial and mining ventilation.
