A high velocity fan is often the best choice for delivering fresh or conditioned air to every destination on a large ventilation circuit. Ventilation is just one possible application for high velocity blowers. Many industrial buildings, large office buildings, very large homes, schools and other large buildings are likely to make use of a high velocity blower.
The automotive industry, metal parts fabrication industries and all other industries in which air quality can be compromised by processes that involve chemicals or hazardous fumes may need high velocity fans to reduce risks to worker health. There is no standard industry measurement or agreement concerning what fan size or construction or speed (in terms of RPM and air speed) constitutes a high-velocity fan.
A high velocity fan in a multi-story building’s HVAC system will look different from a small, portable consumer fan that is marketed as a “high velocity fan.” For this reason, it is important to take note of a fan’s specifications in advance of purchase.
Regardless of the variability of standards in the classification of fans, it is always true that a given high velocity fan will fit into one of two fan configuration categories. Axial fans are air blowers whose blades spin around an axis; their motion is similar to that of hands on a clock. Each fan blade features a degree of curvature. This curvature causes variances in air pressure on either side of the fan as its blades turn.
The differences in pressure cause the movement of air, which naturally tries to equalize imbalances in pressure. This equalization effort creates airflow, and as the fan continuously provides an imbalance of pressure, the air continuously tries to balance it, creating a steady flow of air. High velocity fans do this, but at higher-than-normal speeds and with larger-than-normal fans.
Radial fans function by achieving the same unequal pressure distribution but in a different way. Radial fan blades also spin around a center of gravity, but their motion more resembles the forward spinning of waterwheel panels than clock hands. More importantly, they spin within a circular enclosure, which causes the pressurized air to become subject to centrifugal motion. This added property allows for radial fans to generate more pressure than axial fans when operating at the same speed. For this reason, high velocity fans are often designed in radial configurations.