A plenum chamber is a mechanical ventilation system that is used to equalize irregularly distributed pressure. The chamber contains a gas or a fluid, most commonly air, pressurized higher than the surrounding atmospheric pressure, which expels undesirable air. Some applications include hovercraft, some organs, bagpipes and supercharged piston engines.
A plenum chamber operates on the opposite principle of a vacuum chamber. A positively pressurized plenum chamber naturally forces air out, causing the air outside the chamber to replace it. Plenum chambers are sometimes used in homes and businesses instead of duct-based ventilation systems. They can also be used in engines and machinery to keep things cool.
Supercharged piston engines utilize a large plenum chamber. Because the cylinders in an engine require air in varying increments, direct ducting does not work because the nearest cylinder receives more airflow and becomes cooler. The plenum chamber prevents significant shifts in pressure and provides equal distribution of air to all of the cylinders.
Plenum chambers are used in hovercraft. The plenum chamber takes the air from the lift fans and distributes it evenly. This allows the hovercraft to travel smoothly, regardless of the length of the direct route. Plenum chambers are used in musical instruments, such as certain types of organs and bagpipes that require steady airflow.