Plenum chamber

A plenum chamber is a pressurised housing containing a gas or fluid (typically air) at positive pressure (pressure higher than surroundings). One function of the plenum is to equalise pressure for more even distribution, because of irregular supply or demand. A plenum chamber can also work as an acoustic silencer device.

Examples of plenum chambers include those used with:

• Superchargers
• Hovercraft
• Corliss steam engines
• Raised floors and false ceilings in equipment rooms
• Some organs (to supplement the bellows)^[1]
• A number of aerophones, such as the bag of bagpipes and the slow air chamber of the Native American flute
• Plenum Chamber Anesthetic Vaporizers
• Rocket Motor Combustion Chambers. May have a section near the nozzle that is free of the propellant charge to improve mixing and combustion.

Etymology

Schoolroom ventilation on the plenum system

The term "plenum" derives originally from classical theories and the notion that "Nature abhors a vacuum". These gave rise to the notion of 17th century 'plenum' as the opposite of vacuum, and all things "being either Plenum or Vacuum".^[2]

By the 19th century, the development of mechanical fans and industrial machinery had provided another, more technical use. This referred to "a system of artificial ventilation",^[2] which used a pressure raised slightly above atmospheric pressure, in contrast to the "vacuum system" which used a pressure below atmospheric. At a time when high pressure steam or hydraulic systems were well established, these were a distinct set of systems based on low pressure and high volume flows.

Supercharging

V12 engine (non-supercharged), showing plenum chamber

Supercharged piston engines typically use many cylinders arranged in-line and one or two superchargers. Superchargers deliver air at a relatively constant rate, while cylinders demand it in a varying manner, as the valves open and as piston speed varies through the stroke. Simple direct ducting would give problems where the nearest cylinders received more airflow. The pulsating demand from the cylinders would also show problems of either pressure waves in the duct, or a shortage of inlet air towards the end of the inlet phase.

The solution is to provide a large-volume plenum chamber between the inlet and the cylinders. This has two benefits: it evens out the difference in path restriction between cylinders (distribution across space), secondly it provides a large-volume buffer against pressure changes (distribution over time).

For non-supercharged / normally aspirated engines see Airbox.

Norton Classic

Norton Classic with twin-rotor Wankel engine

The Norton Classic was a motorcycle whose air-cooled twin-rotor Wankel engine was developed by David Garside at BSA.^[3][4] Wankel engines run very hot, so Garside gave this air-cooled motor additional cooling air that was filtered and drawn first through the rotors and then through a large plenum chamber into the combustion chambers via carburetors.^[5] The plenum (a sheet-alloy item which doubled as a semi-monocoque spine frame) partly cooled the air, and the transfer of latent heat during the carburation process further reduced the temperature of this fuel-air mixture, but its volumetric efficiency was still somewhat impaired. The cooling air filter was mounted below the steering head (between the forks) to provide a partial ram air effect. (The engine roller-bearings were lubricated by oil-injection, while the rotor tips were lubricated by the fuel/air mixture which carried an oil mist obtained from passing through the interior of the rotors).

Hovercraft

Hovercraft schematic

Practical hovercraft use a peripheral skirt system, where the air from the lift fans is routed to a narrow slot around the edge of the hull, and bounded by a flexible skirt. Distribution of this air from the fans to the periphery is through a large-volume plenum chamber, so as to provide even distribution of airflow without sensitivity to the length of the direct path.^{[citation needed]}

References