ASHRAE Terminology

transportation of a specified airflow to or from the treated space, by ducts or plenums. Air-treatment devices can be added to the distribution system for the purpose of treating the air (e.g., cleaning, heating, cooling, humidifying or dehumidifying, etc.)

modular air-terminal device designed to diffuse air to the treated space from a pressurized plenum through holes or slots in the ceiling surface or the supporting framework. Also see air terminal device.

makeup air delivered directly to the interior plenum of an exhaust hood such that it is introduced vertically downward, typically from the front edge of the hood. Sometimes this kind of makeup air is referred to as an air curtain.

multiple fans in parallel between two plenum sections in an air distribution system.

in air-handling units, the section containing the filters and the outside air/return air dampers and mixing plenum.

the entire air volume of a space or building in which the ventilation air is distributed, including ductwork and plenums. The volume of indoor furnishings, equipment, and occupants must be subtracted from the gross indoor volume that is based on interior dimensions of the space or building.

(also called plenum box and plenum chamber) (1) a compartment or chamber, to which one or more ducts are connected, that forms a part of the air-distribution system and that is not used for occupancy or storage. A plenum often is formed in part or in total by portions of the building. (2) an air compartment that is attached to, or is an integral part of, a forced-air furnace which is designed to either distribute the heated air after it leaves the heat exchanger in the case of a supply plenum or to collect air that enters the return inlet in the case of a return plenum. (3) component forming an interface between a ductwork and one or more air terminal devices, by virtue of its design or by the inclusion of accessories, it can also be used to equalize the pressure/velocity across the air terminal device.

fan assembly consisting of a single inlet impeller mounted perpendicular to airflow that pressurizes a plenum chamber in an air-distribution system. The impeller is typically an airfoil or backward-inclined design.

layers of air at different temperatures or different velocities flowing through a duct or plenum system and affect fan and/or pump performance in the distribution system.

an air-distribution system that uses an underfloor plenum (open space between the structural concrete slab and the underside of a raised-floor system) to deliver conditioned air into the space, typically through floor diffusers. Air is returned at a level above the occupied zone (typically at the ceiling level). Under cooling operation, UFAD systems produce a stratified environment, similar in principle to that of displacement systems. The primary difference between these systems is that UFAD outlets deliver air at higher velocity, producing greater mixing in the area near the outlet discharge.