In building services engineering and HVAC, an air-mixing plenum (or mixing box) is used for mixing air from different ductwork systems.
The most common application for an air-mixing plenum is the mixing of return air (or extract air) with fresh air to provide a supply air mixture for onward distribution to the building or area which the ventilation system is serving. The air transferred from the return air stream to the supply air stream is termed recirculated air. All air not mixed is rejected to the atmosphere as exhaust air. Air streams are mixed to save energy and improve energy efficiency.
The mixing plenum normally combines two air streams, and includes for three sets of dampers: one for the fresh air, one for the exhaust air, and a mixing damper between the two air streams. The mix of fresh air and recirculated air can thus be adjusted to suit the needs of the building's occupants. Most systems will use motorized dampers to control the air mixing, and controlled by the building management system (BMS), or controls system. Typically as the fresh air and exhaust air dampers are driven from 0% open to 100% open, the mixing damper will in turn be driven from 100% open to 0% open, so as to always ensure a constant volume of supply and extract air.
Air supply to a building is generally performed by an air handling unit. The process may include for filtering, heating, cooling, humidification, or dehumidification, all of which processes consume energy. Since the fresh air demand for the building occupants may be less than that is required for air conditioning purposes, it would be wasteful if 100% fresh air were used, with wholesome treated air rejected to the atmosphere in its place. Therefore mixing is used to balance the needs between the occupants requirements for fresh air and the air conditioning process for the building.
