Ductwork is used for transport of air used for ventilation or air conditioning in buildings. The supply air is typically conditioned (filtered, warmed or cooled, sometimes humidified or dehumidified). It is important that the air is distributed properly in the building. Thus the duct system must be well balanced regarding airflow rates, or have provisions controlling the air distribution. The ducts should have a low leak rate. A primary function is also thermal insulation to protect the heat content of the air.
Another primary function of ductwork can be to transport smoke out of the building in case of fire, or assist in pressurisation of escape routes.
There are many boundary conditions regarding air ducts:
- Energy use. It is important to choose a technical solution that uses a small amount of energy (and exergy) for the air transportation in the duct. Low energy use also means that tight and (where appropriate) well insulated ducts should be used.
- Minimisation of cost. Between different layouts of a duct system, all of which are able to fulfil the primary functions causing the system to be built, the one using least resources, based on the lifetime performance, should be chosen. Provided that the price of different resources as energy, material, building space and flexibility is adequate, this choice can be based on cost minimisation. In practice it is difficult to state that two different layouts of a duct system have the same quality regarding function. Whenever in doubt, proper function and low energy use should be given priority to cost minimisation.
- Fire. The duct system should not spread fire or smoke in the building. This gives restraints regarding e.g. duct system lay-out, duct material, and fire insulation of the ducts.
- Acoustics. Noise, or private conversations in rooms, should not be transmitted through the ductwork. Nor should noise be generated in the ducts so it is transmitted to the rooms. Noise generation is often governing the choice of air velocity in the ducts, resulting in velocities lower than economically optimal.
The present review attempts to summarise the information on all these aspects available in international literature.