Last edited 07 Mar 2017

Indoor air velocity

CFD.jpg

Air velocities impact on the rate of heat transfer between that air and adjacent surfaces. The higher the velocity of the air moving across a surface, the higher the heat transfer will be. Higher velocities can be useful where heat transfer is required, such as in the transfer of heat between the thermal mass of a building and the air within it, but can be undesirable under other circumstances, such as the exchange of heat with the cold internal surface of a window.

This is a dynamic relationship. The greater the temperature difference between a surface and the air next to it, the greater the air velocity is likely to be and so the higher the rate of heat transfer. This is apparent for example in the cold downdraught that can be felt next so a cold window.

The transfer of heat between a surface and the air adjacent to it will depend on the roughness of the surface, the velocity and turbulence of the air, the temperature difference between the air and the surface and the geometry and orientation of the surface.

Indoor air velocities also affect the thermal comfort of people within spaces. The greater the air velocity, the greater the heat exchange between people in a space and the air around them. In some circumstances a higher air velocity may be desirable, for example, a fan might be turned on during hot weather to increase the rate at which the body is able to lose heat to its surroundings. Under other circumstances however, this may be undesirable, for example in colder conditions when higher air velocities may be noticeable as a draught.

Generally air velocities inside buildings are relatively low compared to the outside, however it is possible for the range of air velocities to be quite large. There may be entirely stagnant areas where air velocities are close to 0 m/s, whilst in tall, naturally ventilated spaces, where there is a large difference between the inside and outside air temperatures, or in large mechanically ventilated spaces, internal air velocities can be several m/s. In 1999, the main roof vents were opened in the Millennium Dome in Greenwich, in the UK, and all the perimeter doors closed other than the main entrance, and it was then possible to fly a kite inside the building, the internal air velocities were so high.

However, some more typical ranges are set out below:

0 m/s Stationary air.

Note, minimum air change rates are required to maintain indoor air quality.

0.1 m/s May be used as the assumed internal air velocity in some simple heat transfer calculations.
0.1 to 0.15 m/s and above May be felt as a draught in a cold climate in the winter.
0.3 m/s and above May be felt as a draught in a cold climate in the summer.
0.8 to 1 m/s and above May be felt as a draught in a hot climate.

These values are very rough guides only, as perception by individuals will depend a wide range of parameters, such as their personal preferences, clothing, activity and so on, and other characteristics such as the difference between the air temperature and the temperature of the person, the degree of turbulence in the air, the humidity of the air, the radiant temperature in the space and so on will also affect what may be considered acceptable.

Accurately predicting internal air velocities is very complicated and can involve the use of computational fluid dynamics. Similarly, measuring internal air velocities, particularly where they are very low, is a complex process, with most air movement detectors intended to measure higher velocities found outside, or in specific locations such as in ventilation ducts.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • Approved document F.
  • ASHRAE 55.
  • ISO 7730.