The centrifugal fan, like the axial flow fan, is a constant volume equipment, which means that at a constant fan speed, the centrifugal fan moves a relatively constant air volume rather than a constant mass. This means that the air velocity in the system is fixed, but the actual air quality will vary according to the air density. Changes in density may be caused by changes in incoming air temperature and altitude above sea level, which makes these fans unsuitable for applications that need to provide constant quality air.