Mixed flow fans
The suitability of a particular type of fan for a duty depends more on the relationship between the performance parameters than on their absolute values. This is especially true where there are limits to the size of the unit, and/or where the maximum speed is specified. In Section 1.3.2 the concepts of specific speed and diameter are discussed, and it is noted that there is an area for mixed flow fans between the two traditional types. This type has not been commercially available to any extent until recently.
For HVAC applications, there is a region for which neither centrifugal nor axial fan is ideal but for which a mixed flow fan can be designed. For the centrifugal fan to be of an acceptable size it has to be selected at efficiencies away from its peak; the axial fan has to have a high hub to tip ratio and/or has to be multi-staged to achieve the pressure.
Mixed flow fans should not be confused with in-line radials. Their casing diameter is generally smaller and they run at a spee d intermediate between axials and centrifugals.
Posted in Fans Ventilation A Practical Guide