Considerations Concerning the Possible Fan Efficiency in Real Life Applications with Limited Fan Size

Cordier Guided Design Strategies

For a large number of fan demands with limitation of the fan size, the specifications have been analyzed. The considered fan demands relate to axial, backward- and forward curved centrifugal fans and mixed flow fans, with or without guide vanes or housings. However, the fan type is often not crucial for a fan demand; to a greater degree, achievable efficiency and/or noise level are important and the choice of fan type is sub-ordinated and can be adapted to the needs.
The requirements are separated in two groups, first, the "basic" requirements which are duty point and fan size, and which are easy to fulfil but essential for the working task of the fan. Secondly, we have "quality" requirements as low rotation speed, low energy consumption or low noise level which result of a good fan choice and a high degree of optimization.
From the basic requirements, a non-dimensional parameter δ* is deduced which is a constraint for the fan provider or designer, and which cannot be bypassed. δ* is a binding link between fan application and non-dimensional fan design, however it is not constant for a given fan but depends on the resistance curve at which the fan is operated. δ* has a strong relationship to the specific fan diameter δ, resulting in the fact that the achievable quality, especially efficiency and noise, depend on this non-dimensional parameter δ*.
The efficiency and noise of a fan depend on the value of δ* at which it is operated. This may differ significantly from a region of values of δ* where the fan has good efficiencies. Thus the maximum efficiency of a fan, often used as quality measure, is in general not an indicator of its energetic efficiency in a given application with size limitation.
Illustrated by typical examples, forms of fan size limitations, such as installation space, available drive torque or impeller mass, are presented and discussed. The approach of extending the fan size within a limited installation space in order to achieve higher efficiencies is addressed for a typical centrifugal fan case.
We further present a strategy of clustering the δ* values to a limited number of regions, which can be covered each obtaining good efficiency by one non-dimensional fan design, limiting the number of required fan designs. For some of the clusters, fan designs are characterized, ending up in different axial, mixed flow and centrifugal fan designs.