Maximum Achievable Efficiency of Centrifugal Fans Without Housing

A3 - Numerical Optimisation of Centrifugal Fan Performances

The present paper is focused on centrifugal fan wheels with backward curved blades and a rotating diffuser in free-blowing operating (without housing). Moreover, the consideration is confined to the flow area within impeller.

Such centrifugal fans are very popular for the use in air handling units because of advantages such as compact design and direct drive etc.. The fan efficiency ηr is, however, a little lower than the fan with housing, which converts the dynamic flow energy of the air directly into usable static flow energy. There is no doubt a further potential to improve the efficiency through a further aerodynamic optimization of the wheel geometry. It is, however, exciting to know on which parameters the efficiency of these fans ηr primarily depends and what is the theoretic limit of the efficiency maximum achievable ηr,max.

The basis for the theoretic consideration is the Euler's main equation, which describes the energy conversion in the fan wheel. For an idealized flow within the fan wheel the relation between the fan efficiency ηr and the hydraulic efficiency of the blading (incl. inflow and outflow from the wheel) ηhydr is derived in function of the other technical operating and geometric parameters.
A theoretic estimation of the expected efficiency ηr of an aerodynamically "lossfree" fan can be carried out by ηhydr = 1. Inversely from the really measured/achieved efficiency ηr of a fan, the theoretic, hydraulic efficiency ηhydr can then be estimated as to the introduced formula aiming at evaluating the potential for a realistic improvement.

Three wheel examples with different blade geometries show how much the efficiency of a fan without housing depends on the working point (air volume and pressure) apart from the blade geometry. The limit of the maximum efficiency achievable of a centrifugal fan should in fact be higher than the real value of 73% presently achieved at the working point with a dimensionless volume number: 0.26 and a dimensionless pressure number: 0.57. The margin for a further improvement through an aerodynamic optimization of the flow in the wheel should, however, be highly limited for this wheel due to the high value already achieved.

In order to increase the total efficiency of the fan, the efficiency of other components like motor, control, etc. has to be improved as well.