A Statistical Survey on the Actual State-of-the-Art Performance of Radial Flow Fans Based on Market Data
1.3 Fan Efficiency Metric & Market Surveillance
In the current energy and environmental context, sustainability is emerging as the new paradigm of reference. A recent European Directive establishes a framework for the setting of eco-design requirements for energy-related products, in order to reduce the energy consumption of the products concerned. In this global context, each machine that performs an energy conversion must be characterized by the highest possible efficiency. In the field of turbomachinery, high-performance machines have been realized over the last years thanks to the technological advancements and to the use of sophisticated design methodologies based on advanced CFD techniques. At the outset of the design process of a turbomachine, the main dimensions of the impeller (i.e. the exit diameter and width) need to be evaluated as a function of the overall required performances. For a fan, these performances include the total pressure rise, the volume flow rate and the rotational speed, which are generally expressed as non-dimensional performance parameters (i.e. pressure coefficient and flow coefficient) by means of dimensional analysis.
The preliminary calculation of the main impeller dimensions of a fan is classically performed on the basis of consolidated empirical correlations (e.g. the Cordier diagram), which relate non-dimensional performance parameters and characteristic geometric ratios to the specific speed, defined at the point of maximum efficiency of a turbomachine. The specific speed represents a parameter of great importance in selecting the most appropriate type and size of machine required for a given duty, as its value gives the designer a guide to the type of machine that will provide the normal requirement of high efficiency at the specified design conditions. Since the aforementioned literature correlations have been determined in the middle of the last century upon experimental results obtained from a large number of different type machines, these correlations need to be updated in order to design fans whose performances meet the eco-design requirements set out in the current European Regulation. In this paper, a statistical investigation on the performances and the dimensions of high efficiency existing radial flow fans is reported. Either single or double suction centrifugal fans are analyzed, with backward or forward curved blades. Some correlations between non-dimensional performance parameters and characteristic geometric ratios are found and their connection with literature correlations is pointed out. These relations can be used for a general design of high-efficiency radial flow fans.