Evaluation of Squirrel-Cage Fans for HVAC Applications in Public Transport: Key Parameters and Design Guidelines

D2 - Aerodynamic Design

Small centrifugal and axial fans are used in automobile applications, as it is the case for HVAC systems for public transport. In particular, a specific type of centrifugal fans named "squirrel-cage" is broadly used in these systems. Their main geometrical characteristics are a large number of short chord forward-curved blades and a rotor exit-to-inlet area ratio unusually large. Due to its size and relative high specific speed, these fans are used in applications with requirements of compact size, high flow rate and low cost. Typically, these fans are used to work at high rotation speed (over 4000 rpm) and variable operation conditions, even at extreme off-design points. Flow instabilities, low efficiency and high levels of noise and vibrations appear in these machines due to these working characteristics and to the need of low fabrication costs.

In this work, a morphological and functional characterization of several ventilation equipments used in commercial HVAC systems for public transport, is carried out. They all use an assembly of two squirrel-cage fans with drive motor incorporated.

First, a detailed description of geometric and construction parameters of the available equipments is made: diameters, thicknesses, drive system, building materials, etc. Following, the functional characterization of the machines is carried out, paying attention to aerodynamic, acoustic and energetic aspects: pressure, flow-rate, power consumption, total efficiency, sound power, etc.

All the information collected is expressed in terms of dimensionless parameters, making possible the comparison between the fans, independently of their size and drive speed. The specific speed ns ranges from 1.5 to 2.5 while the specific diameter ds shows values from 1 to 1.4. In general, performance curves with unstable shape have been obtained, with significant pressure and efficiency drop in the medium flow rate zone, coupled with increases in power consumption in this area. Total efficiencies are low in all the cases, in some cases staying in a poor 30% and not exceeding in any case 45%. In terms of acoustic generation, the results have been extremely diverse, with significant differences among the fans, but also in a particular machine when the operating point is varied.

The analysis of the obtained results is applied to select the best alternative among those analyzed, focusing on energy efficiency and minimum noise generation criteria. In this way, the most important design criteria are established in order to get ventilation systems more efficient and less noisy.