Fan-System Interaction and Blockage Effects for HVAC Multi-Fan Units

B2 - HVAC and Condenser Fans

Small fans, both axial and centrifugal, are often installed in the air conditioning systems for public transportation vehicles. These fans are driven at relatively high speeds (around 4000 rpm might be reached), with extremely variable working conditions, probably far from the nominal ones. All these facts, together with the need to reduce fabrication costs, give rise to frequent instability problems, low performance, low efficiency and high levels of vibrations and noise.

Besides, when using multi-fan units, there are important geometrical restrictions and the interactions with the system play an increasingly important role. In the evaporator units of any of the before mentioned vehicles it is common the use of parallel configurations of multi-fan units. In such units, the modern designs restrict the volume of air available around the fan and closer location of the units produce important interaction problems.

The main goal of this work is the optimization of a multi-fan unit, finding the minimum distance between fans to reduce within a reasonable limit the interaction effects, with other fans and with the ventilation system. For such objective, an experimental set-up is built and several experiments have been carried out, particularly measurements to find the fan-wall interaction, the wall-fan-wall interaction and the interaction among three fans have been performed. Rotational speed, power consumption and noise measurements were performed for two different flow rates, by regulating the opening valve in the conduct.

From the experiments, the conclusion is that the minimum distance between any fan and the wall for a given rotational speed is one diameter. For the three fans installation, the results show a limit around 1.6 diameters as minimum distance between two fans. The recorded noise pressure levels were found almost independent of the fans distance.

Through the analysis of the geometric characteristics, aerodynamic and acoustic performance, conducted on a wide range of fans used in HAVC systems, it was found that there is a tendency in the industry to cover the engine located between the two fans in the machine for aesthetic reasons. A study of the effect on the aerodynamic performance of a fan for different covering solutions is also carried out. As is known, the engine causes a blocking effect in the aspirations closer to it, which implies asymmetric flow and other problems. The results show that any attempt to cover the engine area only intensifies this effect and consequently the problems associated with it.