Malfunctioning of Air-Delivering Systems - Examples of Fan Driven Transient and Oscillating Flows

System & Preliminary Design

The paper reports the analysis of two initially malfunctioning industrial air-delivering systems. In both cases the air-delivering fans were the drivers for the undesired performance - not because the fans were per se of poor design but the interplay with the particular system created an unforeseen transient or oscillating flow.
The first example deals with a huge silo for powdered goods in the food industry. The powder is conveyed into the silo via an aerating and fluidizing air stream. During the first start up the silo collapsed. A network analysis of the air path showed that the dimensioning of the air supplying screw compressor, safety valve and exhaust air fan was inappropriate - the comparably small exhaust fan was able to create the damaging negative gauge pressure inside the silo at a certain time instance during the loading of the silo.
The second example is a large central air conditioning system for a clean room laboratory building consisting of two parallel supply and two parallel exhaust air handing units. The pressure in the supply air handling units showed intolerably high amplitude pressure oscillation in the order of 10 Hz with the consequence that the walls of the unit were moving visibly. From measurements in a series of systematic experiments a number of possible known mechanisms were checked: vortex-generated unsteady flow phenomena and standing wave resonance phenomena, interaction of the two parallel supply and/or exhaust air handling units and self-excited surge of the system due operation of the fan on its characteristic with a positive volume flow rate/static pressure rise slope. A possible correlation of frequency and amplitude of the pressure oscillations with the mean flow velocity and the blade passing frequency of the rotor was checked by varying the fan speed. By increasing and reducing provisionally the pressure resistance in the flow path one of the fan's actual characteristic and its operating point were determined experimentally. Moreover, the unsteady volume flow rate through each flow of one of the double-flow fans in one of the supply units was determined. Here, an alternating suction of the two flows of one of the fans was identified which is believed to be a rather unique mechanism. It could be weakened by rectifying grids in the inlet of both flows, with the consequence that the overall pressure oscillations were reduced substantially.