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The true centrifugal compressor operating range: Surge, Stall and Choke
The operating range of any compressor is limited on the high flow side by the choke region and on the low flow side by surge. Sometimes a phenomenon called rotating stall (or diffusor stall) further limits the operating range of the compressor. Due to this, there are often numerous opinions on what constitutes a compressor's operating range.
When a compressor is operated away from its design point, the gas flow into the aerodynamic components (impellers, diffusore and so on) deviates from its design direction. If the angle of deviation (or incidence angle) is large, flow separation occurs.
At higher incidence angles, the flow fully separates at the impeller leading edge or diffusor inlet and is said to be stalled. Rotating stall is a special form of stall, where one or multiple flow regions in the diffusor (or impeller) are stalled but where other regions of the same impeller or diffusor are not stalled yet. The stall regions usually travel in the direction of the rotation at a speed that is fractionally lower than the rotating speed of the compressor. Stall and flow separation may be precursors to surge.
The stability limit of the compressor is reached when the head-flow characteristic becomes horizontal due to the increased losses at higher incidence angles. At this peak on the speed line, any small flow disturbance entering the compressor will lead to surge (as the maximum head capability of the compressor is exceeded). However,...