Polytropic Analysis of Centrifugal Compressors
While compressor applications with low flows and high-pressure outputs necessitate reciprocating compressors, and high-flow, low-pressure outputs call for axial compressors, the moderate conditions required by a majority of industrial applications are best served by centrifugal compressors. Centrifugal compressors demonstrate polytropic gas compression characteristics, which fall between slow isothermal compression that yields heat to its surroundings and rapid adiabatic compression that has less time to lose this heat. Therefore designing centrifugal compressors for optimal performance in an application will employ rigorous analysis according to the polytropic compression model.
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Compressor Performance Parameters
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Centrifugal compressors straddle the definition line of being more air or process gas movers that deliver a flow at a given pressure than pure compressors where pressurized gas is the objective. As such, the fact that they move a lot of gas quickly at a moderate pressure results in the polytropic characteristics they demonstrate. Polytropic compression analysis consists of considering each of the prospective compressor's parameters, so they may be set to deliver the best performance for a given set of operational requirements. These are output flow and pressure, compressor impeller size and blade shape, operating speed, anti-surge considerations and desired efficiency at steady state conditions.
Surge
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Centrifugal compressor surge is a damaging and potentially dangerous condition that occurs when the pressure load at the compressor's output header exceeds the compressor's capability to sustain a net flow through itself and the flow rapidly cycles between positive and negative net flow. This condition can be violent enough to destruct the compressor starting with the impeller or thrust bearing. The flow-pressure curve that defines the threshold of this condition is the line on the compression compressor performance graph known as the surge line. The region to the right of this line on the graph is the region of compression ratios versus flows that define the correct polytropic performance domain of the compressor.
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Operational Complications
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Operational complications for these machines arise out of situations where two or more compressors feed the same header. What may be too much capacity load for one may well be too little for two compressors to operate in their most efficient zone. They also require a run-up period from start which requires close attention until the unit is ready to be brought on line.
Range of Applications
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The polytropic centrifugal compression model has been found to be effective for a range of process gases besides air, which makes them ideal for many applications in refineries and chemical operations. This is according to Lars Brenne, et al, who refer to J.M. Schultz, from his 1962 article "The Polytropic Analysis of Centrifugal Compressors," ASME Journal of Engineering for Power, where the polytropic compression is cited for this very characteristic.
Even Under Water
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The greater utility of centrifugal compressors that run the polytropic model and their comparatively smaller footprint compared to other approaches has made them the prime candidate for oil and gas production facilities, even in rigorous undersea conditions.
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