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COMPRESSED AIR COMPARING COMPRESSORS
The Ingersoll Rand E-Series Oil-Free Rotary Screw Air Compressors 200-355 kW (series) and MSG TURBO-AIR 1500 Centrifugal Air Compressor
Claudia Vignelli, Product Manager, Plant Air Centrifugal Compressors, Ingersoll Rand, explores the variety of factors to consider when selecting the most appropriate oil-free compressed air technology
C
ompressed air purity is essential in numerous process industries. Even the slightest contamination can result in
product spoilage, potential product recall and damage to the manufacturing equipment. High quality compressed air, free from oil
contaminants, is required to power the pneumatic tools of increasingly sophisticated industrial automation systems. Traditionally, oil-free compressors have been selected to contribute to system reliability by avoiding downtime, reducing maintenance costs, and operating expense. The two most frequently specified oil-free
air compressor types for industries requiring compressed air from 20-30 m3 150m3
/min are screw and centrifugal
technologies. But how should operators choose between the two? The answer to this question is not simple as
it depends on the process and the application and requires that multiple aspects be evaluated. The benefits of an oil-free compressed air
system are not only limited to manufacturing high-quality, safe, and compliant products, meeting regulatory compliance requirements, and improving the life of downstream pneumatic equipment and instrumentation. Oil-free compressors also
14 NOVEMBER 2023 | PROCESS & CONTROL /min up to 100-
contribute to system reliability by avoiding downtime caused by air actuators jamming, reducing maintenance costs by eliminating air/oil separation and filtration treatments, and minimising operating expense by reducing these sources of additional pressure drops. Coupled with the unprecedented rise in
energy costs, identifying the most appropriate compressed air solution that will contribute to low cost of ownership is now considered essential.
Analysing compressor technologies There are three options available in the 20- 50m3
/min capacity range and 125-350kW
power ratings: the centrifugal compressor, fixed-speed rotary screw compressor and variable-speed rotary screw compressor. The centrifugal compressor is available in
2-stage and 3-stage configurations and can operate at 3 bar(g) in low pressure or exceed 10 bar(g) in 3-stage configurations. When comparing these compressors, the
two-stage oil-free screw compressor at full load is less efficient than the three-stage centrifugal compressor. To mitigate for this, Ingersoll Rand has developed screw compressors with a nominal power up to 350kW, as above this size the oil-free screw efficiency gap can reach 15-20 per cent and,
investing in higher power compressors would not make sound financial sense, especially considering the current high energy costs. A key decision when selecting the most
appropriate technology for the application is the variation in compressed air demand. A centrifugal compressor reaches its
maximum potential when the demand has a 30-35 per cent variability and operates within the ‘flow regulation’ range. This solution is the most efficient in the 70-100 per cent load range, before air discharge, also called blow-off. Compared with the fixed-speed screw
compressor, the centrifugal compressor also has lower unload power, but allows for a limited number of starts and stops. If there are significant and frequent
fluctuations, with up to 70 per cent demand variability, a variable-speed screw compressor may be the more appropriate choice. It provides the best specific power if it operates at less than 80 per cent of maximum load for the majority of the time. It has the added benefit of having no limit on the number of starts, which makes it the optimal choice as an additional compressor to cover air demand fluctuations and peaks, when coupled to a fixed-speed compressor. It is also important to consider when air variability is required. A recent centrifugal
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