Oil free compressed air: Factors to consider

21 November, 2023

Selecting the appropriate compressor technology

Ingersoll Rand offers three oil-free compression options in the 20-50 m3/min capacity range and 125-350 kW 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 350 kW, as the oil-free screw efficiency gap can reach 15-20 per cent and, above this size, investing in higher power compressors would not make sound financial sense.

Variation in compressed air demand

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% 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% 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.

Air variability

It is also important to consider when air variability is required.

A recent centrifugal compressor example in a glass industry process plant, demonstrates a 50% excess of air flow rate during the mould changing phase, equivalent to about an hour a day.

To avoid wasting this air, an air assessment auditor recommended the replacement of the centrifugal compressor with a 500-kW variable-speed screw compressor, thus eliminating the blow-off and saving around €10,000/year.

However, this solution overlooked the fact that during remaining 23 hours of full load operation, the two-stage screw compressor would have generated operating costs of approximately €100,000/year more than the three-stage centrifugal compressor.

If fluctuations in air demand are minimal, the fixed-speed screw compressor with an adequate buffer tank can be a valid alternative, particularly below 200-250 kW where it requires a lower initial investment, while ensuring good efficiency.

Compressed air configuration

In addition to the air flow and variability requirements, there are a few further considerations.

The centrifugal compressor, with its various pre-configured options enables greater flexibility and there is also the possibility of having additional options engineered on request.