There are rigorous standards in place governing compressed air performance and provision in these environments, ensuring processes remain free from contaminants, while minimising unscheduled downtime and the associated costs, too. Traditionally, oil-free compressors have been specified to ensure high hygiene standards are maintained and to avoid the risk of product contamination – with the additional benefit of low maintenance costs, improved environmental performance and cost of ownership.

Typical oil-free air compressor types for industries requiring compressed air from 20-30 m3/min up to 100-150m3/min are screw and centrifugal technologies. But selecting between the two options requires careful consideration, taking into account multiple aspects to ensure that an appropriate system is specified.

Oil-free compressed air at-a-glance

Oil-free systems use a range of technologies that bypass the need for oil. While oil-lubricated compressors use oil in the compression chamber to cool and seal the compressed air generated, oil-free technology eliminates this risk of compressor-generated contamination while delivering guaranteed air purity.

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 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.

ISO 8573

ISO 8573 is a group of international standards stipulating compressed air purity and quality. ISO 8573 consists of nine separate parts, with part one identifying contamination levels that can be tolerated within a single cubic metre of compressed air. Specifically, part one identifies permitted particle count, pressure dewpoint and amount of oil allowed within this sample.

The other eight parts detail testing methods for a range of contaminants, including solid particles, oil vapour, organic solvents, microbiological contaminants, liquid water and gaseous contaminants, alongside humidity levels and aerosol content. They also ensure that air quality test results are comparable within a given tolerance of measurement.

By design, ISO 8573 is very strict and thorough. However, many decision-makers are going beyond its standards to guarantee high-quality compressed air, instead adopting the principles of the Hazard Analysis and Risk-Based Preventive Control (HARPC). This ensures facilities are complying with hygiene legislation and either eliminating any potential hazards or reducing them to an acceptable level.

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.

In contrast, screw compressors are supplied in standard configuration with a sound enclosure, which guarantees quieter operation in environments requiring a very low noise level. For the centrifugal compressor, the sound enclosure is optional, as the centrifugal technology is inherently quieter.

The design of the centrifugal compressor is compact, enabling simple installation alongside a range of standard options pre-mounted on the machine, including air filter, condensate circuit, water manifold with individual flanged connections and control panel with integrated starter.

The installation of a screw compressor is even easier still, with a plug and play configuration.

Lifetime maintenance

Service costs are typically lower for oil-free solutions than oil-lubricated models. Whole life costs are reduced because businesses able to save on the cost of oil replacement.

Unlike oil-lubricated systems that require frequent oil change intervals throughout their service life, there is also no need to purchase equipment that would clean and separate oil from air, such as oil separators, filtration equipment and condensate treatment.

All oil-free compressor models are now being designed to optimise service and repair costs, with easy access to major components, but compared to rotary screw compressors, a 30-35% saving is estimated on centrifugal technology. This is in part due to the compression unit having fewer components subject to wear and tear, which also guarantees greater performance consistency over time.

Another benefit to consider with oil-free technology is that, with many businesses faced with ambitious sustainability goals, deciding on an oil-free solution provides the most eco-conscious choice too, helping to contribute towards a facility’s green credentials. For example, while oil-lubricated compressors mean oil or oil-contaminated condensate will need to be disposed of, impacting the environment, this is not a concern with oil-free technology

For further information please visit: https://www.ingersollrand.com/

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