COMPRESSED AIR 36 HYDRAULICS & PNEUMATICS September 2023 www.hpmag.co.uk full load is less efficient than the threestage 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 oilfree 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 oilfree technology is that, with many businesses faced with ambitious sustainability goals, deciding on an oilfree solution provides the most ecoconscious choice too, helping to contribute towards a facility’s green credentials. For example, while oillubricated compressors mean oil or oilcontaminated condensate will need to be disposed of, impacting the environment, this is not a concern with oil-free technology.
RkJQdWJsaXNoZXIy MjQ0NzM=