Energy efficiency is currently seen as one of the biggest considerations for the industrial world, and as such, it is one of the most important factors driving product innovation. And, given that compressed air accounts for between 10 and 15% of overall electricity consumption in the industrial sector, this is an area where efficiency improvements can have a hugely significant impact. Although Sliding Vane Rotary Compressors (SVRC) are commonly characterised by high energy efficiency, there is still, much like any other technology, considerable scope for performance improvements. 

Historically, intercooling is an established practice in compressor technology for reducing the power required for operation. This method works by reducing the temperature of the compressed gas between two compression stages. Thanks to the intercooling, the gas reduces its volume, with less power required for the compressor to work efficiently. 

New research by Mattei, however, shows that the similar, but not identical, concept of ‘intracooling’ achieves even better results in two-stage compressors. Part of the amount of lube-oil is exploited for the air cooling between the two stages, and this technology promises to save up to 10% of the energy required by a conventional single-stage compressor for typical air compression applications. 

What is Intracooling?

Intracooling is the cooling of the compressed gas between two compression stages by way of spraying a liquid coolant in the gas flow, without separating that liquid prior to entering the next compression stage. The liquid is then separated at the end of the last compression stage. When applied to an oil-flooded compressor, such as a two-stage sliding-vane air compressor, the lubricant itself can be used as sprayed liquid.

The considered two-stage intracooled compressor is characterised by fewer pieces of equipment and allows for oil to be available at the highest pressure, a condition that may be exploited for a better injection in the first stage of the compression process.

Testing the technology

Mattei recently set about verifying the effectiveness of intracooling processes. A prototype was set up equipped with the instruments required to measure the temperature and pressure of air and lube-oil in all key points of the circuit, as well as the injected oil flow rate and the electrical power. Thirteen different configurations were tested, characterised by different positions of active nozzles in the intracooling duct, but with the same pressure and temperature injection conditions.

The parameter used to evaluate intracooling effectiveness was the temperature difference between the duct inlet and outlet, and the thermal distribution on the intracooling duct was controlled by an infrared camera.

Assessing the results

The experimental investigation demonstrates that a conceptual two stage compressor which utilises intracooling, can reach energy savings of up to 10%, when compared with a single stage system. The low-pressure stage of the intracooled compressor showed a better compression process compared to the intercooled one, due to the higher injection pressure that results in a finer injected oil atomisation. 

This test shows that the measured intracooling effectiveness depends strongly on the number of active nozzles and, consequently, on the oil flow rate as well as on their position in the duct. The highest intracooling performance resulted in a temperature reduction up to 10°C across the duct between the two stages.

With such positive findings, the two-stage intracooled compressor appears to be a viable solution to improve the efficiency of sliding-vane rotary compressors and extend their operating range of pressure. 

In terms of next steps, Mattei will continue to focus on the design of the high pressure stage. Furthermore, a smart system for the nozzle activation control will be developed in order to guarantee the fine-tuning of the temperature at the inlet of the second stage.

Watch this space!

For more information on Mattei and its energy efficient range of compressors and compressed air solutions, visit www.mattei.co.uk.