The industrial world is currently under intense pressure to make energy efficiency savings. This is driving product innovation, particularly in the compressed air sector, which is a heavy user of electricity.

Many industries use compressed air systems as power sources for tools and equipment used for pressurizing, atomising, agitating, and mixing applications. In fact, compressed air is estimated to account for between 10 and 15% of overall electricity consumption in the industrial sector.

When a compressed air system is designed, the key is to get the specification right first time, to make sure that the compressor is going to fit the processes, save energy and to extend its lifespan. In the case of Sliding Vane Rotary Compressors (SVRC) - which are commonly characterised by high energy efficiency - wide room for performance improvement is still possible. As technology advances there are new ways of assessing and measuring compressor efficiency that are challenging established thinking on the subject.

Defining ‘intracooling’

Historically, intercooling is a common 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. With intercooling, the gas reduces its volume, with less power required for the compressor to work efficiently.

The latest research by Mattei, however, shows that the similar, but not identical concept of ‘intracooling’ achieves even better results in two-stage compressors. This technology promises to save up to 10% of the energy required by a conventional single-stage compressor for typical air compression applications.

So, what exactly is intracooling? Andy Jones, managing director at Mattei explains that essentially, it is the process of 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 separated at the end of the last 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 compression process.

New analysis

A recent study was conducted by Mattei to verify the effectiveness of intracooling processes. Jones says that 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 the intracooling effectiveness was the temperature difference between the intracooling duct inlet and outlet. And the thermal distribution on the intracooling duct was controlled by an infrared camera.”

Efficiency gains

Ultimately, the investigation demonstrates that a conceptual two stage compressor which uses intracooling can reach energy savings of up to 10%, when compared with a single stage system. This is because 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.

Jones says that 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.

For further information please visit: www.mattei.co.uk