compression, energy, saving, efficiency, cooling, oil injection, Organic Rankine Cycle
The Compressed Air Sector (CAS) is responsible for a relevant part of energy consumption, accounting for a mean 10% of the world-wide electricity needs. This ensures about the importance of the CAS issue when sustainability, in terms of energy saving and CO2 emissions reduction, is in question. Since the compressors alone account for a mean 15% of the industry overall electricity consumption, it appears vital to pay attention towards machine performances. The paper deals with compressor technology and it discusses the energy consumptions, on the basis of a comprehensive analysis of data for existing machines, mainly provided by the Compressed Air and Gas Institute (CAGI) for the US scenario, and PNEUROP, on the European compressors market. Data referring to different machine technologies, were processed to obtain consistency with fixed reference pressure levels and organized as a function of main operating parameters. Saving directions for different compressor types, screws & rotary vanes, have been analyzed. Main factors affecting overall efficiency have been split and all different efficiency terms (adiabatic, volumetric, mechanical, electrical, organic) considered separately. This has allowed a term-by-term evaluation of both the margin for improvement and the impact of each term on the “step change” in energy saving, leading to the evaluation of how efforts in the CAS contribute to the 20-20-20 policy emissions reduction targets. If a negligible growth in efficiency is achievable by further increase of volumetric and mechanical terms (few tenths percent), wide margins for improvement come from an upgrade of the transformation, through the adoption of a dual-stage intercooled compression. Its potential has been compared to that of an internal cooling strategy, with a fine oil spray injected within the flow: if the former solution requires the use of dedicated cooling devices between stages, the latter has its main drawback in the presence of cooling medium vapor phase within the flow, leading to a growth in compression work. Since the heat provided by oil cooling is available at a temperature (70-90°C range) that allows the conversion into mechanical energy by means of an Organic Rankine Cycle (efficiency range 8-10%) and considering that the thermal power from the oil and the mechanical power absorbed by the compressor are of the same order of magnitude, energy recovery is interesting as well. This measure, coupled with that of a multi-stage compression, has the potential to overcome the globally shared goals on energy and carbon saving.