Used as drive air and process air in industry, compressed air is a multi-talent medium: it is easy to handle, available in large amounts and suitable for a wide range of applications, including cooling and drying tasks. Its only disadvantage is the amount of energy required to compress it. To counter this problem, the Chemnitz-based Steinbeis Innovation Center for Drive and Handling Technology in Mechanical Engineering in cooperation with the mid-sized company Drucklufttechnik Chemnitz and the chair for Technical Thermodynamics at Chemnitz University of Technology developed a technology to accumulate exhaust process air temporally and technologically decoupled to reuse for secondary purposes. The project was supported by the German Federation of Industrial Research Associations and the German Central Innovation Program for SMEs.
Based on examples, the Steinbeis team analyzed the use of compressed air in the cooling process of extrusion blow-molding machines. They determined that compressed air was used at pressures between 5.5 and 9.4 bar, with standard volumetric flows ranging from 53.4 to 550 l/min. Different operating conditions each showed nearly constant pressure levels and volumetric flow rates; in all cases, the compressed air used for blow-mold cooling was regulated via exhaust throttling. For using the compressed air application in the conventional way, the team had to control the exhaust parameters via the compressed air reuse system and ensure they stayed in the ranges determined. Combining a novel accumulator concept with a technology for connecting the accumulator system to compressed air applications ensured this requirement was met. In order to capture exhaust air at constant pressure without affecting the volumetric flow, an accumulator with a variable volume is needed. The accumulator system developed by the Steinbeis team thus uses a bladder accumulator, common in hydraulics, together with an adjustable pressure-limiting valve.
In its initial state, this bladder is completely filled with water. As soon as the exhaust air flows into the accumulator, the water is forced out of the bladder by the pressure limiting valve. The exhaust pressure is thus the same as the switching pressure of the pressure limiting valve, which can therefore be used to control it. A throttle valve in the exhaust duct regulates the exhaust volumetric flow following standard procedures; however, due to the accumulator pressure, there is almost no loss in pressure via the valve, and the volumetric flow can now be primarily controlled via the flow area. The compressed air thus retains its potential of energy. In order to meet the required parameters to reuse the compressed air, a pressure reduction valve ensures the air is extracted at a constant, adjustable pressure.
Should the volume of reused exhaust air prove insufficient for a particular application, the team also plans to add a bypass allowing users to connect to an additional compressed air source. During air extraction, the same volume of water displaced during accumulation must be returned to the accumulator to ensure the accumulator system operates stably and correctly. Therefore two of the accumulator units are connected to each other and used alternately. As one accumulator unit captures incoming exhaust air, the water thereby displaced enters the second accumulator; at the same time, the second accumulator provides the previously accumulated compressed air to the application. Once the first accumulator is completely filled with exhaust air and all of the water is in the second one, the two accumulators switch roles. Now, the second accumulator is filled with exhaust air while the first supplies the application with the previously accumulated compressed air. To realize this accumulator concept, the project team developed a pneumatic circuit – including sensors to detect the right moment for switching the functions of the two accumulators – and built a prototype. Lab experiments and practice tests with extrusion blow-molding machines proved that both the alternating accumulator system and the regulation of exhaust pressure and volumetric flow were effective. The team succeeded in accumulating the exhaust air with a constant pressure and volumetric flow rate and a pressure loss of just 0.5 bar and providing it to secondary applications. They also created a simulation model of the accumulator concept to support the design and the evaluation of future operation modes of the accumulation concept.
Two concepts for re-using the accumulated compressed air in other applications proved beneficial: direct supply, and an additional compression stage. Re-using compressed exhaust air in new applications means the energy otherwise needed to compress air extracted from the atmosphere can be stored in its entirety. However, to do this an adequate pressure and volume of exhaust air is needed. If the exhaust pressure is lower than the pressure needed by the new application, an extra compression stage can be added. As the compressor uses the already partly compressed exhaust air, this represents an energy saving – which depends on the pressure of the compressed exhaust air. Lab experiments in which exhaust air with a pressure of 4 bar was compressed to 10.5 bar showed an energy saving of 42.2% compared to the energy needed to compress air drawn from the atmosphere.
This solution for highly efficient reuse of compressed air enables major energy savings. The example of the extrusion blow-molding machine makes this clear: compressed air at a pressure of 8.5 bar and a volumetric flow of 550 l/min was accumulated, and after a tiny pressure loss of 0.5 bar, it could be provided for reuse at a pressure of 8 bar and an unchanged volumetric flow of 550 l/min. This means that applications can be directly supplied with compressed exhaust air. This translates to potential savings of 4.68 kWh per hour of operation compared with the energy otherwise needed to compress this volume of air.
A triumph for the researchers: they succeeded in developing a new technology to reuse exhaust process air that makes it possible to use compressed air highly efficiently. The next step is to transfer this solution into a marketable product.
Prof. Dr.-Ing. habil. Bernd Platzer, Daniel Zipplies, Verena Loeck, Dr.-Ing. Eberhard Zipplies
Chemnitz University of Technology
Drucklufttechnik Chemnitz GmbH