Light gas, heavy job
Helium not air – compression using BOGE compressors
Helium is a very light gas. The density of air is seven times greater than that of helium. Therefore, the greatest challenge for the BOGE compressed air specialists was to ensure the maximum density of the compressor.
The BOGE solution
The BOGE SLF 101-3 used had to be completely modified. Every little leakage source had to be located and sealed accordingly.
Thanks to the good, close cooperation of Vorbuchner and BOGE, an operationally safe and efficient system for helium compression emerged from a prototype.
Vorbuchner GmbH & Co., KG based in Kirchweidach, is a specialist in the field of low temperature technology. The portfolio includes cryotechnical plants for helium applications – besides recovery, the liquefaction of helium is of the utmost importance. Vorbuchner is one of the few German manufacturers that supplies plants for the liquefaction of helium. Liquid helium produces the lowest temperatures. The refrigerant is used frequently in universities and institutions, for example, for cooling superconductive magnets. Vorbuchner developed a complete system that prepares liquid helium at a temperature of 4.5 K (–268,65 °C) for the University of Giessen. To do this, the cryotechincal specialist required a compressor that compresses helium efficiently and demonstrates the highest possible leakage protection. “The greatest challenge was to seal this machine in such a way that no helium escapes and no air can enter. For that would lead to impurities”, reports Wilhelm Vorbuchner, CEO of the company. The compressed air specialist BOGE set itself this difficult task. The Bielefeld-based company, which has been awarded prizes for its innovations many times, developed a completely new concept for its SLF 101-3 screw compressor. This special construction from plant engineering combines BOGE quality with innovative technology, and thereby fulfils the demand of the company to bring together progress, performance and durability to create particularly efficient solutions.
Gaseous helium becomes liquid
Besides the BOGE SLF 101-3 screw compressor, the newly developed plant for liquefying helium comprises two expansion turbines and a built-in cleaner for treating contaminated helium, as well as a dirt gas dryer, an oil removal system and a buffer tank. The compressor provides the required delivery volume of 3.69 to 10,69 m³/min. and compresses the helium gas from 1.05 to 13 bar. During the compression process, oil is injected in for lubrication and cooling. BOGE uses a vertical oil separator to clean the helium. The refrigeration then takes place in a so-called cold box. Expansion of the helium gas is carried out in two turbines arranged one after the other. The gas is cooled using the counterflow principle and is directed into a helium tank via a Joule-Thomson valve. The valve causes further expansion of the cold gas, which is cooled even more and finally becomes liquid. A temperature of 4.5 K (–268,65 °C) must prevail in this cold box. The treatment of the contaminated helium is carried out with the help of a freezing-out cleaner. Air contents condense and freeze as a result of the cooling processes. Neon and hydrogen traces are removed by the cold adsorber. The result is that the Vorbuchner plant supplies 20 to 33 litres of liquid and pure helium per hour.
The highest demands in terms of impermeability
Helium is a very light gas. The density of air is seven times greater than that of helium. Therefore, the greatest challenge for the BOGE compressed air specialists was to ensure maximum density of the compressor. The compressor used looks like a traditional SLF 101-3 screw compressor, but many of its components have been completely modified in comparison with this. There has therefore been an optimisation of the drive motor with regard to impermeability. A clutch case surrounds the motor. In the event of helium escaping, this therefore ensures that gas collects in the case and can be diverted. The vertically positioned oil separator ensures an optimised process for removal of oil from the helium. BOGE has also sealed the piping and heat exchanger to prevent the gas escaping. “During the development phase, it had to be possible to find even the smallest leakage source”, explains Sebastian Krake, project manager at BOGE. “Thanks to these fairly time-consuming and costly designs we have achieved a high level of process reliability.” The loss of helium is therefore almost excluded, along with the contamination of helium through the ingress of air.
Flexible to use, compact and space-saving
The SLF 101-3 screw compressor can be deployed very flexibly. The built-in frequency converter enables constant adaptation of the volume flow according to the current requirement. This leads to clear increases in efficiency for the system. The screw compressor can be installed to save space due to the compact design. Despite additional components and modifications having been made, the dimensions are identical with that of a traditional screw compressor of this performance class. The components of the entire system have been installed on-site and connected to one another by pipes; the screw compressor stands in a compressor space provided by the factory. All pipes and the heat exchanger consist of stainless steel and therefore ensure a long service life.
From prototype to efficient system
“We were in close contact with BOGE for the entire development period”, says Wilhelm Vorbuchner. “The good cooperation enabled us to successfully implement the plant design.” Thus an efficient system for helium compression that is safe to operate was produced from a prototype. This led to Vorbuchner awarding BOGE another order. A system of the same design was thus planned for use at a university in Poland, whereby a modified BOGE SLF 101-3 screw compressor was deployed. “As there is a major requirement for low temperature systems in the field of research, such systems will also be in demand in the future”, predicts Wilhelm Vorbuchner. BOGE has expanded its expertise in the field of helium compression through its cooperation with Vorbuchner. The compressed air specialist is planning additional projects using the screw compressor as a helium compressor. Thus, BOGE is continuously extending the existing product portfolio of special gas applications to include nitrogen compressors as well as nitrogen and oxygen generators.