Compressors for Cryogenic Gases Ultralight and Extra Cold: Choosing the Right Compressor for Cryogenic Helium
Cryogenic temperatures are extremely demanding for the used equipment — A noble gas as a refrigerant for cryogenic temperatures: Although helium is only be found in very specific applications, light gas is in great demand as a refrigerant in research. However, the low density of the medium does not bode well for the process equipment used. Especially safe sealing becomes a challenge — also and especially for the required compressors.
Many universities and institutions use liquefied helium as a refrigerant gas for their research. The requirement for liquefaction plants is correspondingly high and a leak-free process absolutely vital. That is just one of the specialties of Vorbuchner, a low temperature technology specialist from Kirchweidach, Germany. As the company developed a complete cryogenic system that prepares liquid helium at a temperature of 4.5 K (-268.65 °C) for the University of Giessen, the cryo–specialists required a compressor that handles 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,” remembers CEO Wilhelm Vorbuchner. Enter the compressor experts: Based on their SLF 101-3 screw compressor, manufacturer Boge developed a prototype that compresses helium efficiently. The company could nevertheless not rely on off the shelf solutions and instead had to change the machine design and add components without detracting from the compact construction.
The Right Compressor Makes the Difference
Besides the SLF 101-3 screw compressor, the 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. The refrigeration then takes place in a so-called cold box. Expansion of the helium gas is carried out in two turbines.
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.
Cryogenic Helium: A Challenging Gas
Helium is a very light gas, making the construction of a gas-tight compressor a tough nut. The airend used looks like one in a traditional SLF 101-3 screw compressor, but many of its components have been completely modified for cryogenic helium. Therefore, a clutch case now encapsulates the drive motor, which can collect any gas that escapes the process. 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. The loss of helium is therefore almost excluded, along with the contamination of helium through ingress air.
The Compressor That Came in From the Cold
“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 for a plant of the same design was thus planned for use at Polish university. So the potential of the liquid noble gas is far from exhausted, the partners are sure.