【In-depth Article】How to Develop a Liquid Helium Compressor Capable of Reaching -269 Degrees Celsius
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The following is the transcript of the speech given by Hu Zhongjun, a researcher at the Institute of Physical and Chemical Technology, Chinese Academy of Sciences: Hello everyone. I’m Hu Zhongjun from the Institute of Physical and Chemical Technology, Chinese Academy of Sciences. Today, I would like to share with you some of the “unusual stories” that I have experienced. When I was in middle school, once the teacher asked us what we wanted to do when we grew up I said I want to become an Antarctic researcher. Because at that time I read in the school library Teacher Qin Dahe’s story \"Diary of a Trip Across Antarctica\", which was very touching. From then on, that mysterious and cold world left a deep impression on me. However, Antarctica is not a place that anyone can visit just by wanting to. After graduating from middle school, I went to the beautiful city of Harbin to study general refrigeration technology, that is, commercial refrigeration technology. In class, there was a sentence said by the teacher that I still remember vividly to this day. He said, “Teachers can only teach you how to ‘slaughter pigs’; it is the Chinese Academy of Sciences that is capable of ‘slaying dragons’.” ”
Why is it called “killing pigs”? This is because our ability to enjoy freedom of pork, fruits, seafood, as well as beef and mutton today depends to a large extent on advanced commercial refrigeration technology. So I became very curious: what exactly is the \"secret weapon\" for cooling? Later, I was admitted to graduate school at the Chinese Academy of Sciences. When I was working on my thesis, we **were developing a large rocket**. Now we all know that the name of this large rocket is “Long March 5”. Only this “Eternal Sky Sword” can match the “Dragon Slaying Knife”. In addition to being called “Fat Five,” it has another nickname, which is “Ice Arrow.” Why is it called “Ice Arrow”? We can see that the temperature of some components in the Long March 5 is much lower than that of frozen pork, as low as minus 253°C. Because it needs to turn hydrogen into a liquid in order to launch heavy payloads.
In fact, since the invention of cryogenic refrigeration technology, relevant work has won the Nobel Prize from time to time. Not only is it required for cutting-edge physical research, but magnetic resonance imaging used in medical check-ups also needs such a low-temperature environment. Moreover, the quantum world, which we often hear about, relies on low-temperature technology as well. In the era when we were studying, people were still struggling to meet their basic food needs; food was undoubtedly of the utmost importance, which led to a high demand for refrigeration and freezing technologies. With the progress of society and the advancement of science, we urgently need to develop cutting-edge fields such as aerospace and superconductivity today, and this requires cryogenic equipment to support such research. The older generation of scientists had already developed some smaller cryogenic equipment during the era of the \"Two Bombs, One Satellite\" project. This photo was taken in the 1970s, when scientists were trying to place satellites in an environment that simulated space for testing purposes. The temperature in space is extremely low, around minus 270°C. We need to simulate the low-temperature environment of space on Earth in order to ensure that there are no problems once the satellite is launched into space. This image makes it even clearer; the area with white mist in the middle of the picture is where the liquid hydrogen and liquid oxygen used for refueling the \"Fat Five\" are evaporating. The liquid hydrogen required for this process needs to be produced using cryogenic technology.
What exactly is cryogenic technology? Academically, we use the boiling point of natural gas used for cooking at home, which is 120 K (minus 153.15°C), as a benchmark. Technologies above this temperature are called normal cryogenics, while those below this temperature are called low-temperature technologies. Our Institute of Physical and Chemical Technology, Chinese Academy of Sciences, focuses primarily on cryogenic technologies below the temperature of liquid hydrogen. In particular, helium is the gas that is most difficult to liquefy in the world, and liquid helium is the main focus of our research; it also serves as a key to unlocking the quantum world that goes beyond Newton’s classical mechanics.
This was the device used for liquefying helium at that time; different color blocks indicated different liquids. After pre-cooling liquid air with alcohol, liquid hydrogen is then pre-cooled using that liquid air; only after the liquid hydrogen has been pre-cooled can liquid helium be obtained. This nesting-style design is difficult to implement on a large scale in industry, as well as to operate continuously. We prefer to obtain liquid helium from helium in one step through compression. In fact, whether it is commercial refrigeration or cryogenic technology, the core principle behind them is the same simple concept: heat transfer through phase change. For example, when we run, we sweat, and once the sweat evaporates, we feel cool, because water changes from a liquid state to a gas state, thereby absorbing the heat from our bodies. Some scientists have also discovered that worker bees bring water to the hive during hot summers to help regulate its temperature.