Ultimately, the target goals for this project are to be as follows:
- Keep cost under $5000. Bonus points if the liquefier can be built for less then $2000. This price range allows for local Makerspaces to crowdfund the liquefier. Not only does this reduce the per person price, but it also allows for entire communities to have access to liquid helium.
- Occupy no more then 12 cubic meters. While this is a considerable amount of space, the requirements of the liquefier prevent it from being ultra compact.
- Maintain a substantial production rate, somewhere in the neighborhood of 1-5 liters per hour. At this rate, the needs for common cryogenic projects (superconductors, etc.) should be met.
- Simpler to build then NASA rockets, but still require reasonable mechanical skills. While the experience required may exceed the common DIY skill set, anyone with reasonably developed mechanical and electrical skills should be able to build the liquefier.
Physics and Thermodynamic Principles Behind the Liquefier:
To understand in a basic sense how the liquefier operates, an exploration of a basic refrigeration system is in need.
The follow diagram outlines the flow in a refrigeration loop (image source: Wikipedia):
In this system, the fluid that is circulated, also known as the refrigerant, undergoes a cycle of compression and expansion. Basic thermodynamic theory: when a gas is compressed, it gains thermal energy; when a gas is expanded, it looses thermal energy. Knowing that, if we start at the compressor, the first thing to happen is the refrigerant is compressed, where it now gains thermal energy. While compressed, it passes through a heat exchanger, labeled above as the condenser. When it flows through, thermal energy is lost to the surrounding air and the refrigerant has now become cooler. The cool compressed refrigerant passes through an expansion valve (more on that later) where the gas expands and further looses more thermal energy. The cold expanded gas flows into another heat exchanger, where it adsorbs the thermal energy of the circulating air, thus cooling the air. In the final step, the warm expanded refrigerant is once again compressed and begins the cycle again.
Joule-Thomson Effect + Joule-Thomson Valves:
Now more on the expansion valve. The thermal change in the expansion of a gas through a valve or restriction is due to the Joule-Thomson effect. When a gas passes through a valve (also known as a Joule-Thomson valve when referenced in correlation with gas expansion) the gas looses thermal energy, thus cooling the gas. In the refrigeration loop above, the gas was already cool before it entered the valve, therefore it cooled even further when expanded. However this is not always the case. When expanding helium through a Joule-Thomson valve at room temperature, an interesting phenomenon occurs. Rather then cooling down, the gas instead heats up. This can be traced back to the Joule-Thomson Coefficient. In a basic sense, if the coefficient is positive at a specific temperature for a specific gas then the gas cools when expanded, but if the coefficient is negative, then the gas heats up instead (read more about the coefficient here). In helium's case, the point at which the coefficient becomes positive (also known as the inversion temperature) is below ~44 kelvin.
This poses problems when attempting to liquefy helium. For example: an easy gas to liquefy, such as nitrogen, can be liquefied by simply compressing the gas, cooling to room temperature, and then passing through a Joule-Thompson valve, where it thus converts to liquid. The problem with helium is it must be pre-cooled to a extraordinary 44 kelvin before it is expanded in the Joule-Thompson valve.
This pre-cooling step is what causes most of the complications within the project. There are a few solutions which can be used individually, or combined together, in order to pre-cool the helium to below 44K.
Cooled Vapor Exchange:
During...Read more »