The helium liquefier is a TCF-20, and the nitrogen liquefier is a LINIT-25, both purchased from SULZER. The TCF-20 produces about 11 liter liquid helium per hour (up to 30 liters per hour with liquid nitrogen precooling), and the LINIT-25 produces about 20 liter liquid nitrogen per hour.
the control system
In parallel with the original control systems, a new more intelligent system has been developed by former kryolab manager Henrik Laurila. The liquefier has been rebuilt and improved in several ways through the work of Henrik. The new control system is based on a couple of PLC-systems from Satt Control, and forth-based micro controllers from New Micros inc. And in the center of everything is an i486 PC that coordinates the controllers. Powered by Linux, of course.
The original control system from 1985 has been improved in two steps, first with logging and control of critical parts by a Satt Control PLC-system built by former staff Henrik Laurila around 1996. Then in 2011 Tove Mattsson and present staff Leif Magnusson made a state of the art control system in National Instruments LabVIEW:
how does it work?
For refrigeration and liquefication, two processes are used, the Siemens cycle and the Hampson-Linde cycle.
The Siemens Cycle
In the Siemens cooling cycle (patented by W. Siemens 1857) a gas is compressed and thereby heated in a compressor [A]. Most of the heat is removed by a cooler [B]. The gas does then pass through a heat-exchanger [C]. Energy is then extracted from the gas by letting it do a work in an expansion-machine [D], with the result that the gas is expanded and cooled. Finally, the gas will pass the heat-exchanger [C], to cool the compressed gas stream from the compressor [A]. As the gas stream into the expansion-machine [D] becomes cooler, the gas stream out of it will also be cooler, thereby the temperature of the cold end of the cycle will continuously decrease.
The Hampson-Linde cycle
The Hampson-Linde cycle ( independently patented by W. Hampson and C. von Linde 1895) is very similar to the Siemens cycle, except that the expansion-machine is replaced by a Joule-Thomson orifice in [D]. First a few words about the Joule-Thomson effect. A gas could be cooled by letting it expand freely against the atmosphere. This could be explained by that the gas was doing a work against the atmosphere by lifting and/or heating it, and thereby loosing energy in form of heat. 1852 Joule and Thomson did some experiments to observe a gas, expanding without any work to be done. They let the gas flow through a pipe with a porous plug to restrict the flow. To their surprise the gas was cooled by passing the plug. This could first be explained twenty years later by van der Waals. The gas was not doing an external work, but an internal against the forces between the molecules. What Hampson and von Linde did was to combine Siemens cooling machine with Joules and Thomsons expansion plug, and put together a complete gas liquefication cycle. The disadvantage with the Hampson-Linde cycle is that it is very inefficient in compare to the Siemens cycle at higher temperatures. And depending of the initial temperature and which gas is used, it may not even start to cool down. On the other hand, it has the advantage over the Siemens cycle by not having any moving parts at the cold end.
The helium liquefier (click to enlarge image)
To liquefie helium a Hampson-Linde cycle [blue] is used, boosted by a two stage Siemens cycle [red] doing most of the refrigerating work. As an expansion-machines in the Siemens cycle, turbines are used.
the nitrogen liquefier
The nitrogen liquefier works much like the helium liquefier, but instead of helium the circulating gas is air. After the compression, moist and carbon-dioxide is absorbed in aluminium-oxide beds. Then the air is refrigerated and liquefied by a Siemens and a Hampson-Linde cycle, just as in the helium liquefier. And the last step is to distillate the liquid air to separate the nitrogen from the oxygen.