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ENEA - Fusion division

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Liquefier plant

Some experimental devices require to be cooled at low temperatures.
This is the case, for instance, of superconducting magnets. Sensors of diagnostic equipments in many different applications may also take advantage of the low operating temperature, which results in an improved signal to noise ratio.
Liquid helium, whose normal boiling point is 4.2 K (about -269C), is typically used to refrigerate such equipments. There are many different applications of liquid helium among the research activities pursued at ENEA Frascati, most of which are carried out by the Fusion Department. Experiments with the Frascati Tokamak Upgrade (FTU) require liquid helium to cool down the magnets of the auxiliary heating facility ECRH (Electron Cyclotron Resonance Heating), as well as to freeze solid deuterium pellets to be injected at high speeds in the fusion plasma, and to keep cold plasma diagnostics. The Superconductivity Division also uses large amounts of liquid helium for experiments with large superconducting magnets, as well as to test critical components at low temperatures.
Liquid helium however is rather expensive, so that, provided that the overall consumption is large enough, it may be advantageous to recovery the helium evaporated during experiments. The collected gas can then be liquefied again, subject to prior purification, allowing recycling the same amount of helium many times. In spite of the initial capital investment required to erect the necessary facilities, such a practice may significantly reduce the costs of liquid helium supply.

A recovery facility and a helium liquefier have been operating at ENEA Frascati since many years (the first helium liquefier was installed in 1956). The latest upgrade was carried out by the end of April 1999, with the erection of a new helium liquefier, supplied by Linde Cryogenics Ltd. (UK)., in place of an obsolete plant built by Cryogenic Technologies Inc. (CTI U.S.A.) and installed in the seventies.

Liquefier plantFigure 1 - The coldbox of the Linde TCF20 helium
liquefier (on the right) and the liquid helium
storage dewar (in the middle)
The new Linde helium liquefier consists of a TCF20 cold box (figure 1) equipped with an internal auto purifier and two turbine expanders for gas pre-cooling, a screw driven KAESER DS241 recycling compressor equipped with its oil removal system (figure 2), a line drier, a 7 m3 pure helium buffer tank, a pressure control panel, and an analytical panel equipped with a purity monitor and a moisture meter.

During the process, the liquid helium is collected in a 1000 litres self-pressurized storage dewar (a suitably thermally insulated vessel) by means of a special transfer line. It is then transferred into smaller dewars, by means of a special decant line, to be distributed to the users.
compressore a viteFigure 2 Screw driven compressor KAESER
DS241 (back) with its oil removal system (front)
An Allen Bradley Programmable Logic Controller (PLC) supervises the automatic operation of the plant, whose nominal liquid helium production rates, with or without liquid nitrogen precooling, are 60 and 30 litres/hour respectively. All relevant data are constantly logged in a computer connected to the PLC, in order to monitor the performance of the liquefier. During liquid helium transfers, either to decant it from the storage dewar for distribution purposes or to cool down an experiment, helium vaporizes. The resulting gas is collected by means of a pipeline network, branching out all over the Centre, and conveyed through a manifold to the recovery gasbag placed in the liquefier building (figure 3). Once the gasbag has been inflated with helium, two multistage compressors automatically compress the gas from the bag into the impure helium storage system, which features an overall storage capacity of about 1700 m3NTP (roughly equivalent to 2200 litres of liquid helium). The recovered gas is thus available to be purified and liquefied again. Of course, helium cannot be recovered entirely, so that limited amounts of liquid helium must be periodically purchased to compensate the unavoidable losses.

Figure 3 Schematics of the helium recovery systemSchematics of the helium recovery system

Several tens of thousands litres of liquid helium are distributed to the users every year, with an overall recovery efficiency of about 86%. Table 1 summarizes the activity of the helium recovery and liquefaction facilities during the two years 2001/02.

TABLE 1 Activity of the helium recovery and liquefaction facilities during the two years 2001/02
Year Liquid helium delivered (litres) liquid
transfers for
FTU Super-
2001 9.300 79.918 1.866 91084 15.261 94.040 109.301 20,0% 86,0%
2002 7.829 60.290 315 68.434 11.680 70.441 82.121 20,0% 85,8%
TOT. 17.129 140.208 2.181 159.518 26.941 164.481 191.422 20,0% 85,9%