R134a recovery system for the resistive plate chambers (RPCs) at the Compact Muon Solenoid (CMS) experiment.

A new gas recovery system for the resistive plate chambers (RPCs) at CMS (Compact Muon Solenoid) experiment was installed, commissioned, and tested to find the optimal operating configuration in terms of recovered gas quality and system efficiency. The aim of the system was the R134a recuperation, using a distillation process, to solve both the environmental and the economical issues, considering the high global warming potential, of 1430, and the high specific cost of R134a. The system works with an input mixture composed by 95.2% of R134a (1,1,1,2-Tetrafluoroethane), 4.5% of isobutane (C4H10), and 0.3% of sulfur hexafluoride (SF6). The recovery system was designed in four units: the electric rack, the distillation unit, the pumping unit, and the storage unit. The electric rack contains all the electrical connections, in an ATEX environment, for the system automation. The distillation unit is composed by two racks, closed in an ATEX environment, each one containing two columns for the separation process. Each column develops vertically with two buffers: the top and the bottom buffers. The top buffers are cooled down with a series connected cooling system, whose temperature is controlled by a chiller (Lauda Integral XT 280), while the bottom buffers are kept warm with a series connected cooling system controlled by a second chiller (Huber Ministat 125). The pumping unit (Gas booster 7LG-TS-7) is devoted to the recovered gas compression to the storage unit, which consists of a stainless-steel cylindrical tank with a storage capacity of 400 kg. The system performances were investigated in terms of buffer pressure and temperature, system filling and emptying flowrates, using thermocouples, absolute and differential pressure sensors, and flowmeters. Moreover, the gas quality was analysed quantifying the isobutane concentration in the recovered mixture, through the analytical columns PPU (PoraPlotU) and MS (Molecular Sieve) 5Å of the gas chromatogram. Firstly, tests were performed in manual mode, filling and emptying column by column manually. Secondly, the system managed to run automatically, 24 hours per day, with a fraction of the recovered gas sent to the RPCS mixer, to reduce the flowrate of fresh R134a to the chambers. The system was tested modifying five main operating parameters: filling flowrate (200 l/h, 300 l/h, and 400 l/h), emptying flowrate (400 l/h, 600 l/h, 700 l/h, and 1000 l/h) top buffer pressure (10 mbar, 20 mbar, 30 mbar, and 50 mbar) and temperature (-36.5 °C, -36.2 °C, and -36.0 °C), and bottom buffer temperature (14 °C, 17 °C, 20 °C, and 23 °C). The target efficiency was 80%, with an upper bound isobutane concentration of 1000 ppm.