This gas system is designed to purge the six Cerenkov detectors, recirculate and filter the radiator gas, and recover the gas for storage. The gases used are Nitrogen, as a purge gas and Perfluorbutane (C4F10), as the radiator gas. Liquid Nitrogen is used as the source of the purge gas and is also used as a coolant for the system's distillation unit. C4F10 is a very dense, non-flammable, non-toxic, colorless gas with no discernible odor. It's commercial use is as a fire extinguishing agent. C4F10 is easily liquefied and is delivered as a liquid in 25 psig tanks. This system is split between the Hall B gas shed (96B) and the in-hall equipment mounted on the forward carriage. The system features two supply lines and two return lines. The two supply lines deliver Nitrogen and C4F10 to the hall. A three-way valve upstream of the flow control valve selects which gas is fed to the detector. The outlet from the each detector is fed to a three-way valve which selects whether the effluent from a detector is directed to the recirculation loop or to the distillation system. Flow rate into each detector is controlled by a mass flow control valve downstream of the three-way supply valve. The mass flow control valves are controlled by a MKS 647B controller in the gas shed control room. Pumps, in the gas shed draw the gas from the hall. The system is protected from over and under pressure by three systems. The primary system, two electronic pressure controlled proportional valves downstream of each buffer tank, act as pump throttle valves for the recirculation and distillation lines. The second protection is a set of pressure switches that measure the pressure in both buffer tanks. The position of these pressure switches is shown by indicator lights on the door to the by-pass switch box on the second level of the forward carriage. An overpressure in either tank will shut-off the C4F10 supply in the hall. An underpressure in either buffer tank will close the solenoid valve in the return line from that tank to the gas shed. The third protection is over and under pressure bubblers mounted on the second level of the forward carriage.
In normal operation the detectors are brought on-line one at a time. The detectors are purged of air by flowing nitrogen through the detectors. The nitrogen is vented through vent valves in the hall. Caution: The overpressure pressure switches do not shut-off the nitrogen supply. The nitrogen purge is intended to be used only when the system is manned. After the detector is adequately purged, the flow into the mass flow control valve is switched to the C4F10 supply and the return should be directed to the distillation unit. The distillation pump must be turned on before switching the detector to the distillation return line. The C4F10 will be trapped in the distillation unit, the Nitrogen will pass through it. When the detector is filled with C4F10 the return line should be switched to the recirculation loop. The recirculation pumps must be turned on before switching the detector to the recirculation line.