Ice on the Inner Detector

10 November 2008

Parts under ice, captured by a camera phone

Attempting to get the evaporative cooling system for the Inner Detector to run smoothly has, over recent months and years, become the source of a persistent collective headache for the ID community. Little wonder then that they were rather alarmed to find, one day in October, that large blocks of ice had formed on the racks which distribute cooling fluid throughout the silicon detectors.

Thankfully the problem, this time, was trivial and short-lived. “They aren’t there when we’re running,” explains Steve McMahon who is responsible for the ID environment, “only when we turn off certain circuits.” A simple case of a ‘dripping tap’ effect was to blame for the chunks of ice, which measured around 30 centimetres across and appeared in the cavern when the system was stopped for a few hours. “These things do just leak slightly,” says Steve. “A tiny amount of liquid can still get through internally, and this liquid expands and cools, causing the blocks of ice to form on the outside of the unit.” There is no loss of liquid as the leak is only internal.

Unfortunately, there are bigger problems afoot for the evaporative cooling system. Since August, there has been a series of small leaks on the compressors, which compress the gaseous coolant as it returns from the detector, before it is condensed for recirculation.

“To begin with, [the leaks] were just a nuisance, but then we realised they were actually cracks,” says Steve. “We’re currently running four out of the six compressors, and they’re basically shaking themselves to pieces.”

A report on the problem was released two weeks ago, concluding that the vibrations the compressors experience in their normal ID operation is beyond their design specifications. “We knew from an initial examination that the welding was not up to spec; now we know that the design is not up to spec,” says Steve.

The plan is to deal with these issues at the end of the run on December 12th. At that point, a re-design will take place and the compressors will be rebuilt by the manufacturers according to the new design. “The cost of this will be borne by them, since the failures are all mechanical in origin,” Steve assures.

Concurrently to the refurbishment, there is now talk of relocating the compressors from USA15 to the surface, to take advantage of a handy hydrostatic ‘trick’. Under this arrangement, vaporised coolant would rise to the surface, and post-compression liquid would sink. Currently the compression ratio (the pressure of the gas as it comes in, compared with the pressure as it goes out) is large – as big as 16/0.8 – and is one of the suspected reasons why the compressors are experiencing damaging levels of vibrations. If they were re-located to the surface, this ratio could be reduced somewhat, with the weight of the column of gas/liquid acting to compress the liquid further as it falls to the bottom of the column.

The idea was first proposed a couple of years ago, by Greg Hallewell of the Centre for Particle Physics of Marseille, and it was discussed further at an engineering forum held on October 30th. According to Steve, it is under consideration “on a much longer timescale, perhaps a year” as a possible solution to the problem.

For more on the history of the evaporative cooling issues, see:
ID evaporative heating problems 07/04/08
News from the Pit: ID cooling 09/06/08

Ceri Perkins

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