Early this year, the Lab's Central Helium Liquefier (CHL) was shut down for extended maintenance and equipment upgrades. It was the first prolonged hiatus since CHL operations began in 1991.

Over the past seven years, the CHL's record of availability stands at 98%--a singular accomplishment when compared to industry standards that peg optimal performance to a two-year servicing schedule.

Aerial view of the Central Helium Liquifier on the accelerator site.

After a one-month shutdown, the CHL was back on-line by February 5. By the time work was completed, a new standardized control system was added and technicians had upgraded the CHL's helium-distribution piping. Additional piping to gas storage tanks and the liquefier's charcoal adsorbers were also installed. The charcoal adsorbers purify helium at 80 degrees Kelvin or -320 degrees Fahrenheit. Standby compressors were cross-connected to the existing compressor system. Valves were inspected and repaired as needed, and various strainers and filters inspected and cleaned.

During the outage, the CHL's standby refrigerator kept the accelerator chilled to approximately 4 degrees K. Normal operating temperature is 2 degrees K (-456 degrees F), or 2 degrees above absolute zero.

"This was the big shutdown. And we're happy; it went well," says Claus H. Rode, Accelerator Division deputy division leader. "The next time we do the maintenance it will be easier. We'll already have gone through the learning curve."

The $16 million CHL is thought to be the largest cooling plant of its kind in the world, producing 10 times more superfluid helium than all other superfluid helium refrigeration facilities combined. And it is vital. Without a helium liquefier, superconducting-based physics research at the Laboratory would come to an abrupt halt, idling dozens of experimenters.

"Other systems are more forgiving. Turn off one component and you can work around it," points out Accelerator Division Associate Director Christoph Leemann. "This one cannot hide. Turn off the CHL and it's instant death for beam operation."

Claus Rode gives a full measure of credit for the CHL's extraordinary availability record to Cryogenic Group leader, William C. Chronis, and those who work with him.

"Bill Chronis and his crew have done a fabulous job of achieving excellent availability," Rode says. "CHL operation has been so reliable that I can almost forget about it." He describes the near flawless running of the CHL over the past several years as a testament to the efforts of the Cryogenic Group.

Chronis says the Cryo Group's constant challenge is keeping contamination, i.e., water vapor, air gases or any foreign material, from harming any CHL component. He explains that, at the liquefier's ultra-low operational temperatures, even a small droplet of frozen water allowed to migrate from a heat exchanger to a turbine's gas bearing can "destroy a turbine in a millisecond."

"Contamination is our biggest fear," Chronis says. "Our standards are higher than the ones for sterile air in some hospitals. For water and nitrogen we're talking less than one part per million: micrograms per cubic meter."

Ironically, the CHL's rocky installation by the subcontractor seems to have served as a prelude to its smooth-running present. After repeated project delays and difficulty in meeting strict liquefier design specifications, the Lab was forced to take over CHL construction from the original contractor.

"The construction phase was ... well, let's just call it a real adventure, a cliffhanger," Leemann recalls. "There were moments when I really lost sleep worrying. Until we took over the contract."

More CHL upgrades are scheduled. Chronis and his crew are building a backup two-degree-Kelvin cold box in case of catastrophic failure of the CHL's primary cryogenic compressors. The cold box is scheduled to be commissioned during the January 1999 shutdown. The Cryo Group will modify the CHL system to accommodate the additional cold box.

The Lab is planning to raise the accelerator energy to 6 billion electron volts by late 1999. Such an increase in energy will raise the heat load to the cryogenics plant--and the demand for additional cooling capacity.

In the aftermath of the successful maintenance, Chronis seems satisfied with the pace and scope of liquefier improvements.

But he believes that even better performance can be coaxed from the CHL in the coming months and years. "We think we have a more reliable CHL," he says. "We'd like to be at 100 percent every day. That's obviously an impossible goal. But 99 percent-plus availability is possible."

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Updated June 27, 2003