Heat is a formidable enemy in MR and CT chiller design

by John W. Mitchell, Senior Correspondent | September 09, 2015
From the September 2015 issue of HealthCare Business News magazine

Bill Bonneau, principle engineer, MR heating and cooling at GE Healthcare, said the trend is for their customers to order MRI systems with a chiller package. He said that customers are seeking standardization and a few vendors to work with on major installation, expansion or a new-build program.

Bonneau noted that several recent chiller technology advances reduce chiller load and power consumption. Free cooling is a design that uses ambient air to remove excess heat from the MRI system and room environment prior to using the chiller. Free cooling is typically a standalone optional piece of equipment supplied by the chiller manufacturers and designed to work with the outdoor chiller directly.

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Also, advances in temperature-controlled, variable-speed condenser fans and compressors on the chiller help reduce power consumption. Bonneau noted for chiller manufacturers, the environmental trend to reduce hazardous waste limits the selection of cooling fluids that are currently optimal.

Loeffler at UNM said they do not include chiller packages with their MRI and CT purchases. Instead, they engineer both primary and redundant chilling requirements into their existing in-house chiller systems. This consists of three, large, industrial 300- to 400-ton chillers and three 800-ton cooling towers.

“These units will run 25 years if you take care of them, and another five [to 10] years past that if you don’t overuse them,” Loeffler said. Bob Bachman, president of Advanced Mobility, a subsidiary of KY Trailers, is a man who knows a few things about imaging chillers. The company builds specialty trailers for clients who need everything mobile, from television production studios to mobile MRI and CT scanners. “We have to design specs that meet the OEMs’ specifications for temperature range, chilled water temperature and flow, and present solutions that meet or exceed those specifications,” said Bachman.

He added that the units have to be capable of running 24/7. To engineer this kind of performance, Advanced Mobility has been working with just a few chiller makers for the past decade. He said that with only one exception, there has not been any significant OEM change in the past 10 years in mobile imaging — so familiarity works in their favor. But even so, the chillers in their rigs are designed to operate under extreme and stressful conditions.

“We design-in overcapacity and redundancy,” Bachman said. He noted the chilled water and air-cooling to keep the imaging technicians and patients comfortable in the trailer are designed off the same unit. Depending upon the need, the load between water and air can be weighted one way or the other. “We have two compressors and fans, so if one shuts down the unit will continue to operate,” he said. “Our typical chiller is at 225,000 BTUs, so we over-engineer the design. And, of course, the first priority is to keep the chilled water around the shield collars for the imaging equipment.”

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