Cooling is the life force of any MR unit. Yet this vital function is often treated as an afterthought, setting up expensive consequences.
An MR’s cooling platform consists of three components: chiller, cold head, and compressor. All are equally vital in regulating the chill water and helium that moves heat away from an MR magnet.
MR cooling problems tend to be seasonal. Kim Elmore, operations and business development manager for KR Products, which specializes in building refrigeration equipment, said the toll of delayed preventive maintenance emerges with the spring season. Every year between April and September, she fields a couple of phone calls a week concerning faulty MR chillers.
"When I ask them what the glycol percentage [to water] is, I often find they don’t know what I’m talking about,” said Elmore. “I even find that in some cases, the caller tells me that they refill their reservoirs with straight water.”
This despite warning stickers all over the big chiller units and the owner’s manual about the importance of maintaining the proper ratio of glycol to water. Optimal levels are between 30 to 50 percent glycol, depending upon the latitude of a location. As with the antifreeze in a car, Elmore said colder temperatures call for higher glycol levels in a chiller.
Sumitomo producing more than 2,500 cold heads a year with a warranty return rate of less than 0.5 percent. Photo courtesy of Sumitomo Cryogenics of America
"If it gets below 30 percent, the inhibitor won't work and encourages bacteria and algae in the fluid,” said Dan Kehl, service technician at Dimplex Thermal, a chiller and cooling systems provider. “If that happens, the heat exchangers can become fouled … and it will require two days of downtime to clean the entire fluid system."
To avoid this, Kehl recommends two preventive maintenance checkups per year, one in the spring and one in the fall, to thoroughly inspect the fluids and take refrigeration readings, as well as clean the unit.
“Part of the problem is these big chiller units are often up on the roof, out of sight. No one thinks about them until something starts to go wrong,” said Elmore. “Without the proper glycol level, the heat exchanger will burst, resulting in a compressor failure.”
The cooling challenge defined
MR cooling should be thought of as a trifecta, according to Ken Sourounis, managing partner at CyroSRV, a company that designs and produces MR cold heads. The three components — the cold head, chiller, and compressor — are linked operationally; a failure in any of the trio equals MR trouble. He said the goal of this "cryogenic-sub system” is to run in perfect harmony with the vacuum and liquid helium.
“All we’re trying to do is keep the MR cold enough, so the coils in the magnet have no resistance and therefore are superconductive, creating the electromagnet," said Sourounis. “It’s a three-stage removal of heat. If any of the components are failing the end result is poor cold head performance.”
Filtrine is a manufacturer of MR chillers.
The function of the cryogenics subsystem is to cool the helium gas enough to convert it to a liquid and maintain proper pressure on the magnet. Any extended combination of power loss or cooling capacity, he added, will slowly build pressure and evaporate helium in the system; the magnet will stop working within days. Unfortunately, such a failure is not uncommon.
“Often when we go in for service, it’s after the cold head pressure has gone over the OEM safety relief valve amount,” said Sourounis. “The difference between normal operating pressure and the safety relief valve is how much time you get before you start losing helium.”
In addition, he said, there are two other relief valves. The last valve is the largest and, if activated, it will quench, releasing the helium gas forcefully and loudly — an expensive proposition. Even just a gradual loss of two to three percent of helium a week costs about $2,000 for 99.9 percent ultrahigh purity, alpha helium gas, (medical grade helium is only 99.5 percent pure and should never be used in an MR, Sourounis added).
The first line of defense in MR cooling is staff awareness. Troy Teegerstrom, field service engineer with Cool Pair Plus, a provider of chillers, cold heads and compressors, recalled one instance when an MR tech was listening for the wrong sounds. Via remote monitoring, he was able to see that the site that had lost 10 percent of the helium in its MR.
"I called the site to check-in, and the tech told me the MR had never sounded better," he recalled. “I asked the tech to go into the RF room to listen for the cold head chirping sounds. There was no sound because the cold head had been shut off all weekend. The tech had equated the cold head silence as the 'MR never sounded better’.”
Another important consideration is the variability among different cold heads and how dependable they can be. James Beier, vice president of service for Sumitomo Cryogenics of America Inc., has seen everything from cold heads that were rebuilt using handmade parts, to others that were simply repackaged without being refurbished.
“The installation of the cold head is just as important as the quality of the cold head for long life and minimizing cost,” he added. Sumitomo follows a vigorous quality control program, according to Beier, producing more than 2,500 cold heads a year with a warranty return rate of less than 0.5 percent.
Cool Pair Plus offers daVinci Cryogen Management Systems for remote monitoring.
An MR cooling failure may start with a single component, but left unchecked it can have a cascading effect on a facility’s core mission, cautioned Turner Hansel, process chiller division manager for Filtrine Manufacturing Company, which makes MR chillers. "The loss of a single MR due to a chiller failure can cost a facility tens of thousands of dollars per day (in lost revenue), as well as the loss of Trauma One status."
Prevention, maintenance, redundancy, and monitoring
Because they design redundancy into their chillers with two refrigeration circuits and two pumps, Kehl with Dimplex Thermal said his company’s systems seldom overheat unless there is a site issue, such a power interruption, or the fluid system springs a leak — which is a relatively easy fix. Plus, now that all sites have access to well or city water as an emergency cooling response if need be, quenches have become an increasingly rare event. This is good news, considering the cost of replacing helium after a quench can be between $30-50,000, said Kehl.
For optimal performance, Elmore with KR Products said there are four preventive maintenance steps every MR operator should take to ensure peak chiller MR cooling:
- Check the glycol level and inspect the strainers for rust and algae.
- Inspect the pump for leaks.
- Check the fan and motor for excessive vibrations, which can cause the unit to draw air effectively.
- Wash out the filter screens and condenser, similar to spraying out the outside fan unit on a home air conditioner.
"Every chiller for an MR must have backup cooling in case of a failure," said Hansel. "There are many options for backup cooling that vary in cost. However, any backup cooling is inexpensive insurance for the facility and should not be overlooked."
Remote monitoring is another relatively inexpensive way to reduce magnet maintenance and helium loss, according to Teegerstrom. Cool Pair Plus offers its customers the DaVinci remote monitoring system, a non-invasive stand-alone solution that can predict cooling failure and notify the operator in real-time.
The two most common culprits that shut down an MR cold head have to do with facility water and power. Many times, Beier explained, those systems are in areas where staff are not present. He recommends a remote monitoring service that sends an alert, via text or email, if there's a water or power supply problem.
“We purchase and use a model that supports all three OEMs,” he added. “Be careful who you buy these from, because not all of them comply with FDA requirements.”
MR cooling advancements and the future
One of the most significant advancements in MR design in recent years is the switch from 10K to 4K magnets which require less helium to maintain.
“10K systems require helium top-offs each year, averaging $10,000, whereas the 4K systems, if maintained properly, don’t require top-offs,” said Beier. “This is why there’s been a shift from 10K to 4K.”
The next generation of cold heads offers 25 percent more heat capacity handling, he added. This means longer life between failures and the ability to keep marginal magnets better under control. He believes these two features — less helium use and better heat handling capacity — will save customers money.
Hansel pointed to the many software advancements OEMs have made to improve MR capability, but said the MR heat load has remained about the same. Fortunately, advancements in new refrigerants and electronic expansion valves are allowing the cooling system to respond more quickly to electric load fluctuations.
By relying on pressure-sensitive pulse tubes and solenoids (rather than valves), the next generation of cold heads will last longer and have fewer moving parts, predicted Sourounis. He also thinks that in the future MR units will require even less helium and in some cases the cold head itself may become obsolete, which is being seen now in some smaller extremity magnets.
At least one scanner manufacturer has already built a pre-clinical MR that does not require a traditional cooling jacket, noted Teegerstrom. This could be another indication that in the future, helium top-offs and keep cold will become less important considerations on the day-to-day basis.
Until they become more self-sustaining, Sourounis said the goal of anyone trying to maintain an MR unit is pretty simple: “Identify issues before they become larger — while they are cheaper, easier and faster to repair. That's the takeaway.”