Hot tips for coldheads: five industry professionals share their secrets of success
September 05, 2015
I’ve moved about in the medical equipment industry for a long time. But even after all these years, I still find myself learning. Recently, I reached out to a few friends in the industry to get their top coldhead tips to share with HealthCare Business News readers. As you probably know, coldheads sit on top of the MR magnet and help to keep the magnet cool in order to prevent boil off of the liquid helium. A boil-off can be a real problem, especially if it escalates to a quench. At that point, you have the potential for harm to a patient or staff, loss of revenue from machine downtime and, of course, an added expense since the helium will have to be replaced.
With a reasonable amount of planning and knowledge, it’s fairly easy to avoid coldhead problems. Fortunately, James Beier of Sumitomo, Chris Matthews of Oxford Superconducting Technology, Sean Mykleby of Cool Pair Plus, Larry Knight with Altima Diagnostic Imaging Solutions, LLC, and Marshall Shannon with Image Technology Consulting, LLC, were willing to provide much of the information you’ll need to avoid coldhead problems.
According to Larry Knight, coldheads can be divided into two categories — 10K (ten Kelvin) and 4K (four Kelvin) systems. The two systems have some key differences. The first, the 10K, is typically found on legacy systems like the Siemens Symphony and Harmony, Toshiba’s Excelart, Picker magnets and GE’s pre-LCC magnets, says Larry. These coldheads keep the shields of the magnet cool to limit cryogenic loss, but they do not re-condense the helium.
The fact that these coldheads don’t re-condense the helium means that the liquid helium will turn to gas and boil off over time, with the rate generally being about .7 to 1.2 percent per week. According to Larry, the helium boil-off rate is the best indicator of the 10K coldhead’s performance. If your helium loss is about .1 percent per day, you likely have a healthy coldhead. When you start climbing beyond that and have indications that your shield temperature or magnet pressure is rising, that can be a warning sign that your coldhead needs to be serviced or replaced. Even with a coldhead that’s functioning properly, you can expect to spend money each year recharging your helium and you should budget accordingly.
The 4K coldheads are found on newer systems across the range of OEM MRs. These coldheads re-condense helium and bring it back to a liquid state. These systems are often touted as “zero boil-off.” Even so, many will have a helium loss in the neighborhood of 10 to 200 liters per year. While significantly less than what one should expect with the 10K system, it’s still a factor you should be aware of. Coldhead performance for the 4K is easy to monitor, says Marshall Shannon. “These systems should always have close to the same helium level over time,” he says. In other words, if you take your helium reading on Jan. 1 and see the level is at 78.8 percent, when July comes, you should have a reading of 78.7 percent.
The 4K coldheads depend on constant magnet pressure which facilitates the coldhead to re-condense the gaseous helium. Each OEM has different specifications for this pressure. Siemens specs this pressure to be 15.3 psig, and GE specs this pressure at 4 psig. Deviation from OEM-recommended pressures will result in a higher boil-off of the helium.
Both the 4K and 10K systems depend on a constant flow of water cooled by the chiller to cool the coldhead compressor, so consistent operation of the chiller is critical to good coldhead performance. Both water temperature and the flow rate are critical to the coldhead compressor being able to remove heat from the coldhead circuit.
Too little water flow will result in the compressor overheating and expelling oil into the helium gas, causing poor coldhead performance. Conversely, a water flow rate that is too high will also result in overheating, as the heat exchanger will not have enough time to dissipate the heat into the water, which will also result in compressor overheating.
The adsorber is another component worth checking periodically. It’s like the oil filter in your car, but instead of removing contaminants from your oil, it removes contaminants in the gaseous helium driving the coldhead. Since the coldhead depends on high-purity helium gas to perform well, replacing the adsorber every 18 months will ensure the helium gas is free of contaminants and it will extend the life and performance of the coldhead. The adsorber should be replaced proactively as there are no indicators that an adsorber has reached its limit of capacity to remove oil and contaminants, so it is worthwhile to create a reminder.
The group contributed their thoughts to create a list of 10 items that should be checked in order to get the most life out of your MR unit.
1. Helium level
2. Magnet pressure — as read on the magnet turret gauge
3. Shield temperature(s) - 80K shields should be ~65K, 50K shields should be ~40-45K
4. Chiller water inlet and outlet temperature(s) — Inlet ~65F, Outlet ~78F
5. Compressor pressure while compressor is running. This should run about 295-305 psi
6. Be sure you are replacing the adsorber in the compressor every 18 months
7. Listen to and be aware of how your coldhead sounds when it is running normally. A change in sound, with the coldhead getting louder, can indicate an imminent failure
8. Have your chiller serviced regularly (at least once per year). This service is best performed in the spring, before the greatest heat load arrives during the summer months
9. Clean any strainers or filters in the water circuit regularly to ensure good consistent water flow at the OEM-specified rate
10. Ensure that your compressor is running all the time, 24/7.
Sean Mykleby suggests checking the helium levels along with the magnet pressure, and recording the stats in a log every day. This will help you to track trends and more readily notice any deviation from those trends, thereby potentially getting a warning before serious problems occur. He also suggested that for GE magnets, to take the RUO and re-condenser temps and log them as well. Sean agrees that chilled water temps and water flows are a great indicator of the functionality of the chiller, and a good chiller will extend the operational life of the coldhead and compressor.
Chris Matthews also suggests keeping a log. But he suggests monitoring and logging the compressor and chiller hours. He also suggests checking oil and gas lines for contamination using a black light. According to Chris, it’s a good idea to check the magnet turret for ice or heavy condensation — both indicators of a refrigeration problem. Jim Beier suggested relying on personnel to check the heater duty cycle (HDC) on the magnet. Most magnets have an internal heater.
Without proper control, the coldhead will try to drive the magnet pressure down. The magnet monitoring system will monitor the vessel pressure and send electrical current to the heater whenever the pressure gets too low. The heater will turn on and boil some helium, increasing the vessel pressure. The ratio of on/off of the heater is called the “heater duty cycle.”
Most magnets record the duty cycle. If you download it, you should see a steady state over time. If the system starts to fail (it takes six to eight weeks from the start to a complete failure) the duty cycle will eventually fall to zero. On GE magnets, the duty cycle is anywhere from 20-30 percent. When the system starts to deteriorate, the duty cycle will decrease, and when it hits zero, then the magnet pressure will start to increase. The HDC gives you advance notice, whereas the other, more obvious symptoms show up after the system has failed completely.