by
Philip F. Jacobus, CEO | September 05, 2015
From the September 2015 issue of HealthCare Business News magazine
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
