By nature of these products, the superconductor material normally needs to operate in a temperature range of 15-60K (-258 to -213ºC)., which rules out liquid nitrogen, with a minimum (workable) temperature of 64K, as a coolant. The most common cooling media for these kind of systems is (pressurize) gaseous helium (gHe). Sometimes liquid neon is used, with a boiling temperature of 27K.
As helium has a boiling point of 4K, in gaseous state, it can be applied in a wide temperature range, with (relative) easy handling and limited safety hazards. However compared with a cryogenic liquid its thermal mass (as being gas), and therefore its capability to remove heat, is much less. So it needs to be compressed (pressurized) to create sufficient mass flow, normally at a pressure of 10 to 20 barg.
When helium is circulated from the cold source to the application and back, the rotating superconductor, is cooled to the desired operating temperature. For this purpose, Stirling Cryogenics products make use of impellers manufactured by DH industries other brand CryoZone.
CryoFan
These impellers named CryoFans are specially designed for this (cryogenic) purpose, i.e. to operate in vacuum housings with as low heat input into the cryogen as possible.
In Stirling’s SPC-T's (Stirling Process Cryogenerator) this CryoFan comes together with the Stirling Cryonics two-stage Cryogenerator, creating a system that can remotely cool applications down to 15K with high cooling power. Two-stage Cryogenerators have two cooling loops, one at 80K and one at 20K and both have there individual CryoFan in an independent loop. By using pressurized helium the system transports cooling power to the application and returns slightly warmer gas in a closed loop. The 80K loop can be used to shield the 20K loop or as pre-cooling, therefore reducing the thermal losses for the 20K loop.
The load represents customers application (magnet, generator, motor, chamber etc.). Light blue Q1 is the thermal load on the 80K loop, darker blue the thermal load on the 20K loop.
Gas Cooling Unit
The built up a gas cooling unitt starts with a basic two-stage cryogenerator (or a one-stage cryogenerator if gas cooling in the 50-100K range is required). To this a cold head will be added which contains either one or two cold heat exchangers which extract the heat out of the gas. In this cold head thesee graph). The system has a “zero watt” temperature of 13K.
The included control panel operates and safeguards the cryogenerator and controls the fan(s). The speed of the fan is fully adjustable to control the flow of helium gas in the closed loop adjusting to customers requirement (gasflow, delta T).
Typical applications utilizing the technology are HTS motors and generators where cold (20K) gas is injected in a rotating device, High field (HTS) magnets where the cold gas cools the HTS coils and (non HTS) vacuum/thermal conditioning chambers that needs to be cooled down to 20K by circulating helium gas through the thermal shrouds.
Products
For use in applications as described the following products may be considered: