Controllability and temperature stability as key performance characteristics
Modern applications such as quantum computers, superconducting cables and magnets, cryogenic test benches, and medical systems place high demands on temperature stability. Even slight deviations can compromise measurement accuracy, material properties, or system availability.
Gas-bearing high-speed systems offer several advantages in this context. The contactless bearing arrangement, which uses herringbone gas bearings, creates a stable gas film that prevents mechanical wear while ensuring exceptionally smooth operation. The continuous flow pattern reduces pulsations and facilitates control of the entire system.
The electrical control of the permanent magnet motors enables highly dynamic speed control across a wide temperature range. From startup at ambient temperature to restart under cryogenic conditions below 20 K. “Our systems can start up at room temperature and restart from a cold state,” explains Moser. “This significantly increases operational reliability and supports reproducible process control.”
Measurements also show extremely low levels of microvibration, with peak forces of less than 0.02 newtons. This characteristic is particularly relevant for sensitive applications in aerospace or scientific experiments, where vibrations could interfere with the operation of critical instruments.
Cold compressor and cryofan: Compression on the cold side
In addition to the traditional RTB configuration, compression or recirculation directly in the cold section is becoming increasingly important. In complex distribution networks or in zero-boil-off designs for liquid hydrogen, pressure drops must be overcome, or cold gases must be transported.
This is where the so-called cold compressor comes into play. It operates at cryogenic temperatures inside the cold box and enables an additional increase in pressure without recirculation to the warm side. Alternatively, a cryofan can be used solely for circulating cold gases. “With these components, we significantly expand the system’s flexibility,” says Moser. “The process can not only be cooled, but also actively controlled in the cold zone.”
Spaceflight as a reference application: Zero-boil-off and thermal stability under extreme conditions
The requirements in spaceflight are significantly stricter than those in industrial applications. Every gram of weight saved, and every cubic centimeter of space saved has a direct impact on launch costs and mission planning. At the same time, exceptionally high reliability is required, as maintenance in orbit or during interplanetary missions is virtually impossible. The components used must therefore remain stable, wear-free, and functional over long periods of time under extreme environmental conditions.
A key application is the cooling of cryogenic fuel tanks in launch vehicles or spacecraft. Liquid fuels such as hydrogen or oxygen must be kept at very low temperatures to prevent vaporization. For future long-duration missions, such as those to the Moon or Mars, the so-called zero-boil-off concept continues to gain increasing importance. The goal is to completely prevent evaporation losses and to provide the stored energy reliably over long periods of time.
Typically, RTB-based cooling processes are used for this purpose. A gas, often helium, is compressed, pre-cooled via heat exchangers, and then expanded in an expander, causing it to cool significantly. The resulting cold flow stabilizes the temperature of the tanks or sensitive instruments. This process places extreme demands on controllability, operational safety, and system integration.
Typically, RTB-based cooling processes are used for this purpose. A gas, often helium, is compressed, pre-cooled via heat exchangers, and then expanded in an expander, causing it to cool significantly. The resulting cold flow stabilizes the temperature of the tanks or sensitive instruments. This process places extreme demands on controllability, operational safety, and system integration.
When combined with the appropriate high-frequency converters, rotational speeds can be precisely controlled and adapted to changing thermal loads. This enables precise temperature control even with varying mission profiles. Also particularly relevant are the extremely low microvibrations in the range of less than 0.02 Newton, which eliminate the need for complex mechanical decoupling. Applications such as the cooling of infrared mirrors or highly sensitive detectors benefit directly from this feature.
For more complex architectures, the RTB can be supplemented with an additional cold-side turbo compressor. Such a cold compressor allows individual cryogenic circuits to be decoupled from the main cooling flow or to operate under conditions of increased pressure drop. This enables separate subsystems to be controlled and stabilized independently, which proves to be an advantage in modular satellite platforms or multistage spaceflight systems.
Fields of application with growing momentum
The space industry thus serves as a benchmark for exceptionally high standards of temperature stability, controllability, and reliability. In the space industry, RTB cryocoolers are also used in satellites with high payloads to deliver high thermal performance despite limited radiator surface area. Technologies that perform well under these conditions can be reliably adapted for industrial and scientific applications.
At the same time, the range of applications for controllable cryogenic cooling systems is constantly expanding. In the fields of quantum computing and superconductivity, for example, the focus is on reliably cooling cables, motors, generators, and magnets to extremely low temperatures. Physical research experiments and medical applications also benefit from stable, low-vibration cryogenic systems.
“Many of these applications require not only high cooling capacity but also a precisely defined temperature profile over long periods of time,” Moser emphasizes. “The controllability of our high-speed systems is a key feature for this.”
Oil-free, leak-proof, and low-maintenance – design advantages
In addition, the completely oil-free design, which does not use rotating seals, prevents contamination and significantly reduces the effort required for integration. At the same time, the systems are vacuum-compatible and helium-leak-tight, which is of particular importance in noble gas and hydrogen applications.
Thanks to their wear-free operation, the machines have a long service life and are designed to withstand several hundred thousand start-stop cycles. This reduces maintenance requirements to a minimum, which has a positive impact on total cost of ownership and system availability.
Conclusion
Cryocooling applications are increasingly evolving into highly dynamic, system-integrated process architectures. In addition to efficiency and power density, controllability and temperature stability are becoming key considerations.
Gas-bearing high-speed systems in the Reverse Turbo-Brayton offer a technologically mature solution for this purpose. Separate, electrically controllable compressor and expander stages, as well as optional cold compressors, enable cryogenic processes to be precisely controlled, operated stably, and flexibly scaled. This results in robust yet finely adjustable cryogenic systems that meet the growing demands of hydrogen technology, aerospace, superconductivity, research, and medicine.
