Demanding cooling: With helium at –100 °C
Applications such as physics experiments, detectors or laser sources require helium cooling at temperatures down to -100°C. This demanding task is addressed using two different system approaches and gas-bearing turbomachines from Celeroton.
Helium is suitable as a cooling and process gas for applications that require reliable heat transfer, chemical inertness or contamination-free operation. Because helium remains gaseous within the temperature range considered, from approximately –100 °C to room temperature, it can be cooled and recirculated in a closed gas circuit without a phase transition. Typical fields of application include physics experiments, detectors, semiconductor equipment and certain laser systems.
For helium cooling in the range from approximately –100 °C to room temperature, Celeroton pursues two fundamental system approaches: the recirculation of externally cooled helium and combined cooling and recirculation in a Reverse Turbo-Brayton chiller.
System approach A: Recirculation of externally cooled helium
In the first approach, an existing chiller provides the required temperature and cooling capacity. A specially designed turbo compressor recirculates the cold helium between the chiller and the application to be cooled. The turbo compressor design takes particular account of the low inlet temperature, the required mass flow and the required pressure ratio.
Gas-bearing technology enables oil-free and contamination-free helium recirculation. During operation, the rotor runs without contact on a gas film generated by the process gas. As a result, the system requires neither lubricants nor an oil separator. This is particularly relevant when the helium must remain clean and contact with additional operating fluids must be avoided.
Celeroton can adapt turbo compressors to the respective gas, pressure level, pressure ratio, mass flow and electrical interface. Depending on the operating point, several units can be arranged in series or in parallel. This allows helium recirculation to be tailored to the specific application without using a standard compressor that is significantly oversized for the pressure or mass flow.
This approach is particularly suitable if an appropriate chiller is already available or is to be procured separately. The generation of cooling and helium recirculation remain two independent subsystems.
System approach A: Recirculation of externally cooled helium
System approach B: Combined cooling and recirculation in a Reverse Turbo-Brayton chiller
In the second approach, a closed Reverse Turbo-Brayton cycle provides both cooling and helium recirculation. The central components include a gas-bearing turbo compressor, a recuperator, a gas-bearing turbo expander, the required heat exchangers and the associated inverters.
The turbo compressor first increases the pressure of the helium. The compressed process gas then releases heat at a warm heat exchanger. In the recuperator, the high-pressure stream is precooled by the returning cold helium. The turbo expander then expands the gas, reducing its pressure and temperature. The cold helium absorbs waste heat directly from the application as it flows through it and returns to the turbo compressor via the recuperator.
The turbo expander is equipped with a generator. This allows part of the energy released during expansion to be recovered electrically and used within the system. The inverters, which control both the compressor and the expander, also allow the turbomachines to be controlled independently and thus enable dynamic fine-tuning of the mass flow (cooling capacity) and the expansion ratios (cooling temperature) during operation.
The Reverse Turbo-Brayton approach does not require precooling with a separate refrigerant circuit. Helium serves both as the process gas for generating cooling and for removing heat from the application. This allows cooling and recirculation to be implemented in one integrated system.
System approach B: Combined cooling and recirculation in a Reverse Turbo-Brayton chiller
Which of the two system approaches is technically and economically appropriate depends, among other factors, on the target temperature, the required cooling capacity, the mass flow, the pressure level and the interfaces to the process to be cooled.
From the turbomachine to the complete system
Celeroton combines the development of aerodynamics, gas bearings, electric motors, power electronics, control and thermal design. This system expertise makes it possible to coordinate the turbo compressor, turbo expander and inverters with one another and with the helium circuit.
Which system approach is more suitable cannot be determined solely on the basis of the target temperature. If a chiller is already available, separate helium recirculation may be the appropriate solution. If cooling generation and recirculation are to be combined in one system and a separate refrigerant circuit is to be avoided, a Reverse Turbo-Brayton chiller is an option.
Depending on the requirements, process gases other than helium, such as argon, nitrogen or air, can also be used. Celeroton designs the system on the basis of the specific application and the required operating conditions.
For an application-specific design, contact us at moc.notorelec@ofni or by telephone at +41 44 250 52 20.
