JOULE-THOMSON (JT) CRYOGENIC COOLING

JOULE-THOMSON (JT) CRYOGENIC COOLING

Joule-Thomson cryocoolers, also known as Linde-Hampson cryocoolers, are based on the principle of Joule-Thomson expansion. A schematic view of a JT cryocooler is shown in Figure 1. These cryocoolers function through the continuous flow of refrigerant, expanding from high pressure to low pressure at the cold-head. This expansion drops the refrigerant temperature to the required cryogenic level. The refrigerant is then recompressed to high pressure, passing through an after-cooler to bring it back to ambient temperature. JT cryogenic systems enable the use of lengthy connecting lines between the compressor and cold head. Our previous research demonstrated the successful operation of an IR camera with a JT cryocooler, placing the compressor 25 meters away from the camera. Additionally, these connecting lines can be flexible, allowing for on-gimbal positioning of the cold-head while keeping the compressor off-gimbal. A key advantage of JT cold-heads is their lack of moving parts, eliminating vibration and noise emissions. This design results in high reliability, and longevity, and negates the need for regular maintenance. Moreover, JT cold-heads don’t emit heat as the refrigerant both enters and exits the cold-head at ambient temperatures.

Open cycle JT cryocoolers:

Open-cycle JT cryocoolers are driven by pressure vessels, instead of compressors. Usually, the vessel contains a high-pressure refrigerant (a few hundred bars), which is discharged through the JT cold-head on demand. This allows to obtain cooling without any power consumption on-site because the work is invested in pressurizing the refrigerant at the factory. Open-cycle JT cryocoolers often operate with pure refrigerants, which are determined mainly by the required cooling temperature. This is the only technology that allows extremely fast cool-down of a few seconds only. In previous work (2008² , 2014³ ) we investigated fast cool-down of various open cycle JT cryocoolers. The major drawback of open-cycle systems is their limited operating time. When the vessel is empty, the cooling effect stops.

Mixed-Refrigerants

Mixed refrigerants aim to replace pure refrigerants, to allow lower operating pressures, and therefore, closing the cycle with compressors (instead of pressure vessels), and obtaining a continuous operation (24/7). The mixture composition is determined to comply with the cooling requirements: cooling temperature, cooling power, cold-temperature stability, refrigerant flammability, environmental restrictions, and more. We have much experience in researching and developing mixed refrigerants for different JT cryocoolers (2012a⁴, 2012b⁵, 2014a⁶, 2014b⁷, 2014c⁸, 2016a⁹, 2016b¹⁰).

Advantages of JT cryogenic cooling systems

• Performance:

  cooling temperatures: 70°C down to -200°C, cooling power: from tens of milli-Watts to tens of kilo-Watts. There isn’t a limit for the cooling capacity, because it is a function of the refrigerant’s flow rate, which can be determined to comply with any cooling power demand.

• Cold-Head:

  small dimensions, absence of moving parts (no vibration and no noise), no heat emission, flexible design to comply with the customer's needs.

• Refrigerant:

  pure or mixed, possibility to be optimized for high efficiency, flammability requirements, complying with environmental restrictions.

• Compressor:

  commercial oil-lubricated, self-made oil-free, possibility to redundancy.

• System:

  long distance between compressor and cold head (tens of meters), flexible connecting tubes between compressor and cold head. The absence of moving parts in the cold head makes the compressor the main component that might need maintenance. Since the compressor can be located away from the cooled object, maintenance is simple and fast.

 

Frequently asked questions

1. What is a Joule–Thomson (JT) cryocooler and how does it work?

A JT cryocooler is the Linde-Hampson machine which leverages the Joule–Thomson effect. It consists of a continuous flow of a pressurized gas which expands through a valve or nozzle, where it cools rapidly. A recuperative heat exchanger is incorporated, to obtain cryogenic temperatures. An open cycle JT cryocooler normally operates with pure gases at very high pressures, supplied by a pressure vessel. A closed cycle JT cryocooler usually operate with mixed gases at much lower pressures, supplied by a compressor.

2. What makes JT cryocoolers different from other cooling technologies (Stirling, Pulse Tube, GM)?

- No moving parts at the cold head - no vibration, no acoustic noise.

- Compact and modular design of cold head - suitable to miniature devices, as the compressor can be located far from the cold head, connected by flexible tubing.

- Fast cooldown times - JT cryocoolers is the only technology which can provide cooldown in a few seconds vs. longer ramp-up in Stirling/GM.

- High system availability - JT cryocoolers don’t have any moving parts at the cold head, making the compressor the only maintainable component, which can be replaced in a fast easy procedure. The ability for compressor redundancy, which doesn’t exist in Stirling type cryocoolers, significantly contributes to the system availability.

 

3. What temperature ranges and cooling power can JT systems achieve?

Typical ranges: 70 - 200 K.

 

4. What are the key advantages of JT cryocoolers for mission-critical applications?

- No moving parts at the cold head - no vibration, no acoustic noise.

- Compact and modular design of cold head - suitable to miniature devices, as the compressor can be located far from the cold head, connected by flexible tubing.

- Fast cooldown times - JT cryocoolers is the only technology which can provide cooldown in a few seconds vs. longer ramp-up in Stirling/GM.

- High system availability - JT cryocoolers don’t have any moving parts at the cold head, making the compressor the only maintainable component, which can be replaced in a fast easy procedure. The ability for compressor redundancy, which doesn’t exist in Stirling type cryocoolers, significantly contributes to the system availability.

- Modular integration - compressor can be remote, reducing heat rejection and vibration near payload.

- Scalable - suitable for handheld devices, airborne systems, satellites, or industrial setups.

 

5. What are the main challenges or trade-offs of JT cooling?

- Efficiency: JT is generally less energy-efficient than Stirling or Pulse Tube.

- Open cycle cooling duration: open cycle JT cryocoolers are driven by a pressurized gas from a pressure vessel. Once the vessel is empty, the cooling effect ends. Therefore, these cryocoolers are often used for limited cooling durations. Continuous operation is possible by replacing pressure vessels, a procedure which requires logistic effort and trained technicians.

- Closed cycle technology maturity: an evolving technology which only recently penetrates the market.

 

6. How do open-cycle and closed-cycle JT cryocoolers differ?

- Open-cycle: uses pre-pressurized gas bottles; provides instant cooldown but limited duration.

- Closed-cycle: includes a compressor and refrigerant loop; offers continuous cooling and long operational life.

 

7. Where are JT cryocoolers most valuable today?

- Infrared cameras & detectors (defense, surveillance, scientific instruments).

- Aerospace (satellites, missile seekers, space telescopes).

- Medical devices (cryosurgery, imaging).

- Industrial & R&D (superconductivity, liquefaction processes).

 

8. What about miniaturization and micro-JT coolers?

JT cryocoolers suggest the most miniature cold head, among all cryocooling technologies. This is valid for existing manufacturing methods, and more dramatically for manufacturing technologies that are in R&D stages, such as etched micro channels and 3D printing.

 

9. What future trends are shaping JT cryocooler development?

- Mixed refrigerant optimization

More efficient operation across wider temperature ranges → increasing COP (reducing power consumption).

Compatibility with flammability demands and environmental restrictions.

- Miniaturization - micro-JT coolers for portable devices.

- Sorption JT cryocoolers - combining sorption compressors to obtain a complete active cryogenic cooling system without any moving parts, yielding highest reliability and lifetime.

- New applications - data centers (superconducting circuits), quantum computing, and next-gen aerospace missions.