When liquid helium is used during an experiment it boils off, and, if no recycling system is used, the resulting helium gas is vented to the atmosphere where it cannot be reclaimed. Consequently, without a system that allows for helium recycling, liquid helium must be purchased on a regular schedule from a supplier and can be a high annual cost for a laboratory that relies on low temperature operations. Implementing a recycling system has the benefits of both reducing laboratory costs for liquid helium and allowing the reuse of a finite resource. Recycling can also protect researchers who rely on a steady supply of liquid helium from the occasional, but potentially crippling, helium supply shortages. In addition to systems that provide helium-recycling capabilities, closed-cycle cryostats, or “dry” systems, can eliminate the need for liquid helium altogether.
All cryostats have a process for removing heat from the system. In a wet system, liquid helium either surrounds the cryostat or superconducting magnet in a bath, or the liquid is dynamically transferred into the system to keep it cold. Any heat leaking into the system due to the ambient room temperature or generated by the cryostat/experiment will cause the liquid helium to boil off and the resulting helium gas to slowly escape. In some systems, additional helium losses occur with cooling techniques that rely on pumping helium vapor out of the system to lower the temperature below 4.2 K.
In dry systems, heat is removed using a closed-cycle cooler with a compressor, an initial charge of helium, and a cold head. Closed-cycle systems can use a number of thermodynamic approaches for cooling, including using a Gifford-McMahon cooler or a pulse tube cooler. Many cryocoolers have moving parts at the cold stage, somewhat limiting their use for research applications. However, recently the use of pulse tube coolers with no moving parts at the cold stage and high-cooling power at 4 K has enabled manufacturers to build more advanced dry cryostats. Currently, dry cryostats can be purchased for temperature operation from below 10 mK to 4 K, and dry systems can also be used for superconducting magnet operation.
Wet cryostats are essential in many scientific research laboratories where financial constraints, technical limitations or investments in older wet systems rule out dry systems. For example, some superconducting magnets, nuclear or electron magnetic resonance, low temperature scanning probe systems, systems with very high cooling power requirements, or ultra-low temperature systems cannot be operated with a liquid helium-free system.
Dry systems have a significant advantage in that they do not use liquid helium. However, at this time, purchasing a dry cryostat or magnet is significantly more expensive than the equivalent system that uses liquid helium. In addition, higher vibrations, cooling power limitations and other factors may preclude the use of dry systems for some experiments. For new installations, dry systems might be the right choice, but for laboratories with existing cryogenic equipment or requirements not met by dry systems, recycling helium will remain an important option.
Small-Scale Helium Liquefaction (Recycling) Systems
The small-scale liquefaction systems introduced in 2007, and now implemented by a number of commercial suppliers, liquefy helium using two-stage pulse tube refrigeration and a cooling head. The benefits of this new technology are:
- Smaller scale and much lower cost for the device;
- Operation does not require full-time staff;
- Simple construction with slower moving parts giving high reliability and multiple years between service times;
- Smaller space requirements;
- Significantly lower vibration and noise generation in the area near the liquefier.
In their simplest mode of operation, small-scale liquefaction systems liquefy helium gas directly from the exhaust of the wet system.1
The recovery system is important to the overall cost savings of the helium recycling effort. A high recovery percentage means lower costs in makeup gas. Helium gas can be recovered at greater than 98% when best practices for gas collection are used. To improve the amount of helium gas recovered, several other elements can be added to the recovery system. Such options should be discussed with vendors.
While infrastructure considerations for small-scale liquefaction systems are less significant than for large-scale systems, they must be considered and will add to the overall cost of installation.2 Numbers presented here assume a 25 L/day liquefaction and recovery system. A room with an area of approximately 200 ft2 with 10-feet-high ceilings is capable of holding the gas bag, gas storage tanks, compressor, purifier, heat exchanger and liquefier. Due to compressor noise, the system should not be part of an area that is often occupied. Metal piping from the research instruments using the helium to the recovery gas bag must be configured. The liquefier requires 3-phase power of approximately 10 kW and a chilled water supply with a heat exchanger. Additional 3-phase power for the gas compressor of approximately 4 kW is required. The energy cost will depend on the local energy rate, but will be approximately $1 per liter of helium liquefied.
Helium Liquefaction at Different Scales
Major Helium Users
Large institutions, such as a national laboratory, may serve their researchers’ yearly liquid helium requirements at a level greater than approximately 150,000 L/year with local infrastructure for supply and distribution, including liquefaction and gas recovery. These facilities are multimillion-dollar enterprises. However, there is a paucity of such facilities at national laboratories today. A large-scale facility can be initiated with the more limited objective of establishing a facility for transfilling and liquefaction and on-site distribution without comprehensive gas recovery. As availability of helium tightens, such facilities can move to set-up helium gas recovery from their more isolated laboratories and buildings, approaching the standards that are in place in most countries other than the United States. An example of a national laboratory where an institutional facility like this has been established is the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, an excellent role model for how such large facilities can be implemented to serve a diverse clientele. It is essential to have adequate gas and liquid storage, at the level of several weeks of product use, to ensure better pricing and the availability that comes with regular bulk orders for the resupply that covers facility losses.
Moderate Helium Users
For academic institutions that serve a number of departments, with annual liquid helium usage greater than approximately 30,000 L/year, it could be economically favorable to invest in small-scale liquefiers, which are typically capable of producing 25 to 50 liters of liquid helium per hour. As a starting point, institutions should develop and evaluate an implementation plan for installing a facility-wide liquefaction system with suitable storage to permit regular bulk delivery. Also, gas recovery from remote laboratories should be included, as feasible. Once fully operational, the system’s helium recycling efficiency should exceed 95%. A bare-bones liquefaction system with adequate storage for 3,000 L of liquid, and a corresponding amount for high-pressure gas storage, is an investment of approximately $1.5 million to $4 million. Generally, a full-time operator is employed. At this level of production, the use of an array of Gifford-McMahon or pulse tube liquefiers cannot be used effectively.
Small-scale Helium Users
Single investigators or small groups of laboratories can time-share with 3 or 4 liquefier units in their system including purification. These plants should be adequate for users with requirements of 25 to 80 L/day; initial system costs could be less than $500,000. For individual laboratories, losses can be reduced to being almost negligible. It is advisable to have adequate gas storage (25 L) per unit and liquid storage of 150 L per unit. Low-pressure gas storage at ~400 psi is cheaper to implement and maintain although it occupies much more space than standard high-pressure tanks. For a laboratory with a single cryostat, commercial systems have been developed to effectively reduce the boil-off from a single cryostat. Some scientific/medical instrumentation is delivered with this capability in place, or it can be retrofitted at existing installations.
Helium liquefaction using cold head technology will condense impurities out of the helium gas feed, which buildup over time on the cold head. Eventually, the helium liquefaction will slow and the cold head and associated cryogenic holding tank must be warmed and evacuated. The evacuation of the holding tank is a many-day process during which time the system cannot store or produce liquid helium. If the helium gas feed is taken directly from the output of a superconducting magnet and connected to the liquefier through metal piping, the gas feed may be sufficiently high in purity that evacuation of the liquefaction tank is seldom needed. If the gas feed is from recycled helium that has been used to cool samples, or the makeup gas is of lower purity, or any other uses that might degrade the helium purity, a purification system is required to remove impurities from the helium feed gas. ↩
Metal piping from the research instruments using the helium to the recovery gas bag must be installed. PVC or plastic piping is not recommended due to helium leakage. A neoprene gas bag collects the exhaust gas at atmospheric pressures. A multistage compressor intermittently removes the gas from the bag and pressurizes the gas into storage gas tanks. The output of the compressor has a desiccant trap to remove water from the gas. The pressure in the storage tanks can reach 400 psi (medium-pressure) or 3,000 psi (high-pressure) depending on the storage system. The medium-pressure systems uses lower-pressure compressors, which are less expensive, tend to last longer and have less loss of helium. The disadvantage of the medium-pressure system versus high-pressure is the additional space required for larger holding tanks. The medium-pressure tanks require approximately 8 times greater volume for equal storage capacity of gas. The purifier is a cold trap using liquid nitrogen or a low-temperature refrigerator. Depending on the purity of the helium, the purifier may need weekly or monthly regeneration to purge the contaminants. ↩