Refrigerators and Liquefiers

Two of the most common terms used in cryogenics are “refrigerator” and “liquefier.” These terms describe similar and, as will be seen, in some cases identical components.

A refrigerator provides cooling (that is, absorbs heat) at cryogenic temperatures. Refrigerators typically put a working fluid (such as helium) through one of a variety of thermodynamic cycles (e.g., Collins, Brayton, Claude, Stirling) to provide the cooling.

There are a number of ways to classify refrigerators. Refrigerators may use regenerative heat exchangers or reculperative heat exchangers (see Cold Facts October 2014). Thus, we can speak of regenerative cycles and refrigerators or reculperative cycles and refrigerators. Refrigerators can also be classified as those that have steady flows and those that have oscillating flows. Refrigerators may also be divided by size, with ones producing less than 100 W of cooling frequently referred to as cryocoolers. Cryocoolers typically use oscillating flows and regenerative cycles (see Cold Facts Winter 2009). Most refrigerators use a single pure working fluid but there are mixed gas cycles and refrigerators as well. Lastly, there are specialized refrigerators such as adiabatic demagnetization refrigerators (see Cold Facts Spring 2011) that provide cooling without the use of a working fluid.

Despite the variety of refrigerators available, one feature they have in common is that they are closed cycle devices: All of the working fluid is retained in the system. Refrigerators may create liquid cryogens as part of the process, but the liquid or its associated vapor remains within the refrigeration system.

Liquefiers use similar cycles, working fluids and equipment as refrigerators. While liquefier cycles are optimized to produce liquid cryogens, the principal distinction is that liquefiers are open cycle devices. They produce a cryogenic liquid that is then removed from the system and used elsewhere. The missing mass (of, say, helium or nitrogen) is replaced in the cycle by room temperature gas. This gas may come from any source, including recovered boiloff vapor from the original liquid or helium gas from a natural gas well.

The design and construction of cryogenic refrigerators and liquefiers is well within the state of the art. Reliable, automated systems are available from manufacturers sometimes as catalog items and sometimes as custom designs. There are a number of firms producing liquefiers and refrigerators over a wide range of sizes. Further information may be found in the CSA Buyer’s Guide.

Having made the distinction between refrigerators and liquefiers, it’s worth pointing out that large modern cryogenic plants are frequently designed to operate as both refrigerators and liquefiers simultaneously. It is somewhat more appropriate to talk about liquefaction loads and refrigeration loads. An example of this is given in the accelerator cryoplant currently under design for the European Spallation Source (ESS). This plant will provide up to 3 kW of refrigeration at 2K and 11 kW of refrigeration at 40K. In addition, however, it also will produce up to 9 g/s of liquid helium that will be used to cool the radio frequency power couplers in the accelerator. The gas from this helium returns to the plant at 300K and represents a liquefaction load. Other systems that have both significant refrigeration and liquefaction loads include the ARIEL e-linac cryoplant at TRIUMF, the W7X Stellerator refrigerator and the ESS Test and Instruments cryoplant. These facilities are sometimes referred to as cryoplants in recognition of their combined refrigeration and liquefaction capabilities.

Details on refrigeration and liquefaction cycles and technology may be found in Cryogenic Engineering by R. Barron and Helium Cryogenics by S. W. Van Sciver. Examples of recent large cryogenic refrigerators and liquefiers include “ITER Cryoplant Status and Economics of the LHe Plants,” E. Monneret et al., Proc. ICEC 25 (at press); “Commissioning of the Helium Refrigerator System at JLab for the 12 GeV Upgrade,” V. Ganni et al., Adv. Cryo. Engr. Vol. 59B; “Design, Project Execution, Construction and Commissioning of the 1.8K Superfluid Helium Refrigeration System for SRF Cryomodule Testing,” P. Treite et al., Proc. ICEC 25 (at press); “Ras Laffan Helium Recovery Unit HeRuII Project,” R. Ali Said et al. Proc. ICEC 25 (at press); and “ESS Accelerator Plant Process Design,” X. L. Wang et al., to be presented at the 2015 Cryogenic Engineering Conference.