by Dr. Ray Radebaugh, NIST Fellow Emeritus, email@example.com
Cryocoolers are a type of refrigerator designed to reach cryogenic temperatures. Engineers use the term most often for smaller systems, typically no larger than table-top size, with input powers less than about 20 kW. Some have input powers as low as 2-3 W. In comparison, engineers refer to larger systems, such as those used for cooling the superconducting magnets in particle accelerators, as cryogenic refrigerators. Input powers here can be as high as 1 MW.
In most cases, cryocoolers use a cryogenic fluid as the working substance and employ moving parts to circulate the fluid around a thermodynamic cycle. There are six commonly used cryocooler types, divided into two categories—recuperative cycles and regenerative cycles. Schematics of the three most common recuperative and regenerative cycles are shown respectively in Figures 1 and 2. Compressor work is performed at ambient temperature, with heat being absorbed at the cold end.
Recuperative cycles move the working fluid around a loop in one direction at fixed high and low pressures. The room-temperature compressor can either be a reciprocating-piston compressor with inlet and outlet valves or a unidirectional compressor without valves, such as a scroll or screw compressor. Efficient oil-removal equipment must be used in the high pressure stream at room temperature to eliminate all traces of compressor oil from reaching the cold end and freezing. In a few cases, engineers use oil-free compressors.
The heat exchanger for the recuperative cycles is known as a recuperator or recuperative heat exchanger. It has two separate flow channels—one for the high pressure fluid and one for the low pressure fluid. The low-temperature expansion can be controlled with an orifice, capillary or valve—as in the Joule-Thomson (JT) cycle—or with a reciprocating or turbine expansion engine, as in the Brayton cycle. The Claude cycle is a combination of the two, where engineers use an expansion engine for precooling and JT expansion for the final expansion. Liquefaction often takes place in the final JT expansion.
The JT cycle normally uses a working fluid that is liquefied at the cold end—such as nitrogen for 77 K, hydrogen for 20 K and helium for 4.2 K. For higher temperatures, engineers often use mixed refrigerants of nitrogen and various hydrocarbons to provide higher efficiencies.
Regenerative cycles, in comparison, use oscillating flow and pressure with appropriate phase angles between the flow and pressure to achieve refrigeration at the cold end. Helium gas is almost always the working fluid. The phase of the displacer sets the ideal phase angle, with respect to the piston in both the Stirling and Gifford-McMahon (GM) cycles. The displacer also recovers expansion work at the cold end and reintroduces it at the warm end to reduce the required input power from the compressor (pressure oscillator). Normally, there is no displacer in the pulse tube cryocooler, so the expansion work is lost unless a displacer is used at the warm end of the pulse tube.Engineers can generate the oscillating pressure in regenerative cryocoolers with a valveless compressor (pressure oscillator), as shown in Figure 2 for the Stirling cycle, or with valves that switch the cold head between a low and high pressure source, as shown for the GM cryocooler. Engineers often take GM oil-lubricated air conditioning or refrigeration compressors and modify them for use with helium gas, using them primarily for commercial applications of cryocoolers where low cost is very important. Oil removal equipment can be placed in the high pressure line where there is no pressure oscillation.
Pulse tube cryocoolers can use either source of pressure oscillations, as shown in Figure 2, and they are referred to as either Stirling-type or GM-type depending on the type of compressor used. The Stirling compressors must be oil-free because oil removal equipment cannot be placed in the oscillating pressure region.
The Stirling cycle and Stirling-type pulse tubes typically operate with frequencies in the range of 30-60 Hz, whereas the oscillating pressure in GM and GM-type pulse tubes is usually at 1-2 Hz to achieve longer lifetimes with dry rubbing parts. Average pressures are often in the range of 1.5-3 MPa (15-30 bar) with oscillating pressure amplitudes of 10 to 15 percent of the average pressure.
Engineers refer to the heat exchanger in regenerative cryocoolers as a regenerative heat exchanger or regenerator. It has only one flow channel, in which the flow changes direction every half cycle. The incoming warm stream is precooled during the first half cycle by heat transfer to the regenerator matrix, typically a packed bed of fine screen or packed spheres. The matrix has a high heat capacity to store the heat for a half cycle. During the second half cycle, heat is transferred from the matrix back to the returning cold stream.
Cold end temperatures achieved with regenerative cryocoolers vary from about 3 K up to 300 K, though temperatures below 150 K are most common. The lowest temperatures—of about 3 K—are possible with GM cryocoolers and GM-type pulse tube cryocoolers, while engineers more often use the Stirling and Stirling-type pulse tube cryocoolers for temperatures above 20 K. These have the highest efficiencies of all cryocoolers, in the range of 10 to 25 percent of Carnot at 80 K.
There are many common applications of cryocoolers, including the cooling of infrared sensors to temperatures of 80 to 150 K with Stirling or Stirling-type pulse tube cryocoolers for use in military night vision equipment; the rapid cooldown of infrared sensors in missile guidance systems with nitrogen- or argon-based JT cryocoolers; and the maintenance of superconducting magnet coils in most magnetic resonance imaging systems at 4 K with GM cryocoolers that condense the boiloff from a bath of liquid helium.
Researchers advancing applications at 4 K mostly use GM or GM-type pulse tubes, while many detectors, superconducting electronics or power systems utilize either mixed-refrigerant JT cryocoolers or regenerative cryocoolers. Dozens of cryocoolers, mostly Stirling and Stirling-type pulse tube cryocoolers, now fly in space aboard satellites for the cooling of infrared sensors. Such cryocoolers use flexure or gas bearings to eliminate rubbing contact in the moving parts, and many have operated continuously for over 10 years.
There has been an increased emphasis in the last few years for cryocoolers operating at temperatures in the 80 to 150 K range, with mixed-refrigerant JT, Stirling and Stirling-type pulse tube cryocoolers often filling these needs. Smaller sizes have become especially important to fill various space and portable applications. Microcryocoolers using the JT cycle have been the focus of much recent research, but true microcompressors have been elusive. Researchers are investigating Stirling and pulse tube cryocoolers for operation at frequencies up to 300 Hz to reduce size and weight while still maintaining high efficiency. And large systems with refrigeration powers of 1 kW or more at 80 K are becoming more popular for use with superconducting power systems, often employing Stirling, Stirling-type pulse tubes or GM cryocoolers.
Additional information on cryocoolers is available in R. Radebaugh, “Cryocoolers: the state of the art and recent developments,” Journal of Physics: Condensed Matter, Vol. 21, 2009.