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A cryocooler is a refrigerator that is built to reach cryogenic temperatures. These are temperatures below 120 K (−153 °C). These are most often smaller systems. They are normally small enough to sit on a table. Most cryocoolers have input powers less than about 20 kW. Some can have input powers as low as 2–3 W. Most cryocoolers use a cryogenic fluid and moving parts to move the fluid around a thermodynamic cycle. The fluid is normally compressed at room temperature. It is precooled in a heat exchanger. The fluid is then expanded at some low temperature. When the low-pressure fluid returns, it passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated.

Large systems are more often called cryogenic refrigerators. These are used for cooling the superconducting magnets in particle accelerators. Their input powers can be as high as 1 MW.