Development of a Miniature Fast Cool Down J-T Cryocooler
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One major advantage of Joule-Thomson (J-T) cryocoolers over other cryocoolers is the ability to achieve a very fast cool-down, in the range of only a few seconds. The main fluid is chosen according to the desired cooling temperature and the fast cool-down is usually obtained by allowing high flow rates during this transient process. A primary cooling stage may be added in order to cool the main fluid and reduce the cool-down time, but it has the price of two pressure vessels and a more complex, and bigger, cryocooler. Fast cool-down is usually required when the total cooling time is relatively short, a few seconds up to a few minutes. Thus, fixed-orifice cryocoolers are preferable, according to manufacturing and reliability aspects. However, the flow rate of a fixed-orifice cryocooler is determined by the pressure in the vessel. Thus, the reduction of the pressure in the vessel during operation reduces the flow rate, the pressure in the evaporator varies, and the cooling temperature changes as well. In order to reduce the flow rate immediately after cool-down for depressing the cold temperature variations during the steady operation, a regulation system is required. In this paper we describe the development of a new flow controller, patent pending, for fast cool down cryocoolers that is a benefit of practical considerations. The new flow controller is designed for miniature cryocoolers, has high reliability, is maintenance friendly, and provides fast reduction of the flow rate after cool down.