Development of a compact laminar flow heat exchanger with stainless steel micro-tubes

The present paper describes the design concept and manufacturing of a new c ompact laminar flow heat exchanger with stainless-steel micro-tubes for hel ium refrigerators. In the temperature range of less than 20 K, aluminum pla te fin type heat exchangers exhibit a remarkable fall of performance charac teristics as a compact heat exchanger. We presented in a previous paper that some compact heat exchangers with goo d performance in the temperature range of less than 4 K are required for a subcooled He II refrigerator cycle to be worked with 3He turbo-compressors (F. Doty, et al., A new look at the closed brayton cycle, Proceedings, IECE C-90 Reno, NV, 1991, p. 116). For this requirement, we developed a micro-tu be strip counter flow type heat exchanger, which consists of 12 elements wi th a total of 4800 stainless steel micro-tubes. Each element is formed with 400 tubes and a newly developed vacuum brazing method was applied for the bonding to the side plate. Each tube has an inner diameter of 0.5 mm, an ou ter diameter of 0.7 mm and is 310 mm long. We developed a cladding plate wi th two layers of gold brazing sheet sandwiched inside. In aerodynamic and thermal design of the element, the laminar flow conditio ns were adopted for the flows of inner and outer tubes to keep a high heat transfer rate and a low pressure loss. (C) 2001 Elsevier Science B.V. All r ights reserved.