The invention provides a coil that comprises a plurality of arc-shaped segments having a first end and a second end connected together , wherein, the angle spanning between the first end and the second end of each arc-shaped segment is larger than 180 degree, preferably 186-286 degree; the central lines of two adjacent arc-shaped segments have a common tangency point at the joint of the two adjacent arc-shaped segments; or the central lines of two adjacent the arc-shaped segments are tangentially connected to the central line of a straight segment; in the above design, the bending part of the coil can be as long as possible within the minimum bending radius, and this may ensure the female screw thread or micro-fin structure is not deformed, meanwhile, it also saves space and achieves compact structure. Therefore, a small sized heat exchanger with a coil can be achieved, and more coils can be arranged in a unit volume compared with the prior art, so as to improve the efficiency of heat exchange.
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