PURPOSE: To improve the coefficient of heat transfer, to reduce the sectional area of heat transfer surface and to miniaturize the titled heat exchanger by a method wherein a single reticulated member or a layer of such members having a suitable thickness is inserted into an annular clearance between an inner pipe and an outer pipe of the heat exchanger.

CONSTITUTION: A heat exchanger has a wire netting 4 (as the reticulated member) inserted into the annular clearance 3 formed between an inner pipe 2 and an outer pipe 1. In this case, the wire netting 4 may be inserted into the annular clearance 3 singly or in layers. Consequently, a fluid flowing through the annular clearance 3 in the direction of the arrow B meanders through the gaps among the wires of the wire netting 4 so that violent turbulences are generated in the fluid and the heat transfer coefficient of the fluid is imporved thereby. Further, due to the existence of the wire netting 4, the heat transfer coefficient of the inner pipe 2 is improved to a remarkable degree, the heat exchanger can be miniaturized and the heat transfer efficiency of the heat exchanger can be improved to thereby contribute effectively to the saving of energy and its resources.