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For heat exchange between a medium transporting heat well (e.g. liquid, liquid with phase transition) and a medium transporting heat poorly (e.g. gas with a low density), finned tube heat exchangers are often used. The heat-exchanging surface on the side of the medium transporting heat poorly is enlarged by an arrangement of fins. The external surface per m tube is increased by up to 15 times by the fin arrangement, the transferable amount of heat does not, however, increase to the same extent. This is due to heat conduction through the fins, whereby the external fin sections reach a different temperature from the tube base, i.e. one that is closer to the gas temperature, and therefore the effective temperature difference of the heat transfer is reduced The amount of heat conduction through simply shaped fins, e.g. fins on a flat plate or annular fins of constant thickness around a core tube, may be calculated strictly by mathematical means. It is common practice to consider heat conduction through the fin in terms of the so-called fin efficiency, which results from the following consideration: the heat transferred at the surface element is proportional to the difference in each case between the fin temperature and the gas temperature integrated over the fin surface. It equals the temperature difference between the gas and the core tube multiplied by the fin efficiency