Abstract: The present invention relates to a duct for conducting a flowing primary fluid, having a thermally conductive wall with exterior ribs having an exterior side through which a secondary fluid can pass, at least partially, whereby the duct wall comprises a structured surface on its exterior side in addition to the ribs. The invention further relates to a heat exchanger with an inventive duct and an air condenser, particularly a natural-draught condenser, with inventive heat exchangers.