Abstract: Device for heating fluid, comprising: a first section for heating the fluid in which fuel and air are mixed and combusted; a second section for heating the fluid which is disposed substantially in the line of the combustion section and in which a number of pipes for the fluid extend substantially transversely of the flow direction of the combustion gases; and a third section for heating the fluid which is disposed substantially in the line of the second heating section and in which a number of pipes for the fluid extend substantially transversely of the flow direction of the combustion gases, wherein at least a number of the pipes in the third section are provided with ribs or fins enhancing the heat transfer.