A device for the thermal treatment of yarns comprising at least one inlet opening and at least one outlet opening for at least one transport means, which transports the yarn through the device, and comprising separating elements at the inlet opening and the outlet opening for thermal shielding, in order to minimise the exchange of media to the environment. The separating elements are configured to have a low bending rigidity and are arranged such that a surface load is applied onto the yarn lying on the transport means, for example a conveyor belt, which is not greater than 0.005 kg*cm.sup.2.