GTF tube furnaces with a maximum operating temperature of 1200 C (2190 F) can accept interchangeable work tubes up to 150 mm (6 in.) diameter. The range includes equipment with seven heated lengths from 300 to 1200 mm (12 to 47 in.) in both horizontal and vertical versions with either single-zone or three-zone control. The multizone models provide good temperature uniformity or can be used to create a temperature gradient. The furnaces are heated by semi-embedded, free-radiating elements in vacuum-formed insulation modules, which provide support and increase the working life of the elements.