| 摘要 |
<p>The tube has a circular cross section (1) at its connection end, and flattened portions (2) in their middles. The cross-section of these is adapted to the volume of heating gases passing through the tubes. The middle is preferably produced by pressing the tubes from two sides, producing an oval (4) or rectangular cross-section. For high pressures, the opposite sides comes into contact at the centre. Twists or radial corrugations may also be provided.</p> |
