YUHONG GROUP - Heat exchanger tube and tube plate connection

In the manufacturing process of heat exchangers and other equipment, ‘expand and then weld’ and ‘weld and then expand’ are two different processing sequences, and they have the following differences:

1. Processing sequence and principle
Expansion first, then welding
Expansion first. Expansion is the use of expanders to produce plastic deformation of the tube, while the tube plate hole wall produces elastic deformation, the tube and the tube plate are closely connected together. There is a good fit between the tube and the tube plate after expansion. Welding is then carried out to further strengthen the connection strength between the tube and the tube plate, and at the same time seal any small gaps that may exist after expansion.
Welding before expansion
The tube and tube sheet are first welded to form a strong connection. Welding is mainly done by fusing metals, which provides a high level of strength. This is followed by expansion. The purpose of expansion is to eliminate stress concentrations that may occur during the welding process and to further enhance the sealing between the tube and the tube sheet, resulting in a tighter connection between the tube and the tube sheet.

2, connection quality aspects
Sealability
Expansion before welding: the expansion process can pre-fill the gap between the tube and the tube plate, reducing the possibility of defects such as porosity during welding, which is helpful in improving the sealing. However, if the surface is not cleaned of impurities such as oil and dirt after expansion, it may affect the quality of the weld, which in turn affects the sealability.
Welding before expansion: Expansion after welding can calibrate the welded parts to a certain extent, so that the tube and tube plate fit more closely, effectively eliminating the welded heat-affected zone may produce small cracks and other defects, to enhance the sealing. However, if the welding quality is poor, there are serious welding defects, expansion may not be able to fully compensate for the impact of these defects on the sealing.

3, the scope of application
Expansion before welding:Applicable to the very high requirements for sealing, and the tube and tube sheet material welding performance is good. For example, some small high-precision heat exchangers, in the expansion can ensure a good fit on the basis of welding to further ensure the sealing and connection strength.
Welding before expansion: For equipment subjected to high pressure and vibration loads. By first welding to ensure the basic strength of the connection, and then use the expansion to optimise the distribution of stress and improve the sealing, such as large-scale chemical plants in the heat exchanger equipment, to ensure reliable operation in complex operating conditions.