Pneumatic spraying is used to improve the performance of finned surfaces. Essentially, it involves spraying powder particles onto the fin surface using a high-speed, cold or slightly heated fluid containing particulate matter. This method can be used not only to spray metals but also alloys and ceramics (metal-ceramic mixtures), resulting in surfaces with various properties. In practice, the contact resistance at the fin's base is often a limiting factor in the installation of fins on tubes.
An experimental study was conducted to evaluate finned tube heat exchanger elements. The experiment involved spraying AC-aluminum onto the fin surface with the addition of 24A white electric furnace alumina. The experimental data was then analyzed to assess the contact resistance at the fin's base. Comparing the studied fin efficiency with calculated data, the conclusion was that the contact resistance at the base of the pneumatically sprayed fins has no substantial impact on efficiency. To confirm this, metallographic analysis was performed on the transition zone between the base (tube) and the surface (fin). Analysis of the transition zone specimens showed no microcracks or leaks along the entire length of the connection boundary.
Therefore, pneumatic spraying promotes the formation of branched boundaries between the surface and the basic interaction, and facilitates the penetration of powder particles into the matrix. This results in high adhesion strength, physical contact, and the formation of metal chains. Consequently, pneumatic spraying can be used not only for molding but also for fixing fins manufactured using conventional methods onto the surface of heat exchanger tubes, and for reinforcing the bottom surface of ordinary fins. It is foreseeable that pneumatic spraying will be widely used in the production of compact and efficient heat exchangers.
