How To Size An Expander For Center Tube Sheet Expansion
The design of a chiller can pose some challenges when it comes to tube installation and maintenance. From the center tube sheet to tube enhancements, special considerations need to be made when choosing the correct tooling for the job.
Due to the long length of the vessel, many customers will choose to expand the tube into the center tube sheet. This prevents the tube from vibrating and rattling while in production. A few methods for this are collet style expansion, hydro expansion, and traditional expansion. Collet style expanders are designed to lightly tack the tube into the tube sheet. While this can work on some applications, due to the softness of the tube material, many customers find that the tube may return to its original shape after some time in use. As a result, collet expansion may not have enough power to fully seal the tube into the tube sheet.
Hydro expansion is another alternative involving high pressure water that shoots through a flexible tube that inserts into the expansion area. While it can be effective, it is very costly to buy the equipment and may require special permitting to perform on a job site. Due to the limitations of these methods, traditional expansion is often preferred.
Expanding the center tube sheet can be challenging, regardless of the installation method. This is largely due to the long reach required to get to the center tube sheet. Generally, most chillers have about 10 – 15 ft of space from the front of the vessel to the center, resulting in custom tooling and a challenging set up. Additionally, chillers are often made with enhanced tubing, meaning there is a pattern or form created in the surface of the tube to aid in heat transfer. These enhancements can make sizing tooling for the job more complicated, as the ID of the enhanced portion of the tube is different than the ID of the prime surface where the tube is rolled.
Micro Deformation Enhanced Tube
Before selecting a tool there are a few measurements that will need to be provided. First, the distance from the front of the tube sheet to the center tube sheet. It’s also important to note where that measurement falls- for example, is it to the back of the center tube sheet or the center. This will help in determining the required overall reach of the tool. Second, the ID of both the enhanced portion of the tube and the prime portion of the tube. Since the enhanced portion is often smaller in diameter, the cage of the expander will need to be designed to fit through the tube and maintain the necessary expansion range to hit the desired wall reduction. Lastly, the center tube sheet thickness will help determine what roll type and length will be needed for the expander.
Once you have all this information, a long reach baffle expander can be made to your custom application. When expanding the center tube sheet, it is critical that the tool be held perpendicular to the tube sheet. If the tool is not straight or an operator leans on it, it can cause the tool to break, or the motor may produce a false torque reading. This can result in more money on tooling and potential for the tube to come loose from the tube sheet. Additionally, before rolling, the collar should be adjusted so that the rolls are in the center tube sheet. If the collar is not set correctly, it can result in rolling in front or behind the tube sheet, damaging the tube enhancements. The easiest way to find the right spot is by using a drawing or measuring off the distance.
Overall, expanding the center tube sheet can take a lot of prep work and time to execute correctly. However, the benefit of using traditional expansion can outweigh the need for more accuracy. For example, most customers are already familiar with traditional expansion, as they are also expanding the outer tubes and tube sheets. Additionally, a long reach expander allows for more control over the amount of expansion that takes place, ensuring that the tube does not come loose over time. Lastly, it is much more cost effective when compared to hydro expansion methods.
