A tube sheet is meant to isolate and support tubes in boilers or heat exchangers. It may also be used to support filter elements. In heat exchange applications, tube sheets are used in pairs and they are used singularly when used for supporting elements in cartridge-type filter devices.
Tube sheets can be made of various metals, resin components, or plastic depending on the applications. Plastics and resins are usually made for filter devices while metals like steel are used for heat exchangers as they have to bear high temperature and pressure. A cladding material is used to make it corrosion proof and insulating, the proofing is important so that it can resist corrosive attack by fluids in the heat exchangers. It also has to be electrochemically compatible with the material of the tubes. A galvanic anode is also fitted to the surface to protect the sheet from rust.
Designing of tube sheets
The sheet can be in the form of a plate or bulkhead which is perforated with holes patterned to accept tubes or pipes. Designing sheets to accommodate the exact number of tubes is a quite complex process. The process of determining the exact number of holes and planning the pattern of holes is quite a challenge. It becomes even more difficult to design sheets for large exchangers as they require thousands of tubes to be fitted. However, since the invent of computer-aided design (CAD) software for doing the calculations, the design process has become quite streamlined. The drilling is done to precision with computer numerical control (CNC) machines and Radial Arm Drilling Machine.
In addition, significant thought and calculation has to be made to ensure that the tube sheet will not be deformed due to the impact of temperature variation and stresses associated with different high pressure fluids through the tub. The calculation is necessary to optimize the thickness of the tube sheet.
Tube Sheet Layout
The strength calculation of shell and tube heat exchangers is more difficult than for ordinary pressure vessels. The area through which heat is transferred from one medium to another is calculated on the basis of the length and number of tubes. The required dimensions for correct calculation are the interior diameters of the shell, the outside diameter of the tube, tube pitch pattern, number of passes, distance of tube to shell, distance of pitch from partition groove, and distance of impingement plate.
Repair of tube sheets
Minor defects like steam cuts and out-of-shape holes can be corrected by welding. There are other problems such as galvanic and crevice corrosion, pitting, and reduced heat transfer coefficient that develop over time. These problems cause prolonged outages, reduced efficiency, and environmental damages. They can be resolved by applying polymeric composite to reinforce the steel and smooth out surface damages.
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