Advanced Ceramics

Ceramic Tube Selection

August 1, 2012

Expensive does not necessarily mean better.

The key to obtaining the best results for a given application is to understand what is required. Too often, it is assumed that materials like zirconia or rare earth ceramics will perform better when, in reality, lower cost materials can provide an adequate product.

Ceramic tubes are used in a wide range of industrial applications, including thermocouples and furnace tubes. The key to obtaining the best results is to give the supplier as much information as possible regarding the tube’s operating environment.

Purchasing ceramic tubes can be a daunting task. However, the challenge can be simplified by providing the right information. If the exact material and size of the tube are known based on what has worked in the past, then the process is easy. In situations where the information is not known, the more information the supplier receives, the better chance they will have of meeting the set requirements.

Material Selection

Tubes can be made using a variety of materials. The characteristics of the material affect the performance of the tube, so it is important to choose the right material for the application. If the tube material is known, that information should be given to the supplier. If not, details of the working environment of the tube must be considered instead.

Working environment details should include the maximum ambient temperature that the tube will be working in, as well as the environmental variations in temperature and the time needed for the environment to change from the maximum to minimum temperature. With this information, suppliers will be able to select a material with the most suitable thermal shock characteristics.

Because the chosen material may need to be able to withstand varying amounts of pressure or stress, the environment in which the tube is supported is equally important. In addition, elements (i.e., solids, liquids or gases) that are likely to be in contact with the tube should be considered. These may react with or induce stresses into the ceramic material. Further, the supplier should be aware of the specific application and life expectancy of the tube, as some materials last longer than others in different environments.

Certain application requirements may also determine whether the material should be dense or porous. Porous materials tend to have better thermal shock characteristics but are not suitable where a gas-tight product is required.

Sizes and Shapes

It is vital for the supplier to know the required dimensions of the ceramic tube (i.e., the inside and outside diameters); normal tolerances are approximately ± 5% for diameter and ± 0.3% for straightness. Tighter tolerances can be achieved, but this needs to be known at the enquiry stage as this will require special selection or manufacture, which may affect the price. In addition, any flexibility in the required size could provide more options for finding the most suitable product.

The required quantity of tubes depends on an estimate of how often the tubes will be used in a year and the expected lifespan of each tube. Order quantity can affect the price of the tubes, since certain production methods such as extrusion have a minimum production run. An additional factor is the cost of shipment, which can add significantly to the end cost of the tube. It is also important to remember that standard tube sizes can be purchased in smaller quantities, while specific or irregular sizes may require a minimum order.

Tube configuration should also be considered. This information includes whether or not the tube is open at both ends or closed at one end (often called a sheath), and whether or not the tube is multi-holed. If the tube is closed, the required shape of the closed end can be flat or round. Any further additions, such as flanges at the ends, can be made by contacting the supplier.

The Right Choice

Tubes can be made from a wide selection of different ceramics using a variety of production methods. Both the ceramic material and the way the tubes are made can affect characteristics or performance—and whether or not a tube will be suitable for a specific industrial or technical application. Price will be affected by the method of manufacture, chosen material, operating requirements and order quantity. The more information provided to the supplier, the greater the chance of the final product being suitable for its environment.

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