The coefficient of thermal expansion (CTE) is a measure of the extent to which a material expands upon heating. It describes how the size of an object changes with a change in temperature and measures the fractional change in size per degree change in temperature at a constant pressure. The coefficient of thermal expansion is unique to each material and is based on its other physical properties. The larger this coefficient is for a material, the more it will expand per degree temperature increase. There are several types of coefficients of thermal expansion, including volumetric, area, and linear, and the choice of coefficient depends on the particular application and which dimensions are considered important. For isotropic materials, the area and volumetric thermal expansion coefficient are, respectively, approximately twice and three times larger than the linear thermal expansion coefficient. The coefficient of linear thermal expansion (α) is used to account for a substances additional physical properties. It is defined as the fractional increase in length per unit rise in temperature and is often specified at a precise temperature (true coefficient of thermal expansion or a-bar) or over a temperature range (mean coefficient of thermal expansion or a) . The coefficient of thermal expansion must be considered in components that use a mixture of materials, such as heat exchangers with mild steel shells and austenitic grade tubes.