Comparison of Materials Choices for Heat Exchanger Tubes
October 26, 2018Choosing materials for a chemical shell-and-tube heat exchanger is difficult, even for engineers with many years of experience, simply because there are so many options. In fact, there are more than 75 stainless steel alloys and more than 400 copper alloys, according to ASTM. When expanding to a further array of materials such as titanium, nickel, ferrous and non-ferrous alloys, graphite, glass and ceramics, selection becomes quite challenging. To sort out this problem, it helps to organize material performance in terms of operating temperature range, corrosion resistance and heat-transfer characteristics.
Qualities of Alloys and Ceramics
Metal alloys are typically created to deliver performance that none of their constituent components alone can achieve. For example, austenitic stainless steels combine either chromium, magnesium and nickel, or chromium and nickel, to achieve specific properties. Copper, which has high thermal conductivity, is combined with one or more other metals, each combination producing a specific result.
Inconel achieves its high oxidation and corrosion resistance as well as other benefits by combining nickel and chromium, and Hastelloy combines nickel and molybdenum (and sometimes chromium) to deliver corrosion resistance, strength and performance at high temperatures. In addition, Inco alloy is a nickel-iron-chromium alloy designed for high-temperature applications that is comparatively easy to fabricate, as the process is like stainless steel.
Ceramics are nonmetallic materials and thus tend to be brittle, but they have unique properties that make them well suited for heat exchangers used for fine chemicals. They have high chemical resistance and the ability to withstand high temperatures. Among available ceramics are oxide and non-oxide compositions. Non-oxide ceramics are typically preferred for heat exchanger applications because of their higher thermal conductivity when compared to oxide ceramics.
Which Material Is Best?
If all these materials were equally suited for chemical heat exchangers, the designer’s job would be a snap. Unfortunately, they’re not. The best route to sanity is to narrow the choices based on the characteristics of the application and three key performance metrics — thermal transfer, operating temperature performance and corrosion resistance.
Material selection is basically a process of elimination. For example, a given heat exchanger may require tubes with high corrosion resistance but without the need to operate at high temperatures, which produces one list of candidates. If the corrosiveness of the environment is not severe but the heat exchanger operates at high temperatures, other choices are possible. However, a corrosive environment along with high temperatures produces another list of results. When the best possible heat-transfer rate is a major concern, only a few materials are suitable.
Figure 1. Hexoloy is one of the highest performing materials on the market. Source: Saint-GobainFor applications in which all three metrics are important if not essential, the best choices will be ceramic materials. For heat-exchanger tubes, typically silicon-carbide (SiC)-based ceramics are selected. A particular SiC called Hexoloy is an alpha phase sintered silicon carbide developed and produced by Saint-Gobain. The direct sintering process provides Hexoloy with some of its most significant benefits, many of which are above those achieved by SiC that are bonded with siliconizing or nitriding processes.
Hexoloy SiC has thermal conductivity approaching that of graphite, providing exceptional thermal transfer properties. It has a working temperature up to 1,700° C in air. Hexoloy SiC is a fine-grained and essentially nonporous material, making it resistant to nearly all corrosive liquids and gases, at temperatures up to 400° F. No other material has equivalent properties. It also has a low coefficient of thermal expansion, high thermal shock resistance, and high mechanical strength, all of which are important requirements in a heat exchanger application.
Conclusion
Hexoloy SiC is an excellent choice for heat exchanger tubes and tube sheets for application in fine chemicals and pharmaceutical industries. Due to its advanced properties, it can reduce or eliminate maintenance costs as well as other factors that can cause downtime throughout the life of the heat exchanger. While other materials may compete with one or two performance metrics, Hexoloy SiC represents an optimum choice when several performance metrics are considered.
Take advantage of the unique properties of Hexoloy sintered silicon carbide heat exchanger tubes. Contact Saint-Gobain today.