BEST CORROSION-RESISTANT NICKEL ALLOYS FOR USE IN HEAT EXCHANGERS
- October 10, 2022
- Posted by: vAchESpAVe388ema5hetray8
- Category: BSW blog EN
Heat exchangers are found in both large-scale industrial processing plants and everyday appliances alike. We look at these systems in more detail, as well as the best nickel alloys available for heat-exchanger use.
Some of the most important components in many industrial applications are heat exchangers. These are systems that are designed to transfer heat from one fluid to another for either heating or cooling. They are incredibly common, in everything from large processing plants to everyday appliances, like car engines and fridges.
Heat exchangers enable heat from one liquid or gas to pass to another liquid or gas without the two substances coming into direct contact. For example, a home central-heating system uses heat exchange to function: natural gases are burned, creating heat that is absorbed by water that is passing through a network of tubes. That hot water is then fed to radiators or taps to provide hot water and warmth in homes. This is just a very simple example of a heat exchanger, but the principal remains largely the same regardless of its purpose or scale.
Heat exchangers are found in power plants, chemical processing plants, petrochemical production plants, waste treatment systems, and oil and gas processing, for example. The nuclear industry also uses heat exchangers to help cool or heat its systems.
TYPES OF HEAT EXCHANGER
There are many different types of heat exchanger in industrial use, from simple double-pipe heat exchangers, to more complex setups like shell and tube heat exchangers and plate or plate and shell heat exchangers.
Heat exchangers are often used to help recycle waste products, creating more efficient systems. Those long chimneys and smoke stacks towering above power plants pump out waste products that are a by-product of the industrial processes. However, what you can’t see is that often there are heat exchangers inside these chimneys. As the hot smoke rises, it passes a heat exchanger that helps to warm water, which can be passed back into the plant itself. This can then be used to bring heat directly back into the processing systems as needed, or diverted to provide heat elsewhere, recycling what would otherwise be wasted heat.
These heat exchangers naturally have to operate in tough conditions. There are normally high temperatures involved, as well as direct contact with different fluids and gases that can be quite corrosive. Materials used to create the pipes that are used in heat exchangers therefore need to be very corrosion resistant, good under high pressure and able to withstand both high and low temperature extremes.
It is not always metals that are used in heat exchangers. In some circumstances, ceramics can be used, as can composite materials or plastics. Ceramic tubes are good for exceptionally high-temperature uses, as well as in certain corrosive conditions. Plastic is lightweight and cheap, but they are quite weak and not able to withstand very high-temperature processes. They can sometimes be found in things like swimming pools, where the heat exchange temperature range is less excessive.
Many metals are also used in heat exchangers, especially for industrial applications and mechanical uses. There are a lot of considerations that a design engineer needs to take into consideration when it comes to material selection for heat exchangers and its components.
This includes considering the pressure that components will be subjected to, the temperature ranges of operation, the fluids in the system, the ease of maintenance and repair, and the capacity required. Of course, cost is also a primary factor in material selection for large-scale projects.
While metals such as copper, aluminium, carbon steel, stainless steel and titanium are all popular choices, when it comes to the most corrosive and challenging environmental conditions, nickel alloys are often used due to their strength, corrosion resistance, high-temperature resistance and longevity.
There are a number of high-performance nickel alloys that can be employed in heat exchangers across different industrial applications. These are usually provided as pipes or tubes in a choice of different materials, some of which we have outlined below.
INCONEL alloy 625 is such a versatile nickel alloy that it crops up time and time again for all manner of industrial applications. With its very high strength, excellent fabricability and outstanding corrosion resistance, it’s no surprise to find that it plays its part in the production of heat exchangers too.
Due to its ability to resist very corrosive fluids, it is often employed in seawater-based heat exchangers in refineries, power plants and offshore topside services. It can be used in service temperatures from cryogenic up to 1,800°C, which is perfect for heat exchangers in many environments. With the inclusion of molybdenum and niobium in the alloy’s composition, it is also very strong, making it able to withstand the pressure of a heat exchange system.
This nickel alloy has been around for a long time – over 50 years in fact. In that time, it has gained a solid reputation for its corrosion resistance. Because it has a controlled amount of tungsten in the mix, it is used in severely corrosive environments, particularly those in chemical processes where highly corrosive fluids and gases are present.
This corrosion resistance makes it a great choice for use in heat exchangers in these difficult environments. The material has been utilised for components in chemical processing, food processing and pharmaceutical processing. There is a high level of molybdenum in this alloy, and this helps to resist against more localised corrosion, such as pitting.
This INCONEL alloy is a controlled blend of nickel, chromium and iron, and it’s a standard material within the chemical industry, automotive industry and aerospace industry. It is so popular in these industries because of its varied properties, including good corrosion resistance, good high-temperature resistance, high strength and durability, and good workability.
This all-rounder is often employed in environments where materials are at risk of chloride-ion stress-corrosion cracking, to which it is virtually immune. It also has an impressive operating temperature range from cryogenic to 1,095°C.
INCOLOY alloy 800 is a material that is often used for heat exchangers in power generation. It is a blend of iron, nickel and chromium that is designed to have a low carbon content to prevent carburisation. It is also resistant to oxidation and offers high strength. It shows good resistance to thermal cycling, meaning that it is often exposed to extremes of temperature, making it perfect for use in heat exchangers.
In the challenging environments of chemical and petrochemical processing, especially in the presence of nitric acid and where resistance to chloride-ion stress-corrosion cracking is required, INCOLOY alloy 800 is a choice material for heat exchangers. It’s often used in domestic appliances too, such is its versatility. It’s also used in petroleum processing where heat exchangers are used to air cool the process stream.
Variants of INCOLOY alloy 800, namely 800H and 800HT, are also used for heat exchangers, for example in gas-cooled nuclear reactors where there are exceptionally high temperatures.
MONEL alloy 400 is another nickel alloy that has been around for a long time, while still remaining a standard material of use in certain applications. It was introduced to the market way back in 1905 and its primary use is in petroleum distillation tower internals, pumps and valves.
Its list of applications is ever-growing, and one use is that of heat exchanger piping in a number of different industries. For example, it is used in chemical processing, for process vessels and piping, boiler feedwater heaters and deaerating heaters. The material exhibits high strength and toughness over a wide temperature operating range.