Unique considerations of high temperature shell and tube heat exchangers
High temperature heat exchangers are important in many industries, including chemical and manufacturing. Notably, they are vital to the development of efficient energy generation, which is becoming more critical than ever.
A high-temperature heat exchanger is usually considered to include heat exchangers that handle temperatures between 500-1400°C. A shell and tube heat exchanger can be designed to cope with a range of temperatures, but even when using specialist materials, these rarely exceed 500°C before unique considerations must come into play.
Read: why use a shell and tube heat exchanger.
The challenges with high temperature shell and tube heat exchangers
Designing heat exchangers that function at high temperatures brings a unique set of challenges.
Heat transfer
Heat transfer through radiation can have a more prominent role in high temperature shell and tube heat exchangers compared to lower temperature units. This adds extra complexity to engineering calculations.
There is also more potential for heat loss into the environment from the outside surface of the heat exchanger. This has to be factored into the design of the unit itself, as well as its foundation and any other housing.
Materials
As with all shell and tube heat exchangers, high temperature heat exchangers must be made from appropriate materials. The added challenge is that far fewer materials can handle the increased temperatures, particularly if combined with the requirement to handle corrosive materials. This can lead to restrictions or new challenges, such as added costs or difficulties in material manipulation.
Read more about heat exchanger materials.
Selecting the appropriate material, and thickness, for the specific high temperature shell and tube heat exchanger is based on the temperature demands of the process. The material’s thermal shock-bearing capabilities are also an essential consideration, as well as its corrosion resistance against any dirt, salts, sulphates, acids, alkalis or other materials the heat exchanger will handle.
Material stress analysis is essential in any shell and tube heat exchanger design, but with greater risks and costs associated with high temperature units, it becomes even more vital.
Hastelloy is one example of a material that is used in high temperature shell and tube heat exchangers, especially when chemically aggressive fluids are used.
In addition to Hastelloy, the following materials, but not limited to, are utilised:
- Inconel 617
- Inconel 625
- 800H
- Haynes 230 and 120
The coolant selection is also an added challenge when working with higher temperatures. Steam would produce extreme pressures, requiring much thicker shells and tubes; this is the reason why alternative media such as gases, liquid metals or molten salts must be considered.
Design
In high temperature environments, due to present of hot gasses, a low heat transfer coefficient will be achieved. In a lower temperature design, this can be done using finned tubes which help the product to be more efficient at compact sizes.
However, in high temperature shell and tube units, the gaseous stream usually contains particles that would lead to fouling and fill the space between the fins. This would make the fins themselves no more advantageous than standard designs. In addition, gas velocity can often be too low for fins to make a notable difference.
The pressure drop in high temperature heat exchangers must be kept low. This means tube diameters and pitches often need to be larger, leading to additional challenges with adding fans or blowers that are still cost-effective.
Common types of tube sheets used in lower temperature shell and tube heat exchangers aren’t always suitable in high temperature applications. For example, floating tube sheets, which require a sealing gasket, don’t work, or don’t work effectively, at very high temperatures.
However, it’s still vital that differential expansion is accounted for in high temperature shell and tube heat exchangers. Engineers often have to find more unique solutions to ensure the effective running of the product at high temperatures.
Applications of high temperature shell and tube heat exchangers
High temperature shell and tube heat exchangers have a range of applications. Where high temperature gases need to be produced or processed, a high temperature heat exchanger will be required.
Power generation
Higher operating temperatures are important for improving the efficiency of power generation, which in turn, helps to improve renewable power generation and reduce the world’s dependence on fossil fuel power sources.
Hydrogen has the potential to be a leading form of energy storage and fuel. However, it’s currently not viable on a large scale. As technology continues to explore how this might be made possible, it’s clear that high temperature heat exchangers are likely to play a key part.
For example, the international Generation IV project identified a very high temperature gas-cooled reactor design capable of producing hydrogen without using fossil fuels or emitting greenhouse gases. For this application, an efficient high temperature heat exchanger would be essential.
Industrial waste heat recovery
Industrial waste heat recovery captures hot gases as they exit industrial equipment to use the heat energy. Some of these gases can be extremely hot and need specialist heat exchangers to handle them effectively. These gases can be corrosive.
High temperature heat exchangers used for waste heat recovery could also help to reduce high temperature thermal pollution. Recovering and reusing waste heat energy improves the impact of industrial processes on the environment.
If you’re looking for specialist heat exchange equipment, Sterling TT is your expert. We supply bespoke heat exchanger designs for industrial applications. Get in touch.