Types of shell and tube heat exchanger
Shell and tube heat exchangers come in many forms. They are highly versatile, customisable and effective for applications across many industries. As a result, there are quite a few different you can categorise or classify them.
In this article, we summarise the major types of shell and tube heat exchangers, what differentiates them, the pros and cons of each, and some of the things we consider when choosing a heat exchanger design for our clients.
The types
In broad strokes, there are three main types of shell and tube heat exchangers:
- fixed tube,
- U-tube, and
- floating head.
As with all heat exchangers, you can also classify them accordingly to whether a phase change occurs (liquid to gas, or gas to liquid) or not.
Let’s go into more detail on each the pros and cons of each type. We will look at how they handle temperature differences, their cost, and how easy each is to maintain and clean.
Fixed tube heat exchanger
In a fixed tube heat exchanger, the shell and tube sheet are welded together. This means the tube bundle cannot be removed (hence the name).
With this solution, all tubes in the heat exchanger are straight. It eases maintenance: the operator can clean the tubes with a brush or rod (mechanical cleaning).
However, this benefit does not extend to the shell side of the heat exchanger, which cannot be cleaned mechanically. Instead, on the shell side, a chemical cleaning fluid or manual pressure washer must be used, which takes more time and effort.
Another weakness is how a fixed tube solution deals with temperature differences between the tube and the shell. Take this example. If we have hot fluid on one side of the heat exchanger and cold fluid on the other, there is different thermal expansion of the tubes and the shell. the heat exchanger can fail. The solution is an expansion bellows on the shell side, giving the hot, expanding tubes space to expand into.
In general, a fixed tube is the cheapest shell and tube heat exchanger (though an expansion bellows will increase the cost), and it remains a popular choice.
With a bespoke tube heat exchanger design that addresses the disadvantages of this type (e.g. adding expansion bellows), they can be a very effective and economical option.
U-tube heat exchanger
In a fixed tube bundle, if the tubes need to be allowed to expand, then an expansion bellows must be worked into the heat exchanger. In a U-tube, this is not necessary. The tubes are not connected on any kind of fixed joint, meaning that the tubes can easily expand within the shell. The temperature difference between the shell and tube is not a problem.
However, a temperate difference in the tube sheet is a different matter. Let’s look at another example. For your process, you want to cool 300°C gas to 20°C. The temperature distribution across the tube sheet is 280°C. The result is that the tube sheet will warp. Any gaskets there will be unable to seal, and the heat exchanger will fail.
In such cases, a U–tube cannot be used. U-tube is only suitable for smaller temperature differences between hot inlet and cold outlet (up to 100°C).
When it comes to cleaning, a U-tube traditionally had the opposite problem to a fixed-tube bundle: while the shell side can be mechanically cleaned, the curved tubes cannot be. It was one of the reasons why companies chose not to use U-tube heat exchangers in the past. This has changed recently, as mechanical cleaning technology has progressed to allow the cleaning of the tubes.
There are some other disadvantages to the curve of the tube in this type of heat exchanger. You cannot put as many tubes within the same shell size as with a fixed sheet. Extra room is required for the bend (you need at least 3x the diameter of the tube to bend it). It means there is less tubular surface area, leading to a heat exchanger that is less efficient for its size compared to a fixed tube design.
Bending tubes adds to the manufacturing costs of the U-tube heat exchanger. It can increase the costs by up to 30% compared to the straight tubes of a fixed-tube exchanger.
Floating head heat exchanger
The main characteristic of the floating head design is that you can withdraw the bundle out of the shell. From there, the shell and tubes can be separately inspected. This is useful if you want to examine the heat exchanger for any corrosion.
Because you can dismantle the floating head design, cleaning is straightforward: you can clean the shell side, the tube side, and clean the inside and outside of the tubes. Everything is accessible for cleaning.
Floating head designs are the standard in many industries, including oil and gas. The reason is that they have to handle dirty water to cool dirty oil. Therefore, the cleaning advantages of the floating head shell and tube exchangers are highly valuable, and they have become the standard in the industry.
You can see that the floating head heat exchanger is a common type because, in the design software used to create heat exchangers, floating head designs are often the default option.
The floating head heat exchangers must be designed according to the thermal challenges they face. If there is a single pass on the tube side, like a fixed tube bundle, then we would potentially need to install an expansion bellows.
Though they come with many advantages over the fixed tube and U-tube designs, those advantages come at an additional cost.
Classification by process and phase change
As with all heat exchangers, you can also group them according to the process they are involved in.
Coolers and heaters: When there is temperature change but not phase change (e.g. liquid to gas, gas to liquid). For example, when you have hot water on one side and cold water on the other side, they don’t change phase. Similarly, when you use hot air to cool cold air, or water on one side to cool argon.
Condensers: Where the process fluid will condense, phase changing from a gas to a liquid.
Reboilers: Where there is hot fluid (steam, for example) which will condense on one side, and on the other side you have fluid which will boil off. In other words, there is liquid coming at one end, and at the other end you will have 100% vapour (kettle reboiler) or vapour/liquid mixture (thermosyphon reboilers).
Working with clients to choose a shell and tube type
At Sterling Thermal Technology, we work with a variety of customers with varying needs:
EPC clients
Sometimes our client is an EPC (Engineering, procurement, and construction companies). Let’s say an expansion of plant needed. The power plant will appoint an EPC company to design the development. As engineering professionals, they know what they are looking for. Our involvement is on the production side, not engineering design, and they have already chosen the designs, specific tube types etc.
However, even in these cases, Sterling TT might say a change is cheaper, more efficient, and more optimal.
End-user and SMEs
With end-user, we work slightly differently, getting more involved in the design from an early stage. They will give us a specification, then ask us for advice on the best option.
A startup or SME that builds a new system probably knows what it wants to achieve but needs to learn about specific solutions.. They come to us with proposals, and we educate and advise them on the best solutions.
When a client wants to build a plant, they either hire an EPC company or come to us directly. They explain the required duties and specifications of the heat exchanger, and we will come back to them with a quote.
Summary
Let’s summarise the main shell and tube heat exchanger types and their pros and cons:
For dirty processes like oil and gas, a floating head heat exchanger that can be cleaned in all areas easily is essential, but it is the most expensive option. A fixed tube design might need an expansion bellows if the tubes are going to expand, but they are the cheapest option. A U-tube can deal with temperature differences, but only to a point.
Whether you are an EPC, an established heat exchanger user, or a new company building your system from scratch, Sterling TT can help you through the process designing, manufacturing, or replacing a shell and tube heat exchanger.
For a more detailed look on how we design shell and tube heat exchangers across multiple industries, read our blog: unique considerations of high-temperature shell and tube heat exchangers
For more information, contact us today