I summarized the main parts and their functions of shell and tube heat exchanger.
An overview of the parts will be explained using a basic horizontal, fixed shell- and-tube heat exchanger as an example.
The shell and tube heat exchanger is a key piece of equipment for plant mechanical engineers.
The structure is a little complicated for a chemical plant, but I’d like to understand it properly if I’m an electrical engineer.
For beginners, it may be difficult to understand the drawings if they stare at them from the beginning.
Outline of shell and tube heat exchanger
The structure of the shell-and-tube heat exchanger looks like the following.
A shell and tube heat exchanger is roughly divided into two parts: the bonnet and the shell.
In many cases, the process liquid is applied to the bonnet and the cooling liquid is applied to the shell, but there are actually several patterns.
| bonnet | shell | Usage |
|---|---|---|
| process | coolant | I want to cool the process liquid |
| coolant | process | I want to cool the process liquid (clean process liquid) |
| process | heating liquid | I want to heat the process liquid |
| heating liquid | process | I want to warm the process liquid (clean process liquid) |
In batch chemical plants, heat exchangers often cool process liquids , but they rarely heat them.
It is often used as a distillation condenser.
If it is a continuous system, it is heated for applications such as a reboiler.
Tubes and tubesheets are the first part of shell and tube heat exchangers
Tubes are the main component of heat exchangers.
Shell-and-tube heat exchangers are called shell-and-tube heat exchangers, and these ” tubes ” are the backbone of the heat exchange system.
It has a structure in which many tubes are arranged according to a certain rule and both ends are fixed with a tube sheet.
When assuming the case of cooling the process liquid, heat is exchanged between the process liquid and the cooling liquid through a partition called a tube.
Heat is transferred by conductive heat transfer in the tube and convective heat transfer on the liquid side.
The heat transfer area, which is the main index of shell and tube heat exchangers, is determined by the surface area of these tubes.
The diameter of the tube should be as small as possible, and the thickness should be as thin as possible.
Here, we will decide the size considering the strength of the tube.
The tubes are welded to the tubesheet.
Welding is relatively difficult because many thin tubes are arranged.
It may be done with an automatic welding machine.
Welding creates a seal, but leaves a gap between the tube and the tubesheet.
In order to prevent crevice corrosion, we want to make the gap as small as possible, so it is common to expand the tube.
bonnet
The bonnet is the inlet part for the process to pass through the tube.
Normally, the diameter of the tubesheet is much larger than the diameter of the process piping, so it can be considered as a component for increasing the diameter.
When considering pressure resistance, the head plate shape is preferable, but a reducer may be used instead.
The bonnet and tubesheet are connected by a flange.
Since the flange diameter is large, a gasket is used for sealing.
Due to its large size, it is easy to misplace the gasket, so it is a good idea to make a groove on the flange surface to fix the position.
There is absolutely no need to stick to JIS10k for this flange.
This means that since the piping is JIS 10k, the nozzle is JIS 10k, and the tube sheet is not necessarily JIS 10k.
It will be OK if the flange thickness is enough to secure the pressure resistance.
Due to its structure, the bonnet tends to have a structure in which liquid accumulates.
If the bonnet does not have a bottom drain nozzle, the liquid will collect at the position shown in the figure below.
This is unavoidable because the ends of the tubes in the tubesheet are higher than the bottom of the bonnet.
If you don’t like liquid pooling, attach a bottom nozzle.
It looks like the bonnet on the right side of the above figure.
Shell is the first part of shell and tube heat exchanger
The shell is a part that replaces the piping for passing liquids and gases to the outside of the tube.
A pattern in which the tubesheet and shell are welded together is called a fixed type.
There are various patterns other than the fixed type, but the fixed type has the simplest structure.
Since it is fixed, there is a problem that it is difficult to clean even if there is dirt on the outside of the tube.
Therefore, in the case of a fixed type, it is basic to pour clean things as much as possible on the shell side.
It’s case by case.
In the case of condensers, the process liquid is often cleaner than the cooling liquid, and we would like to put the process liquid on the shell side.
But in many cases it is highly corrosive and we have no choice but to put it on the tube side.
This is the case when the tube is carbon or glass.
In the case of stainless steel heat exchangers, the options will be much wider.
If the shell is made from simple piping, the efficiency of heat exchange will be poor.
Therefore, a baffle is attached to forcibly change the flow on the shell side.
By design, this baffle spacing is variable.
The baffle is not 100% the same shape as the tubesheet, but is partially notched.
The notch will be the path.
There are two patterns of notch placement: horizontal and vertical.
This area tends to be a world of hobbies.
The baffle is not connected with the tube.
For that reason, the baffle interval is an important indicator, so fixing is an important factor.
As shown in this figure, fix the baffle with parts called tie rods and spacers.
Place spacers between the baffles to keep tie rods from dropping or displacing spacers.
One side of the tie rod is connected to the tube sheet with a screw or the like, and the other side is fixed with a baffle and a nut.
This will fix the baffle to the tube.
reference
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lastly
I explained the main parts of the shell and tube heat exchanger.
Shells, tubes, bonnets, flanges, gaskets, baffles, tie rods, spacers, etc. are important.
There may be cases where the details are written in the drawing, but it should be difficult to see because it is detailed, so it is better to understand the outline first and then look at the drawing.
This knowledge is essential for plant mechanical engineers.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)
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