Multi-tubular heat exchanger (Shell and Tube) is the standard equipment for chemical plants.
It can be said that mechanical engineers are the most rewarding equipment to design.
We use heat exchanger handbooks and, more recently, specialized analysis software, but before that, we should understand the basics.
I don’t know where to refer to from a handbook with a huge amount of information, or I can’t verify the results by leaving it to the software…
We are facing a problem that is not new.
In order to avoid this, the basics are still important.
This time, we will explain the basics of design for condensers, which are commonplace in batch-type chemical plants.
However, the basic content is the same for liquid-liquid heat exchange.
If you only understand the contents of this article, you should be able to handle the design of shell and tube heat exchangers for batch chemical plants.
Shell-and-tube heat exchangers are basically stationary
There are many types of shell and tube heat exchangers.
At first glance, it is easy to lose motivation.
However, the basics are few.
Especially for batch chemical plants, it is sufficient to understand only the fixed type.
This type of heat exchanger accounts for more than 99% of the factory.
There are several reasons for using a fixed formula.
- little change in temperature
- Less need to clean the shell side
- Narrow installation space
- Good sealing
For example, the purpose of the floating head type is to clean the shell side instead of the fixed type, isn’t it?
The expression “maintenance space for pulling out tubes” is taken into account in the design of batch chemical plants as well.
This is an image of a floating head.
With the fixed type, the tube cannot be pulled out in the first place, but when inserting a camera for inspecting the inside of the tube, maintenance space is required in the same way as pulling out the tube.
Whether it’s a fixed type or a floating head type, it doesn’t mean that there will be a big impact on equipment design and construction design.
The point is that the fixed type has a higher sealing performance than the floating head type because it does not use a gland seal.
That’s why if you understand only the basic fixed type, you can fully withstand the application!
inside or outside
Even if the specifications are limited to a condenser-type shell-and-tube heat exchanger, there is still the problem of cooling inside or outside the tubes.
The following factors should be considered when choosing between in-tube cooling and out-of-tube cooling.
How to choose between in-tube cooling and out-of-tube cooling
- Material
- Cleanliness of process liquids and cooling liquids
- heat exchange
Let’s look at these in detail.
Design items in the pipe
Shell and tube heat exchangers are designed with respect to two elements: inside and outside the tubes.
I’ve organized what to think about each.
Material
The first thing to consider is the material.
Some people may be unaware of this.
That’s because it’s decided at the basic design stage before the detailed design of the equipment.
I would like to emphasize that this material decision plays a major role in the design of shell and tube heat exchangers.
Subsequent discussions will be divided depending on whether it is a special material such as glass lining or a general-purpose material such as stainless steel.
Batch-type chemical plants require special high-grade materials such as glass lining and carbon.
At this moment, it is decided to pass the process liquid inside the pipe and the utility outside the pipe.
Since the heat transfer area is also roughly determined, the material is a powerful design element that automatically completes the design without thinking about most of the subsequent design elements.
In stainless steel, the material of the tube is determined by the basic design, but it is not possible to decide whether to cool inside or outside the tube.
In either case, the material of the tube itself does not change, but in the case of a stainless steel shell and tube heat exchanger,
- Channel cover made of stainless steel for in-pipe cooling
- Stainless steel shell and baffle for external cooling
And the parts that should be made of stainless steel change.
velocity
Flow velocity is important as a design element inside the pipe.
The flow velocity is the first thing that must be determined when performing thermal calculations such as the U value.
Conversely, once the heat transfer area and flow velocity are determined, calculating the U value is like matching answers.
If it is a special material, there is no need to even calculate the flow velocity.
This is because it is unrealistic to change the tube size, the number of tubes, and the number of passes as elements that change the flow velocity.
As long as the heat transfer area is determined, it is sufficient to design from the manufacturer’s standard.
Even with general-purpose materials, there are only design elements that change the number of passes.
Changing the tube size and the number of tubes means doing a large-scale design, and it will be limited to super special equipment where you want to increase the effect even if it takes time to optimize.
I have never experienced one in over 15 years.
The cross-sectional area inside the pipe is
$$ A = N\frac{πd^2}{4} $$
So the flow velocity is
$$ v = \frac{Q}{A} = \frac{4Q}{nd^2} $$
number of passes
The number of passes is a direct design factor for determining flow velocity.
Specifically, a partition is installed on the channel cover.
This cannot be manufactured with special materials such as glass lining.
That is why the inside of the pipe is limited to one pass for special materials.
For general-purpose materials, the flow velocity can be easily changed by increasing the number of passes.
After that, you should look at the balance with pressure loss.
However, in the case of condenser applications, it is actually impossible to have more than two passes in the pipe.
Condensers target gas condensation.
At the moment the process liquid reaches the other channel cover after passing the tube through one channel cover.
It is inefficient unless gas and liquid are separated.
It is actually quite difficult to separate the gas and pass only the remaining liquid through the two-pass route.
Even if the gas is forced to condense in two passes, the pressure loss will increase and the heat transfer effect will not be improved, so it is meaningless in terms of cost effectiveness.
In such a case, it is safer to simply divide the heat exchanger into two units.
Design items outside the pipe
The design items outside the pipe are not much different from those inside the pipe.
It doesn’t have to be difficult.
Material
The material outside the tube is limited to iron-based materials such as SS400 in the case of special materials .
In the case of special materials, the outside of the tube is limited to utilities.
For general-purpose materials, in the case of pipe cooling, it is the same as for special materials, SS400 series.
After all, it is decided whether to pass the utility or the process liquid.
When passing the process liquid outside the pipe, the shell and baffle should be made of a corrosion-resistant material such as stainless steel.
velocity
The basis of shell and tube heat exchangers is to design the flow velocity both inside and outside the tube.
In fact, in the case of gas condensation outside the tube, flow velocity design is not considered.
Flow rate design is important when passing liquid outside the tube.
The aim is to increase the flow velocity to improve the heat exchange efficiency, and to create a facility that can withstand operation even with a small heat transfer area.
Baffle spacing
Design baffle spacing to increase flow velocity.
The shorter the baffle spacing, the smaller the area through which the fluid passes, resulting in a design that increases the flow velocity.
It is the same treatment as increasing the number of passes in the case of jurisdiction.
It is better to think of the fluid passage area outside the pipe as the following index.
(Shell diameter – tube outer diameter x number of tubes) x baffle spacing
This is not a strict transit area. treated as a mere indicator.
This is because the passage area of the fluid outside the tube changes depending on the location.
The baffle cutout is much narrower than the above indicator.
It will be limited to ultra-high load equipment that considers thermal and strength issues with detailed thermal and fluid calculations.
Discussing this issue would be unthinkable for a batch-type chemical plant.
The shorter the baffle interval, the greater the pressure loss, so it is necessary to consider the balance with the pump design.
Normally, it would be safe to have an internal standard for the baffle interval and not change it unless there is an extraordinary reason.
Even if gas condensation does not affect the design, it is safe to use the same baffle spacing as for liquid cooling.
It’s safe, so it doesn’t mean you can’t change it.
If you change it instead, you need to consider it.
Baffle cutout
The baffle notch is also a design element.
The notch opening is generally 25%, so designing that would change this would not normally be done.
This is also the same as the baffle interval, so if you change it, you need to consider it.
There are different design concepts for the orientation of the notch.
- Liquids with no vertical notch phase change
- Liquids with horizontal notched phase change
It’s said like this, but it’s a pretty good element either way.
With a 25% vertical notch, the top and bottom of the shell are almost in contact with the shell on any baffle.
This is the cause of gas and liquid pooling, and the interpretation is that if you don’t like pooling, you should avoid vertical notches.
But the baffle and shell aren’t welded together, and the baffle and tube have gaps as well.
In some cases, the top and bottom of the baffle are intentionally notched with triangular notches.
It’s a comfort level.
It is not true that a horizontal notch can completely solve the pooling that occurs with a vertical notch.
In batch-type chemical plants, vertical notches are often used due to concerns about rust and accumulation of foreign matter due to utility fluids that are not highly clean.
Conversely, only clean process liquids and utilities pass through! In that case, the horizontal notch can be slightly more effective.
To what extent should cleaning be considered?
Textbooks say that the detergency of the liquid determines whether it is inside or outside the pipe , but this article only covers the baffle notch outside the pipe.
I don’t pay much attention to it.
The reason for this is that batch-type chemical plants have the following background.
- Mild usage conditions
- multipurpose use
- Requires highly corrosion-resistant materials
Especially in the case of heat exchangers such as glass lined and carbon, it is naturally specified.
Based on this idea, we can create a royal road equipment configuration for condensers, such as in-tube condensation and out-of-tube utilities.
If it is close to a dedicated facility that is not multi-purpose, it is possible to conceive of inside-pipe utility and outside-pipe condensation, but it is extremely rare.
This is because the number of cases where the process liquid is pure gas is gradually decreasing.
Originally, it was a process that allows clean gas to pass through, but as a result of adding foreign substances and additives such as catalysts,
It is a big problem if the solid content reaches the condenser and clogs it.
Even though it is a fixed type and sacrifices cleaning performance to increase sealing performance, if you suffer from clogging problems, you are putting the cart before the horse.
If you don’t manage the utility, rust will accumulate inside the shell as the water deteriorates, but even so, it will last for about 20 years, so some companies may have the idea of regular replacement.
The decision is whether to invest day-to-day costs in utility water quality management, or to invest in medium- to long-term equipment.
Equipment cleaning is one of the important issues in plant operation.
reference
Heat exchangers are already an established technology and undoubtedly tend to use the same construction.
On the other hand, if you know the basic role, you can see the parts that can be improved.
I really want to understand the basics.
Related article
Related information
lastly
I explained the design system of shell and tube heat exchangers.
In the batch system, it is recommended to use it as a condensate inside the tube and as a utility outside the tube for the purpose of a condenser with a fixed structure.
Considering the material and flow velocity as design elements inside and outside the pipe, most of them can be handled by standardization, so there are surprisingly few factors to consider.
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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