Shell and tube heat exchangers are a common site throughout the engineering world and are one of the two most common types of heat exchanger; the other common type being the plate heat exchanger.
Shell and tube heat exchangers have a simple design, robust characteristics and relatively low purchase and maintenance costs. They also have a very high heat transfer rate although they require more space than a plate heat exchanger of similar thermal exchange capacity.
Shell and Tube Heat Exchangers
A shell and tube heat exchanger consists of a series of tubes housed within a cylindrical container known as a ‘shell’. All tubes within the shell are collectively termed a ‘tube bundle’ or ‘tube nest’). Each tube passes through a series of baffles and tube sheets (also known as ‘tube stacks’). One of the tube sheets is fixed and one is free to move, this allows for thermal expansion as the heat exchanger is heated.
Shell and Tube Heat Exchanger Components
The flowing medium within the tubes is known as the ‘tube side’ medium. The flowing medium outside of the tubes is known as the ‘shell side’ medium. Each medium has one entry and one discharge.
The tube side medium is usually selected for the high pressure fluid as each tube can act as a small pressure vessel; it is also more cost effective to produce high pressure rated tubes than it is to produce a high pressure rated shell.
A shell heat exchanger uses water to cool oil. Oil is the shell side medium whilst water is the tube side medium. The oil enters through the top left inlet and flows through the heat exchanger until reaching the lower right discharge. Water flows through the tubes from the right inlet to the left discharge.
Single Pass Heat Exchanger
How do shell and tube heat exchangers work?
The shell and tube heat exchanger is split into two main systems, referred to as the shell side and tube side. Each system has one associated flowing medium. In our example, we will assume the shell side contains hot mineral oil that must be cooled, whilst the tube side contains cooling water.
The cooling water enters the heat exchanger and flows through the tubes. The mineral oil enters the heat exchanger and flows in the shell surrounding the tubes. The two fluids do not mix as the wall of the tubes prevents this. Because the fluids do not directly mix, indirect cooling occurs (not direct cooling).
Turbulent flow increases the heat transfer rate of the heat exchanger and also reduces the likelihood of dissolved solids accumulating on the tube and shell walls (turbulent flow has a self-cleaning effect).
Turbulent flow within the tubes is created by inserting tube inserts (also known as ‘turbulators‘) into each of the tubes. Turbulent flow within the shell is created by baffles, which are used to direct the water across the tubes multiple times as it travels through the heat exchanger.
Tube Inserts (black line in middle of tube)
Heat is exchanged between the two fluids because they are in thermal contact with each other. The oil leaves the heat exchanger cooler and the water leaves the heat exchanger warmer.
Parallel, Counter and Cross Flow
Parallel, Counter and Cross Flow
Heat exchangers are available in many shapes and sizes. To make classification of heat exchangers easier, they are often split into groups based upon design and operating characteristics. One such characteristic is the flow type.
There are three main flow types, these are parallel, counter and cross flow. Due to design considerations and the applications of heat exchangers, it is rare that a heat exchanger be only one of these flow types, usually they are a combination of several flow types e.g. counter cross flow.
Parallel flow occurs when both the shell side and tube side mediums enter the heat exchanger from the same end of the heat exchanger and flow to the opposite end of the heat exchanger. The temperature change (delta T/ΔT) across the two mediums is equal for both i.e. they both increase or reduce by a certain amount. Notice that the output temperature for both mediums tend to converge and it is not possible to cool below this point even though the colder fluid inlet temperature is lower than the convergence temperature (the convergence temperature on the graph below is approximately 80°C).
Parallel Flow Heat Exchanger
Counter flow (also known as contra-flow) heat exchangers have two flowing mediums that are flowing in a counter direction (180° apart) to each other. Each flowing medium enters the heat exchanger at opposite ends and is discharged at opposite ends. Because the cooler medium exits the counter flow heat exchanger at the end where the hot medium enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid; this makes the potential delta T far greater than that of a parallel flow heat exchanger. Counter flow heat exchangers are the most efficient type of heat exchanger.
Counter Flow Heat Exchanger
Cross flow heat exchangers have one medium flowing that flows perpendicular (at 90°) across the other. Cross flow heat exchangers are usually found in applications where one of the fluids changes state (2-phase flow). For example, a steam system’s condenser, in which the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the tubes absorbs the heat from the steam, condensing it into water. Large volumes of vapour may be condensed using this type of heat exchanger flow.
Cross Flow Heat Exchanger
Single and Multi-Pass
An economical and efficient way of increasing a heat exchangers efficiency is to bring the flowing mediums into contact with each other several times. Each time one medium passes over the other, heat is exchanged.
When one flowing medium passes over the other only once, it is termed a ‘single pass’ heat exchanger.
Single Pass Heat Exchanger Design
When one flowing medium passes over the other more than once, it is termed a ‘multi-pass’ heat exchanger.
Multi-Pass Heat Exchanger Design
Multi-Pass In The Tubes
Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more sets of “U” bends in the tubes. The “U” bends allow the fluid to flow back and forth across the length of the heat exchanger. This type of heat exchanger is known as a U-tube shell and tube heat exchanger.
U-Shape Heat Exchanger
It is also possible to reverse the flow through the tubes by using the lower or upper side of the tube bundle for one pass, and the opposite side for the next pass. Thus each half of a tube bundle equates to one pass.
Multi-Pass In The Shell
A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger. These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect.
Multi-Pass Heat Exchanger
Advantages and Disadvantages
- Cheap compared to plate heat exchangers.
- Relatively simple design and easy to maintain.
- Suitable for higher pressures and temperatures compared to plate heat exchangers.
- Pressure drop (delta P/ΔP) is less than a plate heat exchanger.
- Easy to find and isolate leaking tubes.
- Tubes can be ‘double walled’ to reduce the likelihood of the shell side fluid leaking into the tube side fluid (or vice versa).
- Easy to install sacrificial anodes.
- Do not foul as easily as plate heat exchangers.
- Less efficient than plate heat exchangers.
- Require more space to open and remove tubes.
- Cooling capacity can not be increased, but a plate heat exchanger’s can be.
3D Model Annotations
The partition plate separates the lower and upper halves of the heat exchanger. The partition diverts the flowing medium through the tubes.
Inlet / Discharge
Inlet or discharge of the fluid medium that flows through the tubes or shell of the heat exchanger.
The housing/shell is used to contain the flowing medium and house internal parts. It also serves as a strong structural piece upon which other pieces can be attached.
The cover plate is used to seal one end of the shell and prevent leakage.
A gasket is placed between two metal surfaces. The gasket is usually constructed of paper or rubber and is ‘squeezed’ between the metals to create a seal. The seal prevents leakage.
The shape of the gasket also prevents leakage around the partition plate.
The tubesheet sits within the shell and supports the ends of the tubes. The weight of the tubes is then further supported by the baffles (depending upon the design).
Baffles are used to change the directional flow of the fluid medium. Changing the direction ensures an even heat distribution throughout the heat exchanger. Efficiency decreases when flow through the heat exchanger is not evenly distributed.
Nuts and bolts are used for securing parts of the heat exchanger. Chosen bolts should have suitable tensile strength and corrosion resistance characteristics.
Bolts are the ‘male’ part of a nut and bolt assembly.
Nuts and bolts are used for securing parts of the heat exchanger. Chosen nuts should have suitable tensile strength and corrosion resistance characteristics.
Nuts are the ‘female’ part of a nut and bolt assembly.
Tie bars are used as guides for the baffles to ensure no rotational or axial movement of the baffles occurs.
One of the fluid mediums flows directly through the tubes whilst the other flows turbulently on the outside. Heat is exchanged between the two mediums due to proximity (heat is exchanged via conduction to the tube walls and then further to the outside medium).
The tubes, baffles and tie bars are all housed within the shell (housing). It is the shell and tube construct which gives this type of heat exchanger its name.
This article has discussed all of a shell and tube heat exchanger’s main parts, how it works, its design features, and the advantages and disadvantages associated with this type of heat exchanger. Be sure to check out our Plate Heat Exchanger Fundamentals video course and Introduction to Heat Exchangers course if you would like to learn more about heat exchangers!
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- [Narrator] Shell and tube. The most basic and the most common type of heat exchanger construction is the shell and tube. This type of heat exchanger consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube-side fluid, and the fluid flowing on the outside of the tubes is the shell-side fluid. At the end of the tubes, the tube-side fluid is separated from the shell-side fluid by the tube sheets. The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak-tight seal. So, the shell and tube type heat exchanger is essentially a series of pipes that will pass through a heat exchanger. And we'll have one medium flowing through the pipes, and one medium flowing on the outside of the pipes. And we're gonna have a look at an example in a moment. In systems where the two fluids are at vastly different pressures, the higher-pressure fluid is typically directed through the tubes, and the lower-pressure fluid is circulated on the shell side. This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates shown below act as baffles to direct the flow of fluid within the shell back and forth across the tubes. So what we mean here is, when we are pumping a fluid or a medium through a heat exchanger, the medium at a higher pressure is going to go through the tubes. Sometimes if you have two mediums such as oil and water, where the water is cooling down the oil, it may be very important that the oil does not leak out into the water. This is especially true if we're using something like river water or lake water to cool down the oil. We don't want oil leaking out through the tubes and going back into the river or the lake, or even the ocean. So what we'll do, we'll have what they call a double-walled tube. And we will have a tube that has two walls. And if oil was to leak out of the inner wall of the tube, it will go into the middle between the outer and the inner wall. And it will set off a leak alarm. So that way we know that one of the tubes is leaking, and we have an alarm. And it doesn't leak out directly into the shell and into the water. Let's just load up a model here so I can show you more detail, how the heat exchanger works. Okay so here we have a standard shell and tube heat exchanger. Just do a little spin. We can see there's two pipes on the top and two pipes on the bottom. We've got a stand, and that is for installing the heat exchanger. Do a spin around this side. We can see the nuts and bolts on the end here, we will undo those to open up the end cover and get inside the heat exchanger. Or do an inspection or maintenance. Just take a cross-section. Okay so we've got a cross-section now of the heat exchanger. We can see here tube-side fluid out, tube-side fluid in. When we say tube-side, we mean the medium that is flowing through the tubes. The opposite of tube-side is shell-side. We'll have shell-side fluid in, and shell-side fluid out. That is this lower section here or the lower pipe. Now let's get to an overview so we can follow the flow through the heat exchanger. Okay, so we have got a cold fluid flowing in from the bottom. It is then flowing along through the heat exchanger through these tubes. It is doing a u-turn, this is actually called a u-turn shell and tube type heat exchanger. A u-turn here, and then it is flowing back that way, and it is coming out of the top. Let me just spin around this side, we can see the entrance points. And we can see there are the tubes, and the tubes are going off into the distance. So the fluid is gonna be flowing directly into these tubes. We can also see a plate, which is used for separating the fluid as it flows in from the bottom and then out of the top. If we were to move this plate or take it out, we would actually just have a fluid that flows in from the bottom and directly out the top. It's gonna choose the path of least resistance. But because the plate's there, it's coming in and flowing through the bottom tubes. And we'll just follow it along, along these tubes. And we can see here, this is where it suddenly turns around. It's coming, all traveling to the right on the lower section, around the tubes, around this u-turn, and back the other way. And it's gonna keep going all the way until it comes out of the top again, or the top section of the heat exchanger. What's actually happened is it's gone in the bottom and out of the top, and it has absorbed some of that heat. And it's then gonna take away that heat and distribute it somewhere else, perhaps to ambient air. Or perhaps it'll just go back to a reservoir. Sometimes they'll even use some of the warmer fluid for a later stage in the process. It's a good way to recycle the heat, rather than just waste it. Because essentially when you're removing the heat, that is a efficiency or an energy loss. So if you can use it early or late in the process again, you're recovering some of that energy. And you're increasing the process efficiency. Let's have a look at the fluid that comes in at the top. The shell-side fluid in, in at the top. Now it does not, unlike the tubular flow, which flows relatively direct. The fluid that is flowing in on the shell side is gonna flow around a series of baffle plates. It's gonna come around here and be forced to turn. It's gonna turn again, it's gonna turn again, and it's gonna keep doing that all the way along. And then it is going to exit at the bottom of the heat exchanger. I have to say, it would be slightly better if this discharge port from the heat exchanger was more to the right in order that it could flow through the heat exchanger and down on the right-hand side rather than here. But that is how the heat exchanger is being built in 3-D here. The reason for this criss-crossing pattern, this is actually called cross flow, is because we wanna maximize the heat transfer between the two flowing fluids. And we do this by having a cross flow pattern. There's no point in the fluid entering in the top flowing directly here and then dropping out of the bottom. Because if we do it like that, we've had very little turbulent flow. And there's not gonna be as much heat transfer between the two mediums compared to when we do this cross flow pattern. And although you can't actually see it, inside these tubes there is normally a thin piece of copper or plastic, and it will slide into each and every one of these tubes. Now this thin piece of copper, or perhaps plastic or other form of metal, it depends on the system that you're actually using it for, is similar to a very thin strip, a flat bar. A thin flat bar of copper, for example. And the idea is that as the fluid is flowing through the tubes, it does not get to flow in a straight laminar direction. It is going to come into contact with this thin perforated copper bar, and then it is gonna be forced to flow over and under the copper bar. In other words, it's gonna have a very very turbulent path through the tubes. And this is what we want, we want turbulent flow. Because this is gonna increase our heat transfer. The other benefit with turbulent flow is simply that if we have suspended bodies within the fluid that may stick to the sides of the tubes, they won't be able to stick to the sides of the tubes as easily. If they do stick to the sides of the tubes, that is going to reduce our heat transfer rate, or our heat transfer capacity. So by having this turbulent flow, we're preventing them or reducing the risk that they're gonna be able to stick to the sides of the tubes. And this will maintain our heat transfer capacity. Now if you don't know what I'm talking about when I talk about things sticking to the sides or surfaces of a heat exchanger, go and have a look at your kettle. Now if you boil your kettle a thousand times using standard tap water, it's very likely you'll notice a thin white powdery substance building up within the kettle. Now this is actually reducing your heat transfer. This white powdery substance comes from the water itself, the minerals and suspended bodies within the water. And over time, it will stick to the inside metal surfaces of your kettle, and it will reduce the heat transfer rate. And that's exactly what can also happen in a heat exchanger. Another example is a boiler. With boilers, they go to great lengths to ensure that the water quality is as clean as possible. And the reason again, is that any suspended bodies that stick to the surface of the boiler tubes will reduce the heat transfer rate. And in severe conditions, this can actually cause the piping to melt. So it's very important that you keep the contact surface areas within your heat exchanger as clean as possible. So that is a u-type shell and tube heat exchanger. If we were to click here, we could actually have a look at some of the more specific pieces. For example, let's just have a look at the tubes. And we can see our tubes. Right, do a full version again. We can see all of our tubes there and the way they are installed. We can also highlight the baffles. And the baffles now shown. We will see our flow comes in and around the baffles like so. So the baffles, we'll just highlight them for you, are those pieces here. And they'll be designed to be installed in this pattern. So that we get this cross flow. If that was all a bit quick, don't worry. We are going to go through this in more detail later in the course with some different examples. I just think that was a nice introduction to what a shell and tube type heat exchanger is. Now let's go on to the next lesson.