What are shell and tube heat exchangers?
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
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Shell and Tube Heat Exchanger Components
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.
Example: Heat Exchangers
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.
How do shell and tube heat exchangers work?
The below video is an extract from our Heat Exchangers Online Video Course.
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.
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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
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
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
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.
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 of Shell and Tube Heat Exchangers
Advantages
- 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.
Disadvantages
- 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.
Shell and Tube Heat Exchanger Parts
Partition Plate
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.
Housing/Shell
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. Cover Plate The cover plate is used to seal one end of the shell and prevent leakage.
Gasket
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.
Stationary Tubesheet
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
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.
Bolt
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.
Nut
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
Tie bars are used as guides for the baffles to ensure no rotational or axial movement of the baffles occurs.
Tubes
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).
Shell
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.
Additional Resources
http://www.mcraeeng.com/how-shell-and-tube-heat-exchangers-work/page-2/blog.html
https://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger
https://www.explainthatstuff.com/how-heat-exchangers-work.html