Shell and Tube Heat Exchanger
Shell and tube heat exchangers are a type of heat exchanger. The most common type of heat exchanger in oil refineries and other large chemical processes, it can withstand higher pressures. Heat exchangers of this type consist of a shell (a large pressure vessel) with a bundle of tubes inside. One fluid flows through the tubes and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. Tube bundles are composed of several types of tubes, including plain tubes, longitudinally finned tubes, etc.
The heat exchanger flows two fluids, each of which has a different starting temperature. Both flow through the tubes (the tube side) and one flows outside the tubes but inside the shell (the shell side). Through the tube walls, heat is transferred from one fluid to another, either from tube to shell or vice versa.
Fluids can either be liquids or gases on the shell or tube side. To transfer heat efficiently, there must be a large heat transfer area, which leads to the use of many tubes. By doing so, waste heat can be utilised. Heat exchangers with only one phase (liquid or gas) on each side are called one-phase or single-phase heat exchangers.
These dual-phase heat exchangers can either be used to heat a liquid to boil it into a gas (vapour) or to cool the vapour to condense it into a liquid (condensers), with the phase change usually occurring on the shell side of the exchanger. Steam engine locomotive boilers are usually large, cylindrical shell-and-tube heat exchangers. Shell-and-tube surface condensers are used in large power plants with steam-driven turbines to condense exhaust steam exiting the turbine into condensate water that is then recycled back into steam in the turbine.
They are also used in liquid-cooled chillers to transfer heat between the refrigerant and the water in both the evaporator and condenser, as well as in air-cooled chillers for the evaporator only.
How do shell and tube heat exchangers work?
The typical shell and tube heat exchanger configuration, with labels for easy reading. As previously explained, the main function of shell and tube heat exchangers is to pass a hot fluid through a cold fluid without mixing them, so that only their heat is transferred. The diagram above shows two inlets and two outlets, with each fluid entering at its respective inlet and exiting at its outlet.
side flow passes through the tube bundle (secured by metal plates called tubesheets or tubeplates) and exits at the tube outlet. Similarly, the shell-side flow starts at the shell’s inlet, passes over these tubes, and exits at the shell’s outlet. There are headers on either side of the tube bundle that create reservoirs for the tube-side flow and can be divided according to the type of heat exchanger.
Types of shell and tube heat exchangers
TIPIC sets the standards for shell and tube heat exchangers. Heat exchangers are divided into three parts: the front end header, the shell, and the rear end header. Because there are many types of each component, this article will only focus on the three most common TEMA standard heat exchangers. Three types of heat exchangers exist: U-tube, fixed tube sheet, and floating head.
U-tube heat exchanger
The typical layout of U-tube exchangers and how they got their name. The tube bundle is made of continuous tubes that bend into a U shape, and is secured to the shell using one tubeplate (shown above). A multi-pass design is created by the coolant flowing from the top half of the header, through the u-tubes, and then out of the bottom half of the header. As the bend side is free-floating within the shell and has room to expand/contract, no expansion joints are needed to allow for thermal expansion. They are excellent for high-temperature differences where expansion is expected. Their only major drawback is that they are hard to clean due to the bends.
Fixed tube sheet exchanger
Two stationary tube sheets (labelled above) are welded directly to the shell of the fixed tube sheet exchanger. As they are the easiest to manufacture, they are the most cost-effective version of the shell and tube design. However, since the tubes are rigidly attached to the shell via the tube sheets, Expansion that is limited must be prevented. If there is a high temperature difference between the tube-side and shell-side flows, operators risk expansion and damage, so the temperature difference must be kept small.
Another disadvantage of fixed tube sheet models is that the outside of their tubes cannot be accessed for cleaning. The shell-side fluid being used must not foul the outside of the tubes, or the heat exchanger’s efficiency will decrease.
Floating head exchanger
The floating head exchanger combines the best features of both previous designs. A fixed tubesheet holds one end of the tubes stationary to the housing, while the other end is free to expand because of a floating tubesheet. This part allows the tubes to expand with increased temperatures, without having to bend the pipes. In addition to being able to clean the tubes easily, operators can also create a high temperature difference without fear of breaking the devices. Therefore, the floating head exchanger is the best heat exchanger in terms of efficiency and maintenance, but comes at a higher price.
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