Shell and Tube Type Heat Exchanger

Shell and tube heat exchangers are popular exchangers, they have a simple design and strong characteristics. They transfer energy in the form of heat. They have a high heat exchange rate but require more space than other heat exchangers like plate heat exchangers. They are used in the petrochemical industry and power industry as feedwater heaters. Shell and tube heat exchangers differ from the other heat exchangers making it more preferable for the heat exchange process. This article covers what shell and tube heat exchangers are, the working principle behind them, and their different types. 

Components of shell and tube heat exchanger

Shell and tube heat exchanger contains a shell and a bunch of tubes, between which heat exchange occurs. The bunch of tubes are collectively known as tube bundle or tube nest. All these tubes pass through tube sheets on either side, one of them is fixed while the other is free to move to allow expansion during the heat transfer. The flow that happens inside the tube is called tube side flow and the flow that happens outside the tube is called shell-side flow. The shell and tube heat exchanger has two inlets and two outlets, where fluids run through the tubes inside the shell. The inlets and outlets differed as shell side and tube side. Tube side medium is preferred to pass the liquid with high pressure as they are more cost-efficient to produce the high pressurised tube flow than shell flow.

Working of shell and tube heat exchanger

The principle involved behind them is similar to any other heat exchangers, a hot fluid flowing inside a tube over colder fluid and they exchange its heat towards the colder side. In shell and tube exchanger design, it puts the two liquids which are at different temperatures, in thermal contact, in between its cylindrical shell. The fluid flows through the tube and the shell. The pathway that separates the tube side and the shell side is usually made of highly conductive metals which allows easy transfer of heat. The tube side flow starts at the tube inlet, runs through the bundle of tubes inside the shell, and ends at the tube outlet. Similar to tube side flow, shell side flow starts at shell inlet, passes over the tubes, and ends at shell outlet. It has headers on either side of the exchanger which acts as a reservoir for the fluid in tube side flow. The two fluids do not mix, as the conductive walls prevent them. 

Turbulent flow increases the rate of heat exchange and the chances of accumulation of dissolved solids on the tubes and shell walls are low. They are created by placing turbulators inside each tube. Baffles are used inside the shell to create turbulent flow. It directs the water inside the shell multiple times across the tube for a better exchange of heat. 

Types of flow:

There are three different types of flows, these are parallel, cross, and counter flow. Most of the heat exchangers use a combination of several types of these flows.

• Parallel flow:

Parallel flow occurs when the fluid enters through the same side of inlets, tube side, and shell side and exits through the side of outlets on the opposite side. The temperature change across the two fluids is equal, it either reduces or increases at an equal rate. Both the temperature converges to a certain temperature and it is impossible to increase or decrease beyond this point. 

• Counterflow:

Counterflow occurs when fluid enters through the opposite sides of the inlets. The fluid enters through the opposite sides and they are discharged at opposite ends. Here the hot fluid makes thermal contact with the cooler fluid first, and the heat exchange occurs between them steadily at every part of the tube. This type of flow is the most efficient type. 

• Crossflow:

In cross-flow heat exchangers, one fluid flows in perpendicular to the other one. This type of flow is used when fluids inside the application change their state. 

Different types of shell and tube exchangers: 

• U – Tube heat exchanger:

The tube bundle is made of continuous tubes in the shape of U and all of the tubes end at one tube plate. The fluid enters through the top half of the header, through the tubes, and exits through the bottom end of the header. The bend in this type of heat exchanger allows thermal expansion to happen without relying on any thermal joints. They are efficient to use when the fluids have a high-temperature difference. 

• Fixed tube sheet exchanger:

The tube sheets are attached directly to the shell. These are the most cost-efficient type of heat exchanger as it is easier to manufacture. Since the tube sheet is rigidly attached, heat exchange should be prevented. The temperature difference between the fluid should be less. 

Conclusion

Shell and tube exchangers are most popular, as they are cheap and easy to maintain. They are preferred and most suitable for high pressure and temperature compared to the plate heat exchangers. The exchangers can be modified according to the requirement and heat exchange between them can be controlled and monitored.