Shell and tube exchangers are the most popular heat transfer equipment used in process industry. This type of heat exchangers consists of metal tubes passing through another metal enclosure, which is referred to as the 'shell'.
So typically we have a fluid on shell side and anther fluid on the tube side. Heat transfer between the two fluids occurs across the tube walls.
Shell & tube exchanger design
When you design a new shell & tube heat exchanger, you have two fluids - one hot and one cold. You need to decide which one should go to shellside and which one should be put through the tube side.
There are a few important factors that influence the design of a shell and tube heat exchanger.
- Achieving the required heat transfer while maintaining the equipment cost below a certain limit
- Ensuring that the new heat exchanger is easy to maintain
- Making sure that exchanger pressure drop values for shell and tube sides, don't result in excess pumping costs
These factors must also be considered when deciding the fluid allocation in a shell & tube exchanger - where to put the hot and cold fluids, shell side or tube side. For example, if the hot fluid is very viscous, putting it on the tube side will result is excessive pressure drop. That will put excess load on the corresponding pump. Hence the more viscous fluids are generally put through the shell side of a shell and tube exchanger.
Should the fouling service go to tubeside or shellside?
If a fluid is particularly dirty, corrosive or causes heat exchanger fouling, it has the biggest impact the 'maintainability' aspect from the 3 important factors listed above.
Hence a dirty, corrosive or fouling fluid is generally put on the tube side of a shell & tube heat exchanger, because -
- It is easier to mechanically clean the inner surface of the tubes, in a shell & tube exchanger.
- Mechanical cleaning of the inner shell surface or outer walls of the tubes is much more difficult in comparison.
- Tubes are relatively cheaper to fabricate than the shell side. Hence even if a corrosive fluid eats up the inside of those tubes in the long run, they can be replaced at a lower cost than the shell side material.
- Tubes made of expensive corrosion resistant material can be cheaper than the entire shell built out of special material.
Mechanical cleaning of the tubes
In two out of three categories of shell & tube exchangers, the tube bundle can be easily cleaned from the inside by removing the read end head. In fixed tubesheet and floating head type of exchangers this is easily possible. Hence these types are preferred when there is a dirty or corrosive service involved.
Mechanical cleaning of tube bundle from inside
Other fluids recommended to be kept on tube side
Apart from the corrosive or dirty fluids, it is also preferred to put the following types of fluid services on the tube side of an exchanger.
- High pressure fluids are kept on the tubeside. Tubes of higher pressure rating are cheaper than the shell side of a higher pressure rating.
- Toxic fluids are also kept on the tubeside to minimize the possibility of leakage to environment
- Streams with low allowable pressure drops are put on the tubeside, where turbulance and resulting head loss can be minimized.