A shell & tube heat exchanger consists of metal tubes passing through another metal enclosure, which is referred to as the 'shell'. Correspondingly, we have 2 distinct spaces in a shell and tube exchanger - shell side and tube side. The hot of the cold fluids are passed through these spaces, each one through either the shell or tube side. And then heat transfer occurs between the two fluids.
Shell & tube heat exchanger fluid allocation
But each time when you design for a new shell and tube heat exchanger, you will need to decide where the hot / cold fluid streams must be places. Which should go through shellside and which will go through the tubes. This decision is commonly referred to as fluid allocation for the shell & tube heat exchanger.
There are three important aspects that need to be considered when designing a new 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 pressure drop values for the fluids on shell and tube sides, don't result in excess pumping costs
These factors must be considered when deciding about the fluid allocation in shell & tube heat exchanger. Here are some guidelines to help you decide.
Preferred fluids on the tube side
Following important characteristics of the tube bundle distinguish it from the shell side.
- It is easier to mechanically clean the inner tube surface than to clean the outer tube surface and inner shell surface. This is true for fixed tubesheet and floating heat type exchangers which are quite common to handle dirty services that cause heat exchanger fouling.
- Tubes are relatively cheaper to fabricate than the shell body.
- Tubes facilitate a streamlined flow, lower turbulence. They can allow flow with lower pressure drop.
Mechanical cleaning of the tube bundle from inside.
Considering these important features of the tube bundle in an exchanger, following fluid services are better suited for the tube side.
- Fouling or corrosive fluid service is kept on the tube side, since mechanical cleaning of the inner tube surface is easier.
- It is cheaper to replace corroded tubes, than the shell cover. Hence corrosive service must be kept on the tubeside.
- High pressure fluid is also kept on the tubeside, since tubes with higher pressure rating are cheaper than the shell with high pressure rating material.
- Toxic services are kept on the tubeside to increase their containment.
- Fluids where you have a low allowable pressure drop are also kept on the tubeside.
- Cooling water is generally kept on the tubeside, because it tends to corrode carbon steel and form scale.
Preferred stream on the shell side
Following factors need to be considered when putting a fluid on the shell side.
- Shell is more expensive to fabricate
- It is more difficult to mechanically clean the inner shell surface and the outer surface of the tube bundle.
- Shell side is commonly equipped with baffles. The shell side fluid needs to make way across the tube bundle. Correspondingly, there is more turbulence on the shell side and larger pressure drop.
With these factors in mind, following fluid services are better kept on the shell side.
- When more viscous fluid is kept on the shell side, the increased turbulence will result in increased heat transfer coefficient and improved overall heat transfer.
- Fluid with higher flow rate is generally preferred to be kept on the shell side.
- Service where a large temperature change is desired is also kept on the shell side, where temperature change can be achiever by increasing the length of the exchanger.