The question always comes up of which fluid to put in the tubes and which fluid to put in the shell. Consider placing a fluid through the tubes when:
1. Special alloy materials are required for corrosion control and high temperatures.
2. Fluid is at high pressures.
3. Fluid contains vapors and non-condensable gases.
4. Fluid is scale forming.
Consider placing a fluid through the shell when:
1. Small pressure drops are desired.
2. Fluid is viscous.
3. Fluid is non-fouling.
4. Boiling service is desired.
5. Fluid has low film rate and is non-fouling (finned tubes can be used).
Placing the fluid through the tubes is a consideration when special alloy materials are needed for corrosion control, because the materials would be needed only on the tubes. If the corrosive material is in the shell, both the tubes and the shell would need to be protected with special alloy. If the fluid is at high pressure, it should be put in tubes because tubes can contain high pressure much more cheaply as they are much smaller in diameter than the shell. The low-pressure fluid would be in the shell. If the fluid contains vapor and non-condensable gases, heat transfer will be greater if it is placed in the tubes. If the fluid is scale forming it should be in the tubes, which can be reamed out.
Similarly, the fluid should be put in the shell when small pressure drops are desired. There is less of a pressure drop going through the shell than there is in going through the tubes. If the fluid is very viscous, pressure drop will be less and heat transfer improved if it is placed in the shell. It is harder to clean the shell than it is the tubes, so the non-fouling fluid should be put in the shell. If boiling service is desired, a kettle design should be used and the boiling fluid should be put in the shell, not inside the tubes. If the fluid has a low film rate and is non-fouling, it can be put in the shell, and finned tubes can be used to add to the U-value. In this case, the fluid would have to be a non-fouling fluid; otherwise, it
would plug the fins.