When designing a shell-and-tube exchanger, one of the first issues is deciding which fluid should go on the shell side and which on the tube side. So, let's look at some rules-of-thumb for several key factors, realizing such rough guidelines won't cover all cases.
High pressure. Put a high-pressure fluid on the tube side. This usually minimizes exchanger cost. The smaller tube diameter has a higher pressure rating for the same metal thickness compared to the larger diameter shell.
Fouling. A fluid with a tendency to foul generally should go on the tube side. Cleaning straight tubes normally is easier than cleaning the shell — even if a relatively large tube pitch or a square tube pattern is used to make the shell side easier to clean. However, the exchanger configuration significantly influences the choice. Using a fixed tubesheet mandates putting a clean fluid on the shell side; unless expected fouling is easily removed by chemical cleaning, the fixed tubesheet makes the shell side impossible to clean. In contrast, U-tubes are more difficult than straight tubes to clean. So, sending a normally fouling service through the shell side may be better if fouling reduction steps, such as installation of helical baffles, are suitable.
Expensive materials. Put a corrosive fluid on the tube side. That way, only the tubes, tubesheets, heads and channels will need expensive corrosion-resistant alloys. In contrast, a corrosive fluid on the shell side requires the entire exchanger to use the materials.
Low pressure drop. The fluid should go on the shell side. An appropriate combination of baffle cut, spacing and type (segmental, double segmental, rod-baffle, etc.) can accommodate nearly any pressure-drop requirement. Services under vacuum almost always are on the shell side because of pressure drop sensitivity.
Vapor services. Because a vapor normally has a higher volume and lower heat-transfer coefficient than a liquid, allocate it to the shell side. This reduces pressure drop for a given volume and typically provides a higher heat-transfer coefficient.
Condensing services. A condensing fluid most often goes on the shell side — but the choice demands careful evaluation. If the shell-side velocity is low enough, the vapor and liquid can separate inside the exchanger. The liquid dropping out makes the vapor leaner, reducing the temperature required to get more liquid to condense from the remaining vapor. For relatively pure mixtures, this effect is unimportant.
For wide-condensing-range mixtures, ensure the overall flow pattern inside the exchanger keeps the liquid and vapor mixed. This may necessitate having the shell-side fluid leave from the bottom (forcing the liquid and vapor to mix) or determine the choice of baffling inside the exchanger (horizontal versus vertical or 45° baffle cut).
Viscous services. Here, the tradeoffs are complex. A viscous fluid on the tube side tends to have high pressure drop and low heat transfer. That favors shell-side allocation. However, high pressure drop on the shell side can prompt significant flow bypassing around baffles, reducing heat transfer. A shell side with a high pressure drop also may suffer from vibration damage; shell-side modifications (assuming the user is aware of the need for them) can reduce such damage.
Solidifying services. Generally avoid a shell-and-tube exchanger for any service with a high risk of solidification or freezing. However, if you must use such an exchanger, I suggest putting the fluid with a risk of solidification in the tubes. If the fluid solidifies, you usually can pull out the tube bundle and replace it. In contrast, if the solid is on the shell side, it's often impossible to remove the tube bundle. The entire exchanger may require replacement.
However, allocating to the shell side a fluid that may freeze may offer advantages. In some cases, externally heating the shell with electric tracing may enable melting the fluid enough to get the exchanger back into service.
In either allocation, a freezing fluid can create local plugs. If its velocity drops, the fluid may approach the temperature of the other side of the exchanger, which may cause the fluid to set solid. This can occur on the shell side next to baffles on the shell edge. It also can happen in tubes. Tubes may differ in flow rates. A tube with a low flow rate may reach a lower temperature, making it more likely to freeze or set up. Often, exchangers with freezing fluids may have large areas where the exchanger has set solid.
ANDREW SLOLEY is a Chemical Procssing Contributing Editor. You can e-mail him at ASloley@putman.net