There are many factors to consider when designing a shell and tube heat exchanger. These include cost, application and the limitations of the facility in which it will operate. One decision every manufacturer will have to make is whether it will be a straight tube or a U-tube exchanger. The tube design is critical. If a manufacturer chooses a tube design that isn't right for the application, it could result in exchanger damage or fouling that is difficult to clean. Both are commonly used in many industries, including food and beverage, chemical and pharmaceuticals, and each offers its own advantages and disadvantages.
The Tubular Exchanger Manufacturers Association classifies exchangers into different types. According to Process Heating, the two most common are the BEU model, a U-tube design and the BEM model, a straight tube design. They are identical except for the tube design and the rear bonnet. The B-type front bonnet allows access to the tube sheet for cleaning after it has been removed from the piping, Chemical Processing said. Plus, it is relatively inexpensive. The E-type shell is a single-pass design and represents more than half of all designs manufactured, Chemical Engineering Progress explained.
Advantages of a straight-tube design
One of the biggest benefits to the straight tube design is the simplicity, Chemical Engineering Progress said.
"Many people like straight tube exchangers because of their versatility."
Process Heating also said many people like straight tube exchangers because of their versatility. The BEM model can be used by virtually any industry, for any application. While some companies may opt for different models for higher efficiency, the BEM design will usually work in another exchanger design's place.
Straight tube exchangers allow for pure countercurrent flow within the exchanger, usually without requiring a second one to be connected in a series to the first. In these cases, an F-type two-pass shell with a longitudinal baffle is preferred over the E-type. The baffle separates the two streams, Thermopedia explained.
Countercurrent flow occurs when the cold stream and the hot stream move in two different directions. The hot stream should be warmer than the cold stream at all points throughout the exchanger, though the cold stream's exit temperature is permitted to be higher than that of the hot stream.
On the other hand, cocurrent flow describes the movement of the hot and cold streams moving in the same direction. In this configuration, the cold stream must always be lower than the hot stream. This means the outlet temperature of the cold stream needs to be, to some degree, cooler than the other. This is difficult to do when the streams are moving in the same direction, so many manufacturers choose to avoid designs that would be cocurrent.
Cleaning is another major consideration for manufacturers choosing an exchanger's design. Straight tubes are easiest to clean, as there are no bends to work around. However, some straight tube designs make inspection and cleaning of the shell more difficult because in some designs, it is impossible to remove the tubes from the shell.
Advantages of a U-tube design
While there are many benefits to the straight tube design, it can fall short in some areas. This is why the U-tube design is so popular. While a simple straight tube design is simpler because the tubing doesn't have to be bent, as with U-tubes, it can become quite pricey when other essential additions are taken into account. For instance, U-tubes only require one tubesheet and bonnet, cutting down substantially on cost.
"U-tubes allow for thermal expansion without putting the rest of the exchanger at risk."
Straight tubes run the risk of damage due to thermal expansion. When the tubes heat at different temperatures and rates, they don't always expand in accordance with one another. This can harm the tubesheet and shell in a straight tube exchanger, as the tubes are connected to these other essential components. An expansion joint can ease this issue, but these additions are not cheap. On the other hand, a U-tube exchanger is only connected to the tubesheet and shell on one end, allowing for thermal expansion without harm to the rest of the machine.
U-tube designs also allow for the tube bundles to be easily removed from the exchanger. This aids in easy inspection and cleaning of the shell and outside of the tube bundle.
If you have questions about the most appropriate type of exchanger for your application, contact the helpful experts at Enerquip for advice on the best option to meet your needs.