Lord Fin Tube-Choosing between a straight tube and a U-tube design heat exchanger
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 isnt 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.
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 designs 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 streams 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 exchangers 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 doesnt 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.
Straight tubes run the risk of damage due to thermal expansion. When the tubes heat at different temperatures and rates, they dont 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.
Shell & Tube Heat Exchangers
The shell is constructed either from pipe or rolled plate metal. For economic reasons, steel is the most commonly used material, and when applications involving extreme temperatures and corrosion resistance, others metals or alloys are specified. Using off-the-shelf pope reduces manufacturing costs and lead time to deliver to the end customer. A consistent inner shell diameter or ‘roundness’ is need to minimize the baffle spacing on the outside edge, excessive space reduces performance as the fluid tends to channel and bypasses the core. Roundness is increased typically by using a mandrel and expanding the shell around it, or by double rolling the shell after welding the longitudinal seam. In some cases, although extreme, the shell is cast and then bored out until the correct inner diameter is achieved.
When fluid velocity at the nozzle is high, an ‘impingement’ plate is specified to distribute fluid evenly in the tubes, thereby preventing fluid-induced erosion, vibration and cavitation. Impingement plates effectively eliminate the need to configure a full tube bundle, which would otherwise provide less available surface. An impingement plate can also be installed above the shell thereby allowing a full tube count and therefore maximizing shell space.
Heat exchangers with shell diameters of 10 inches to more than 100 are typically manufactured to industry standards. Commonly, 0.625 to 1.5" tubing used in exchangers is made from low carbon steel, Admiralty, copper, copper-nickel, stainless steel, Hastelloy, Inconel, or titanium.
Tubes can be drawn and thus seamless, or welded. High quality electro resistance welded tubes display good grain structure at the weld joints. Extruded tubes with fins and interior rifling are sometimes specified for certain heat transfer applications. Often, surface enhancements are added to increase the available surface or aid in fluid turbulence, thereby increasing the operative heat transfer rate. Finned tubes are recommended when the shell-side fluid have a considerably lower heat transfer coefficient than the tube-side fluid. Note, the diameter of the finned tube is slightly smaller than the un-finned areas thus allowing the tubes to be installed easily through the baffles and tube supports during assembly while minimizing fluid bypass.
A U-tube design finds itself in applications when the thermal difference between the fluid flows would otherwise result in excessive thermal expansion of the tubes. Typical U-tube bundles contain less tube surface area as traditional straight tube bundles due to the bended end radius, on the curved ends and thus cannot be cleaned easily. Furthermore, the interior tubes on a U-tube design are difficult to replace and often requiring the removal of additional tubes on the outer layer; typical solutions to this are to simply plug the failed tubes.