Construction Basics of Shell and Tube Heat Exchangers
Shell and tube heat exchangers represent the most widely used vehicle for heat transfer in process applications. They frequently are selected for duties such as:
- Process liquid or gas cooling.
- Process or refrigerant vapor or steam condensing.
- Process liquid, steam or refrigerant evaporation.
- Process heat removal and preheating of feedwater.
- Thermal energy conservation efforts and heat recovery.
Although there exists a wide range of designs and materials, some components are common to all. In all shell and tube heat exchangers, the tubes are mechanically attached to tube sheets, which are contained inside a shell with ports for inlet and outlet fluid or gas. They are designed to prevent the liquid flowing inside the tubes from mixing with the fluid outside the tubes. Tube sheets can be fixed to the shell or allowed to expand and contract with thermal stresses. In the latter design, an expansion bellows is used or one tube sheet is allowed to float inside the shell. The nonfixed tube sheet approach allows the entire tube bundle assembly to be pulled from the shell to allow cleaning of the shell circuit.
Fluid Stream AllocationsThere are a number of practical guidelines that, if followed, can lead to the optimum design of a given heat exchanger. Remembering that the heat exchanger's primary responsibility is to perform its thermal duty with the lowest cost yet provide in-service reliability, the selection of fluid stream allocations should be of primary concern to the designer. When designing, keep the following points in mind:
- The higher pressure fluid normally flows through the tube side. With their small diameter and nominal wall thicknesses, the tubes are better able to accept high pressures, and this approach avoids having to design more expensive, larger diameter components for high pressure. If it is necessary to put the higher pressure fluid stream in the shell, it should be placed in a small diameter, long shell.
- All other items being equal, place corrosive fluids in the tubes. It is much less expensive to use the special alloys designed to resist corrosion for the tubes than for the shell. Other tube-side components can be clad with corrosion-resistant materials or epoxy coated.
- Flow higher fouling fluids through the tubes.Tubes are easier to clean using common mechanical methods than the shell.
- Many possible designs and configurations - affecting tube pitch, baffle use and spacing, and multiple nozzles, to name a few - can be used when laying out the shell circuit. Because of this, it is best to place fluids requiring low pressure drops in the shell circuit.
- The fluid with the lower heat transfer coeffi cient normally goes in the shell circuit. With this setup, low-fin tubing, which will increase available surface area, can be used to offset the low heat transfer rate.
TubesHeat exchangers with shell diameters of 10 to more than 100" typically are manufactured to the standards set forth by the Tubular Exchangers Manufacturers Association. Generally, the 0.625 to 1.5" tubing used in TEMA-sized exchangers is made from low carbon steel, copper, Admiralty, copper-nickel, stainless steel, Hastalloy, Inconel, titanium or other materials.
Tubes are either drawn and seamless, or welded. High quality electroresistance welded tubes exhibit good grain structure at the weld. Extruded tube with low fins and interior rifling is specified for certain applications. Surface enhancements are used to increase the available metal surface or aid in fluid turbulence, thereby increasing the effective heat transfer rate. Finned tubing is recommended when the shell-side fluid has a substantially lower heat transfer coefficient than the tube-side fluid. Finned tubing is not finned in its landing areas, where it contacts the tube sheets. Also, the outside diameter of the finned portions of this tube design is slightly smaller than the unfinned areas. These features allow the tubes to be slid easily through the baffles and tube supports during assembly while still minimizing fluid bypass.
U-tube designs are specified when the thermal difference between the fluids and flows would result in excessive thermal expansion of the tubes. U-tube bundles do not have as much tube surface as straight tube bundles due to the bending radius, and the curved ends cannot be easily cleaned. Additionally, interior tubes are difficult to replace and often requiring the removal of outer layers or simply plugging the tubes. To ease manufacturing and service, it is common to use a removable tube bundle design when specifying U-tubes.
Tube SheetsTube sheets usually are made from a round, flat piece of metal. Holes are drilled for the tube ends in a precise location and pattern relative to one another. Tube sheets are manufactured from the same range of materials as tubes. Tubes are attached to the tube sheet by pneumatic or hydraulic pressure, or by roller expansion. If needed, tube holes can be drilled and reamed, or they can be machined with one or more grooves. This greatly increases tube joint strength (figure 1).
The tube sheet is in contact with both fluids, so it must have corrosion resistance allowances and metallurgical and electrochemical properties appropriate for the fluids and velocities. Low carbon steel tube sheets can include a layer of a higher alloy metal bonded to the surface to provide more effective corrosion resistance without the expense of using the solid alloy.
The tube hole pattern, or "pitch," varies the distance from one tube to the other as well as the angle of the tubes relative to each other and to the direction of flow. This allows the fluid velocities and pressure drop to be manipulated to provide the maximum amount of turbulence and tube surface contact for effective heat transfer.
Where the tube and tube sheet materials are joinable weldable metals, the tube joint can be further strengthened by applying a seal weld or strength weld to the joint. In a strength weld, a tube is slightly recessed inside the tube hole or slightly extended beyond the tube sheet. The weld adds metal to the resulting lip. A seal weld is specified to help prevent the shell and tube liquids from intermixing. In this treatment, the tube is flush with the tube sheet surface. The weld does not add metal but rather fuses the two materials. In cases where it is critical to avoid fluid intermixing, a double tube sheet can be provided. In this design, the outer tube sheet is outside the shell circuit, virtually eliminating the chance of fluid intermixing. The inner tube sheet is vented to atmosphere, so any fluid leak is detected easily.
Shell AssemblyThe shell is constructed either from pipe up to 24" or rolled and welded plate metal. For reasons of economy, low carbon steel is in common use, but other materials suitable for extreme temperature or corrosion resistance often are specified. Using commonly available shell pipe to 24" dia. results in reduced cost and ease of manufacturing, partly because they generally are more perfectly round than rolled and welded shells. Roundness and consistent shell inner diameter are necessary to minimize the space between the baffle outside edge and the shell, as excessive space allows fluid bypass and reduces performance. Roundness can be increased by expanding the shell around a mandrel or double rolling after welding the longitudinal seam. In extreme cases, the shell can be cast and then bored to the correct inner diameter.
In applications where the fluid velocity for the nozzle diameter is high, an impingement plate is specified to distribute the fluid evenly to the tubes and prevent fluid-induced erosion, cavitation and vibration. An impingement plate can be installed inside the shell, eliminating the need to install a full tube bundle, which would provide less available surface. Alternatively, the impingement plate can be installed in a domed area (either be reducing coupling or a fabricated dome) above the shell. This style allows a full tube count and therefore maximizes utilization of shell space (figure 2).
End Channels and BonnetsUsed to control the flow of the tube-side fluid in the tube circuit, end channels or bonnets typically are fabricated or cast. They are attached to the tube sheets by bolting with a gasket between the two metal surfaces. In some cases, effective sealing can be obtained by installing an O-ring in a machined groove in the tube sheet.
The head may have pass ribs that dictate whether the tube fluid makes one or more passes through the tube bundle sections (figure 3). Front and rear head pass ribs and gaskets are matched to provide effective fluid velocities by forcing the flow through various numbers of tubes at a time. Generally, passes are designed to provide roughly equal tube-number access and to ensure even fluid velocity and pressure drop throughout the bundle. Even fluid velocities also affect the film coefficients and heat transfer rate so that accurate prediction of performance can be readily made.
Designs for up to six tube passes are common. Pass ribs for cast heads are integrally cast, then machined flat while pass ribs for fabricated heads are welded into place. The tube sheets and tube layout in multipass heat exchangers must have provision for the pass ribs. This requires either removing tubes to allow a low cost straight pass rib, or machining the pass rib with curves around the tubes, which is more costly to manufacture. Where a full bundle tube count is required to satisfy the thermal requirements, the machined pass rib approach may prevent having to consider the next larger shell diameter.
Cast head materials typically are used in smaller diameters to around 14" and are made from iron, ductile iron, steel, bronze or stainless steel. Typically, they have pipe-thread connections. Cast heads and tube side piping must be removed to service tubes. Fabricated heads can be made in a range of configurations, including metal cover designs that allow servicing the tubes without disturbing the shell or tube piping. Heads can have axially or tangentially oriented nozzles, which typically are ANSI flanges.
BafflesBaffles serve two important functions. First, they support the tubes during assembly and operation and help prevent vibration from flow-induced eddies. Second, they direct the shell-side fluid back and forth across the tube bundle to provide effective velocity and heat transfer rates.
A baffle must have a slightly smaller inside diameter than the shell's inside diameter to allow assembly, but it must be close enough to avoid the substantial performance penalty caused by fluid bypass around the baffles. Shell roundness is important to achieve effective sealing against excessive bypass. Baffles can be punched or machined from any common heat exchanger material compatible with the shell side fluid. Some punched baffle designs have a lip around the tube hole to provide more surface against the tube and eliminate tube wall cutting from the baffle edge. The tube holes must be precise enough to allow easy assembly and field tube replacement yet minimize the chance of fluid flowing between the tube wall and baffle hole.
Baffles do not extend edge to edge but have a cut that allows shell-side fluid to flow to the next baffled chamber (figure 4). For most liquid applications, the cuts areas represent 20 to 25% of the shell diameter. For gases, where a lower pressure drop is desirable, baffle cuts of 40 to 45% are common. Baffles must overlap at least one tube row in order to provide adequate tube support. They are spaced somewhat evenly throughout the tube bundle to provide even fluid velocity and pressure drop in each baffled tube section.Single-segmental baffles force the fluid or gas across the entire tube count, where it changes direction as dictated by the baffle cut and spacing. This can result in excessive pressure loss in high velocity gases. To effect heat transfer yet reduce the pressure drop, double-segmental baffles can be used. This approach retains the structural effectiveness of the tube bundle yet allows the gas to flow between alternating tube sections in a straighter overall direction, thereby reducing the effect of numerous direction changes. This approach takes full advantage of the available tube surface. But, reduced performance should be expected due to a reduced heat transfer rate. Because pressure drop varies with velocity, cutting the velocity in half using double-segmental baffles results in roughly one-quarter of the pressure drop seen in a single-segmental baffle space over the same tube surface.