Shell and tube heat exchanger

  Heat exchanger designs are regularly incorporated in the flow sheets of process equipment in a wide range of industrial plants, refineries, and factories and allow the transfer or exchange of heat in industrial process equipment. What are Shell and Tube Heat Exchangers? Let’s take a look into the Design and Analysis of Shell and Heat exchangers. Shell and tube heat exchangers are one of the most widely used heat exchangers in the processing industries (they account for 65% of the market, according to H. S. Lee’s book, Thermal Design, and are suitable for a wide range of operating temperatures and pressures. STHE’s (Shell and Tube heat exchangers) comprise a cylindrical shell (a pressure vessel) enclosed with a bundle of hollow tubes fitted inside. These tubes are composed of thermally conductive elements, which enable the exchange of heat between the hot fluids flowing outside the tubes and the coolant flowing through the tubes. Baffles are implemented in the shell to direct the fluid flow and support the tubes. Support rods and spacers hold the baffle and tube assembly together. Shell and tube heat exchangers are regarded as one of the most efficient kinds of heat exchangers and are widely used across industries like refining, power, chemical, marine, Oil & Gas, Food and many more. Primary Components of Shell and Tube Heat Exchangers 1. Tubes Shell and tube heat exchangers aren’t complete without tubes. Tubes help in providing a heat transfer surface between a fluid flowing inside the tube and a fluid flowing around the tube’s outside. Tubes are usually made of copper or steel alloys and can be seamless or welded. For specific applications, other nickel, titanium, or aluminum alloys can be needed. Extrusion produces seamless tubing; welding produces welded tubing by rolling a strip into a cylinder and welding the seam. Welded tubing is usually more economical.  The most common tube diameters are 5/8”, 34”, and 1”. Smaller tubes may be used, but cleaning them mechanically is more difficult. Larger diameter tubes are often used to allow mechanical cleaning or to reduce pressure drop. The average tube wall thickness is 12 to 16 BWG (from 0.109 inches to 0.065 inches thick). Thinner-walled tubes (18 to 20 BWG) are used. 2. Tubesheets The tubes are held in place by inserting them into holes in the tubesheets and then expanding them into grooves cut into the holes or welding them to the tubesheet where the tube extrudes from the surface. This prevents the mixing of the fluid on the shell side and the fluid on the side of the tube. The pipe sheet usually consists of a round single metal board that was properly drilled and ground to take the pipes, joints, spacer rods, and the circle of the bolt where the pipe has been attached to the shell. The tube pitch is the distance between the center of the tube holes; the tube pitch is typically 1.25 times the outside diameter of the tubes. Other tube pitches are commonly used to control the velocity of the shell side fluid as it flows across the tube bundle and reduces shell side pressure drop. Triangular pitch is most commonly used due to the increased heat transfer and compactness it provides. The square pitch makes mechanical cleaning of the tubes’ exteriors easier. Except for U-tube bundles, two tubesheets are required. A rolled joint is a tube-to-tube sheet joint formed by mechanical expansion of the tube against the tubesheet. This joint is most commonly made with roller expanders, hence the name “rolled joint.” Hydraulic processes are used less frequently to expand tubes to affect a mechanical bond. Tubes can also be welded to the tubesheets front or inboard face. Strength welding denotes that the mechanical strength of the joint is primarily provided by the welding procedure, and the tubes are only lightly expanded against the tubesheet to eliminate the crevice that would otherwise exist. Seal welding denotes that the mechanical strength of the joint is primarily provided by tube expansion, with the tubes welded to the tubesheet for improved leak protection. Increased reliability, lower maintenance costs, and fewer process leaks are commonly used to justify the cost of seal-welded joints. When using clad tubesheets, tubes with wall thicknesses less than 16 BWG (0.065 inches), and some metals that cannot be adequately expanded to achieve an acceptable mechanical bond, seal welded joints are required (titanium and Alloy 2205 for instance). A double tubesheet can be used to prevent mixing between the two fluids. Because the space between the tubesheets is exposed to the atmosphere, any leakage of either fluid should be detected as soon as possible. Aside from mechanical requirements, the tubesheet must be corrosive resistant to both fluids in the heat exchanger and electrochemically compatible with the tube and all tube-side material. 3. Tube-Side Channel and Nozzles The tube-side channel and nozzles simply control the flow of tube-side fluid into and out of the exchanger’s tubes. Because the tube side fluid is typically more corrosive, these channels and nozzles are frequently made of alloy materials (compatible with the tubes and tubesheet of course). Instead of solid alloy, they could be clad. Channel covers are provided at the channel ends. They are round plates that bolt to the channel flanges and can be removed for tube inspection without causing any disruption to the piping on the tube side. Bonnets with flanged nozzles or threaded connections for the tube-side piping are frequently used instead of channels and channel covers in smaller heat exchangers. 4. Shell and Shell-Side Nozzles The shell is simply a container for the fluid on the shell’s side, and the nozzles are the inlet and exit ports. The shell has a circular cross-section and is typically made by rolling a metal plate of appropriate dimensions into a cylinder and welding the longitudinal joint together. Small diameter shells can be made by cutting the desired diameter pipe to the appropriate length. The roundness of the shell is critical in determining the maximum diameter of