How to Design a Shell and Tube Heat Exchanger
The shell and tube heat exchanger is one of the most ubiquitous types of heat exchangers in the world. Used in many industries from chemical processing to energy systems, its versatility and efficiency are unmatched. But, how do you design one?
At its core, a shell and tube heat exchanger consists of a series of tubes through which one fluid flows, housed within a shell where another fluid flows. The design’s primary objective is to allow efficient heat transfer between the two fluids without them mixing.
The Tubes of the Exchanger
Tubes are the core of a shell & tube heat exchanger. They must be selected with consideration, taking into account factors such as thermal conductivity, corrosion resistance, and mechanical strength. The tube material, diameter, length, wall thickness (gauge), pitch, and layout (triangular, square, etc.) all influence the exchanger’s performance. Materials like carbon steel, duplex, and stainless steels are selected for their thermal conductivity, resistance to corrosion, and economics.
Tubesheets are the thick plates to which the tubes are fixed. They must bear the stress loads of the tubes (axial stresses due to thermal expansion), resist the pressures of the processes, and maintain isolation between process fluids. Thus, tubesheets need to be robust, of suitable metallurgy and appropriately designed. Their configuration also plays a role in the ease of maintenance and the overall longevity of the exchanger, for example, fixed bundles or removable bundles.
Tubesheet joints are a key consideration when designing a shell & tube heat exchanger. Typical configurations are 2-ring-groove and roller expanded, seal-welded, and strength-welded.
2-ring-groove (aka 2RG) is the most economical and is suitable for most services. A groove is machined into the tube-hole in the tubesheet, and then the tube is mechanically expanded into the groove to both physically secure the tube and also provide the seal.
Strength-welded tube-to-tubesheet joints consist of two weld passes, and are typically selected when tubes experience higher axial stresses due to fixed bundles (NEN, AEL, BEM), when design/operating pressures are high, or in critical services like hydrogen or sour service. The tube is lightly expanded into the tubesheet (no ring-groove), and then TIG-welded. Liquid penetrant inspection (LPI) is performed on the first weld pass to ensure complete fusion before a second TIG-weld pass is performed.
Seal-welding via a TIG process is often added to 2-ring-groove to provide additional protection against leaking tubesheet joints. This is often specified when it’s critical that the process streams cannot leak into one another.
Tie Rods, Spacers, and Baffles
Tie-rods and spacers are used to secure baffles and keep the tube bundle together, giving it stability within the shell.
Baffles serve a dual purpose: they support tubes, preventing vibrations, and they also direct the shell-side fluid across the tube bundle, increasing turbulence and, consequently, the heat transfer rate. The baffle cut, spacing, and cut-orientation (horizontal or vertical) are all design elements that greatly affect heat exchanger performance.
Tube-Side Channel and Nozzle Design
The tube-side channel and nozzle are where the tube-side fluid enters and exits the heat exchanger. Proper design is important for uniform distribution and smooth flow. Key considerations include the fluid’s properties, the required flow rate, pressure drop, and potential issues such as erosion or corrosion. The nozzle size and orientation can also impact the tube-side fluid’s velocity and distribution. In heat exchangers with multiple tubing passes, pass-plates are installed in channels to direct flow into the specific passes.
Shells and Shell-Side Nozzle Design
The shell is the exchanger’s outer container, and its design influences the pressure drop and fluid distribution. It must be robustand accommodate the tube bundle and baffles. The shell-side nozzles facilitate the inlet and outlet of the shell-side fluid. Like the tube-side nozzle, their size, location, and orientation can significantly affect the exchanger’s overall performance, and the exchanger designer must consider the rho-v^2 (reference TEMA) for nozzle sizing and if impingement protection (impingement plates or rods) is needed. .
Support saddles are usually welded to the shell and must support the complete exchanger, as well as the forces and stresses caused by piping loads, wind-loads, and transportation loads.
Contact Altex Industries for Shell and Tube Heat Exchanger Design and Manufacturing Services
Now that you understand the complexity of designing a shell and tube heat exchanger, the next step is manufacturing. For top-tier design and manufacturing services, contact Altex Industries. With years of experience and a reputation for excellence, we can provide the expertise needed to make sure your heat exchanger is efficient, reliable, and durable.