An Expert Guide to Shell & Tube Heat Exchanger Design
The shell & tube heat exchanger is a key component in many industrial processes. It used for the transfer of heat between two process fluids. Let’s explore the fundamentals of designing shell & tube heat exchangers, highlighting the key construction elements, and how to find the right manufacturer.
Shell & Tube Heat Exchangers: The Fundamentals
At its core, a shell & tube heat exchanger is comprised of numerous tubes housed within a larger cylindrical shell. This design facilitates the efficient exchange of heat as one fluid flows inside the tubes while another fluid flows outside the tubes within the shell. The design is versatile and can cater to a wide range of processes, temperatures, and pressures.
Construction Elements
The design of shell & tube heat exchangers includes several key components:
Channels
Channels in heat exchangers serve as entry and exit points for the tube-side fluids. They are designed to ensure uniform distribution of the fluid through the tube bundle to maximize heat transfer efficiency. Pass-partition plates are used to direct and separate flows between each tube-side pass.
Tubes
Tubes are the primary surface through which heat is exchanged. Materials like carbon steel, stainless steel, duplex, and high-nickel alloys (Inconel, Hasteloy) are often used, typically selected based on factors like resistance to corrosion, temperature suitability, and overall cost . Low-finned tubes and other enhancements are available to increase the heat transfer coefficient as needed in certain services. Tube internals like twisted tape and wire inserts are also available and best used when exchanger performance or cost can be improved upon.
Tube Sheet
The tubesheet is a critical component that holds (fixes) the tubes in place, sealing and supporting them at the ends. Tubes are often attached to the tubesheet(s) via mechanical means using two-ring-grooves machined into the tubesheet holes, and then hydraulically (or pneumatically) expanding the tube into the tubesheet and grooves (aka, 2RG & Expanded). Another common method is via a welded joint, typically considered a two-pass strength weld. Single-pass seal-welds are sometimes added to expanded joints to minimize the risk of one process stream leaking into the other.
Tube Pitch and Layout
Tube pitch and layout is crucial, as it affects the overall heat transfer and pressure drop. Tube pitch is the distance between adjacent tubes (often 1.25x tube OD), and tube layout is the arrangement of the tubes relative to shell-side baffle flow, either triangular (30° or 60°), square (90°), or rotated square (45°). Triangular layouts are cost effective and maximize performance relative to cost, but are more difficult to clean in shell-side services with heavy fouling. Square pitch allows for shell-side mechanical cleaning, but generally is the most costly layout. Rotated square layouts have some of the performance benefits of a triangular pitch, but with the accessibility of a square pitch. Tube pitch is often adjusted to balance heat transfer efficiency and pressure drop within the heat exchanger.
Shell
The shell is the pressure envelope of the heat exchanger. It contains the fluid that flows over (outside) the tubes. The shell’s configuration, including its diameter and length, is mostly a function of the bundle design.
Tie Rods and Spacers
Tie rods and spacers maintain the tube bundle’s alignment and support the baffle structure (aka, skeleton). They play a vital role in ensuring the structural integrity of the heat exchanger, but do not contribute to heat transfer.
Baffles
Baffles direct the shell-side fluid across the tube bundle, enhancing the heat transfer coefficient. They also help in supporting the tubes, preventing damaging vibration and wear. Single-segmental baffles are the most common, easily directing flow back and forth across the tubes. They can be cut and oriented so that shell-side flow is going side to side or up and down. When shell-side pressure drop (and velocities) are too high, double-segmental baffles can be used to reduce them both. Double-segmental baffles are almost always cut in the vertical direction in order to provide proper tube support. Helical baffles via a licensed technology like Lummus Helixchangers can be used to leverage the available pressure drop, achieve higher shell-side performance, higher average velocities, reduced fouling, and reduced costs.
Different TEMA Designations
The Tubular Exchanger Manufacturers Association (TEMA) designations categorize heat exchangers based on their construction and application. These designations, such as BEU, BEM, AES, and NEN, indicate specific configurations and design details. TEMA also classifies exchangers by letters C, B, and R. TEMA C is for general service and has the lowest requirements. TEMA B is for chemical service. TEMA R is for refinery service and has the most stringent requirements.
Codes and Standards for the Design of Shell & Tube Heat Exchangers
Several codes and standards guide the design of shell & tube heat exchangers. These include the ASME Boiler and Pressure Vessel Code, TEMA standards, and API (American Petroleum Institute) standards. Adhering to these ensures safety, reliability, and efficiency. Using HTRI Thermal Design Software Designing a shell and tube heat exchanger using HTRI involves a detailed process. Engineers start by defining key process requirements such as fluid types, flow rates, and temperatures. They then select appropriate configurations, including shell passes, tube layouts, and materials. Detailed thermal calculations are performed to determine heat transfer coefficients for both shell and tube sides, factoring in fluid dynamics and thermal properties. Hydraulic calculations follow to evaluate pressure drops, ensuring operational efficiency. Engineers iteratively adjust tube lengths, diameters, and layouts, balancing thermal efficiency with mechanical constraints. The final design undergoes rigorous validation against industry standards to ensure safety and reliability.
Contact Altex Industries for Shell & Tube Heat Exchanger Design and Manufacturing
For expert guidance and quality manufacturing of shell & tube heat exchangers, consider reaching out to Altex Industries. With our extensive experience and adherence to industry standards, they can provide tailored solutions that meet specific industrial requirements.
