Shell-and-tube heat exchanger
A shell-and-tube heat exchanger is a type of heat exchanger designed to transfer thermal energy between two fluids without mixing them, consisting of a bundle of tubes enclosed within a cylindrical shell where one fluid flows through the tubes and the other circulates around the exterior of the tubes in the shell space.[1][2] The heat transfer occurs primarily through convection from the fluids to the tube walls and conduction across the tube material, with the overall process governed by an overall heat transfer coefficient that accounts for these mechanisms.[2] Baffles are typically installed within the shell to direct the shell-side fluid flow, enhancing turbulence and improving heat transfer efficiency while also supporting the tubes.[1] These heat exchangers are the most common type used in industrial applications due to their robustness, flexibility in handling a wide range of pressures, temperatures, and fluid properties, and ease of maintenance such as cleaning and tube replacement.[1] They can accommodate phase changes like condensation or boiling on either the tube or shell side and support configurations with multiple tube passes for counterflow or parallel flow arrangements to optimize performance.[1][2] Materials commonly include carbon steel for the shell and tubes, with options like stainless steel or alloys for corrosive environments.[1] Shell-and-tube heat exchangers find extensive use in chemical processing, petroleum refining, power generation, refrigeration, air conditioning, and petrochemical industries, where they handle duties from small-scale cooling to large-scale steam generation.[1][3] Design standards such as those from the Tubular Exchanger Manufacturers Association (TEMA) and the American Society of Mechanical Engineers (ASME) ensure mechanical integrity and performance, with classifications like fixed tube sheet, U-tube, or floating head based on thermal expansion needs.[1][3] Originally developed in the early 1900s for high-pressure power plant applications, their designs have evolved to meet demands in the oil industry and beyond, incorporating methods like the log mean temperature difference (LMTD) or effectiveness-NTU for performance prediction.[3][2]Fundamental Principles
Heat Transfer Mechanisms
In shell-and-tube heat exchangers, heat transfer occurs through a combination of convection on both the tube side and shell side, along with conduction across the tube walls separating the two fluids. On the tube side, where one fluid flows through the interior of the tubes, convective heat transfer dominates as the fluid exchanges thermal energy with the inner tube surface, driven by the fluid's velocity and properties such as viscosity and thermal conductivity. Similarly, on the shell side, the second fluid flows across the exterior of the tube bundle, promoting convection through turbulent or cross-flow patterns that enhance mixing and heat dissipation from the outer tube surfaces. Conduction, meanwhile, facilitates heat flow radially through the thin metallic tube walls, governed by Fourier's law, where the heat flux is proportional to the temperature gradient and the material's thermal conductivity. These mechanisms collectively enable efficient thermal exchange while minimizing direct fluid mixing, with convection typically representing the rate-limiting step due to fluid-film resistances.[2][1][4] The overall heat transfer coefficient, denoted as U, quantifies the combined resistance to heat flow from these mechanisms and is derived from the basic principles of steady-state conduction and convection. Starting from the heat flux equality across the interfaces—where the convective flux on the tube side q = h_i (T_{fluid,i} - T_{wall,i}), the conductive flux through the wall q = \frac{k}{x} (T_{wall,i} - T_{wall,o}), and the convective flux on the shell side q = h_o (T_{wall,o} - T_{fluid,o})—the overall flux is expressed as q = U (T_{fluid,i} - T_{fluid,o}). Solving for U yields the reciprocal form: \frac{1}{U} = \frac{1}{h_i} + \frac{x}{k} + \frac{1}{h_o} + R_{f,i} + R_{f,o}, where h_i and h_o are the tube-side and shell-side convective coefficients, respectively; x is the wall thickness; k is the wall thermal conductivity; and R_{f,i} and R_{f,o} are fouling resistances accounting for scale buildup that impedes heat transfer over time. This derivation assumes negligible radial temperature variation in thin-walled tubes and constant properties, with U typically referenced to a specific surface area (e.g., outer tube area). Fouling factors are empirically determined and added as additional thermal resistances to reflect real-world degradation in performance.[1][4][2] To calculate the total heat transfer rate Q in a shell-and-tube exchanger, the log mean temperature difference (LMTD) accounts for the varying temperature gradient along the exchanger length, assuming constant U and fluid specific heats. Derived from an energy balance on differential elements—where dQ = U dA \Delta T and integrating under steady-state conditions—the LMTD for counterflow (common in single-pass tube designs) or parallel flow is: \Delta T_{lm} = \frac{\Delta T_1 - \Delta T_2}{\ln(\Delta T_1 / \Delta T_2)}, with \Delta T_1 and \Delta T_2 as the temperature differences at the inlet and outlet ends, respectively. Thus, Q = U A \Delta T_{lm}, where A is the effective heat transfer area. In counterflow configurations, LMTD is larger than in parallel flow for the same terminal temperatures, yielding higher efficiency; however, shell-and-tube designs often use multipass arrangements approximating counterflow to maximize \Delta T_{lm}. A correction factor F is applied for non-ideal multipass flows, but the core LMTD concept remains foundational for thermal sizing.[2][5] For complex shell-and-tube configurations with multiple passes or unmixed fluids, the effectiveness-NTU (ε-NTU) method provides an alternative to LMTD, particularly when outlet temperatures are unknown. Here, effectiveness \epsilon is the ratio of actual heat transfer to the maximum possible (\epsilon = Q / (C_{min} (T_{h,in} - T_{c,in})), where C_{min} is the smaller fluid heat capacity rate), and the number of transfer units NTU is NTU = U A / C_{min}. The relationship is \epsilon = f(NTU, C_r), with capacity ratio C_r = C_{min}/C_{max}; for counterflow, \epsilon = \frac{1 - \exp[-NTU(1 - C_r)]}{1 - C_r \exp[-NTU(1 - C_r)]}, while cross-flow approximations (relevant to shell-side patterns) use series expansions or charts. This dimensionless approach simplifies design by decoupling temperature specifics, enabling rapid evaluation of performance across varied geometries without iterative LMTD corrections.[5][6]Operating Principles
In a shell-and-tube heat exchanger, heat transfer occurs between two fluids separated by tube walls, with one fluid flowing through the tubes (tube-side) and the other through the surrounding shell (shell-side). The operating principles revolve around the controlled flow paths that maximize contact time and temperature gradients while minimizing pressure losses. Single-pass configurations involve the tube-side fluid entering at one end and exiting at the other in a straight path, promoting straightforward unidirectional flow. In contrast, multi-pass designs divide the tubes into multiple segments using headers, allowing the fluid to reverse direction several times, which increases velocity and enhances heat transfer efficiency for applications with limited temperature differences. On the shell side, flow is typically perpendicular to the tubes, creating a cross-flow pattern that is directed by baffles to prevent short-circuiting and ensure even distribution, directed by baffles that prevent short-circuiting, ensure even distribution across the tube bundle, promote turbulence to enhance heat transfer, and provide structural support to the tubes. The thermodynamic operation follows the principle of energy conservation, where the heat lost by the hot fluid equals the heat gained by the cold fluid, expressed as Q = \dot{m}_h c_{p,h} (T_{h,in} - T_{h,out}) = \dot{m}_c c_{p,c} (T_{c,out} - T_{c,in}), with Q as the heat transfer rate, \dot{m} as mass flow rate, c_p as specific heat capacity, and subscripts denoting hot (h) or cold (c) fluid and inlet/outlet temperatures. This balance drives the system's performance, assuming negligible heat losses to the environment and constant fluid properties. Flow configurations influence this cycle: counterflow arrangements, where hot and cold streams move in opposite directions, maintain a more consistent temperature difference along the exchanger length, approaching the maximum theoretical efficiency. Parallel flow, conversely, results in converging temperature profiles that diminish the driving force for heat transfer toward the outlet, reducing overall effectiveness. Temperature profiles along the exchanger length are critical to operational efficiency, as they determine the local heat flux via the overall heat transfer coefficient U, which encapsulates convective and conductive resistances. In multi-pass setups, repeated direction changes can flatten these profiles, optimizing energy recovery in processes like steam condensation or refrigerant evaporation. The impact of flow direction is evident in the logarithmic mean temperature difference (LMTD), which quantifies the average driving force and is higher in counterflow than in parallel or cross-flow, allowing for more compact designs without sacrificing performance. Shell-and-tube exchangers excel in handling high-pressure differentials (up to 100 bar) and large fluid volumes (thousands of m³/h), attributes that make them preferable over plate or finned-tube alternatives for industrial-scale operations in petrochemical and power generation sectors.Components and Configuration
Shell Components
The shell in a shell-and-tube heat exchanger serves as the primary outer enclosure, containing the tube bundle and the shell-side fluid while withstanding internal pressures and facilitating overall structural integrity.[7] It is typically cylindrical in shape, with diameters ranging from 152 mm to 3000 mm, constructed from standard pipe sizes up to 610 mm or rolled plates for larger dimensions to accommodate varying process requirements.[7] The length of the shell aligns with the tube length, often extended to minimize costs for a given heat transfer area by allowing longer tubes.[7] Key variations in shell design address thermal expansion differences between the shell and tubes, including fixed tube sheet types where the tubesheet is welded directly to the shell for simple, economical construction without bundle removal capability; floating head designs that permit axial movement of the tube bundle relative to the shell for easier maintenance and expansion accommodation; and U-tube configurations where tubes are bent into a U-shape within the shell to maximize flexibility under temperature changes.[8] These types, classified under TEMA standards such as E (cylindrical for general use), F (for countercurrent flow), and others like K for reboilers, ensure compatibility with diverse applications while containing the shell-side fluid.[7] Inlet and outlet nozzles on the shell, positioned to optimize flow entry and exit, influence shell-side velocity, with design limits such as ρv² ≤ 5950 kg/m·s² to prevent excessive pressure drops.[9] Construction features include bolted flanges at the shell ends for connecting to channel or rear heads, which may be dished for pressure resistance or flat for simpler assemblies, enhancing the shell's ability to seal the shell-side fluid path.[7] Supports such as saddles or brackets are integrated for horizontal or vertical orientations, providing stability against gravitational and operational loads.[7] These elements collectively enable the shell to direct shell-side flow around the tubes, promoting effective containment and pressure management.[7] Historically, shell-and-tube designs emerged in the early 20th century for high-pressure power plant applications, evolving from basic cylindrical enclosures inspired by steam engine condensers to accommodate brass and steel materials for corrosion and pressure handling.[10] By the early 20th century, standardized pressure vessel codes like ASME and TEMA refined shell construction for safety and efficiency in industrial applications, incorporating rigorous tolerances for flanges, heads, and nozzles.[8] Modern shells prioritize material compatibility with aggressive fluids to extend service life, adhering to these evolved standards.[11]Tube Components
In shell-and-tube heat exchangers, the tubes serve as the primary conduits for one fluid, facilitating direct heat transfer across their walls to the shell-side fluid. Tube geometries are standardized to balance heat transfer efficiency, mechanical strength, and manufacturability, with outer diameters typically ranging from 12.7 mm (0.5 in) to 50.8 mm (2 in), though the most common sizes are 19.05 mm (0.75 in) and 25.4 mm (1 in). Wall thickness is specified using Birmingham Wire Gauge (BWG) standards, often 16 to 18 BWG for general applications, providing adequate pressure resistance while minimizing material use. Tubes may be straight, allowing for multi-pass configurations with headers, or U-bent, where each tube forms a semicircular loop to connect inlet and outlet in a single pass; U-bent designs accommodate thermal expansion without additional joints but limit cleaning to the straight sections. For applications requiring enhanced heat transfer, particularly with low shell-side coefficients, finned tubes incorporate low-profile integral fins on the exterior, increasing surface area by up to 2.5 times compared to plain tubes while maintaining compatibility with standard bundle assemblies. Tube arrangement patterns significantly influence the bundle's compactness and flow dynamics. The triangular pitch, with tubes aligned at 30° angles, maximizes tube density for a given shell diameter, enabling higher tube counts and greater heat transfer area but reducing the cross-flow area on the shell side. In contrast, the square pitch at 90° provides aligned rows for easier mechanical cleaning of the exterior but accommodates fewer tubes, resulting in a larger bundle diameter for equivalent capacity. The rotated square pitch at 45° offers a compromise, achieving about 85% of the triangular pattern's tube density while improving shell-side flow distribution and access for maintenance. These patterns are defined by the tube pitch, the center-to-center distance between adjacent tubes, typically 1.25 to 1.33 times the tube outer diameter, ensuring a minimum clearance of 6.35 mm (0.25 in) for square layouts to facilitate cleaning. Tube sheets anchor the tubes at each end of the bundle, providing a sealed barrier between the tube-side and shell-side fluids while supporting structural integrity. Fixed tube sheets are rigidly welded or bolted to the shell at both ends, offering a compact design suitable for low differential thermal expansion but requiring careful stress analysis to prevent distortion. Floating tube sheets secure the tubes at one end while allowing the opposite sheet to move axially within the shell, accommodating greater temperature differences through sliding or bellows mechanisms without compromising the seal. Clamped or U-tube configurations, where tubes are bent into U-shapes and rolled or welded into a single stationary sheet, eliminate the need for a rear header and permit unlimited tube expansion, though they restrict tube-side passes to even numbers and complicate bundle removal. Standard tube counts for bundles are determined by the shell inside diameter, tube outer diameter, pitch, and arrangement pattern, often guided by empirical layouts to optimize space utilization. For preliminary design, tube counts can be estimated using geometric formulas that account for the bundle's circular cross-section and pass partitions. The following table provides representative tube counts for common 1-pass configurations with 25.4 mm (1 in) outer diameter tubes on a 1.25-inch triangular pitch, excluding space for nozzles:| Shell Inside Diameter (mm) | Approximate Tube Count (Triangular Pitch) | Approximate Tube Count (Square Pitch) |
|---|---|---|
| 381 (15 in) | 88 | 74 |
| 610 (24 in) | 244 | 196 |
| 914 (36 in) | 586 | 484 |
| 1219 (48 in) | 1064 | 859 |