A shell and tube heat exchanger (STHE) transfers heat between two fluids of differing temperatures without allowing them to mix, as they are separated by a solid physical barrier. This widely-used industrial equipment facilitates heating or cooling processes across numerous industries. It achieves highly efficient thermal exchange due to its robust design and the large surface area it provides for heat flow.
Core Components and Structure
A shell and tube heat exchanger consists of a central bundle of tubes housed within a larger, sealed cylindrical container known as the shell. The tubes are typically made of materials like copper, stainless steel, or titanium, chosen for their thermal conductivity and corrosion resistance. One fluid flows inside these tubes, creating the “tube side” flow path.
Thick, perforated metal plates called tube sheets secure and seal the ends of the tube bundle. These sheets are fastened to the shell, separating the tube-side fluid from the fluid flowing through the shell. The second fluid, which flows around the outside of the tubes within the cylindrical shell, is referred to as the “shell side” fluid.
Within the shell, metal plates called baffles are strategically placed perpendicular to the tubes. These baffles serve a dual function: they provide structural support to the long tube bundle, preventing vibration-induced damage, and they direct the shell-side fluid flow. By forcing the fluid to flow back and forth across the tubes instead of just lengthwise, the baffles increase fluid velocity and turbulence, which significantly improves the rate of heat exchange.
How the Heat Exchange Process Works
Heat transfer within the exchanger is driven by thermal energy moving from a region of higher temperature to one of lower temperature. The two fluids, entering the device at different temperatures, flow through their respective, separated paths. One fluid flows inside the tubes, while the other is forced to flow in a zigzag pattern around the exterior within the shell.
The process of heat transfer involves a combination of convection and conduction. Heat transfers from the hotter fluid to the inner tube wall via convection. The thermal energy then conducts through the solid metal wall of the tube. Finally, the heat transfers from the outer tube wall to the cooler fluid on the shell side, again through convection.
To maximize the heat transfer efficiency, the flow is often arranged in a counterflow configuration, where the two fluids enter the exchanger from opposite ends and flow in opposite directions. The presence of baffles is crucial for disrupting the smooth, laminar flow of the shell-side fluid, creating turbulence. This turbulence continuously mixes the fluid layers, preventing the formation of an insulating thermal boundary layer and thus increasing the overall heat transfer coefficient.
Common Configurations and Design Types
Shell and tube heat exchangers are categorized into several design types, largely based on the configuration of the tube bundle and how it is secured, which impacts maintenance and thermal stress management. The Fixed Tube Sheet design is the simplest and most economical, featuring tube sheets welded directly to the shell. This robust design is ideal for applications where the shell-side fluid is clean, though it makes mechanical cleaning of the shell side difficult.
The U-Tube design incorporates tubes bent into a ‘U’ shape, with both ends terminating at the same tube sheet. This arrangement allows the entire tube bundle to expand and contract freely, accommodating the large temperature differences that cause thermal expansion without requiring additional expansion joints in the shell. However, the curved tubes are challenging to clean internally compared to straight tubes.
The Floating Head design features one tube sheet fixed to the shell and another, the “floating head,” free to move within the shell. This movable head manages thermal expansion and allows the entire tube bundle to be removed. The removable bundle is a significant advantage in applications involving fouling fluids, as it permits easy mechanical cleaning of both the tube and shell sides.
Widespread Industrial Uses
Shell and tube heat exchangers handle a wide range of operating temperatures and pressures across industrial processes. They are a primary component in the oil and gas industry, used for preheating crude oil before refining and for condensing various gases. Their robust construction is well-suited for the demanding, high-pressure environments of refineries and petrochemical plants. In power generation, these exchangers are used extensively as surface condensers, efficiently cooling and converting spent steam back into water for reuse in the steam cycle. Chemical processing plants rely on them for precise temperature control during reactions, distillation, and evaporation of various fluids.
These fluids often include corrosive substances. Furthermore, they are incorporated into large-scale HVAC systems as chillers. They are also utilized in the food and beverage industry for processes like pasteurization and rapid cooling.