Shell and tube heat exchangers are the most common type of heat exchanger present in industrial applications due to their extreme versatility in fluid styles and process conditions, high flexibility in customization and specification towards certain applications, and the ability to handle high pressures and temperatures. While other styles of heat exchangers have had their designs iterated on in recent decades, shell and tube heat exchangers have been used in virtually every industry since their inception without major changes in design principle. This article will explore structural and design fundamentals of shell and tube heat exchangers, as well as how designs vary by industry, specific industrial applications, and maintenance practices across sectors like power generation, chemical production, food & beverage production, pharmaceuticals, and oil & gas processing.
Materials and Construction
In principle, all shell and tube heat exchangers consist of a cylindrical shell containing the ‘shell-side fluid’ and a bundle of tubes through which the ‘tube-side fluid’ flows; this allows for the indirect exchange of heat between two fluids. Internal baffles redirect shell-side flow, enhancing turbulence to improve heat transfer, while tube sheets secure the tubes in place. Common flow arrangements include counter-current, where fluids move in opposite directions for maximum efficiency, and co-current, used in specific design scenarios. Units can be single-pass or multi-pass on either of the tube or shell sides depending on thermal duty and design considerations. Tube bundles can be fixed to the shell or ‘floating’ to allow for thermal expansion and ease of maintenance; a floating tube bundle can be removed from the shell for simpler maintenance and cleaning of both the tubes and the shell.
Material selection for industrial shell and tube heat exchangers takes the following into consideration: the corrosiveness of the fluids, necessary thermal conductivity to perform the duty, mechanical strength (temperature and pressure limits), cost, availability, and how easy it is to maintain. The following is a list of common construction materials, their use cases, and examples of industries where they are applied:
1. Carbon Steel
- Relatively strong mechanically
- Affordable and widely available
- Only suitable for non-corrosive fluids and moderate temperatures
- Often used as a shell material for simple applications using non-corrosive fluids such as steam or light hydrocarbons in the power generation, chemical and petrochemical industries
- Applications include steam condensing in thermal power plants and heat recovery in lube-oil systems
2. Stainless Steel
- Widely available in many grades such as 304, 316, 316L, 2205, and 2207 to suit various design needs
- Moderate cost for standard designs and higher costs for specialized fabrication
- Corrosion resistant, relatively strong mechanically, and higher temperature resistance
- Widely used as a tube and shell material and virtually every industrial application outside of those with high risk of corrosion
- Applications include food-grade units for food processing and pre-heaters or coolers in pharmaceutical manufacturing
3. Titanium
- Lightweight and suitable for aggressive chemicals and fluids such as seawater
- Expensive and potentially difficult to fabricate or procure
- Suitable for the most corrosive industrial processes
- Commonly used as a tube material in marine industries or those using seawater for cooling
- Applications include heat exchange in desalination plants and sea-water cooled condensers for offshore oil platforms
4. Nickel Alloys
- Specialized material offered in several grades such as Inconel 600 and Hastelloy C276
- Expensive and potentially difficult to fabricate or procure
- Extremely corrosion and temperature-resistant with high general durability
- Often used as a tube material in the petrochemical industry
- Applications include acid gas condensers in sulfur recovery processes and reboilers in high-pressure hydrogenation processes
Applications and Performance Considerations
As mentioned above, shell and tube heat exchangers are ubiquitous in industry due to their abilities to handle virtually all process fluids at a wide range of temperatures, pressures, and flow rates. The following are just some of the applications that shell and tube heat exchangers are used for in industrial processes:
- Power Plants: Condensation of steam to reuse water in boiler cycle.
- Petrochemical & Oil Refineries: Preheating of naphtha feedstock for steam cracking in benzene production.
- Chemical Processing: Condensation of acetic acid vapour post distillation.
- Pulp & Paper: Heating of white liquor entering digesters.
- HVAC: Chilled water cooling coils for commercial air conditioning units.
- Pharmaceutical: Recovery of ethyl acetate after downstream crystallization of ampicillin
- Food & Beverage: Pasteurization of beverages such as beer, juice, and milk.
Shell and tube heat exchangers are able to handle higher heat loads by increasing the surface area for heat exchange, which in this case is the surface of the tubes. By increasing the number and length of tubes, higher heat loads can be achieved in a single shell and tube unit. Other design considerations will also alter the performance of a shell and tube heat exchanger, such as: number, shape, and positioning of baffles to promote turbulent fluid flow, inclusion of fins on outside of tubes, and the physical arrangement of tubes (ex. triangular vs. square pitch). Having tighter clearances will promote better performance at the cost of increasing fouling risk and increased pressure drops; both of these factors lead to higher operating costs through maintenance and power costs respectively. Clean applications using non-fouling fluids such as cooling water in chillers can take advantage of tight clearances, while dirty applications involving slurries, wastewater, etc. such as in a pulp and paper plant must ensure the unit is properly designed to balance downtime to clean fouling and efficiency during operation.
Maintenance, Fouling, and Lifecycle Costs
High pressure drops may result in reduced flow rates that can comprise system performance or demand more expensive operational and capital costs associated with pumps. Shell and tube heat exchangers can be customized to cope with specialized applications; for example, in high-pressure and/or high-temperature applications, thicker tube sheets, reinforced shells, and expansion joints are used. In condensers or evaporators, two-phase flow requires careful control of velocity and orientation (e.g., vertical mounting to aid phase separation) and wider tube diameters and lower flow velocities prevent fouling when handling viscous or high-fouling potential fluids. Depending on the application, cleaning can be performed either mechanically using brushes, etc. or chemically; regular cleaning must be performed to remove fouling/scaling as this lowers heat transfer efficiency relative to a clean unit. Designs with removable tube bundles further simplify maintenance by allowing easier access for cleaning and repairs. Total lifetime costs for industrial heat exchangers include initial purchase and installation, routine maintenance and cleaning including cleaning chemical products, energy consumption via fluid pumping, and productivity loss during downtime.
Shell and tube heat exchangers remain the standard in virtually all industrial heat exchange processes —they are a proven solution in a wide variety of applications with sufficient thermal efficiency and exceptional durability to suit a large range of process conditions. Industrial shell and tube heat exchanger designs are adapted to everything from power generation and chemical refining to food processing and marine cooling and with a wide range of materials, configurations, and maintenance options, there is almost always a suitable shell and tube heat exchanger for a given industrial process.