Key Factors in Shell-and-Tube Heat Exchanger Design

In industrial production, heat exchangers play an indispensable role. They function as efficient "heat transfer mediators," ensuring smooth energy exchange between different fluids and maintaining the stable operation of production processes. Among them, the shell-and-tube heat exchanger, with its simple structure and strong adaptability, is widely applied. However, designing a shell-and-tube heat exchanger that is both efficient and economical is no easy task; it requires a comprehensive consideration of numerous factors. This article will delve into the critical elements that must be addressed in shell-and-tube heat exchanger design, helping readers fully understand this complex and essential process.

The Importance of Heat Exchanger Selection

There is a wide variety of heat exchange equipment, from the commonly used shell-and-tube heat exchanger to plate heat exchangers, and even more specialized designs such as spiral plate heat exchangers. Each has its unique structure and working principle. For every specific heat transfer condition, the optimal selection of equipment can identify the most suitable type. This “most suitable” option is not chosen arbitrarily but is based on multiple considerations, including heat transfer efficiency, equipment cost, and ease of operation and maintenance.

For example, when dealing with high-temperature and high-pressure steam conditions, the shell-and-tube heat exchanger is often the first choice due to its excellent pressure-bearing capacity and stable heat transfer performance. By contrast, in scenarios where rapid, high-efficiency heat transfer is required but installation space is limited, a plate heat exchanger may be more appropriate.

However, applying an exchanger optimized for one condition to another can lead to significant differences in performance. It is much like equipping a race car with tires designed only for straight-line speed—once the vehicle enters a curve, performance inevitably declines. Therefore, selecting the proper type of exchanger for a specific condition is both crucial and complex. Designers must thoroughly understand the various operating conditions of heat transfer and be familiar with the performance characteristics of different exchangers in order to make the best choice.

Flow Velocity: A Key Variable in Heat Exchanger Design

Flow velocity holds a decisive role in shell-and-tube heat exchanger design. It is like the "circulatory speed of blood" in the heat transfer process, directly influencing heat transfer efficiency, pressure drop, and economic performance.

1. Relationship Between Flow Velocity and Heat Transfer Coefficient

Increasing flow velocity can significantly improve the heat transfer coefficient. This coefficient, a key indicator of exchanger performance, reflects how much heat can be transferred per unit time per unit area. As flow velocity rises, the relative movement between the fluid and tube wall accelerates, allowing heat to move more rapidly from hot fluid to cold fluid, thereby increasing efficiency. For instance, in a shell-and-tube exchanger, increasing the velocity from 1 m/s to 2 m/s may improve the heat transfer coefficient by 30%–50%.

2. Impact of Flow Velocity on Pressure Drop and Power Consumption

However, higher flow velocity comes at a cost. As velocity increases, pressure drop and power consumption also rise. Pressure drop refers to the loss in pressure due to friction and other factors as the fluid flows through the exchanger. A higher velocity increases friction against tube walls, leading to greater pressure drop. To maintain fluid flow, more energy is required to overcome this drop, resulting in higher power consumption.

When pumping fluids, it is preferable to allow pressure drop to occur mainly across the exchanger rather than across a control valve. This is because pressure drop in the exchanger can be harnessed to enhance heat transfer, while pressure drop across a valve contributes nothing to efficiency. By rationally allocating pressure drop, designers can improve heat transfer while minimizing power use.

3. Economic Comparison and Determination of Suitable Velocity

Higher velocities increase the overall heat transfer coefficient, reduce required heat transfer area, and help minimize fouling on tube surfaces. At the same time, however, resistance and energy costs increase. This makes an economic comparison essential—balancing equipment cost and operational cost to determine the best velocity.

For example, in a small exchanger, raising flow velocity may significantly reduce surface area and thus lower equipment cost. Even with higher power consumption, the economics may still favor higher velocity. In contrast, for a large exchanger with already high equipment costs, reducing operating costs through controlled velocity may be more important.

4. Structural Limitations on Flow Velocity

Structural considerations must also be factored in. To avoid excessive wear, calculated velocities should not exceed the maximum permissible values based on experience. Excessive velocity can subject tube walls to severe erosion and impact, reducing service life.

While higher pressure drop can help increase velocity and thus reduce surface area, it also raises pumping costs. The optimal balance is achieved by adjusting exchanger dimensions until the annual total cost reaches a minimum.

Rational Fluid Distribution: Tube Side vs. Shell Side

In a shell-and-tube heat exchanger, fluids can be arranged to flow either in the tube side or the shell side. Fluid properties largely determine the appropriate path, which directly affects performance and economics.

1. Fluids Suitable for the Tube Side

Fluids best suited for the tube side include water, steam, highly corrosive fluids, toxic fluids, fouling-prone fluids, and those at high temperature or high pressure.

For instance, water and steam have favorable flow and heat transfer characteristics; in the tube side, they maintain good contact with tube walls, enabling efficient exchange. Corrosive fluids are safer in tubes since suitable materials can be selected for corrosion resistance. Toxic fluids in the tube side reduce leakage risk. Fouling-prone fluids also benefit from higher velocities in smaller tube diameters, which reduce fouling. High-temperature and high-pressure fluids utilize the tube side’s higher pressure capacity to ensure safety.

2. Fluids Suitable for the Shell Side

Shell side applications are more appropriate for fluids such as column overhead condensates, hydrocarbon condensation and reboiling, pressure-drop-sensitive fluids, and viscous fluids.

For example, in condensation and reboiling, vapor distribution benefits from the shell side’s spacious geometry. Fluids requiring pressure drop control are better managed in the shell, lowering pumping costs. Highly viscous fluids also flow more effectively in the shell due to its larger cross-sectional area, which promotes turbulence and enhances heat transfer while easing pressure drop management.

3. Consideration of Heat Transfer Coefficient and Pressure Drop

Excluding the above special cases, path selection should prioritize improved heat transfer coefficients and effective use of pressure drop. Since turbulence (Re ≥ 100) is more easily achieved in the shell side, viscous or low-flow fluids (low Reynolds number) are often better placed there. Conversely, if turbulence can be reached in tubes, assigning the fluid to the tube side is more rational.

From a pressure drop perspective, low Reynolds number fluids typically favor the shell side, where larger space mitigates the impact of their high viscosity or low flow rate.

Determination of Terminal Temperatures

Terminal temperatures are generally dictated by process needs. In some cases, they can be adjusted, and their values greatly influence economic feasibility.

If the hot and cold outlet temperatures are equal, thermal efficiency is maximized. However, the effective temperature difference is minimized, requiring a larger surface area and thus higher equipment cost. Practical design therefore requires balancing process requirements with cost considerations.

Moreover, designers must avoid temperature crossover—where the hot fluid outlet temperature is lower than the cold fluid outlet. This phenomenon leads to inefficient and sometimes infeasible heat transfer, potentially disrupting production.

Choice of Heat Exchanger Structure

For given process conditions, the structural form of the exchanger must be determined, such as fixed tube sheet, floating head, or U-tube. Each has distinct features and applicable scenarios.

1. Fixed Tube Sheet Heat Exchanger

This type features a simple structure and low cost, making it suitable where the shell side fluid is clean and non-fouling. The tube sheets are welded directly to the shell, forming a sealed unit.

The advantage lies in ease of manufacturing, installation, and maintenance. However, when temperature differences are large, thermal stress may occur between the tube bundle and shell, leading to damage. Thus, fixed tube sheet exchangers are best for small temperature differences.

2. Floating Head Heat Exchanger

This design eliminates the thermal stress issue. One end of the tube bundle is connected to a floating head, which can move freely inside the shell, relieving stress.

Floating head exchangers are suitable for larger temperature differences or fouling shell side fluids. However, they are more complex, costlier, and require higher sealing reliability, otherwise leakage may occur.

3. U-Tube Heat Exchanger

Here, tubes are bent into U-shapes, both ends fixed to a tube sheet. The U-shape allows thermal expansion and contraction, eliminating thermal stress.

They are ideal for high-temperature, high-pressure, or large temperature-difference conditions. Tube interiors are easier to clean, but drawbacks include low tube sheet utilization, weaker bundle strength, and susceptibility to shell-side flow impact at higher velocities.

Application of Heat Transfer Enhancement Technologies

Enhancing heat transfer is a key goal in exchanger design. By using advanced technologies, size can be reduced for a given duty, performance improved, fluid temperature differences minimized, or pumping power lowered.

1. Extended Surface Technology

This approach increases effective heat transfer area. For example, threading or finning tubes expands area by 1.6–2.7 times compared to plain tubes. While pressure drop slightly rises due to smaller diameter, heat resistance on the shell side is reduced, improving efficiency.

2. Flow Regime Modification Technology

By altering fluid flow states, heat transfer can be enhanced. Corrugated tubes, for example, are manufactured to produce internal ribs, disrupting stagnant layers and reducing resistance. Inserts inside tubes also change flow direction and velocity distribution, boosting efficiency.

3. Optimization of Shell-Side Supports

Improved support structures also enhance heat transfer. Examples include rod-baffle exchangers, double-segmental baffles, ring-type, and swirl-flow exchangers. These designs reduce resistance, increase velocity, or redirect flow to improve performance. Rod-baffle exchangers, for instance, provide strong anti-vibration stability while enhancing heat transfer and durability.

Conclusion

Designing a shell-and-tube heat exchanger is a complex and meticulous process requiring consideration of multiple factors. From selecting the right exchanger type to determining flow velocity, fluid allocation, terminal temperatures, structural form, and enhanced heat transfer methods, every step is critical. Only through scientific, rational analysis and design can exchangers meet process requirements while achieving efficient, economical, and stable operation. It is hoped that this comprehensive analysis of shell-and-tube exchanger design will serve as a valuable reference for designers, guiding them in producing higher-quality exchanger systems for practical applications.