An Overview of High-Pressure Heat Exchangers

High-pressure heat exchangers are vital equipment in the chemical, petroleum, and energy industries. They are mainly used at the reactor outlet to transfer heat from the reactor to the hydrogen feedstock. These exchangers must be designed to withstand high temperatures and pressures to ensure efficient heat exchange under harsh conditions. Currently, two main types of high-pressure heat exchangers are commonly used: flanged and threaded locking ring designs. Each has unique structural features and applications, making the choice of design crucial for optimizing heat exchange efficiency and ensuring safety.

Flanged Heat Exchangers

Flanged heat exchangers typically use a flat gasket structure, connecting the flanges of the tube box and shell through a metal flat gasket to form a sealed heat exchange passage. This design is widely applied in many industrial settings.

1. Major Advantages

Simple Structure: The manufacturing process of flanged heat exchangers is relatively straightforward, making production and assembly easier and reducing overall costs.
Strong Adaptability: The flat gasket structure offers some adaptability when the thermal expansion coefficients of the flanges and gaskets differ, which is particularly beneficial in large diameter piping applications.

2. Disadvantages

Inadequate Sealing Performance: The flat gasket has limited compensation for axial rebound, making flanged heat exchangers prone to leakage under temperature differences. This issue is especially critical in high-temperature and high-pressure conditions, posing safety risks.
Complex Maintenance: In high-temperature and high-pressure environments, gasket materials can age and deform due to temperature fluctuations and pressure changes, leading to decreased sealing effectiveness. Regular inspection and replacement of gaskets are necessary to ensure normal operation.

To improve sealing performance, modern high-pressure heat exchangers increasingly use octagonal gaskets. While these gaskets provide better sealing, they may still cause leaks in large diameter applications due to differences in thermal expansion. The main reason is the mismatch between the thermal expansion coefficients of the gasket material and the flange, leading to stress concentration in high-temperature environments. Additionally, the sealing surfaces of octagonal gaskets need to be repaired during maintenance, increasing complexity and costs.

To address these issues, many manufacturers have adopted Ω-ring structures. The benefits of Ω-rings include:

Excellent Sealing Capability: Ω-rings effectively compensate for sealing failures caused by bolt stretching, significantly reducing leakage risks, especially under extreme temperature and pressure conditions.
Safety and Reliability: This design ensures safe use in high-temperature and high-pressure environments, making it suitable for applications with stringent sealing requirements.

However, Ω-rings have the drawback of complex disassembly and maintenance; they need to be welded again after use and cannot be reused. This requires operators to have a high level of skill and experience during maintenance.

Threaded Locking Ring Heat Exchangers

Threaded locking ring heat exchangers feature a complete shell design, where the tube side and shell side share a single body. This provides more reliable sealing performance and effectively prevents internal and external leaks.

1. Structural Features

Integrated Design: One end of the shell is welded with a head, while the other end has a threaded pressure ring and cover, ensuring the strength and sealing performance of the entire system. This design simplifies the overall structure of the heat exchanger and enhances its pressure-bearing capacity.
Compression Mechanism: The tube bundle is placed inside the shell, with gaskets between the tube sheet and the internal steps of the shell, creating an effective separation between the tube and shell sides. The gaskets can be tightly compressed using retaining rings and pressure bolts, ensuring stable operation of the heat exchanger in high-pressure environments.
In practice, if a leak is detected at the tube sheet side, it can be quickly resolved by tightening the inner bolts, ensuring the efficient operation of the equipment. Additionally, the threaded pressure ring and cover work together to bear internal pressure loads and the compressive force of the gaskets, maintaining the overall structure's stability and extending the equipment's lifespan.

2. Usage Considerations

Timing for Tightening Inner Bolts: It is crucial to tighten the inner bolts at the right moment, usually when the temperature reaches its peak and an internal leak is detected. This prevents material fatigue or damage from premature tightening.
Monitoring for External Leaks: Checking observation ports before starting is vital to ensure no external leaks, which is a key step in ensuring the safe operation of the equipment.
Regular Maintenance: During maintenance, gaskets on both sides of the tube bundle should be replaced to prevent shell side short circuits, and tightening torque must be applied according to the specifications to prevent damage to large threads.

Conclusion

Selecting the appropriate structural form for high-pressure heat exchangers is crucial for their safety and efficiency when operating in high-temperature and high-pressure environments. Both flanged and threaded locking ring designs have their advantages and disadvantages, and the choice should be based on specific operating conditions. Regardless of the design, adhering to safety regulations during operation and maintenance is essential to prevent leaks and other potential risks. With proper design, careful operation, and regular maintenance, high-pressure heat exchangers can function reliably under demanding conditions, providing effective heat exchange solutions for industrial production.