Plate-fin heat exchangers (PFHEs) are key components in many cryogenic processing plants. Having a very compact design and small footprint, they can cope with complex flow arrangements. A single PFHE can handle up to 20 process streams and very small temperature approaches. With such a versatile range of capabilities, PFHEs have become well established in various cryogenic processing plants for almost 40 years. Because their reliable performance is essential to plant operation, specialists at Linde Engineering have developed methods for assessing the life expectancy of PFHEs, as well as how to extend this through appropriate operation and maintenance.
Plate-fin heat exchangers (PFHEs) are key components in many cryogenic processing plants.
Every PFHE is unique
Linde Engineering uses PFHEs in many types of process plants. These include air separation, petrochemical and gas treatment plants, as well as natural gas and helium liquefaction plants. Among other components, Linde’s PFHEs consist of flat and corrugated plates that are brazed in a Linde-designed vacuum furnace in a process patented by Linde. “We build plate-fin heat exchangers with a length of up to 8.2 metres,” says Pascal Freko, specialist in Static Equipment Process Design & Technology at Linde. “In recent decades, the sizes of our plants and equipment have increased – as the example of natural gas liquefaction shows. This is accompanied by clear increases in mass flows, as well as the amount of heat transferred.” A process stream in a single PFHE block can carry up to 20 MW of heat duty – that corresponds to the output of a small natural-gas power plant.
In total, Linde Engineering has produced more than 12,000 PFHEs, which are in operation around the world. They are designed to optimally meet customer’s individual process requirements.“ We develop custom-engineered solutions for every plant,” the Linde expert says.
Thermal stress reduces lifetime
In everyday operation, plate-fin heat exchangers are subject to frequent load changes. During those changes or when plant systems are turned on and off or during load changes, mass flows, pressure and temperature distribution change. As a result, this can lead to unfavourable temperature gradients that thermally stress the material, and can create microscopic damage, or leaks in the heat exchanger unit. Linde’s engineers have evaluated PFHEs under realistic operating conditions. Their main conclusion: Fatigue resulting from thermal stress conditions can significantly shorten the product lifetime and lead to equipment failures.
“When we reduce thermal stress we can significantly extend the lifetime of heat exchangers,” says Dr. Reinhold Hölzl, senior expert for structural mechanics at Linde. However, no one-size-fits-all solution for this exists, as every plant and each heat exchanger is unique. Here, an interdisciplinary team of Linde experts has an answer. By using the HAZAN (Hazard Analysis) workflow they identify critical scenarios and operating conditions. With high-end simulation tools they examine thermal loads, which enables them to estimate the expected service life and to optimise it through suitable intervention.
PFHE are fabricated at Linde’s own plants in Schalchen, Germany and Dalian, China.
Better understanding plant dynamics
“For decades, we have investigated thermal stress and its effects on plant equipment,” says Reinhold Hölzl. Based on this experience, Linde’s engineers have developed simulation tools that provide insights into temperature distributions and the resulting thermal stress. Using these results operators can extract quantitative information on various processes such as plant start-up, as well as situations like critical upset scenarios or transitions between various modes of operation.
Such simulations are also used during the planning phase of a new plant, and help the engineers to optimise their concept. Thanks to detailed insight by simulation tools, plant design can proactively become more precise and sustainable. Additionally, the use of dynamic models has been a big step forward in understanding the individual challenges of real-life operation.
Lifetime-prolonging measures for heat exchangers
The expertise gained through simulation and analysis helps Linde’s engineers to prolong the lifetime of heat exchangers that are already in operation. The basis for their work is analysing assessed PFHE lifetime. Specialists in structural mechanics, process engineering and other departments define typical scenarios that specific heat exchangers face during operation. After performing various simulations for critical operation conditions, the specialists compile an evaluation of the stresses. Based on this, they suggest ways to improve operating conditions in order to prolong the lifetime of the heat exchanger.
All plant operators can benefit from this know-how. While detailed lifetime-estimation analysis is only possible for Linde’s heat exchangers, the company also offers “lifetime screening” for PFHEs from other manufacturers. “Our service ensures that plants remain reliable and trouble-free. In this way, we help customers to save costs,” summarises Pascal Freko.