material degradation. Chlorides may cause localised corrosion such as pitting or stress corrosion cracking, while particulate matter in the flue gas can contribute to erosion and fouling. In practice, these effects often reinforce each other: fouling creates local concentration differences, erosion removes protective layers, and corrosion accelerates mechanical weakening. For this reason, conventional heat recovery systems are usually designed to remain above the acid dew point. This protects equipment lifetime and plant availability, but it also imposes a strict lower limit on flue gas cooling. As a consequence, a substantial share of low-temperature heat remains unrecovered. Overcoming this boundary requires heat exchanger materials and system designs that can tolerate prolonged contact with acidic condensates without sacrificing performance, reliability, or maintainability. Materials challenge: Why conventional heat exchangers fail Conventional heat exchangers have enabled reliable heat recovery in industrial plants for decades. However, most established materials were not designed for continuous operation in acidic condensates below the dew point. In this operating window, the challenge is not only heat transfer performance but also long-term resistance to corrosion, humidity, particulate loading, and thermal cycling. Carbon steel is widely used because of its low cost, mechanical strength, and good thermal conductivity. Yet once exposed to acidic condensates, it can degrade rapidly. Sulphuric and hydrochloric acids formed during condensation may cause uniform corrosion, wall thinning, and ultimately mechanical failure. Even limited operation below the acid dew point can significantly reduce equipment lifetime. Stainless steels offer improved corrosion resistance in many industrial environments, but their performance can be limited in flue gases containing chlorides and sulphur compounds. Localised corrosion mechanisms such as pitting or stress corrosion cracking may occur, particularly in the presence of condensates and deposits. These failure modes are difficult to predict and can lead to sudden, critical damage.
Other material concepts address parts of the problem but introduce different limitations. Glass- or enamel-lined heat exchangers provide high chemical resistance, but their brittleness makes them vulnerable to thermal shock, mechanical stress, and particle impact. Polymer-coated metals, such as perfluoroalkoxy (PFA)-lined systems, depend on the long-term integrity of the coating. Defects, permeation, or mechanical damage can expose the underlying metal and initiate corrosion. In practice, these limitations often lead to higher maintenance effort, unplanned shutdowns, and conservative operating strategies. Operators may accept reduced heat recovery to protect equipment reliability. This underlines the need for heat exchanger materials specifically designed for the combined effects of acid condensation, temperature variation, and mechanical stress in flue gas environments. Enabling technology: Corrosion-resistant heat transfer materials Overcoming the limitations of conventional heat recovery below the acid dew point requires a shift in material selection. Instead of protecting metallic surfaces from corrosion, the objective is to use inherently corrosion-resistant materials that can operate in direct contact with acidic condensates over long periods. One approach is the use of polymer-based heat exchanger materials engineered for industrial heat transfer applications. Technoform has developed solutions based on PPS-GR, a graphite-filled polyphenylene sulphide material designed for corrosive heat recovery environments. The polyphenylene sulphide matrix provides high chemical resistance against acids commonly found in flue gas condensates, including sulphuric and hydrochloric acid. The graphite filler increases thermal conductivity, making the material suitable for heat transfer while maintaining its resistance to corrosive media. This combination of properties enables stable operation in environments where conventional metals are exposed to critical degradation mechanisms. Unlike metallic materials, PPS- GR does not rely on passive oxide layers for protection and is therefore not susceptible to
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