Understanding fouling in heat exchangers – Part one

Heat exchangers (HE) play a crucial role in various industries, including power generation, chemical processing, and HVAC systems. However, over time, these essential devices can suffer from fouling, the accumulation of unwanted deposits on heat transfer surfaces, reducing the efficiency of heat exchangers and increasing energy consumption. In this article, we will delve into the types, causes, effects, cost, and mitigation strategies for fouling in heat exchangers.

By Omari Hussein Sabuni, Mechanical Engineer – Kinyerezi Power Plant

Types of fouling in heat exchangers and their impact on heat transfer efficiency

Understanding the different types of fouling and their effects on heat transfer is crucial for effective maintenance and optimization of these vital heat exchange devices. In this article, we will explore the various types of fouling commonly encountered in heat exchangers and delve into their detrimental effects on heat transfer efficiency.

Scaling: Scaling occurs when dissolved salts and minerals present in the fluid precipitate and form a hard crystalline layer on heat transfer surfaces. The scale deposits act as insulators, reducing heat transfer efficiency. As the scaling layer thickens, it restricts fluid flow and increases pressure drop, leading to reduced heat exchanger performance. Furthermore, scaling can promote corrosion under the deposits, exacerbating equipment damage.

Corrosion: Corrosion refers to the degradation of heat exchanger surfaces due to chemical reactions between the fluid and the material of construction. Corrosion can lead to the formation of fouling deposits, such as iron oxide or rust, which inhibit heat transfer. The presence of corrosion products on heat exchange surfaces not only reduces thermal efficiency but also increases the likelihood of equipment failure due to material degradation.

Particulate Fouling: Particulate fouling occurs when solid particles present in the fluid settle on the heat exchanger surfaces. These particles can originate from suspended solids in water, dust in air streams, or process contaminants. Particulate fouling reduces heat transfer efficiency by creating an additional thermal resistance layer. It also increases pressure drop, requiring higher pumping power to maintain fluid flow. Over time, thick layers of particulate fouling can severely impede heat exchanger performance and necessitate cleaning or maintenance interventions.

Biological Fouling: Biological fouling involves the growth of microorganisms, such as bacteria, algae, and fungi, on heat exchange surfaces. These microorganisms form biofilms, which act as insulating layers, reducing heat transfer efficiency. Biofilms also promote other types of fouling, such as scaling and particulate deposition. Moreover, the metabolic byproducts of microorganisms can contribute to corrosion, further impacting heat exchanger performance and durability.

Corrosion on heat exchanger head.

Effects of fouling on heat transfer efficiency

The presence of fouling deposits in heat exchangers has several adverse effects on heat transfer efficiency, including:

Reduced Heat Transfer Coefficient: Fouling layers act as thermal barriers, hindering the efficient exchange of heat between the fluid and the heat exchanger surfaces. The reduction in heat transfer coefficient leads to decreased overall heat transfer efficiency, resulting in increased energy consumption to achieve the desired temperature exchange.

Increased Thermal Resistance: Fouling deposits increase the thermal resistance at the heat exchanger surfaces, impairing the rate of heat transfer. This increased resistance reduces the effectiveness of heat exchangers and requires higher temperature differentials to achieve the desired heat transfer, thereby diminishing system efficiency.

Higher Energy Consumption: As fouling deposits accumulate, the heat exchanger’s thermal performance declines, necessitating increased energy input to maintain the desired heat transfer rate. This elevated energy consumption results in higher operating costs and reduces overall system efficiency.

Increased Maintenance and Downtime: Fouling-related issues often require frequent maintenance interventions, such as cleaning or repairs, to restore heat exchanger performance. These maintenance activities lead to downtime and increased operational costs, negatively impacting productivity and profitability.

The cost of fouling on heat exchangers

The costs associated with fouling are twofold: the energy inefficiencies resulting from reduced heat transfer and the expenses incurred for maintenance and cleaning. In this article, we will explore the economic implications of fouling on heat exchangers and highlight the importance of mitigating fouling to minimize costs.

Energy Inefficiencies: Fouling in heat exchangers significantly affects energy efficiency and increases operational costs. The following are the primary ways in which fouling impacts energy consumption:

  • Increased energy demand: Fouling deposits act as insulating layers, reducing the heat transfer coefficient. As a result, heat exchangers require higher energy inputs to achieve the desired heat exchange. The additional energy demand translates into increased operating costs and reduced energy efficiency.
  • Higher pumping power: Fouling leads to increased pressure drop across heat exchanger surfaces due to restricted fluid flow. To maintain the required flow rates, additional pumping power is necessary, resulting in elevated energy consumption and associated expenses.
  • Reduced overall system efficiency: With fouling present, heat exchangers experience decreased overall efficiency. This necessitates compensatory measures such as higher temperature differentials or extended operation times to achieve the desired heat transfer. These adjustments further escalate energy consumption and costs.

Maintenance and cleaning expenses: Fouling imposes substantial maintenance and cleaning requirements, leading to additional expenses. The following factors contribute to the cost of fouling-related maintenance:

  • Frequent cleaning intervals: Fouling deposits necessitate more frequent cleaning or maintenance interventions to restore heat exchanger performance. This often involves downtime, labor costs, and the use of cleaning agents or equipment. The more severe the fouling, the more frequent and intensive the cleaning operations, resulting in increased maintenance expenses.
  • Maintenance labor and resources: Cleaning fouling deposits from heat exchangers requires skilled labor and appropriate tools or chemicals. The costs associated with acquiring or outsourcing these resources contribute to the overall expenses of fouling management.
  • Equipment damage and repair: Fouling can lead to equipment damage, such as corrosion, leaks, or mechanical stress on heat exchanger surfaces. Repairing or replacing damaged components adds to the maintenance costs, including the costs of spare parts and potential system downtime.
  • Extended equipment lifespan: The presence of fouling can accelerate equipment degradation, reducing the lifespan of heat exchangers. Premature equipment replacement entails significant capital investments and disrupts operations, resulting in substantial financial implications.
Boiler tube bundle opened for maintenance

Effective mitigation strategies for fouling in heat exchangers

There are various approaches to prevent and mitigate fouling in heat exchangers, ensuring improved energy efficiency, reduced maintenance costs, and extended equipment lifespan.

Chemical treatment: Chemical treatments can be highly effective in mitigating fouling in heat exchangers. The following chemical additives are commonly used:

  • Scale and corrosion inhibitors: These chemicals help prevent scaling and corrosion by modifying the chemical composition of the fluid, inhibiting the formation of deposits and reducing the risk of equipment damage.
  • Dispersants: Dispersants improve the fluid’s ability to carry suspended particles and prevent their deposition on heat exchanger surfaces. They help keep particulate matter
    in a dispersed state, reducing the chances of fouling.
  • Biocides: Biofilms formed by microorganisms can contribute to fouling. Biocides help control biological growth, preventing the formation of biofilms and reducing the risk of associated fouling.

Mechanical cleaning: Regular mechanical cleaning is essential for maintaining heat exchanger performance. The following cleaning methods are commonly employed:

  • Brushing and scrubbing: Manual brushing or scrubbing with appropriate tools can remove fouling deposits from heat exchanger surfaces. This preventive maintenance approach helps restore heat transfer efficiency and prevents the accumulation of stubborn deposits.
  • High-pressure water jetting: Water jetting at high pressures can effectively dislodge fouling deposits from heat exchanger surfaces. This method is particularly useful for removing more tenacious or hard-to-reach deposits.
  • Mechanical scraping: For severe fouling cases, mechanical scraping using specialized tools employed physically to remove the deposits. This method is highly effective but requires expertise to avoid damage to heat exchanger surfaces.
Technician cleaning a heat exchanger tube bundle with high-pressure water

Heat exchanger design: Optimizing heat exchanger design can help minimize fouling tendencies. Consider the following design factors:

  • Surface Roughness: Smooth surfaces are less prone to fouling than rough surfaces. Choosing materials with low surface roughness or employing surface treatments can reduce fouling potential.
  • Flow Velocities and Turbulence: Higher fluid velocities and increased turbulence can help prevent the deposition of fouling deposits. Designing heat exchangers with appropriate flow rates and flowenhancing features can discourage fouling.
  • Material Selection: Selecting materials resistant to fouling and corrosion is crucial. Compatibility between the fluid and heat exchanger materials can help prevent fouling caused by chemical reactions.
Clean heat exchanger

Filtration and pre-treatment: Installing filtration systems and pre-treatment equipment is an effective measure to reduce fouling potential. Consider the following options:

  • Strainers and filters: By installing strainers or filters upstream of the heat exchanger, solid particles and impurities can be removed from the fluid, minimizing the risk of particulate fouling.
  • Separators: Separators, such as centrifugal separators or cyclones, can remove larger particles or contaminants from the fluid before it enters the heat exchanger, reducing the likelihood of fouling.
  • Water treatment: In the case of water-based systems, implementing appropriate water treatment processes, such as softening or demineralization, can help prevent scaling and reduce fouling potential.

Conclusion

Now you are aware that fouling in heat exchangers can lead to decreased efficiency and increased energy costs, so preventing fouling in heat exchangers is a proactive approach to ensure long-term efficiency and reduce maintenance costs. Understanding how fouling occurs is crucial for maintaining optimal performance. Do not miss part two of our educational series where we dive into the details of detecting fouling and effective cleaning methods. Stay tuned for more valuable insights.

About the author

Omari Hussein Sabuni is an experienced mechanical engineer at Kinyerezi Gas Power Plant, specializing in heat exchanger design, optimization, troubleshooting and providing practical solutions for various heat exchanger problems. He is skilled in analyzing thermal systems and developing innovative solutions to enhance heat transfer efficiency and adept at conducting feasibility studies, performing risk assessments, and ensuring compliance with industry standards.

About this Featured Story

This Featured Story was first published in Heat Exchanger World Magazine in December 2023. To read more Featured Stories and many other articles, subscribe to our print magazine.

Featured Stories are regularly shared with our Heat Exchanger World community. Join us and share your own Featured Story on Heat Exchanger World online and in print.

Previous articleHeat exchanger fouling in practice – understand & mitigate – Part 4
Next articleInstallation of boreholes with sensors in Cheshire
Heat Exchanger World is a leading international magazine covering all aspects of the product supply chain of heat exchangers. Heat Exchanger World is part of the KCI Group of Companies. We are a leading knowledge, communication and information company connecting business-to-business professionals by building and sustaining global communities, solving their information needs and helping them to develop their professional life and friendships.