Understanding fouling in heat exchangers – Part 2

Heat exchanger shell and tube at offshore oil and gas central facility.

In part one of this article (published in Heat Exchanger World October issue), we discussed the types, causes, effects, cost, and mitigation strategies for fouling in heat exchangers. Now we will explore the detection methods to assist in fouling intervention.

By Omari Hussein Sabuni, Mechanical Engineer – Kinyerezi Power Plant

Detecting fouling in its early stages is crucial for timely intervention and effective mitigation. There are various methods for detecting fouling in heat exchangers, enabling proactive maintenance and optimal performance.

Thermal analysis

Thermal analysis techniques provide valuable insights into the presence and severity of fouling in heat exchangers. The following methods are commonly employed:

Temperature difference monitoring: By measuring the temperature difference between the hot and cold fluid sides of the heat exchanger, any deviations from the expected values can indicate the presence of fouling. An increasing temperature difference suggests reduced heat transfer efficiency, signaling the need for further investigation.
Heat transfer coefficient calculations: Calculating the heat transfer coefficient using heat exchanger performance data can help identify changes in heat transfer efficiency caused by fouling. A decline in the heat transfer coefficient over time indicates the presence of fouling and the need for remedial action.
Infrared thermography: Infrared thermography involves using thermal imaging cameras to detect temperature variations across heat exchanger surfaces. Hotspots or uneven temperature distribution can indicate areas of fouling or blockage, enabling targeted maintenance interventions.

Pressure drop monitoring

Monitoring the pressure drop across the heat exchanger can serve as an effective indicator of fouling. Fouling deposits restrict fluid flow, resulting in increased pressure drop. The following methods aid in pressure drop analysis:

Differential pressure measurement: By measuring the pressure difference between the inlet and outlet of the heat exchanger, any substantial increase in pressure drop indicates the presence of fouling. Regular monitoring of pressure differentials allows for timely fouling detection and appropriate maintenance actions.
Flow metre analysis: Analyzing flow rates using flow meters installed in the heat exchanger system can help identify changes in fluid flow caused by fouling. Reduced flow rates or fluctuations may suggest fouling issues, prompting further investigation.

Visual inspection

Visual inspection remains a fundamental method for detecting fouling in heat exchangers. Direct observation of heat exchanger surfaces can reveal visible signs of fouling, such as deposits, scaling, or corrosion. Key visual inspection approaches include:

Borescopic examination: Borescopes, equipped with cameras or fiber optics, allow for visual inspection of internal heat exchanger components. This non-destructive method enables inspection of hard-to-reach areas for fouling identifi cation and assessment.
Tube and surface inspections: Disassembling heat exchanger components, such as tubes or plates, and visually examining them can provide direct evidence of fouling. Visual inspections should include checks for scaling, deposits, biofilms, or any other visible signs of fouling.

Analytical techniques

Analytical techniques can provide detailed information about fouling composition and characteristics. These techniques are useful for understanding fouling mechanisms and developing targeted mitigation strategies. Commonly employed methods include:

Fouling deposit analysis: Analyzing fouling deposits through techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), or Fourier-transform infrared spectroscopy (FTIR) can identify the composition, morphology, and structure of fouling deposits. This information aids in determining appropriate cleaning methods and preventive measures.
Fluid analysis: Analyzing the fluid circulating in the heat exchanger can provide insights into fouling precursors, such as dissolved solids, particulate matter, or microbial activity. Regular fluid sampling and analysis help monitor fouling potential and identify trends that require attention. Early detection of fouling in heat exchangers is crucial for maintaining optimal performance and minimising associated costs.

Effective cleaning techniques to ensure optimal performance

Regular cleaning of heat exchangers is essential to maintain efficient heat transfer and prevent the negative impacts of fouling. Cleaning techniques tailored to the specific type of fouling and heat exchanger design can help remove deposits and restore optimal performance. There are various effective cleaning methods for heat exchangers, ensuring their longevity and optimal operation.

Mechanical cleaning: Mechanical cleaning methods involve physical removal of fouling deposits from heat exchanger surfaces. Common techniques include:

a) Brushing and scrubbing: Manual brushing or scrubbing with appropriate tools, such as wire brushes or scrub pads, can effectively remove loose or lightly adhered deposits. This method is suitable for accessible surfaces and areas with less severe fouling.
b) High-pressure water jetting: Water jetting at high pressures can dislodge and remove more tenacious deposits from heat exchanger surfaces. The forceful water stream breaks down fouling and flushes it away, restoring heat transfer efficiency. Care should be taken to avoid damaging sensitive components or thin-walled tubes.
c) Chemical cleaning: Chemicals can aid in loosening and dissolving fouling deposits, facilitating their removal. Acid-based or alkaline cleaning solutions are commonly used, depending on the nature of the fouling. Chemical cleaning requires careful handling, proper dilution, and adherence to safety guidelines to protect personnel and prevent equipment damage.

Ultrasonic cleaning: Ultrasonic cleaning is a process that uses high frequency sound waves, typically above 20 kHz, to clean objects. The process involves immersing the object to be cleaned in a tank filled with a cleaning solution and then applying ultrasonic waves to the solution. These waves create high-pressure bubbles in the cleaning solution, which implode when they are exposed to the surface of the object to be cleaned. This implosion creates tiny cavitation bubbles that remove dirt, grease, and other contaminants from the surface of the object.

Chemical cleaning: Chemical cleaning involves the use of specialized cleaning agents or solutions to dissolve and remove fouling deposits. Different types of fouling may require specific chemical treatments:

a) Acid cleaning: Acid-based solutions, such as citric acid, hydrochloric acid, or phosphoric acid, are effective in removing scale deposits and mineral-based fouling. Acid cleaning dissolves the deposits, allowing them to be flushed away with water. Proper safety precautions and neutralization of residual acids are essential to prevent equipment damage and ensure worker safety.
b) Alkaline cleaning: Alkaline cleaners, such as sodium hydroxide or potassium hydroxide, are effective for removing organic fouling, oils, greases, and biofilms. Alkaline cleaning solutions emulsify and dissolve organic matter, making it easier to flush out of the heat exchanger. Appropriate safety measures and proper rinsing are necessary to neutralize any remaining alkaline residues.

Steam cleaning: Steam cleaning utilizes high-temperature steam to remove fouling deposits. The heat and moisture in the steam soften and loosen the deposits, allowing them to be easily removed. Steam cleaning is suitable for removing greases, oils, and some types of organic fouling. Care should be taken to prevent damage to sensitive components or materials that may be affected by high temperatures.
Electrochemical cleaning: Electrochemical cleaning methods, such as electrolysis or electrochemical dissolution, use an electric current to remove fouling deposits. This technique is effective for removing mineral-based scale deposits and can be applied to heat exchanger components immersed in a suitable electrolyte solution. Electrochemical cleaning requires expertise and proper electrical safety precautions.

Mechanic is cleaning condenser tube of Chiller – HVAC System.

Strategies for long-term efficiency

Preventing fouling is key to maintaining optimal performance and prolonging the lifespan of heat exchangers. There are various effective strategies for preventing fouling, ensuring long-term efficiency and reducing maintenance requirements.

Proper fluid treatment: Proper treatment of fluids circulating through heat exchangers is crucial in preventing fouling. Implementing the following measures can help minimize fouling potential:

a) Filtration: Installing appropriate filters to remove particulate matter and suspended solids from the fluid can significantly reduce fouling. Filters should be regularly inspected and cleaned or replaced as necessary to maintain their effectiveness.
b) Water treatment: Treating water to control the levels of dissolved minerals, such as calcium and magnesium, can prevent scale formation. Techniques like water softening reverse osmosis, or ion exchange can effectively reduce mineral concentrations, minimizing scale deposition on heat exchanger surfaces.
c) Biocide treatment: Incorporating biocides or antimicrobial additives in the fluid can help control microbial growth and prevent biofilm formation. This is particularly important in applications where biological fouling is a common issue.

Adequate flow velocity: Maintaining an appropriate flow velocity within the heat exchanger is essential for preventing fouling. Sufficient flow rates help deter the deposition of fouling deposits on heat transfer surfaces by keeping the fluid in a turbulent state. This turbulent flow disrupts the adhesion of particles and inhibits fouling formation.

Surface modification: Surface modifications can be employed to enhance the resistance of heat exchanger surfaces to fouling. Some effective strategies include:

a) Surface coatings: Applying specialized coatings, such as anti-fouling paints or polymer coatings can create a protective barrier that resists fouling adhesion. These coatings reduce surface roughness and promote easy release of deposits, making cleaning more efficient.
b) Surface energy alteration: Modifying the surface energy of heat exchanger materials can discourage fouling. For example, hydrophilic coatings or surface treatments can prevent the formation of hydrophobic fouling deposits.
c) Some common types of coatings that are used to prevent fouling in heat exchanger components are:

i. Non-stick coatings: Non-stick coatings are designed to make the surface of the component resistant to adhesion, making it difficult for fouling agents to stick to the surface. These coatings are typically made of polymers such as polytetrafluoroethylene (PTFE), commonly known as Teflon, and can be applied by techniques such as spraying or electroplating. Non-stick coatings have low surface energy, which means they are easy to clean and prevent fouling caused by sticky or viscous fluids.
ii. Hydrophobic coatings: Hydrophobic coatings repel water and other fluids, which reduces the surface tension and makes it difficult for foulants to stick to the surface. These coatings are typically made of silicone or fluoropolymer materials and can be applied by techniques such as spray coating or dip coating. Hydrophobic coatings are effective in preventing fouling caused by water-based fluids.
iii. Anti-fouling coatings: Anti-fouling coatings are specifically designed to prevent fouling in heat exchanger components. These coatings typically have low surface energy and are resistant to chemicals and high temperatures. Anti-fouling coatings can be made of different materials such as polymers, ceramics, or metals, and can be applied by techniques such as spray coating, electroplating, or chemical vapor deposition. These coatings prevent fouling by creating a barrier between the component and the fluid flowing through it.

Regular cleaning and maintenance: Implementing a proactive cleaning and maintenance schedule is vital for preventing fouling. Regular inspections, cleaning, and maintenance procedures should be conducted to remove any initial fouling deposits before they become more stubborn and difficult to remove. Monitoring performance indicators, such as pressure drop and temperature differentials, can help identify signs of fouling and trigger timely maintenance interventions.

New but not yet painted mounted heat exchanger is connected to piping in a plant for refining.

Conclusion

Taking preventive measures not only improves heat transfer efficiency but also extends the lifespan of heat exchangers, resulting in cost savings and enhanced operational reliability. By prioritizing fouling prevention strategies, industries can optimize heat exchanger performance and maintain efficient operations for years to come.

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 February 2024. To read more Featured Stories and many other articles, subscribe to our print magazine.

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