Fouling in Heat Exchangers: Detection, Prevention and Removal
Understanding the causes and types of fouling in heat exchangers is key to selecting the appropriate preventive, control and cleaning strategies.
Heat exchangers, which are used to transfer heat from one fluid to another, are critical equipment in various industrial processes.
Over time, the buildup of unwanted materials on heat exchanger surfaces, also known as fouling, can lead to decreased heat transfer efficiency, increased energy consumption, increased pressure drop, equipment failure and reduced production capacity.
This article explores the phenomenon of heat exchanger fouling in detail, from detection to prevention and removal.
Causes of Fouling in Heat Exchangers
Fouling in heat exchangers can develop due to several primary factors, including:
- The process fluid's properties. The process fluid's composition influences most fouling phenomena in heat exchangers. Heat exchangers with process fluids containing high levels of minerals, chemical contaminants, suspended solids and pH are more susceptible to fouling.
- The heat exchanger's operating conditions. Improper operation of the heat exchanger, such as poor flow distribution, inadequate velocity or insufficient temperature control, can contribute to fouling.
- The heat exchanger's design. Inadequate design parameters, such as material selection, surface roughness, the size and shape of heat transfer surfaces, and the availability of a flow path for the process fluid, can affect the severity of fouling during the heat exchanger's operation.
Types of Heat Exchanger Fouling
In general, the fouling found in heat exchangers can be categorized into several main types based on the main phenomena occurring.
Major types of fouling in heat exchangers include:
- Particulate (deposition/sedimentation) fouling. This fouling is caused by accumulating suspended solids or particles (e.g., dirt, sand, sediments, organic materials) in the process fluid. Particulate fouling results in the formation of a layer of solid material on the heat exchanger surfaces and can reduce heat transfer efficiency and increase pressure drop.
- Scaling fouling. Scaling fouling occurs when minerals or other inorganic materials in the process fluid precipitate out and adhere to the heat exchanger surfaces, forming a hard, crusty layer. Scaling can reduce heat transfer efficiency and increase pressure drop.
- Corrosion fouling. Corrosion fouling develops due to the process fluid's reaction with the heat exchanger materials. Corrosion can result in the formation of local pits or other irregularities on the heat exchanger surfaces, which can trap process fluids and lead to fouling.
- Biological fouling. Biological fouling occurs when microorganisms, such as bacteria or algae, grow on the heat exchanger surfaces. This type of fouling can result in the formation of a layer of organic material, which eventually reduces overall heat transfer efficiency and increases pressure drop. (To learn more about how microorganisms can participate in corrosion, read: MIC Corrosion: How Can Microorganisms Eat Holes in a Metal?)
- Chemical fouling. Chemical fouling occurs when chemical reactions between the chemical contaminants in the process fluid and the heat exchanger materials result in the formation of unwanted compounds or deposits. There are several types of chemical fouling, including scaling fouling (described above). Chemical fouling can reduce heat transfer efficiency, increase pressure drop and cause equipment failure.
The Fouling Factor
The fouling factor measures the extent to which fouling in a heat exchanger influences the process fluid's resistance to heat transfer. In other words, it represents how much additional thermal resistance the fouling layer adds to the heat transfer surfaces. The fouling factor is expressed in units of (m².K)/W.
The fouling factor can be calculated by dividing the thermal resistance due to fouling by the heat exchanger's overall heat transfer coefficient (U). The reciprocal of the overall heat transfer coefficient is the sum of the individual resistances to heat transfer, including the resistances of the fouling layer (Rf), the resistance of the heat transfer surfaces (Rw) and the resistance of the fluid film (Rh, Rc).
This can be expressed mathematically as:
1/U = Rh + Rf,h + Rw + Rf,c + Rc
When designing heat exchangers, the fouling factor can be used to calculate the appropriate fouling allowance -- the maximum amount of fouling a heat exchanger can shoulder without compromising the machine's efficiency -- and to determine the optimal cleaning frequency and maintenance schedule. (For more on controlling corrosion through design, read: How to Control Corrosion by Improving Design.)
How to Detect Fouling in Heat Exchangers
Some of the most common methods to detect fouling in heat exchangers are:
- Pressure drop monitoring. As fouling builds up on the heat transfer surfaces, it increases flow resistance and causes an increase in pressure drop across the heat exchanger. Therefore, the pressure drop is a sound performance indicator for detecting changes in fouling levels so cleaning or maintenance can be scheduled accordingly.
- Heat transfer coefficient measurement. The heat transfer coefficient, which decreases as fouling accumulates, directly indicates the heat transfer efficiency. Thus, measuring this coefficient can be used to identify when corrective actions are necessary.
- Process fluid analysis. Chemical analysis of the process fluid can reveal the presence of fouling agents, such as scale or corrosion products, and provide insight into the fouling's severity.
- Non-destructive inspection and testing methods. Techniques such as ultrasonic testing (UT) or radiographic testing have been widely applied to detect and quantify the thickness of fouling deposits on heat transfer surfaces. Infrared thermography can also be used to detect temperature variations on heat exchanger surfaces. Borescoping and endoscopy can be used to inspect the interior of tubes and other difficult-to-access areas.
- Predictive maintenance. By analyzing data from various sensors and instruments, predictive maintenance techniques (built through modeling or digital twins, for example) can be used to detect and predict the onset of fouling and schedule maintenance or cleaning before it impacts the heat exchanger's performance. (For more on using technology to monitor and prevent corrosion, watch: Robotic Corrosion Management: What to Know Before Investing.)
How to Remove Fouling in Heat Exchangers
Besides fouling detection, cleaning and routine maintenance activities are crucial to maintain heat exchangers' performance.
The following methods will help remove fouling and are beneficial to keep heat exchangers clean:
- Mechanical cleaning. This involves physically removing fouling deposits from the heat transfer surfaces using tools such as scrapers, brushes or high-pressure water jets. Mechanical cleaning removes hard and stubborn fouling, such as scale or corrosion products.
- Chemical cleaning. Chemical cleaning involves introducing chemicals to dissolve and remove fouling deposits from the heat transfer surfaces. Acidic or alkaline cleaning agents are commonly used for chemical cleaning, depending on the type of fouling present. Chemical cleaning effectively removes soft and sticky fouling, such as biological growth or organic deposits. Scale inhibitors and chlorine are often added to control fouling formation (in the sense of delaying or preventing it).
- Radiation/sterilization cleaning. The most common example of this method is ultraviolet (UV)-based radiation. UV treatment is an effective non-chemical and non-invasive method to prevent fouling growth and reproduction.
- Steam cleaning. Steam cleaning involves using high-pressure steam to remove fouling deposits from the heat transfer surfaces. The steam's high temperature and pressure help soften and loosen the fouling, which can then be easily removed. (For more on industrial uses for steam, read: All Steamed Up: The Many Types and Uses of Steam.)
- Acoustic cleaning. Acoustic-based methods, such as ultrasonic cleaning, involves using high-frequency sound waves to remove fouling deposits from the heat transfer surfaces. The sound waves create microscopic bubbles in the cleaning solution, which implode upon contact with the fouling, causing it to loosen and dislodge from the surfaces.
Fouling in heat exchangers is a very typical, common and generally inevitable challenge.
However, understanding the causes and types of fouling allows one to select the appropriate preventive, control and cleaning strategies to maintain the heat exchanger's efficiency and performance.