Causes of Fouling
1.Fluid Flow Velocity:
The velocity of the fluid affects fouling through its impact on heat and mass transfer and mechanical forces, making the process quite complex. In fact, the effect of velocity on different types of fouling varies, and its impact on different types of heat exchangers also differs. In graphite heat exchangers, the influence of flow velocity on fouling should consider both deposition and erosion. For all types of fouling, the increase in erosion rate due to higher flow velocity is generally more significant than the increase in deposition rate, thus the fouling growth rate decreases with increasing flow velocity. However, in practical operations, increasing flow velocity will increase energy consumption, so it is not always better to have a higher flow velocity. Both energy consumption and fouling need to be considered comprehensively.
2.Fluid Properties:
The properties of the fluid include the nature of the fluid itself and the characteristics of various substances insoluble in the fluid or carried by the fluid. In cooling water systems, water quality characteristics play a crucial role in fouling deposition. If the water contains salts and other substances, they may crystallize due to changes in temperature or concentration. If it contains insoluble gases, it may affect corrosion on metal surfaces. Additionally, the presence of microorganisms and nutrients can impact biological fouling.
3.Temperature of Heat Transfer Surface:
The temperature of the fluid and its heat transfer coefficient determine the interface temperature. The rate of chemical reactions depends on temperature, as does biological fouling. Generally, an increase in fluid temperature leads to higher rates of chemical reactions and biological fouling, thus affecting the amount of fouling deposition and increasing the fouling growth rate.
4.Heat Exchanger Parameters:
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- Heat Transfer Surface Material: Fouling conditions are often related to the material used. Studies have shown that copper alloy materials inhibit biological fouling, while common carbon steel and stainless steel may be affected by fouling due to corrosion products. Using corrosion-resistant materials like graphite or ceramics reduces the likelihood of fouling.
- State of Heat Transfer Surface: The surface quality of the heat transfer material influences fouling formation and deposition. A rougher surface promotes fouling deposition.
- Structure of Graphite Heat Exchanger: Experience indicates that plate-type graphite heat exchangers and spiral plate graphite heat exchangers have better anti-fouling performance than shell-and-tube graphite heat exchangers.
Types of Fouling
For common graphite heat exchangers, fouling is generally categorized based on fouling mechanisms into the following types:
1.Crystallization Fouling: Fouling formed by the deposition of dissolved inorganic salts from supersaturated fluids onto the surface of the graphite heat exchanger. Scale is the most common type of fouling in industrial equipment, where supersaturated calcium and magnesium salts in cooling water crystallize due to changes in temperature or pH, forming scale on the surface of the graphite heat exchanger.
2.Particulate Fouling: Fouling formed by the accumulation of suspended solid particles such as sand, dust, and carbon black in the fluid system on the heat transfer surface.
3.Chemical Reaction Fouling: Deposits formed due to oxidation, polymerization, or other chemical reactions between the heated surface and the fluid.
4.Corrosion Fouling: Fouling formed by the deposition of corrosion products on the heat transfer surface due to the corrosive nature of the fluid or the presence of corrosive impurities.
5.Biological Fouling: Adhesive deposits formed by microbial colonies and their excretions along with chemical pollutants and sludge on the walls of heat exchanger tubes and pipes.
6.Solidification Fouling: Fouling formed by the deposition of high-solubility components from supercooled clean fluids or multi-component solutions on the heat transfer surface.
The above classification indicates that a primary process is responsible for forming a particular type of fouling. However, fouling is often the result of the combined effects of multiple processes. In practical scenarios, fouling on heat transfer surfaces usually comprises a mixture of various types.
For the sake of simplifying research, it is necessary to study each type of fouling individually.
Descaling Methods
Mechanical Cleaning
Mechanical cleaning involves applying a force greater than the adhesion force of the fouling to remove it from the surface. This method can remove carbonized fouling and hard scale that chemical methods cannot. Mechanical cleaning methods can be classified into two types:
1.Intensive Cleaning: This method uses jet equipment to spray the medium with high impact force into the tube and shell sides of the graphite heat exchanger to remove fouling. Common intensive cleaning methods include shot blasting, high-pressure water jet cleaning, air blasting, sandblasting, and strong pipe cleaning tools. High-pressure water jet cleaning is often used to remove carbonized or hard fouling. For fouling that requires heat to loosen it, steam jet cleaning is used.
2.Soft Mechanical Cleaning: This method relies on the movement of inserts inside the tubes to contact and remove fouling from the inner surface. This is also known as online mechanical cleaning. Common methods include rotating spiral wire, liquid-solid fluidization, rotating belt, spiral spring vibration, and online cleaning with sponge rubber balls. The sponge ball method involves squeezing slightly larger diameter sponge balls into the tubes to remove fouling. Steel brushes can also be used to clean fouling with lower hardness.
Chemical Cleaning
Chemical cleaning uses chemical cleaning solutions to induce chemical reactions that dissolve, detach, or peel off scales and other deposits on the heat transfer tubes of graphite heat exchangers.
This method is characterized by short cleaning times, simple operation, and thorough removal of fouling. It is one of the most widely used and effective cleaning methods. Chemical cleaning can be performed on-site, has lower labor intensity than mechanical cleaning, and provides more thorough cleaning, reaching places that mechanical cleaning cannot. It also avoids potential mechanical damage to heat transfer surfaces from mechanical cleaning. Chemical cleaning is advantageous for shell-and-tube heat exchangers that cannot be disassembled.
Before cleaning, it is essential to understand the structure, material, fouling distribution, thickness, and composition of the equipment to select appropriate cleaning agents, inhibitors, and additives. The correct amounts, concentrations, speeds, temperatures, and cleaning durations must be chosen. Proper handling and disposal of cleaning waste are necessary to avoid environmental impact.
Physical Cleaning
Physical cleaning involves using various mechanical forces and energy forms to crush, separate, and detach fouling from surfaces, achieving the cleaning effect. Common methods include ultrasonic descaling, PIG (pipeline inspection gauge) cleaning technology, and electrostatic descaling technology. Ultrasonic descaling uses the cavitation, activation, shearing, and inhibition effects of ultrasound to remove fouling. The key to ultrasonic descaling is selecting appropriate ultrasonic power, frequency, and cleaning fluid temperature.
Microbial Cleaning
With increasing hydraulic retention time (HRT), the COD removal rate gradually increases. When HRT exceeds 5 minutes, the COD removal rate stabilizes at about 75%. In electrochemical reactors, fluid flow and gas stirring significantly increase particle collisions and growth opportunities. The average bubble size produced by electroflotation is 20-70 μm, providing a large specific surface area that offers more adsorption and bonding centers for flocs, making them float more easily due to the presence of gas within the flocs. Thus, satisfactory treatment results can be achieved in a short time.
Impact of Current Intensity
The removal rates of turbidity, COD, and MBAS in laundry wastewater are related to current intensity. As the current intensity increases, the removal rates of these indicators also increase.
According to Faraday’s electrolysis law, the electrochemical dissolution of Al and the electrolysis of water are proportional to the amount of current supplied (I/t). Theoretically, when passing 1F (26.8 Ah) of current, 9 g of Al³⁺ can be dissolved, releasing 0.0224 Nm³ of H₂ and O₂, which is much higher than the amount of gas released in DAF. Higher current intensity produces smaller bubbles, benefiting the flotation separation process.
By integrating electrocoagulation, electroflotation, and electrochemical oxidation, a new type of electrochemical reactor has been developed. This reactor effectively removes surfactants, SS, COD, and phosphates from laundry wastewater.
Cleaning Frequency
In industrial production, various situations can cause fouling and blockage in graphite heat exchangers or pipelines, affecting the heat exchange performance. During heat exchange, cooling water forms hard scale on the surface of graphite heat exchangers, impacting heat transfer. Severe fouling can lead to insufficient flow and pressure drop, hindering normal operation. Some companies only consider cleaning when severe fouling affects production, not realizing that fouling under load operation damages equipment and increases failure rates.
Graphite heat exchangers need cleaning based on fouling severity. When heat exchange performance is unsatisfactory, energy consumption increases, the heat exchanger has been in use for a long time, or water quality is poor, it is time to consider cleaning. Professional cleaning companies should be hired for the task.
Cleaning graphite heat exchangers ensures normal operation, restores production, and prevents hazards.