Various heat exchanger cleaning techniques

Thread Content

A heat exchanger is a compact and efficient heat exchange device, serving as an excellent tool for applications such as heating, cooling, heat recovery, and rapid sterilization. However, due to the long-term operation of the heat exchanger, as well as differences in the purity of the fluids used for cooling or heating and the inherent properties of the process media, scaling of the heat exchanger becomes inevitable. This reduces the heat exchange efficiency of the heat exchanger, thereby affecting the normal progress of production and the safety of the equipment. Therefore, the heat exchanger should be cleaned regularly to remove dirt, ensuring efficient heat exchange and the smooth progress of production. Causes, types, and hazards of fouling in heat exchangers. The three main causes of fouling in heat exchangers are as follows: (1) Since most common heat exchangers operate using water as the heat transfer medium, certain salts precipitate out of the water when the temperature rises and the concentration becomes high; these deposits attach to the surface of the heat exchange tubes, forming scale. Over time, as usage continues and the frequency of operation increases, this scale layer becomes thicker and harder, adhering tightly to the surface of the heat exchange tubes. (2) Similar to scale, the fluid on the other side of the heat exchanger may also produce non-scale solid deposits due to the properties of the substances present. If left unaddressed, these deposits will accumulate on the surface of the heat exchange tubes over time. (3) When the fluid contains a large amount of mechanical impurities and organic matter, and the flow rate is low, some of these impurities or organic substances will deposit within the heat exchanger, forming loose, porous, or gel-like contaminants. The six main scaling processes in heat exchangers, for commonly used heat exchangers. Based on the mechanisms of scaling, we generally classify scaling into the following categories: (1) Crystallization scaling: In water cooling systems, supersaturated calcium and magnesium salts in the water crystallize and deposit on the surface of heat exchangers due to changes in temperature, pH, etc., thus forming scale; (2) Particle scaling: The accumulation of solid particles suspended in the fluid on the heat exchange surface; (3) Chemical reaction scaling: Deposition of solids resulting from chemical reactions; (4) Corrosion scaling: The corrosion of the heat exchange surface by the medium in use, with corrosion products depositing on the heated surface and forming fouling; (5) Biological scaling: In common cooling water systems, industrial water often contains microorganisms and the nutrients they require. These microorganisms multiply, and they along with their excretions form biological scale on the heat exchange surface; (6) Deposition scaling: On subcooled heat exchange surfaces, highly soluble components of pure liquids or multi-component solutions deposit as solids. The above classifications merely indicate that a certain process is a key factor in the formation of that type of fouling. Scaling is often the result of multiple processes acting together; therefore, the actual fouling on the heat exchange surface is typically a mixture of various types of fouling. Dangers of not cleaning scale (1) Scale significantly reduces the heat exchange efficiency of equipment, leads to increased energy consumption, and raises production costs ； (2) Scaling deteriorates the heat conduction conditions in heat exchange equipment, causing the heat transfer surfaces to overheat, which leads to safety incidents such as bulging, cracks, and tube failures ； (3) Scaling can cause corrosion beneath the scale, leading to equipment perforation and leakage as well as reducing the equipment’s service life. It can also disrupt the production process, affect product quality, and result in quality issues. To ensure product quality and production safety, heat exchangers must be descaled and cleaned regularly. Selection of heat exchanger cleaning methods: Depending on the cleaning approach, the main methods are physical cleaning and chemical cleaning. Chemical cleaning: Chemical cleaning involves using chemical cleaners to initiate a chemical reaction that dissolves, removes, or peels off the scale and other deposits on the surface of the heat exchanger’s heat transfer tubes. Chemical cleaning does not require disassembling the heat exchanger, which simplifies the cleaning process and reduces the labor involved. Its drawback is that if the chemical cleaning solution is not chosen properly, it can corrode and damage the substrate being cleaned, resulting in losses. Chemical cleaning methods ◉ Circulation method: The cleaning solution is forced to circulate using a pump to carry out the cleaning. ◉ Impregnation method: Fill the equipment with the cleaning solution and let it sit for a certain period of time. ◉ Surge method: Fill the equipment with cleaning solution, then periodically remove a portion of the solution from the bottom and pour it back into the equipment to achieve thorough mixing and cleaning. Chemical circulation cleaning steps: (1) Isolate the equipment and drain all the water from the heat exchanger. (2) Clean the pipes of impurities using high-pressure water and seal the system. (3) A ball valve is installed between the isolation valve and the exchanger; a transfer pump and pipes are connected, with the cleaning agent being pumped in from the bottom of the heat exchanger and flowing out from the top. (4) Inject the required cleaning agent and perform repeated cyclic cleaning. (5) Release gas at any time and inject appropriate water. (6) Use a pH test strip to determine the effectiveness of the cleaner. (7) Recover the cleaning solution and rinse it repeatedly with clean water until the pH becomes neutral. Physical cleaning involves using various mechanical forces and energies to break down dirt, separate it from the surface of an object, and remove it, thereby achieving a cleaning effect. Physical cleaning methods all share one common feature: they are efficient, non-corrosive, safe, and environmentally friendly. Its drawback is that when cleaning the interior of devices with complex structures, its force sometimes cannot reach all areas evenly, resulting in \"dead zones\". Common methods include ultrasonic descaling, PIG pigging technology, and electric field descaling technology. High-pressure water jet cleaning uses high-pressure water generated by a piston pump, which is directed at the scale layer through special nozzles; it provides thorough scale removal and high efficiency, but it requires large storage containers and consumes a lot of water. Ultrasonic descaling primarily utilizes an ultrasonic sound field to treat the fluid; under the action of this sound field, the scaling substances present in the fluid undergo a series of changes in their physical structure and chemical properties, which causes them to disperse, break down, become loose, and fail to adhere to the pipe walls, thus preventing the formation of scale. Removal of scale using mobile descaling equipment inside pipes: The new type of mobile descaling equipment for use inside pipes is highly efficient and of good quality; it is suitable for removing scale from oil and gas transmission pipelines, as well as from pipelines used for transporting chemical liquids and water. It fails based on the driving method. Typical pipe-mounted mobile descaling equipment is classified as follows: A. Electrically driven mobile descaling equipment ； B. Hydraulically driven mobile descaling equipment ； C. Compressed air-driven mobile descaling equipment. Mechanical cleaning: It relies on mechanical action to exert a force greater than the adhesion force of the dirt, thereby removing the dirt from the heat exchange surface. This method can remove carbonized deposits and hard scale that cannot be removed by chemical methods, but it generally takes 5–6 applications to clean the scale inside the pipes, and sometimes even up to 10 applications; as a result, the cleaning efficiency is low and the quality of cleaning is poor. Microbial cleaning is a method that uses microorganisms to break down the oil stains attached to the surface of equipment, converting them into non-toxic and harmless water-soluble substances. This cleaning method completely breaks down pollutants such as oils and organic substances, making it a truly environmentally friendly cleaning technology. Both physical cleaning and chemical cleaning have their own advantages and disadvantages, yet they complement each other well. In practical applications, both are usually used together to achieve a better cleaning effect. For chemical cleaning methods, the choice of cleaning agent has a significant impact on the cleaning efficiency. Selection of cleaning agents: Principles for choosing industrial cleaning agents ➱ Good dirt-removal ability; ➱ No adverse effects on the items to be cleaned; ➱ Stable quality; ➱ Low cost. Commonly used chemical cleaners: (1) Cleaners that remove dirt through dissolution (including water and organic solvents) ； (2) Surfactant cleaners that use surfactant action for cleaning (such as cationic, anionic, non-ionic, and amphoteric surfactants) ； (3) Chemical cleaners that use chemical reactions for cleaning (such as acids, bases, salts, oxidants, etc.). Method for selecting cleaning agents for heat exchangers: When cleaning a heat exchanger, it is first necessary to identify the area to be cleaned and determine the material of the heat exchanger. After taking samples for analysis, the appropriate agent is selected based on the material of the heat exchanger and the degree of scaling. ➱ For carbon steel surfaces where carbonate scale and rust are the main issues, hydrochloric acid is generally the best choice as the primary cleaning agent; for safety reasons, organic acids such as aminosulfonic acid can also be used as the main cleaning agent. ➱ For stainless steel, nitric acid is usually the most suitable cleaning agent; again, for safety reasons or depending on the specific circumstances, aminosulfonic acid, which has a milder acidity, can also be used as the primary cleaning agent. ➱ When cleaning copper-based heat exchangers, it is essential to pay attention to what type of copper material is used. Special attention should be paid to brass; its main component is copper, with a fairly high content of zinc as well. To prevent zinc loss, it is necessary to use acid wash solutions with as low a concentration as possible. Generally, corrosion inhibitors do not provide effective protection for both copper and zinc simultaneously. Therefore, during the operation process, a gentle cleaning method should be used, that is, low concentration, short duration, low flow rate, and cleaning at room temperature is preferable. Generally, Lan-826 can be used as the corrosion inhibitor; for other additives such as surfactants, sludge removers, and foaming agents, they can be selected based on the principles for choosing cleaning agents and taking into account specific circumstances. In some special cases, mainly when the cleaning material may have defects, be relatively thin, or present other special conditions, careful consideration should be given to the choice of chemicals. Cleaning methods for plate heat exchangers: There are three methods for cleaning the plates, namely the backwashing method (cleaning without disassembling), the manual cleaning method (cleaning by disassembling), and the chemical cleaning method (cleaning without disassembling). Cleaning method: (1) Manual cleaning method. When the scale thickness on the heat exchange plates is thin and insoluble in water, they can be disassembled, and each plate can be cleaned by spraying with pressurized water (0.1–0.2 MPa) or low-pressure steam containing water. For deposits that are difficult to remove using water, soft fiber brushes or bristle brushes can be used for cleaning. (2) Chemical cleaning method. On the surface of the heat exchange plates, especially in the areas where the fluid flows, there are hard deposits (oxides or carbides), and it is difficult to remove them using manual cleaning methods. Different chemical solvents can be used for cleaning, depending on the material of the heat exchange plates. Selection of cleaning agents: Currently, acid cleaning is commonly used, which includes organic acids and inorganic acids. The main organic acids include oxalic acid, formic acid, etc. Inorganic acids mainly include hydrochloric acid, nitric acid, etc. Cleaning process: (1) Rinsing: Before acid cleaning, the heat exchanger is first rinsed openly to remove impurities such as mud and scale from its interior. This not only improves the effectiveness of the acid cleaning but also reduces the amount of acid required. (2) Pour the cleaning solution into the cleaning equipment, and then inject it into the heat exchanger. (3) Acid cleaning: Immerse the heat exchanger filled with acid solution statically for 2 hours. Then, it undergoes continuous dynamic cycling for 3–4 hours. Every 0.5 hours in between. Clean alternately in both directions. After the pickling process is completed, if the pH value of the acid solution is greater than 2, it can be reused; otherwise, it should be diluted and neutralized before being discarded. (4) Alkali cleaning: After acid cleaning, a solution prepared from NaOH, Na3PO4, and softened water in specific proportions is used to carry out alkali cleaning of the heat exchanger through dynamic circulation, thereby achieving acid-base neutralization and preventing further corrosion of the heat exchanger plates. (5) Water washing: After the alkaline cleaning is complete, use clean softened water to rinse the heat exchanger repeatedly for 0.5 hours, in order to thoroughly remove any residues remaining inside it. (6) Recording: During the cleaning process, the time taken for each step should be carefully recorded in order to assess the effectiveness of the cleaning. In short, after the cleaning is complete, a pressure test must be conducted on the heat exchanger. It can be used only after passing the inspection. Measures to prevent scaling: (1) Strictly control water quality during operation; it is necessary to conduct thorough water quality tests on the water in the system as well as on the softened water in the softening tanks, and only water that passes these tests may be introduced into the pipeline network. (2) When the new system is put into operation, the heat exchanger should be separated from the system; after circulating for a period of time, the heat exchanger should then be connected to the system again. This is done to prevent impurities in the piping network from entering the heat exchanger. (3) Throughout the entire system, in addition to the occasional cleaning of dirt removers and filters, it is also necessary to maintain cleanliness in the piping network to prevent blockages in the heat exchangers.