Cleaning Technology of Heat Exchanger
Heat exchanger is a compact and efficient heat exchanger, which is an excellent equipment for heating, cooling, heat recovery, rapid sterilization and other purposes. However, due to the long-term operation of heat exchangers, the different purity of cooling or heating side and the different nature of process media, scaling of heat exchanger has become inevitable, resulting in the reduction of heat exchange efficiency of heat exchangers, thus affecting the normal operation of production and the safety of equipment. Therefore, the tubesheet of heat exchanger should be cleaned regularly to remove dirt, so as to ensure the efficient heat transfer and normal production of heat exchanger.
Causes, types and hazards of fouling in heat exchangers
Three Causes of Scaling in Heat Exchangers
- (1) Because most of the common heat exchangers are heat exchangers with water as the heat carrier, some salts precipitate from the water when the temperature rises and the concentration is high, and adhere to the surface of heat exchanger tubes, forming scales. With the increase of time and frequency, the scaling layer becomes thicker and harder, and adheres tightly to the surface of heat exchanger tubes.
- (2) Like scale, the fluid on the other side of the heat exchanger may produce non-scale solid precipitates due to the nature of the substance itself, which will accumulate more and more on the surface of the heat exchanger if it is not treated for a long time.
- (3) When there are more mechanical impurities in the fluid and the flow rate of the fluid is low, some mechanical impurities or organic matter will deposit in the heat exchanger, forming loose, porous or colloidal fouling.
Six Main Scaling Processes of Heat Exchangers
For commonly used heat exchangers. According to scaling mechanism, scaling is generally divided into the following categories:
- (1) Quasi-crystalline scaling: For example, in water cooling system, the supersaturated calcium and magnesium salts deposit on the surface of heat exchanger due to the change of temperature and pH.
- (2) Particle scaling: the accumulation of suspended homologous particles in the fluid on the heat transfer surface;
- (3) Chemical reaction scaling: homologous deposition caused by chemical reaction;
- (4) Corrosion and scaling: the heat transfer medium corrodes the heat transfer surface and produces corrosion products deposited on the heating surface to form fouling;
- (5) Biofouling: For commonly used cooling water systems, industrial towels often contain microorganisms and the nutrients they need. These microorganisms multiply and their excreta and slurry form biofouling on the heat exchange surface.
- (6) Coagulation and scaling: On the supercooled heat exchange surface, the highly soluble components of pure liquid or multi-component solution coagulate and deposit together.
The above classification only indicates that a process is a major process for the formation of such fouling. Scaling is often the result of the interaction of various processes, so the actual fouling on the heat exchange surface is often mixed with various fouling.
The Harm of Uncleached Scaling
- (1) Scaling greatly reduces the heat exchange efficiency of equipment, increases energy consumption and increases production cost.
- (2) Scaling aggravates the heat conduction condition of heat exchanger and causes safety accidents such as blister, crack and tube burst due to overheating of heat transfer surface.
- (3) Scaling can cause corrosion damage under scale, cause equipment perforation and leakage, shorten the service life of equipment scaling will make the production process unstable, affect product quality, and cause quality accidents.
- In order to ensure product quality and production safety, the heat exchanger must be cleaned regularly.
Selection of Cleaning Method for Heat Exchanger
According to the different cleaning methods, the main cleaning methods are physical cleaning and chemical cleaning.
Chemical cleaning is to dissolve, peel off or peel scales and other sediments on the surface of heat exchanger tubes by chemical reaction of cleaning liquid. Chemical cleaning does not need to dismantle the heat exchanger, which simplifies the cleaning process and reduces the labor level of cleaning. The disadvantage is that improper selection of chemical cleaning solution will cause corrosion damage to the matrix of the cleaning material and cause losses.
Chemical cleaning method
- Circulation method: Pump forced cleaning liquid circulation, cleaning.
- Impregnation: Fill the cleaning liquid with equipment and hold it for a certain time.
- Surge method: The cleaning fluid is filled with equipment, and a part of the cleaning fluid is discharged from the bottom at regular intervals, then the discharged liquid is loaded back into the equipment to achieve the purpose of mixing and cleaning.
Physical cleaning is to use various mechanical external forces and energy to make the dirt smash, separate and peel off the surface of the object, so as to achieve the cleaning effect. Physical cleaning methods have one thing in common: high efficiency, no corrosion, safety and environmental protection. The disadvantage of this method is that it sometimes fails to reach all parts evenly when cleaning the equipment with complex structure.
Common methods include ultrasonic descaling, PIG pigging and electric field descaling.
High pressure water jet cleaning
The high-pressure water produced by the plunger pump is sprayed to the scale layer of the tube sheet through a special nozzle. The scale removal is thorough and efficient, but the installed container of the plunger pump is large and consumes much water.
The main purpose is to treat the fluid with ultrasonic sound field, so that the scaling substances of the fluid can change their physical and chemical properties in a series of ways under the action of ultrasonic field, making them disperse, crush, loose and not easy to adhere to the pipe wall to form scaling.
Scale removal by movable scale remover in pipeline
The new type of mobile scale remover in pipeline has high efficiency and good quality. It is suitable for scale removal in oil and gas pipelines, chemical liquids and water pipelines.
According to the driving mode, the typical movable scale remover in pipeline can be divided into:
- A. Electrically driven mobile scale remover;
- B. Hydraulically driven mobile scale remover;
- C. Compressed air driven mobile scale remover.
It provides a force greater than the adhesion force of fouling by mechanical action to make fouling fall off from the heat exchange surface. This method can remove carbonized dirt and hard dirt which can not be removed by chemical method, but it usually takes 5-6 times to clean the dirt layer in the pipe, sometimes up to 10 times, and the pigging efficiency is low and the quality is poor.
Microbial cleaning is a method of using microorganisms to decompose the oil contamination attached to the surface of equipment and transform it into non-toxic and harmless water-soluble substances. This kind of cleaning, which completely decomposes pollutants (such as oil) and organic matter, is a truly environmentally friendly cleaning technology.
Physical cleaning and chemical cleaning have their own advantages and disadvantages, but also have good complementarity. In the actual application process, usually the combination of the two is used to obtain better cleaning effect.
For chemical cleaning methods, the choice of cleaning agent has a significant impact on the cleaning effect.
Selection of cleaning agent
Selection Principles of Industrial Cleaning Agents
- Good decontamination ability;
- No adverse effect on cleaning objects;
- Stable quality;
- Low price.
Commonly used chemical cleaning agents
- (1) Cleaning agents (including water and organic solvents) for decontamination by dissolution;
- (2) Surfactant cleaning agents (such as cationic, anionic, non-ionic and amphoteric surfactants) that use surface activity to decontaminate.
- (3) Chemical cleaning agents (such as acids, alkalis, salts, oxidants, etc.) for decontamination by chemical reaction.
- Selection of cleaning agents for heat exchangers.
When cleaning the heat exchanger, the cleaning position and the material of heat exchanger should be determined first. After sampling and analysis, the reagent should be selected according to the material of heat exchanger and the degree of scaling.
When carbonate scale and rust are the main detergents for carbon steel, hydrochloric acid is generally chosen as the main acid detergent, and organic acid sulfamic acid can be chosen as the main acid detergent for safety consideration.
For stainless steel, nitric acid is generally chosen as the best acid cleaning agent. Similarly, mild acid sulfamic acid can be chosen as the main acid cleaning agent for safety reasons.
When cleaning copper material heat exchanger, we must pay attention to which copper material is.
In particular, it should be noted that the main component of brass is copper, followed by zinc content is quite high. In order to prevent the phenomenon of dezincification, the concentration of pickling solution should be as low as possible. Generally, corrosion inhibitors can not protect both copper and zinc metals at the same time. Therefore, in the process of operation, mild cleaning method is adopted, that is, low concentration, short time, small flow rate, room temperature cleaning is better.
Lan-826 is the general choice of corrosion inhibitor. For other additives, such as surfactant, clay stripping agent, foaming agent, etc., it can be selected according to the selection principle of cleaning agent and specific conditions. In some special cases, mainly refers to the cleaning materials may be defective or thin or other special circumstances, should be carefully considered the choice of agents.
Cleaning Method of Plate Heat Exchanger
There are three kinds of cleaning methods for plate, i. e. backwash (no disassembly cleaning), manual cleaning (disassembly cleaning), and chemical cleaning (no disassembly cleaning).
- (1) Hand cleaning. When the tube sheet of heat exchanger is very thin and insoluble in water, it can be dismantled. The tube sheet of heat exchanger can be sprayed with pressurized water (0.1-0.2 MPa) or low pressure steam with water. For sediments which are difficult to be washed by water, soft fiber brushes and bristle brushes can be used to wash them.
- (2) Chemical cleaning method. Hard deposits (oxides or carbides) on the surface of heat exchanger plates, especially at the dead corner of medium flow, are difficult to be solved by manual cleaning method. Different chemical solvents can be used to clean heat exchanger plates according to their material.
Selection of cleaning agent
At present, acid pickling is commonly used, which includes organic acid and inorganic acid. Organic acids mainly include oxalic acid, formic acid and so on. Inorganic acids mainly include hydrochloric acid, nitric acid, etc.
- (1) Flushing: Before pickling, the heat exchanger should be flushed in an open way, so that there are no mud, dirt and other impurities in the heat exchanger, which can not only improve the effect of pickling, but also reduce the acid consumption of pickling.
- (2) pour the cleaning liquid into the cleaning equipment and then inject it into the heat exchanger.
- (3) pickling: the heat exchanger filled with acid solution is immersed in static state for 2 hours. Then the continuous dynamic cycle lasted for 3 to 4 hours. The interval was 0.5 hours. Alternate positive and negative cleaning. After pickling, if the pH value of the acid solution is greater than 2, the acid solution can be reused. Otherwise, the acid solution should be diluted, neutralized and discharged.
- (4) Alkali washing: After pickling, NaOH, Na, PO and softening water are prepared according to a certain proportion. The heat exchanger is alkali washed by dynamic circulation to achieve acid-alkali neutralization, so that the heat exchanger plate is no longer corroded.
- (5) Washing: After alkali washing, rinse the heat exchanger repeatedly for 0.5 hours with clean softening water, and thoroughly rinse the residue in the heat exchanger.
- (6) Recording: During the cleaning process, the time of each step should be strictly recorded to check the cleaning effect.
- In a word, after cleaning, the pressure test of heat exchanger should be carried out. It can be used only after it is qualified.
Measures to Prevent Scaling
- (1) Water quality must be strictly controlled in operation. Water in the system and softened water in the softening tank must be strictly tested for water quality before being injected into the pipeline network.
- (2) When the new system is put into operation, the heat exchanger should be separated from the system. After a period of cycle, the heat exchanger should be merged into the system to avoid impurities entering the heat exchanger in the pipe network.
- (3) In the whole system, besides the irregular cleaning of dirty appliances and filters, the cleaning of the pipe network should also be maintained to prevent the blockage of heat exchangers.
Source: China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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