Mechanical cleaning:
Mechanical cleaning is to provide a force greater than the adhesion force of dirt to remove the dirt attached to the surface. This cleaning method can remove carbonized dirt and hard dirt that cannot be removed by chemical methods. Mechanical cleaning methods can be divided into the following two categories:
1. Strong cleaning. The strong cleaning method uses the spraying equipment to spray the medium into the tube side and shell side of the heat exchanger with a very high impact force, so as to achieve the purpose of descaling. Common strong cleaning methods include shot blasting, high-pressure water jet cleaning, air jet cleaning, sand blasting cleaning, strong pig, etc. Among them, high-pressure water jet cleaning is mostly used to remove carbonized scale or hard scale, while steam jet cleaning is used for dirt that cannot be removed only by impact force and must be loosened by heat.
2. Soft mechanical cleaning. This cleaning method relies on the movement of the insert in the tube to contact the inner surface of the tube to achieve the effect of removing dirt.
This soft mechanical cleaning is also called online mechanical cleaning [7]. Common methods include rotating helix method, Liquid-Solid Fluidization method, rotating tie method, spiral spring vibration method, sponge rubber ball online cleaning method, etc. There are various types of inserts, among which the sponge rubber ball method is to squeeze the sponge ball with a diameter slightly larger than the inner diameter of the pipe into the pipe for the purpose of descaling, and a wire brush can also be used to clean the dirt with lower hardness.
Chemical cleaning:
Chemical cleaning is the use of chemical cleaning fluid to produce a certain chemical reaction, so that the scale and other deposits on the surface of the heat transfer tube of the heat exchanger dissolve, fall off or peel off.
This method has the advantages of short cleaning time, simple operation and thorough descaling. It is one of the most widely used and effective cleaning methods at present. Chemical cleaning can be completed on site. The labor intensity is lower and the cleaning is more complete than mechanical cleaning. It can clean places that mechanical cleaning cannot reach, and avoid mechanical damage to the heat exchange surface caused by mechanical cleaning; Moreover, chemical cleaning does not need to disassemble the equipment, which has incomparable advantages over mechanical cleaning for shell and tube heat exchange equipment that cannot be disassembled.
Before cleaning, it is necessary to understand the structure, material, dirt distribution, thickness and composition of the equipment to be cleaned, so as to reasonably select the main cleaning agent, corrosion inhibitor and auxiliary agent, and select the appropriate amount, concentration, speed, temperature and time of cleaning agent [8]. Finally, it is necessary to do a good job in the treatment and discharge of cleaning waste liquid to avoid impact on the environment.
Physical cleaning:
Physical cleaning is to smash, separate and peel the dirt away from the object surface with the help of various external mechanical forces and energy, so as to achieve the effect of cleaning. Common methods include ultrasonic descaling, pig pigging technology, electric field descaling technology, etc. Ultrasonic descaling is to use the cavitation effect, activation effect, shear effect and inhibition effect of ultrasonic to achieve the effect of descaling. The key of ultrasonic descaling technology is to choose the appropriate ultrasonic power and frequency and the temperature of cleaning solution.
Microbial cleaning:
With the increase of HRT, the COD removal rate increases gradually. When hrt>5min, the COD removal rate basically tends to be stable, and the COD removal rate reaches about 75 In the electrochemical reactor, due to the flow of fluid and the stirring of gas, the chance of particle collision and growth is greatly increased. The average bubble particle size produced by electric flotation is 20 ~ 70 μ m. It has a relatively large specific surface area, which can provide more adsorption and bonding centers for flocs, so that there is gas in flocs, which is more conducive to flocs floating. Therefore, a satisfactory treatment effect can be obtained in a short time.
Influence of current intensity:
Relationship between turbidity, COD and MBAs removal rate of laundry wastewater and current intensity. With the increase of current intensity, the removal rate of these indicators increases gradually.
According to Faraday's Electrolysis law, the electrochemical dissolution of Al and the electrolysis of water are proportional to the amount of electricity provided (i/t). When 1F (26.8ah) electricity is passed, 9gal3 can be dissolved theoretically, and 0.0224nm3h2 and O2 can be released at the same time, which is far greater than the amount of gas released in DAF. At the same time, increasing the current intensity can obtain smaller bubbles, which is very beneficial to the air flotation separation process.
Summary of heat exchanger cleaning:
Combining electrocoagulation, electroflotation and electrochemical oxidation, a new electrochemical reactor is developed, which integrates the high-efficiency flocculation of Al3 produced by electrocoagulation and its hydrolytic polymerization products, the flotation of tiny bubbles produced by insoluble electrodes and the electrochemical oxidation of catalytic oxidation electrodes. Using this reactor to treat laundry wastewater can effectively remove surfactants, SS, COD and phosphate from the wastewater.