Graphite heat exchanger fouling factor and cleaning method

Cause of scaling

(1) The flow velocity of the fluid. The flow rate of the fluid can be affected by the influence of heat and mass transfer and the mechanical force, which is very complicated. In fact, the effect of flow rate on different types of fouling is different, and the degree of influence on the scaling of different types of heat exchange equipment is also different. In graphite heat exchangers, the effect of flow rate on fouling should also consider its effect on fouling and scale erosion. For all types of fouling, the increase in erosion rate due to increased flow rate is more pronounced than the rate of fouling deposition. Therefore, the growth rate of dirt decreases as the flow rate increases. However, in actual operation, the increase of the flow rate will increase the energy consumption. Therefore, the flow rate is not as high as possible, and should be considered in terms of energy consumption and dirt.

(2) Fluid properties. The properties of the fluid include the nature of the fluid itself and the properties of the various materials that are insoluble or entrained by the fluid. In the cooling water system, the water quality characteristics play a key role in the deposition of dirt. If salt and other substances are contained, it may crystallize due to changes in temperature or concentration; if it contains insoluble gas, it will affect the corrosion of the metal surface; if it contains microorganisms and nutrients It also has an effect on biofouling.

(3) The temperature of the heat transfer wall. The temperature of the fluid and its heat transfer coefficient determine the interface temperature. The rate of chemical reaction depends on the temperature, and the biofouling also depends on the temperature. The increase in the temperature of the fluid generally leads to an increase in the rate of chemical reaction and the rate of biofouling, thereby affecting the amount of fouling deposited, resulting in an increase in the growth rate of the fouling.

(4) Parameters of heat exchange equipment. First, the heat exchange surface material: usually the fouling situation has a great relationship with the material. Studies have found that copper alloy materials inhibit biofouling. For other commonly used carbon steels and stainless steels, scaling is only affected by the deposition of corrosion products, and if non-metallic materials such as graphite or ceramics with good corrosion resistance are used, scaling is less likely to occur. The second is the state of the heat exchange surface: the surface quality of the heat exchange surface material will affect the formation and deposition of dirt, and the greater the surface roughness, the more favorable for the formation and deposition of dirt. Third, the structure of the graphite heat exchanger: experience shows that the anti-scaling performance of the general plate-type graphite heat exchanger and the spiral-plate graphite heat exchanger is better than that of the shell-and-tube graphite heat exchanger.

Type of dirt

For the commonly used graphite heat exchangers, according to the fouling mechanism, the dirt is generally classified into the following categories:

(1) Crystallized fouling: refers to the fouling of the inorganic salt dissolved in the supersaturated flowing liquid and deposited on the surface of the graphite heat exchanger, which is called crystallization fouling. Scale is a common foul in industrial equipment. In water-cooled systems, due to supersaturated calcium in water, magnesium salts are crystallized from water in the surface of the graphite heat exchanger due to changes in temperature, pH, etc., and scale is formed.

(2) Particulate type dirt: solid particles suspended in a fluid system such as sand particles, dust, carbon black, and dirt formed on the heat transfer surface.

(3) Chemical reaction fouling: formation of deposits between the heated surface and the fluid due to auto-oxidation and polymerization, ie, chemical reactions.

(4) Corrosive type dirt: The heat exchange surface is corroded due to corrosive or corrosive impurities, and corrosion products are deposited on the heat exchange surface to form dirt.

(5) Bio-type fouling: It is a kind of sticky deposit formed by the microbial population and its excreta and chemical pollutants, mud and other components adhering to the wall of heat exchange tubes, pipes, etc., called bio-type dirt.

(6) Solidified soil: On the super-cooled heat exchange surface, the solid formed by the cleaning liquid or the highly dissolved component of the multi-component solution is solidified and deposited.

The above classification only indicates that a certain process is a major process for the formation of such dirt. Fouling is often the result of a combination of multiple processes and interacts with each other. In the actual dirt on the heat transfer surface, a variety of soils are often mixed. together.

However, for the simplification of the research, it is necessary to study the single dirt first.

Descaling measures

Mechanical cleaning

Mechanical cleaning provides a force greater than the adhesion of the dirt to remove dirt adhering to the surface. This cleaning method removes carbonized and hard scales that cannot be removed by chemical methods. The methods of mechanical cleaning can be divided into the following two categories:

(1) Strong cleaning. The strong cleaning method uses the spraying device to spray the medium into the tube side and the shell side of the graphite heat exchanger with a very high impact force, which serves the purpose of descaling. Common strong cleaning methods include shot blasting, high pressure water jet cleaning, jet cleaning, sand blast cleaning, and strong pigs. Among them, the high-pressure water jet cleaning is mostly used for removing carbonized scale or hard scale, and for the dirt which cannot be removed by relying only on the impact force and relies on heat to loosen it, steam jet cleaning is used.

(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. Common methods include rotary spiral method, liquid-solid fluidization method, rotary link method, spiral spring vibration method, and sponge rubber ball online cleaning method. There are many types of inserts. The sponge ball method is to squeeze a sponge ball with a diameter slightly larger than the inner diameter of the tube into the tube for descaling. A wire brush can also be used to clean the dirt with lower hardness.

Chemical cleaning

Chemical cleaning is the use of a chemical cleaning solution to produce a chemical reaction that dissolves, detaches or peels off scale and other deposits on the surface of the heat transfer tubes of the graphite heat exchanger.

The method has the advantages of short cleaning time, simple operation, thorough descaling, and is one of the widely used and effective cleaning methods. Chemical cleaning can be done on site, labor intensity is lower than mechanical cleaning and cleaning is more complete, it can clean the places that mechanical cleaning can not reach, and can avoid mechanical damage caused by mechanical cleaning on the heat exchange surface; and chemical cleaning can be done without disassembly The equipment has advantages that cannot be compared with the shell-and-tube heat exchange equipment that cannot be disassembled.

Before cleaning, you should know the structure, material, distribution and thickness of the cleaning equipment and its composition, so as to reasonably choose the cleaning agent, corrosion inhibitor, and additives, and choose the appropriate amount of cleaning agent, concentration, speed. , temperature and time, should be done in the discharge of cleaning waste liquid to avoid environmental impact.

Physical cleaning

Physical cleaning is the use of various mechanical external forces and energy to smash the dirt, separate and peel off the surface of the object to achieve the cleaning effect. Common methods include ultrasonic descaling, PIG pigging technology, and electric field descaling technology. Ultrasonic descaling utilizes the cavitation effect, activation effect, shearing effect and suppression effect of ultrasonic waves to achieve the effect of descaling. The key to ultrasonic descaling technology is to choose the appropriate ultrasonic power and frequency and the temperature of the cleaning solution.

Microbial cleaning

As the HRT increases, the COD removal rate gradually increases. When HRT>5min, the COD removal rate is basically stable, and the COD removal rate reaches about 75. In the electrochemical reactor, the collision and growth opportunities of the particles are greatly increased due to the flow of the fluid and the agitation of the gas. The average bubble size produced by the electrical float is 20-70 μm, which has a relatively large specific surface area, so that the floc can be provided with more adsorption and bonding centers, and the gas inside the floc is more favorable for the flocs to float. Therefore, a satisfactory processing effect can be obtained in a short time.

Current intensity

Washing wastewater turbidity, COD and MBAS removal rate and current intensity. As the current intensity increases, the removal rate of these indicators gradually increases.

According to Faraday's law of electrolysis, the electrochemical dissolution of Al and the electrolysis of water are directly proportional to the amount of electricity supplied (I/t). When passing 1F (26.8 Ah), 9 g of Al3 can be theoretically dissolved, and 0.0224 Nm3H2 and O2 can be released, which is much larger than the amount of gas released in DAF. At the same time, the current intensity can be increased to obtain smaller bubbles, which is very advantageous for the air flotation separation process.

Combining electrocoagulation, electro-floating and electrochemical oxidation, integrating electrocoagulation to produce high-efficiency flocculation of Al3 and its hydrolyzed polymerization products, flotation of extremely small bubbles generated by insoluble electrodes and electrochemical oxidation of catalytic oxidation electrodes Role, developed a new type of electrochemical reactor. The reactor is used to treat laundry wastewater, which can effectively remove surfactants, SS, COD and phosphate in wastewater.

How often do you clean it?

In the process of industrial production, there are many conditions that will constitute the graphite heat exchanger or pipeline fouling and blockage, affecting the heat transfer of the graphite heat exchanger. During the heat exchange process, the cooling water forms a solid scale on the surface of the graphite heat exchanger. It affects the heat exchange effect. The lack of flow and the depressurization of the cooling water during severe weather will make the production unable to operate normally. In order to save the cleaning cost, some companies will think about cleaning when the graphite heat exchanger is severely scaled and affected the production. However, it is not known that the graphite heat exchanger will damage the equipment under the condition of load operation and increase the failure rate of the equipment. The spiral plate graphite heat exchanger is primarily used in the application of plate heat exchangers, such as cleaning, because there may be a variety of dust in the plate graphite heat exchanger, such as oil residues. There are many kinds of asphalt and fat, hydrocarbon deposits, etc., and these dusts are all cleaned up.

The cleaning cycle of the graphite heat exchanger should be determined according to the degree of fouling of the equipment. If the heat transfer of the graphite heat exchanger is not ambiguous, the heat transfer can not meet the production demand, the energy consumption increases, and the graphite heat exchanger has been used for a long time. When the temperature of the cooling water is poor, and the like, it is time to think about cleaning. When cleaning, you must find a professional cleaning company.

Clean the graphite heat exchanger to keep the equipment running, return to production, and prevent dangerous attacks.