Why a shell and heat exchanger scaling?

- Aug 15, 2018-

First, the scaling mechanism and state of shell-and-tube heat exchanger:

Because shell-and-tube heat exchangers mostly use water as the heat transfer system of the heat carrier, some salts are crystallized out from the water when the temperature rises. Adhered to the surface of the heat exchange tube to form scale. The addition of a polyphosphate buffer to the cooling water can also cause scale formation when the pH of the water is high. The scale formed in the initial stage is relatively soft. However, as the scale layer is formed, the heat transfer conditions are deteriorated, the crystal water in the scale is gradually lost, and the scale layer is hardened and firmly adhered to the surface of the heat transfer tube. In addition, as with scale, when the operating conditions of the heat exchanger are suitable for the solution to produce rust. A scale layer formed by rust crystals can be accumulated on the surface of the heat exchange tube. When the fluid contains more organic impurities and the flow velocity of the fluid is small, some mechanical impurities or organic matter will also deposit in the heat exchanger to form loose, porous or colloidal dirt. 

Second, the shell and tube heat exchanger descaling and cleaning necessary:

The shell-and-tube heat exchanger is widely used in production and is also a typical partition wall heat exchanger. Its structure is mainly composed of shell, tube bundle, tube sheet, baffle baffle and head. The utility model has the advantages that the heat transfer area provided by the unit volume device is large, the heat transfer effect is good, the structure is simple, the operation flexibility is large, and the plurality of materials can be manufactured. But the heat exchanger is in use. Dirt deposits will inevitably appear on the heat transfer surface, fouling will occur, and the thermal resistance of the scale deposits will be higher, greatly reducing the thermal conductivity. At the same time, since the fouling reduces the fluid flow, the fluid medium flow resistance increases and the energy consumption increases. For many years, most of the shell-and-tube heat exchangers used by various units have problems in the scaling of pipes and casings, which have affected the use effect, resulting in a series of economic losses. According to statistics, the annual economic losses caused by the increase in energy consumption caused by heat exchanger fouling in China are as high as 10 billion yuan. 

Therefore, it is necessary to clean and descale the shell-and-tube heat exchanger. The quality of the heat exchanger operation and the length of time are inseparable from the daily maintenance, cleaning and maintenance. For fluids that are prone to fouling, cleaning and descaling can be carried out in a suitable manner at the specified time. In recent years, in order to better clean the heat exchanger fouling, we have made authoritative tests on the cleaning and scaling of shell-and-tube heat exchangers many times, providing customers with solutions. 

Third, the shell and tube heat exchanger fouling as long as the factors:

There are many factors affecting fouling. Fluid velocity, fluid flow state, composition and content of fluid components, and heat exchanger structure all have certain influence on the formation of fouling. From the application point of view, we only find out the main factors. In order to solve the scaling problem effectively. For a particular fluid, the main factors affecting the fouling of the heat exchanger are as follows:

Flow velocity of the fluid: In the heat exchanger, the influence of the flow rate on the fouling should also consider its effect on fouling and scale erosion. For all types of fouling, the increase in the rate of erosion caused by the increase in flow rate is greater than the rate of fouling. More significant, so the growth rate of fouling decreases as the flow rate increases. However, in the actual operation of the heat exchanger, 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. However, in the actual operation of the heat exchanger, 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. 

Temperature of the heat transfer wall: Temperature plays an important role in chemical reaction scale and salt crystallization. The increase of fluid temperature generally leads to an increase in chemical reaction rate and crystallization rate. This has an effect on the amount of fouling deposited, resulting in an increase in the growth rate of the fouling.

Heat exchange surface material and surface quality: for commonly used carbon steel, stainless steel heat exchangers. The fouling is only affected by the deposition of corrosion products. If a non-metallic material such as graphite or ceramic having good corrosion resistance is used, scaling is less likely to occur. The surface quality of the heat transfer surface material will affect the formation and deposition of dirt. The greater the surface roughness. The more favorable it is to the formation and deposition of dirt. 

Fourth, shell and shell heat exchanger scale composition analysis:

Crystalline scale: In a water-cooling system, scales are formed by crystallizing super-saturated calcium and magnesium salts from water crystals deposited on the surface of the heat exchanger due to changes in temperature and pH. 

Particle scale: accumulation of concomitant particles suspended in a fluid on a heat exchange surface.

Chemical reaction scale: Isotropic deposition due to chemical reactions.

Corrosive scale: the heat exchange medium corrodes the heat transfer surface, causing corrosion products to deposit on the heated surface to form dirt.

Bioscale: For the commonly used cooling water system, industrial water towels often contain microorganisms and their required nutrients. These microbial populations multiply, and their groups and their excreta form biofouling on the heat exchange surface with mud and the like.

Mixed scale: On the supercooled heat transfer surface, the highly dissolved components of the pure liquid or multi-component solution are deposited together. The above classification is only indicated. A process is a major process for the formation of such soils. Fouling is often the result of a combination of processes. Therefore, the actual dirt on the heat exchange surface is often mixed with a variety of soils.

5. Pipe and shell heat exchanger pipe cleaning and descaling method:

(1) Physical cleaning methods:

Jet cleaning method: Jet cleaning is a powerful cleaning method. It uses the flushing and spraying equipment to spray the medium with the extremely high impact force on the tube side and the shell side of the heat exchanger to achieve the purpose of descaling. Commonly used media are water, steam or quartz sand. For the dirt that cannot be removed by relying solely on the impact force and must rely on heat to loosen it, it is also known as the steam jet cleaning method.

Object tube insertion cleaning method: This method can only be used to remove the dirt inside the tube, which relies on the movement of the insert inside the tube. It is in contact with the inner surface of the pipe to achieve the effect of removing dirt. There are many types of inserts, such as a scraper or drill bit attached to the end of the flexible shaft. Wire brushes can also be used to clean low hardness dirt.

Physical cleaning For the dirt in the heat exchanger tube, we have used the above two heat exchanger cleaning methods. We believe that the effect of cleaning the inside of the tube is better than that of the direct jet cleaning, but the labor of the insert cleaning is relatively large. We tried to combine the two methods, using a high temperature water jet while cleaning with the insert. The result is greatly improved. However, the disadvantage is that the cleaning equipment needs to be assembled and disassembled, which is easy to cause mechanical damage to the equipment.

(2) Chemical cleaning methods: 

The cleaning equipment is connected with the shell-and-tube heat exchanger to form an independent water circulation system, and a high-efficiency chemical cleaning agent is added into the water storage tank, and the chemical cleaning can be completed on site, and the cleaning intensity is low. But the cleaning is more thorough, you can clean the places that mechanical cleaning can not reach. Mechanical cleaning can be avoided to cause certain mechanical damage to the heat exchange surface. Moreover, the chemical cleaning can be carried out without disassembling the equipment, and the shell-and-tube heat exchange equipment that cannot be disassembled has the advantage that mechanical cleaning cannot match.

(3) Ultrasonic descaling method

In addition to the above several common cleaning methods, according to the data. There is now a new heat exchanger cleaning technology - ultrasonic descaling. It utilizes the cavitation effect, activation effect, shear effect and inhibition effect of ultrasonic descaling. Ultrasonic descaling technology has been applied in some industries such as petrochemical and sugar production, and has achieved good results. There are still many blank areas of this new technology that need to be studied. The key to ultrasonic descaling technology is to select the appropriate ultrasonic power and frequency for different materials, different types of charging and heat transfer area.

Sixth, the importance of shell and tube heat exchanger cleaning:

Fouling is very harmful to heat exchangers and it is necessary to actively prevent and periodically remove descaling. There are many factors that affect scaling. Only by finding out the main factors can the problem be simplified. For different types of scales, targeted cleaning and descaling measures should be taken to effectively control the scaling problem of the heat exchanger to improve the heat transfer efficiency and operating life of the heat exchanger.