The shell-and-tube heat exchanger is composed of shell, heat transfer tube bundle, tube sheet, baffle (baffle) and tube box. The shell is mostly cylindrical, with a tube bundle inside, and both ends of the tube bundle are fixed on the tube plate. Two kinds of cold and hot fluids that exchange heat, one flowing in the tube is called the tube-side fluid; the other is flowing outside the tube and is called the shell-side fluid. In order to improve the heat transfer coefficient of the fluid outside the tube, several baffles are usually installed in the housing. The baffle plate can increase the fluid velocity on the shell side, forcing the fluid to pass through the tube bundle multiple times in a prescribed way, and enhance the degree of fluid turbulence. The heat exchange tubes can be arranged in an equilateral triangle or square on the tube plate. The equilateral triangle arrangement is relatively compact, the fluid outside the tube has a high degree of turbulence, and the heat transfer coefficient is large; the square arrangement makes it easy to clean the outside of the tube and is suitable for fluids that are prone to fouling.
Each time the fluid passes through the tube bundle is called a tube side; each time it passes through the shell is called a shell side. The picture shows the simplest single-shell single-tube heat exchanger, referred to as 1-1 heat exchanger for short. In order to increase the fluid velocity in the pipe, partitions can be set in the pipe boxes at both ends to divide all the pipes into several groups. In this way, the fluid only passes through a part of the tube at a time, so it goes back and forth multiple times in the tube bundle. This is called multi-pass. Similarly, in order to increase the flow rate outside the tube, a vertical baffle can also be installed in the housing to force the fluid to pass through the housing space multiple times, called the multi-shell side. Multi-tube and multi-shell can be used together.
The materials of shell and tube heat exchangers are mainly carbon steel, stainless steel and copper. When the tube sheet made of carbon steel is used as a cooler, the welds between the tube sheet and the column tube often have corrosion leakage, and the leakage enters The cooling water system pollutes the environment and causes material waste.
In the manufacture of shell-and-tube heat exchangers, manual arc welding is generally used for the welding of the tube sheet and the column tube. The shape of the weld seam has different degrees of defects, such as depressions, pores, and slag inclusions. When in use, the tube sheet part is generally in contact with industrial cooling water, and impurities, salts, gases, and microorganisms in the industrial cooling water will constitute corrosion of the tube sheet and the weld, which is what we often say is electrochemical corrosion. Research shows that industrial water, whether it is fresh water or sea water, will have various ions and dissolved oxygen, and the changes in the concentration of chloride ions and oxygen play an important role in the corrosion shape of metals. In addition, the complexity of the metal structure will also affect the corrosion morphology. Therefore, the corrosion of the weld of the tube sheet and the column tube is mainly caused by pitting corrosion and crevice corrosion. From the outside, there will be many corrosion products and deposits on the surface of the tube sheet, with pits of different sizes distributed. Galvanic corrosion will also occur when using seawater as the medium. Chemical corrosion is the corrosion of the medium. If the heat exchanger tube sheet contacts various chemical media, it will be corroded by the chemical media. In addition, there will be some bimetallic corrosion between the heat exchanger tube sheet and the heat exchange tube.
In summary, the main factors affecting the corrosion of shell and tube heat exchangers are:
(1) Medium composition and concentration: The influence of concentration is different. For example, in hydrochloric acid, the greater the concentration, the more severe the corrosion. Carbon steel and stainless steel have the most severe corrosion in sulfuric acid with a concentration of about 50%, but when the concentration increases to more than 60%, the corrosion decreases sharply;
(2) Impurities: Harmful impurities include chloride ions, sulfur ions, cyanide ions, ammonia ions, etc. These impurities can cause severe corrosion in some cases
(3) Temperature: Corrosion is a chemical reaction. When the temperature is increased by 10°C, the corrosion rate is increased by about 1 to 3 times, but there are exceptions;
(4) Ph value: Generally, the smaller the pH value, the greater the corrosion of the metal;
(5) Velocity: In most cases, the greater the velocity, the greater the corrosion.
corrosion protection
In view of the anti-corrosion problem of cooling towers, the traditional method is mainly repair welding, but repair welding is easy to cause internal stress in the tube sheet, which is difficult to eliminate, and may cause the weld of the tube sheet of the cooling tower to leak again. Currently, most Western countries adopt the method of polymer composite materials for protection. Among them, the most widely used products are Mega Wah technology products. It has excellent adhesion performance, temperature resistance and chemical corrosion resistance. It can be used safely in a closed environment without shrinking, especially good isolation bimetallic corrosion and erosion resistance, which basically eliminates the corrosion of repaired parts Leakage provides a long-lasting protective coating for the cooling tower.
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