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What is an air cooler

What is an air cooler?

An air cooler is the abbreviation of air cooler, which is the most widely used heat exchange equipment for condensation and cooling in petrochemical industry and oil and gas processing. Air cooler is generally composed of tube bundle, tube box, fan, shutter and frame.

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Air cooler is a kind of equipment which uses ambient air as cooling medium and sweeps outside the finned tube to cool or condense the high temperature process fluid in the tube. It is also called “air cooled heat exchanger” for short.
Air cooler, also known as fin fan, is often used to replace the cooling medium of water-cooled shell and tube heat exchanger.
With the increasing use of induction heating equipment, due to the different conditions of users, some do not pay enough attention to the water cooling system of the equipment, and many users do not use distilled water according to the regulations, but use ordinary well water or tap water. In practical work, the water quality has a great impact on the water cooling system and components of the equipment.

Main classification of air cooler

Air cooler can be divided into the following different forms because of its different structure, installation form, cooling and ventilation mode.

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  • a. According to the arrangement and installation form of tube bundle, it can be divided into horizontal air cooler and inclined top air cooler. The former is suitable for cooling, while the latter is suitable for all kinds of condensing cooling.
  • b. According to different cooling methods, it can be divided into dry air cooler and wet air cooler. The former relies on continuous air supply by fan; The latter is enhanced by water spray or atomization. The latter has higher cooling efficiency than the former, but it is not widely used because it is easy to cause corrosion of tube bundle and affect the service life of air cooler.
  • c. According to different ventilation modes, it can be divided into forced ventilation air cooler and induced ventilation air cooler. The former fan is installed at the lower part of the tube bundle, and axial flow fan is used to supply air to the tube bundle; The latter fan is installed on the upper part of the tube bundle, and the air flows from top to bottom. The latter consumes more power and costs more than the former, and its application is not as common as the former.

Application scope of air cooler

It is mainly used in petroleum, chemical industry, metallurgy and other air coolers, and has the advantages of large air volume, high efficiency, small vibration, low noise, convenient use and maintenance.
There are different types of fans, such as high efficiency and energy saving, low noise, anti-corrosion and so on. The air volume regulation mode of fans can be divided into three types: self-regulation, semi self-regulation and manual regulation; Cooling methods can be divided into blast type and induced draft type.

Main advantages of air cooler

Improve the service life of electron tube (high frequency equipment). The common water is easy to scale and needs to be cleaned regularly. Once the scale is formed, the emission power and life of the electron tube will be affected. In serious cases, the anode of the electron tube is easy to burn through.

  1. Save electricity( Taking 100kW high frequency electronic tube equipment as an example, the power consumption of anode water circuit to the ground under DC high voltage is about 8kw for ordinary water and 70W for distilled water.
  2. Reduce electrolytic corrosion. The high voltage of anode forms direct current to ground through water resistance. The corrosion rate of common water is more than 100 times that of distilled water.
  3. The water storage tank of the air cooler is about 0.4 ~ 2.0m3 according to the power of the equipment, and the price of pure water per ton is between 250 yuan and 300 yuan. Because of the use of closed cycle, the consumption of distilled water is very low, which creates conditions for the use of distilled water. At the same time, if antifreeze products are added according to a certain proportion, the problem of freezing in winter of high-frequency waterway can be completely solved. The use of distilled water not only improves the service life of equipment and components, but also reduces maintenance costs, so as to achieve the purpose of improving productivity and saving electricity.
  4. Through the heat exchange between air and softened water, the air cooler discharges the heat from electric furnace inductor, silicon controlled rectifier, electronic tube and other electrical components into the air. The heat transfer tubes with high thermal conductivity form high heat flux after combination, so the cooling efficiency of the device is very high. Air continuous heat exchange is used to transfer the heat to the air and remove the accumulated heat, which meets the temperature requirements of the induction heating system.

The cooling system uses distilled water in the air cooler and induction heating equipment closed-circuit circulation, the water quality is clean, so that the electrical system is cooled for a long time without scaling, not blocked by foreign matter. In winter, antifreeze can be added into the cooling water, which can eliminate the failure of the water way being damaged by freezing and reduce the failure of the induction heating equipment. The cooling system does not need a pool and a cooling tower.

Main structure of air cooler

Coil

There are two kinds of coil materials of high efficiency air cooler: aluminum tube and aluminum sheet, steel tube and steel sheet. The expansion process is adopted between the fin and the heat exchange tube of the aluminum tube and aluminum sheet air cooler, which has the advantages of close contact, small contact thermal resistance, high heat transfer efficiency and good anti-corrosion performance.
The integral hot-dip galvanizing process is adopted for the fin and heat exchange tube of steel tube and steel sheet air cooler, which makes the segment contact more closely, has small contact thermal resistance, high heat transfer efficiency and strong anti-corrosion ability. The fins are integral corrugated porous aluminum or steel.
The fin combination of fixed pitch (8,10,12mm) and variable pitch (8 / 16,10 / 20,12 / 24mm) can be used. The fin is a combination of deep extension sleeve and fixed distance fin, which has the advantages of accurate positioning. The coil can be delivered after 2.0MPa air pressure test.

Fan

The cooling fans all use the special axial flow fan for cooling fan, which has the characteristics of high efficiency, low noise and long service life. The blades are made of die-casting aluminum alloy wing type forward swept twisted blades, with high strength and clean surface. Use temperature – 40 ℃ ~ – 50 ℃.

Tube shell

There are four kinds of high quality steel plate spray paint or electrostatic spray paint, stainless steel plate and aluminum plate to choose from. Fan panel adopts unique design, strong rigidity, beautiful appearance, good anti-corrosion performance and long service life.

Common corrosion types of air cooler tube bundle

(1) Pitting corrosion, the pressure bearing material of air cooler tube bundle in the medium containing dissolved oxygen and harmful anions (mainly Cl ions), after a certain period of time, most of the surface does not corrode or the corrosion is light, but there are holes or pits in individual points or small areas on the surface, with the passage of time, the corrosion holes continue to develop in depth, forming small hole like corrosion pits, It is pitting corrosion, because the serious pitting can make the equipment perforated, so it is also called pitting corrosion or pitting corrosion.
In the design and manufacture of air-cooled heat exchanger tube bundle, the important factors include:
a. Effect of aging temperature of heat treatment: for stainless steel tube bundle, generally the main pressure parts are not heat treated after welding, but austenitic stainless steel has the best pitting corrosion resistance after solid fusion treatment. For other stainless steel materials, the precipitation phase will be produced by annealing or tempering at some temperatures, which will increase the tendency of pitting corrosion.
b. With the improvement of the surface finish of the tube bundle metal material, the pitting corrosion resistance will be enhanced, but the pitting corrosion resistance will be reduced when the cold work hardening occurs on the metal surface. Therefore, the evaluation of pitting corrosion test for non commonly used materials and service conditions can be generally divided into chemical immersion method, electrochemical measurement method and field test method. The evaluation of pitting corrosion resistance includes pitting depth, pitting density, corrosion rate, corrosion area, etc. The corrosion rate is combined with the distribution, shape, size, density and depth of the etched holes.
(2) Interstitial corrosion is caused by the formation of a small gap (generally 0.025-0.1mm) between metals or between metals and nonmetals, the heat exchange medium remains in the gap, and there are harmful anions in this medium. The corrosion results in the decrease of material strength, the increase of local additional stress and the decrease of material bearing capacity.
In the air-cooled heat exchanger, the gap corrosion usually occurs at the plate lap, flange sealing surface connection, gasket sealing, between the base tube and the liner, or rust layer, among which the gap corrosion is most likely to occur between the base tube and the liner. Therefore, in product design, the base tube and liner are often expanded in the whole process to eliminate the gap between them.
For air-cooled heat exchanger, the sealing structure of screw plug is usually adopted, which is sealed by metal gasket. The two sides of gasket contact with screw plug and tubesheet respectively. There is a certain gap in the micro level, and the gasket and tubesheet are subject to heat exchange medium corrosion. Due to pressure, temperature, vibration and other factors, there is gap corrosion at the sealing tip, which easily leads to gasket seal failure.
(3) Erosion corrosion, including erosion and abrasion corrosion, is a kind of metal damage caused by high-speed relative motion between metal surface and fluid medium. The eroded metal surface is generally in the shape of groove, concave Valley and tear drop, and the surface is bright without corrosion deposit.
Compared with the corrosion under other stresses, the influence factors of erosion corrosion are more complex. In addition to the chemical composition, microstructure, mechanical properties, surface roughness and corrosion resistance of the material itself, it also has a great influence on the medium temperature, pH value, dissolved oxygen content, solid particle size and hardness, as well as the shape, structure, flow velocity and flow pattern of the flowing parts.
In the air-cooled heat exchanger, the most vulnerable part to erosion corrosion is the base tube and tube sheet welding. Because the base tube and tubesheet are generally single-layer or double-layer welding, and the height of the welding leg is between 1.5 ~ 2mm, long-time erosion will make the weld thinner until leakage. Therefore, reasonable design of the heat exchanger structure can effectively reduce the erosion corrosion rate, Usually, the flow path of fluid is changed in the form of return pipe box, or the flow stop band is added in the base pipe. The flow stop band can be used as a buffer component to reduce the flow rate of medium and change the flow process of medium.
(4) Intergranular corrosion is a kind of local selective corrosion of metal materials along grain boundaries in a specific corrosive medium. Grain boundary is the boundary between different grains. Due to the different orientations of the grains, the arrangement of atoms at the junction must gradually transition from one orientation to another. Therefore, the grain boundary is actually a structural defect with incomplete “surface”.
The conditions of intergranular corrosion are as follows:

  • a. There are impurities in the metal or alloy, or the second phase precipitates along the grain boundary. 
  • b. Due to the difference of chemical composition between grain boundary and grain, the corrosion cell is formed in suitable medium. The grain boundary is anode and the grain is cathode, and the selective dissolution occurs at the grain boundary. 
  • c. There is a specific corrosive medium. Severe intergranular corrosion often occurs in some alloy medium systems. For example, austenitic stainless steel may have serious intergranular corrosion in weak oxidizing medium or strong oxidizing medium.

Intergranular corrosion test method and its evaluation method: at present, there are five basic methods listed in the standard: oxalic acid electrolytic etching method, boiling 65% nitric acid method, boiling sulfuric acid ferric sulfate method, boiling sulfuric acid copper sulfate method and nitric acid hydrofluoric acid method.
(5) Stress corrosion, the damage of materials caused by the combined action of stress and corrosion environment, is called stress corrosion. As a brittle failure mode, it has great harm. The damage of materials under the single action of stress factors belongs to mechanical fracture or mechanical fatigue fracture; When the material is corroded by medium alone, it belongs to environmental corrosion. Under the joint action of stress factors and corrosive environment, the corrosion damage leads to fracture and cracking, which becomes corrosion cracking.
The most common corrosion cracking is hydrogen sulfide stress corrosion cracking (SSCC), which refers to the brittle fracture of pressure bearing material of tube bundle under tensile stress in sulfide medium:

  • a. The metal has the tendency of stress corrosion cracking in this medium;
  • b. The structure composed of pressure bearing materials is in contact or in selective corrosive medium;
  • c. The tensile stress is higher than a certain level.

Corrosion rate test of air cooler

Corrosion rate is an important index to evaluate the corrosion resistance of metals. In the process of metal corrosion, the weight, structure, dimension, surface state and mechanical properties of metal will change to some extent. The macro and micro change rate can be used to reflect the degree of metal corrosion, so there are different corrosion rate test methods. The common methods for air cooler tube bundle are resistance method, weight loss method and linear polarization method.
The resistance method is based on the principle that the cross-sectional area of metal sample is reduced due to corrosion, which leads to the increase of resistance. This method can be used to continuously monitor the corrosion status of the equipment during the operation of the air cooler tube bundle. It can accurately reflect the corrosion rate and its changes at various stages of the equipment operation, and can be applied to various media. The resistance method is fast, sensitive and convenient, and can be used to monitor the corrosion rate at high conditions.
Linear polarization method is a very suitable method for monitoring. It has the characteristics of high sensitivity and fast response to the changes of corrosion conditions. It can obtain instantaneous corrosion rate and reflect the changes of tube bundle working conditions in time. However, this method is based on the steady-state conditions to obtain the corrosion rate, so the measured object is uniform corrosion or overall corrosion, which is not suitable for local corrosion.
Weight loss method is a common method to determine corrosion rate, which is simple and reliable. Generally, coupon test is adopted. The material of coupon is consistent with the pressure bearing material of the main body of air-cooled heat exchanger. The shape and size of coupon can be determined according to the sample and the test conditions and needs. Before the test, the surface of the coupon should be treated. First, it should be polished with sandpaper, degreased with acetone and other solvents, washed with distilled water, and dried at 50 ℃. The coupon was immersed in the corrosion solution, and the corrosion rate was measured by detecting the weight of the coupon.

Corrosion protection measures of air cooler

Reasonable selection of materials

Correct material selection is one of the key links to control corrosion, and the following principles should be followed:

  • (1) The corrosion resistance of the material should meet the requirements of the service environment of the equipment. According to the corrosion type, sensitivity, corrosion rate and other characteristics of the medium, the service environment should be analyzed and the material selection should be controlled reasonably;
  • (2) The physical properties, mechanical properties and processing performance of materials shall meet the design and processing requirements of equipment, such as necessary strength, hardness, impact toughness, fatigue performance, heat resistance and weldability. For air-cooled tube bundle, nickel based alloy and duplex stainless steel are usually used in hydrogen and sulfur medium. Duplex stainless steel is a new type of steel with excellent corrosion resistance, high strength and easy manufacturing and processing. Its physical properties are between austenitic stainless steel and ferritic stainless steel, It is closer to ferritic stainless steel and carbon steel.

Reasonable design of structure

Product design must start from the perspective of corrosion protection, according to the material and the environment, strictly calculate and determine the use of stress, reasonable anti-corrosion structure design. It needs to pay attention to the following points: 

  • (1) simple shape;
  • (2) Prevent ponding and dust accumulation;
  • (3) Try to avoid sharp corners, grooves and gaps;
  • (4) Try to avoid metal connection with large potential difference;
  • (5) Avoid superposition of stress, assembly stress and residual stress in the same direction.

Manufacturing process improvement

Materials in processing and assembly engineering will also cause corrosion or leave corrosion hazards,
For example:

  • (1) The machining process is easy to produce residual stress;
  • (2) The heat treatment specification should be carefully selected to avoid intergranular corrosion, stress corrosion and hydrogen embrittlement caused by improper heat treatment. The heat preservation in the sensitized temperature zone should be avoided as far as possible. For the heat treatment process that may produce large residual stress, measures should be taken to eliminate the residual stress;
  • (3) In order to reduce the stress after welding, attention should be paid to the design of welding sequence to minimize the deformation of the workpiece. After welding, the residue at the weld should be cleaned in time to avoid local corrosion;
  • (4) Degreasing and cleaning treatment should be carried out after the welding and processing of the workpiece, so as to avoid the corrosion of the workpiece at night. Under specific working conditions, nickel phosphorus plating method can be used for internal anti-corrosion treatment of the air cooler tube bundle;
  • (5) During the equipment assembly, it should not cause excessive assembly stress, improve the accuracy and reduce the tolerance in the design, and adopt reasonable assembly methods to avoid stress concentration.

Conclusion

Only by mastering the corrosion types of equipment, controlling them effectively and taking protective measures, can the safe and stable operation of air cooler tube bundle be effectively guaranteed and the service life be improved. Therefore, the corrosion research of materials and equipment is one of the important research topics in the industry.

Source: Network Arrangement – China Steel Pipe 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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