Necessity analysis of air compressor interstage cooler fin coating film Hangzhou Hangyang Heat Exchange Equipment Co., Ltd. / Feng Miagen Li Jingjing At present, in the air compressor interstage cooler, in order to enhance heat transfer, improve the heat transfer coefficient of the gas side Various types and specifications of fins are being used as heat transfer elements in large quantities; at the same time, in order to reduce manufacturing costs and increase market competitiveness, fin materials are also using aluminum in large quantities instead of expensive copper materials due to aluminum fins. When working under wet conditions or dry and wet conditions, the surface will form oxide powder! ! :0), bringing about problems in the life of the cooler and compressor and environmental pollution. In addition, when working under wet conditions, the moisture in the air condenses and adheres to the fins to form a "water bridge", resulting in increased gas resistance and increased energy consumption. Fin surface coating treatment is an effective method to solve the problem. Surface coating treatment technology is a new technology developed in the 1990s. It is widely used in air conditioning heat exchangers, and the technology is becoming more mature, mainly with corrosion resistance. Both film coating treatment and hydrophilic coating treatment.
1.1 Corrosion resistance The metal aluminum itself can form a dense oxide film in the air. This oxide film protects the inner metal aluminum from corrosion, but the oxide film is an amphoteric oxide. In an acid, alkali or oxidizing environment, the oxide film is liable to cause rapid oxidation or electric corrosion, shortening the service life of the metal aluminum. When the air compressor is working, due to the action of condensed water, the interstage cooler fins are in wet condition, and when the machine is shut down, it is in dry condition. This wet condition or continuous alternating wet and dry conditions makes the surface untreated. The fins are easily corroded. When dry, the corrosion layer is cracked and peeled off. It is white powder (the main component is al2o3). This is called "white powder". It pollutes the environment and is harmful to health. Therefore, metal aluminum fins that have corrosion resistance requirements or are used under wet conditions and dry and wet conditions need to be surface treated to obtain high corrosion resistance.
1.2 During the operation of the hydrophilic air compressor interstage cooler, since the temperature of the fin surface is lower than the dew point of the air, the moisture in the air condenses on the surface of the fin, and the semicircular small water droplet adheres to the surface of the fin. When the fin spacing is small, the water droplets "bridge" between the fins to block the flow of air, so that the resistance and energy consumption are increased. To solve the "water bridge" problem, it is required that the fins have the ability to cause the attached condensed water to quickly flow away along its surface. Applying a hydrophilic coating on the surface of the fin to form a hydrophilic coating film, so that the water droplets spread quickly on the surface of the coating and flow away. This method is a trend of general research and development at home and abroad. The coating thickness of the surface is not rigidly specified, and the price of the coating is generally high, so the thinner the coating thickness is, the lower the production cost is, provided that the performance requirements are met. The thickness of the coating directly affects the performance of the coating, so the thickness of the coating on the surface of the aluminum strip is required to be consistent.
2.2 Coating Adhesion The adhesion of a coating is an indicator of the adhesion of an aluminum strip to its surface coating. If the coating adhesion is too small, the coated aluminum strip will be easily peeled off during further processing and use, which in turn affects the weight of the coated aluminum strip, so the coating adhesion is required to be as good as possible. Coating adhesion is generally only qualitatively tested. The main testing methods are friction resistance test, cross-cut test and cupping method.
2.3 Hydrophilic properties After a period of use, the hydrophilic aluminum strip will affect the hydrophilic properties of the surface due to various environmental factors. Therefore, the hydrophilic properties are generally classified into initial hydrophilicity and hydrophilic properties after environmental resistance test. The hydrophilicity is mainly measured by the contact angle a. The initial hydrophilicity of the general product requires "<1Â°, and the hydrophilicity requirement after the environmental test is a <25Â°. The contact angle a can be measured by using a special contact angle measuring instrument, or by calculating the area occupied by a certain amount of water droplets on the surface of the aluminum strip.
2.4 Corrosion resistance Corrosion resistance is mainly reflected in three aspects: First, it is resistant to alkali. Since the lubricant on the surface of the heat exchange sheet needs to be removed with an alkaline cleaning agent, the functional coating on the surface of the aluminum strip must have A certain degree of alkali resistance, generally required to soak in 20% sodium hydroxide solution for 3 minutes without foaming. The second is salt spray corrosion resistance. Generally, it is required to have no corrosion spots for 500 hours in a salt spray environment with a test condition of 35 Â° C and 3%. The corrosion resistance of salt spray is directly related to the service life of the heat exchanger. In coastal areas, due to the high salt content in the air, there is a high requirement for salt spray corrosion resistance of heat exchange sheets. The third is the resistance to humidity and heat, that is, the corrosion resistance which is placed for a long time at a certain temperature and a certain humidity. This property is also an important indicator for measuring the weather resistance of the heat exchange sheet.
The technology and manufacturing process of the fin coating film is mainly based on the substrate, after the anti-corrosion treatment of the bottom layer, and then the hydrophobic and hydrophilic coating process is carried out, thereby obtaining the required corrosion resistance and prolonging the use. Lifetime, which can effectively prevent condensation of condensation water on the surface of the fin, to reduce the air resistance of the composite film with good coating properties (as shown).
Hydrophobic coating layer Hydrophobic coating layer The bottom layer of the substrate is treated by a coating process. The typical coating process is as follows: (1) The bottom treatment layer: prior to the formation of the bottom treatment layer, priority is given to aluminum or aluminum. The surface of the alloy substrate is washed with an aqueous alkali solution, and the surface of the substrate is degreased in advance to improve the adhesion between the substrate and the underlying layer, and the fin material is also provided with corrosion resistance. Then, a chemical conversion treatment liquid is applied onto the surface of the substrate by a method such as spraying, and a chromium phosphate salt is surface-treated to form a primer layer composed of an inorganic oxide or an organic-inorganic composite compound.
(2) Hydrophobic coating layer: one of a resin solution of a polyurethane resin, an epoxy resin, a polyester resin, and a polyacrylic resin is applied and baked on the formed undercoat layer, at the bottom A hydrophobic coating layer is formed on the treatment layer. By forming such a hydrophobic coating film layer, the fin material can be prevented from coming into contact with the substrate by the dew condensation water penetrating the hydrophilic coating film layer in a humid environment under an acidic atmosphere, thereby suppressing aluminum oxide caused by oxidation of the substrate. Produce and improve corrosion resistance.
(3) Hydrophilic coating layer: A resin solution of a hydrophilic resin is applied and baked on the surface of the formed hydrophobic coating layer, and a hydrophilic coating layer is formed on the hydrophobic coating layer. The hydrophilic coating layer is composed of a hydrophilic resin containing a cross-acid group, a carboxyl group, and a hydroxyl group to exhibit hydrophilicity, thereby improving the fluidity of the dew condensation water on the surface of the fin material.
The fin material of the coated film layer is finally formed by the above-described underlayer treatment and coating treatment on an aluminum or aluminum alloy substrate.
For air compressor interstage coolers, laminated coolers are a common form of construction, while corrugated fins and slotted fins are the most widely used type of fin. When the surface temperature of the fin is lower than the dew point of the air, the air side will decompose, and the precipitated condensed water will adhere to the surface of the fin, thereby affecting the heat exchange effect and increasing the pressure drop on the air side. In order to reduce this adverse effect, the coating treatment is usually performed on the surface of the fin, and the coating layer can effectively improve the discharge of the condensed water and reduce the pressure drop on the air side.
Researchers have studied the air side characteristics of hydrophilic layer corrugated fins (as shown), louver fins (as shown) and slotted fins (as shown) under moisture conditions. .
The results show that after the surface treatment of the fins, there is almost no influence on the heat transfer performance on the air side, and the resistance characteristics on the air side are greatly improved. This is because the hydrophilic layer makes the condensed water condensed in the wings. The surface of the sheet forms a water film and flows down quickly, reducing the "water bridge"
The blocking effect causes the air side resistance to drop. The specific performances are as follows: (1) K. Hong and RL Webb found through experiments on louver and corrugated fins that the hydrophilic coated film fins are in wet conditions at 2.5 m/s oncoming wind speed. The pressure drop loss is reduced by 45% and 15%, respectively. Therefore, the influence of the coating layer on the louver fins is greater than that of the corrugated fins.] Zhang Yuanming and the like, through the seven hydrophilic layers of the corrugated finned tube The study of the air flow characteristics of the heat exchanger shows that the heat transfer and drag factors on the air side increase with the decrease of the fin spacing, but when the Reynolds number increases to a certain value, the resistance factor to the fin spacing The change is not sensitive.
When the pipe diameter increases, the heat transfer and drag factors are also affected by the fin spacing.
In 1987, a fin-and-tube heat exchanger with a hydrophilic layer was studied. It was found that the air-side heat transfer coefficient of the surface-coated heat exchanger and the uncoated heat exchanger is similar, but the pressure drop is Compared with heat exchangers without coating film, it is 20% smaller. (4) In 1997, Wang et al. carried out experimental research on 11 different structural parameters of flat fins and louver fins covering the hydrophilic layer. The results show that: in wet work Under the condition, the influence of the hydrophilic layer on the heat transfer characteristics of the heat exchanger is negligible, and the pressure drop loss is 15%~40% smaller than that of the heat exchanger without the hydrophilic layer. (5) 2000, Wang et al. A comparative experimental study was carried out on eight kinds of compact finned tube heat exchangers with a hydrophilic layer and a hydrophilic layer. The results show that the air side of the heat exchanger covering the hydrophilic layer is in the same inlet air state. The heat transfer coefficient is slightly lower than that of the heat exchanger without the hydrophilic layer, but when the fin pitch is reduced to 1.2 mm, the lower value will reach the heat exchange of the heat exchanger without the hydrophilic layer. 20% of the coefficient. (1) Under wet conditions, the resistance of the slotted fins increases more than in the dry conditions, so The fin spacing should not be too small, and corrugated fins can be considered at this time.
(2) When the fin heat exchanger is operated under wet conditions or dry and wet alternate conditions, a hydrophilic coating film may be added on the surface of the fin, which has little influence on heat transfer performance, but can greatly reduce wetness. Under the condition, the flow resistance of the air is better for the louver fins and the slit fins.
Designers should be fully aware of the fin form of the selected heat exchanger and treat it differently depending on the application.
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