Analysis and countermeasures of failure of compressor intercooler
An investigation of the interstage coolers of gas compressors used in the petrochemical and chemical machinery industry found that although my country has listed compressors as one of 21 key energy-saving products, and most manufacturers have also formulated standards and have serialized production, The cooler associated with it is still very messy. The main reason is that there is no complete series of products that are matched with the mainframe. Professional manufacturers who do not match the mainframe are not paying attention to their special research work.
As an important auxiliary equipment for the compressor, the heat transfer performance of the interstage cooler not only affects the energy consumption of the entire compressor, but also the poor cooling effect also affects the lubrication and safety of the equipment. There have been many reports on the waste of water and electricity caused by poor cooling, and even accidents that lead to the burning of the cooler. Investigation and research on the gas compressors purchased from abroad by large petrochemical companies in recent years have also found that the compressors produced by some famous manufacturers in the world There are also many problems with the inter-stage cooler of the machine. While its design philosophy attaches great importance to compactness and light weight, it ignores the durability and sufficient operating life of the cooler. This article will analyze it through two concrete examples, and propose improvement countermeasures at the same time.
1 The heat transfer element form of the foreign compressor intercooler 1.1 Smooth tube The material of the smooth tube is carbon steel, stainless steel, copper, etc. The gas has a tube pass or a shell pass. This classic design can still be seen in the intercoolers of large compressors purchased in recent years. Maoming Petrochemical Company purchased a hydrocarbon gas compressor intercooler manufactured by Ketema in the United States in 1995, and its heat transfer tube is also a smooth tube. Many nitrogen fertilizer plants originally imitated Soviet-made large H compressor intercooler heat transfer tubes are also made of smooth tubes. For an intercooler with gas passing through the tube, if a heat transfer tube with externally rolled internal ribs, such as a threaded grooved tube or a cross-grained tube, is used, the heat transfer area can be reduced by 20-30 compared with the use of a smooth tube, and the pressure drop can be controlled at Within 0.1 MPa.
1.2 Low-rib tube, mid-rib tube The heat transfer element of the air compressor intercooler produced by Hitachi, Japan is a low-rib tube. This type of tube has also been used in some domestic units, and it is rolled with a tube with a wall thickness of 2.5 to 3 mm.
Because the heat transfer resistance of air-water heat exchangers is on the air side, for shell-and-tube intercoolers that are designed with water in the tube and air out of the tube, the use of this tube can not only increase the air side of the tube Since the fin and the tube wall are a whole, the heat transfer effect of the smooth tube is improved by 50-60. Compared with the intercooler with the tube through the fin, it can also avoid the loosening of the fin tube. The problem of increased contact thermal resistance, and the compactness of the heat exchanger is also superior to that of the tube-through fin heat exchanger.
However, the ribbing coefficient of this low-ribbed tube is about 2.5. If rolled into a middle-ribbed tube with a height of more than 3 mm, the required wall thickness of the embryo tube is between 2.5 and 3 mm, which is due to the large number of consumables and the complexity of the process. The main shortcoming, so it was not accepted by the manufacturer later.
1.3 The purpose of the tube passing through the fin cover is also to increase the heat transfer area on the air side. Because the projected area of the elliptical tube or the drip-shaped tube in the airflow direction is small, the air circulation area is large, the pressure drop is small, and the heat transfer effect is good, so The intercoolers of some foreign compressors also adopt this tube type, and the rectangular sleeves are tightly sleeved on the tubes by means of thermal expansion sleeves. The domestically produced DA 350 series centrifugal compressor is a Soviet-like product in the 1960s. There are two problems. The cold pressing of the round tube into an elliptical tube will cause microcracks to occur at the transition between the circle and the ellipse after deformation. The crack compressor technology expanded and leaks occurred. The reason for the poor cooling capacity of this kind of cooler is related to the copying of the set when imitating it at that time. Because this kind of cooler is designed according to the higher latitude of the former Soviet Union, and my country's latitude is lower, especially in the southern region, where the inlet water temperature is higher in summer, which leads to the poor effect of the cooler. At present, there are still nearly 100 such units in operation in China. Therefore, the Ministry of Chemical Industry decided to replace this finned cooler with a huge smooth tube cooler.
1.4 chrysanthemum-shaped finned tube Ingersoll Rand centrifugal compressor produced in the United States is the choice of this tube type intercooler. There are two types of cooling water inside the tube, air outside the tube and cooling water air outside the tube, and the band-shaped winding sheet is used to increase the heat transfer area on the air side. However, the flow resistance of this kind of heat exchanger is very large. When Re=10 000, the flow pressure drop per meter of tube length is as high as 8000 Pa.
1.5 copper tube sleeve aluminum integral fins. In this heat exchanger cooling water pipe, the gas flows between the fins to exchange heat, and the ribbing coefficient is between 10-12.
The small diameter copper tube and the aluminum sheet are tightly combined after being expanded and connected. Since the linear expansion coefficient of aluminum is 3 times higher than that of copper, the punching process of the aluminum sheet orifice is required to be high, and it is required to be punched into a double-turn There should be no cracks at the flange. When the pipe is expanded, the aluminum hole has a shrinking tightening force. Since the air inlet temperature of the air compressor intercooler is mostly around 150°C, the interference of the expanded pipe should consider the linear expansion coefficient of these two metals. Loosening caused by differences.
The air film thermal resistance of the heat sink accounts for 80 of the total thermal resistance. The water film thermal resistance only accounts for 10. In order to enhance heat transfer, the fins will use wavy fins or slits, or expand the heat transfer area on the air side . Reducing the fin distance will inevitably increase the heat transfer area on the fin side and improve the compactness of the heat exchanger. However, a too small fin distance will inevitably increase the flow resistance, and even cause the boundary layers of two adjacent fins to overlap. Or cause the condensation of compressed air to precipitate and cause bridging. At this time, not only will the air flow resistance increase, but also the heat transfer surface of the fin will not be in contact with the cooled gas due to the condensation water covering the heat transfer surface of the fin, and the heat transfer function will be lost. Basic heat transfer formula: K total heat transfer coefficient, Wm logarithmic average temperature, K air side heat supply coefficient, Wm total heat sink efficiency δ tube wall thickness, mm air side total heat transfer area, mλ pipe thermal conductivity, WmK Heat transfer area on the water side, m heat transfer coefficient on the water side, Wm. For this tube-fin heat exchanger, since δλF is relatively small to be negligible, Equation 2 is simplified to η under normal conditions. It can be seen that the cooling of the intercooler The performance of 85 to 90 is determined by the heat transfer coefficient of the air side. The convective heat transfer coefficient of the air side of the fin material, the size of the thermal resistance of the contact gap between the fin and the tube, the size of the heat transfer resistance caused by dirt or water film on the fin, etc., directly affect the heat transfer performance of this intercooler .
2 Reasons for failure of the centrifugal air compressor intercooler of JOY company in the United States. The model of this machine is TA 48, which is a stage electric centrifugal compressor. The outlet pressure is 0.7 MPa and the compressed gas flow rate is 8 000 Nm. There are two inter-stage cooling. The cooling device is placed under the main engine. The structure of the unit is arranged 2. The core of the cooler is a copper tube through an integral aluminum fin. The copper tube used is 10×0.5 mm, the thickness of the fin is 0.2 mm, and the pitch is 1.52 mm. There are a total of 198 copper tubes, the ribbing coefficient is 10, the water side is 6 passes, the water flow speed is 2 ms, the circulating cooling water is used as the cooling medium, and the compressed air is used as the process air for the polypropylene vehicle.
According to the plant's process and equipment management personnel, since the start of the installation in 1996, the heat exchange effect of the intercooler of the TA 48 unit has declined after a period of operation, and it has been damaged and scrapped after more than 10,000 hours of operation. The main reason is that the aluminum fins of the heat exchanger core are severely oxidized and corroded. At the same time, the lubricating oil is coking at high temperature due to insufficient cooling, and some pipes also leak due to corrosion. The reasons for the failure of the intercooler of the TA 48 unit will be analyzed below.
2.2 Analysis of Causes of Failure of TA48 Unit Intercooler Although the intercooler of this unit is highly compact, due to the small distance between the sheets, when the compressor is working under conditions of high relative humidity, especially in Guangdong, there are 3 times per year. In the rainy season of more than a month, after the air compressor technology is compressed and cooled, when the gas temperature is lower than its dew point temperature, the moisture in the air will be precipitated. In addition, the air in the chemical plant contains SO and other corrosive gases and condensed water. It not only covers part of the heat exchanger surface, increases the heat transfer resistance of the heat exchanger, but also corrodes the fins, causing large-area damage to the fins. The following is a more advanced analysis from these two aspects.
When the fluid flows between the fins, its flow boundary layer and heat transfer boundary layer. 3. When the fluid is forced to flow in the X direction, the continuity equation of the fluid can be written. Momentum and heat transfer equations. When the fluid flows in the X direction, the equation It can be simplified as follows. The boundary condition is y = 0V. When the operating Re number of the fluid is Re 2500, the velocity distribution of the boundary layer is V, so that the thickness of the flowing boundary layer is when the operating Re number of the fluid is Re 10 At 000, the velocity distribution of the boundary layer conforms to the law of 17. It is obtained that because the flow boundary layer and the heat transfer boundary layer have the following relationship for air P = 0.699, the thickness of the heat transfer boundary layer δx can be considered for the TA 48 unit , The thickness of the boundary layer at different Re numbers is as 1.
The thickness of the flow boundary layer and the heat transfer boundary layer. The flow core area mm can be seen from the above. When the gas velocity is low, the heat transfer boundary layer between the fins is easy to overlap, especially when there is condensation on the fin surface. When the gas flow rate is larger, the boundary layer overlap will also occur, which greatly increases the heat transfer resistance. Therefore, it can be said that too small fin spacing is the main reason for the insufficient heat exchange capacity of the intercooler. Of course, too small fin spacing is the main reason for condensate bridging. The reason for bridge bridging by condensation will be analyzed below.
The maximum cooling temperature and dew point temperature of the compressor are related to the relative humidity of the inlet gas parameters and the discharge pressure. It can be found from 4 that when the inlet temperature of the TA 48 unit is about 20°C and the relative humidity is between 80 and 90, its recooling temperature is between 40 and 43°C. We can also further calculate the total humidity in the formula of H, H'absolute humidity before and after compression, the saturated vapor pressure of water at the inlet and outlet temperature of kgkg dry air gas, and the mass flow rate of Pa G gas. , The boiling point of kgh water and the latent heat of vaporization of saturated vapor water at the boiling point of water, the absolute humidity of the gas in the saturated state of Jmol φ the relative humidity of the air, the inlet pressure of the compressed gas, and the gas constant of Pa R can be obtained by the above calculation. The result of 2.
Dew point temperature and total exhaust moisture of the intercooler, inlet temperature, relative humidity, cooling outlet temperature °C, dew point temperature, total moisture exhaust, and the above data. It can be seen that the total exhaust moisture of the gas compressor is quite large, especially in Jiangnan During the rainy season in South China from March to May, the cooling water temperature is between 27 and 30°C, and the relative humidity in the air is between 80 and 95. However, the intercooler fin spacing of the TA 48 unit is 1.52 mm, which is condensation. Dew provides bridging conditions. In addition, the air contains SO and other corrosive gases. SO and water work together to severely corrode aluminum fins and copper tubes. On the one hand, the heat transfer performance of the fins is reduced quickly, and on the other hand, it may cause the leakage of the copper tube. The precipitation of condensed water will not only corrode the equipment, but also can form a water film on the fin surface, thereby increasing the thickness of the heat transfer boundary layer, thereby increasing the heat transfer resistance, making the heat transfer performance of the cooler drop quickly. Therefore, it can be said that the root cause of the TA 48 unit failure is that the fin spacing is too small, and the direct cause of its failure is the precipitation of condensation.
2.3 Treatment countermeasures According to the above analysis, the fin spacing is too small is the root cause of the damage and failure of the intercooler of the TA 48 unit. The calculation shows that the fin spacing is between 2.5 and 3.5 mm is the best choice.
In addition, the precipitation of condensed water is the most important and direct cause of damage to the heat exchanger and low heat exchange efficiency. Starting from these two basic points, we propose the following improvement measures for the TA 48 unit intercooler: 1 Increase the fin pitch to 2.5 mm 2 Choose aluminum fins with better anti-corrosion effect 3 The fin surface is treated with a hydrophilic film 4 Use double-flanged wavy fins.
3C902A1B Roots air compressor intercooler failure reason analysis and countermeasures Compressor technology C902A1B is the product of Italy ROBUSOM 1 factory. The compressor has a displacement of 3806 Nm h and a discharge pressure of 0.94 M Pa. It adopts stage compression and is equipped with two intercoolers. The core of the cooler adopts a copper tube to pass through the integral fin, and its structure is also a copper tube to pass through the fin. The structure of the cooler is shown, and 6 is the core of the cooler.
The following problems occurred shortly after the C902AlB unit was put into use. From the real photos of the heat exchanger core removed on site, it can be seen that the manufacturing process is rough, and it can be seen that the U-shaped tube end has been corroded and the traces left by the factory with epoxy resin plugging.
3.2 C902A1B unit intercooler failure analysis can be seen from 5 and tube fin parameters, C902A1B unit intercooler and TA 48 unit have many similarities. If copper tubes are used to pass aluminum fins, the compressed working medium is air, the exhaust pressure is equivalent, and the fin spacing is basically the same. Therefore, the reasons for their failure are basically the same, that is, the intercooler fails if the fin spacing is too small. The root cause, and the precipitation of condensate is one of the direct reasons leading to the failure of the intercooler. Of course, the C902A1B unit also has other design defects. For example, the heat exchange tube adopts a U-shaped tube. Due to the low flow rate of the cooling water, the tube is prone to fouling, and it is difficult to clean it after fouling. Also, due to the use of highly ribbed fins for heat transfer, even though the air-side heat transfer coefficient is small, the total thermal resistance is still biased toward the water side. Therefore, as far as possible to avoid scaling in the tube, increasing the water-side heat transfer coefficient will improve the overall heat exchanger’s performance. The heat transfer performance is very beneficial. In addition, due to the use of tube-piercing fins, it has poor integrity and poor resistance to mechanical vibration.
In short, the reason for the failure of the intercooler of the C902A1B unit can be attributed to 1 rough process, uneven and irregular chip spacing, poor anti-vibration ability, 2 small gaps, which can easily cause condensed water to bridge and increase airflow resistance, and Cause the core to corrode 3 single tube plate structure, the other end of the water pipe is connected with a U-shaped pipe, which makes it difficult to clean dirt and difficult to repair leaks.
3.3 Measures to improve the intercooler of C902A1B unit For the reasons analyzed above, the following two improvement measures are proposed for C902A1B unit 3.3.1 In view of the unstable airflow, poor heat transfer performance, and poor mechanical vibration resistance, the overall shape can be adopted. The petal-shaped finned tube PF heat transfer tube is used to enhance heat transfer, or the low-fin fin tube is used to enhance heat transfer.
PF heat transfer tube is a new type of enhanced heat transfer tube jointly researched and developed by Professor Wang Shiping Lin Peisen from the Institute of Chemical Engineering of South China University of Technology. Its structure 7. The PF tube can adapt to single-phase flow and multi-phase flow under various conditions Enhanced heat transfer of the flow. In addition, many scientific researchers in the Institute of Chemical Industry have used a variety of working media to study the enhanced heat transfer performance of PF tubes. Regarding gas, Zhan Qingliu used air as the medium in 1996 to study the heat transfer performance and flow resistance performance of PF tubes with a variety of flow methods in transverse and longitudinal directions, and proposed a relatively complete calculation method for heat transfer coefficient and flow resistance6 , And has been used in D 1007 air compressor.
The strengthening mechanism of the PF tube includes three aspects. It greatly increases the heat exchange area, and its ribbing coefficient is between 2.3 and 2.5. Therefore, the use of PF tubes can greatly reduce the amount of metal. PF tubes are a kind of dimension.