Intercooler is fully called "compressor intercooler", "compressor intercooler", also called "interstage cooler", or "intercooler" for short. It is between two adjacent sections of compressor.AndAftercoolerIn contrast, it has the characteristics of low gas pressure and temperature, relatively large volume, water cooling for most and wind cooling for a few.[1]
The intercooler is generally installed only afterTurboI can only see the car.Because the intercooler is actually a matching part of turbocharging, its role is to improve the air exchange efficiency of the engine.One of the reasons why turbocharged engines have more power than ordinary engines is that their air exchange efficiency is higher than the natural intake of ordinary engines.When the air enters the turbocharging, its temperature will increase greatly and its density will decrease accordingly. The intercooler plays the role of cooling the air. The high-temperature air is cooled by the intercooler and then enters the engine.If the supercharged high temperature air enters the engine directly due to the lack of intercooler, the engine will be damaged or even misfire due to the high air temperature.aboutSupercharged engineThe intercooler is an important component of the supercharging system.[2]
During multi-stage compression, the cooling of refrigerant gas between high and low pressure stages is called intermediate cooling.There are two types of intermediate complete cooling and incomplete cooling. The former cools the low-pressure exhaust to the dry saturated steam state at intermediate pressure. This method is often used for two-stage compression of ammonia. The latter is used to mix the low-pressure exhaust with the steam evaporated in the intercooler, reducing the temperature, but not reaching the dry saturated steam state at intermediate pressure. It is commonly used in two-stage compression of R12 and R22.
The ratio between the absolute pressure after compression and the absolute pressure before compression is called compression ratio.Also called "pressure ratio".In a refrigerator, condensation pressure, absolute pressure, and evaporation pressure,Absolute pressure, instead of the ratio.Single stage refrigeration compressor, usually ammonia compressor, with compression ratio not more than 8, R12 and R22 not more than 10.Otherwise, the gas transmission capacity of the compressor will be reduced and the exhaust temperature will be increased,cryogenThe increase of throttling loss is unfavorable to the reliability and economy of the refrigerator.Under normal circumstances, the condensing pressure of the refrigerator generally changes little. The main reason for the increase of the compression ratio is that the evaporation pressure decreases due to the low evaporation temperature.When the compression ratio exceeds the above limit, two-stage compression shall be adopted.When the compressor exhaust temperature rises, the cylinder wall temperature rises. On the one hand, the temperature of the suction steam rises, and the specific volume increases, which reduces the suction capacity. On the other hand, it worsens the lubrication conditions and makes the compressor difficult to operate.For example, when the condensation temperature is 40 ℃ and the evaporation temperature is 30 ℃, the exhaust temperature of the single-stage ammonia compressor can reach more than 160 ℃.Obviously, such a high exhaust temperature is not allowed.Generally, the discharge temperature of compressor shall be limited as follows: R717 (NH3)<140 ℃<R12<100 ℃<R22<115 ℃.
Intercooler is used to cool the compressed gas or steam between two compression stages.The intercooler of the refrigeration system can reduce the exhaust temperature of the low-pressure compressor (i.e. the suction temperature of the high-pressure compressor) to avoid the exhaust temperature of the high-pressure compressor being too high. It can also make the refrigerant liquid entering the evaporator supercooled, reduce the flash gas in the pipe, and thus improve the refrigeration capacity of the compressor.It is used in the two-stage or multi-stage compression refrigeration system of Freon or ammonia, and connected between the exhaust pipe of the low-pressure stage and the suction pipe of the high-pressure stage.[3]
principle
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To facilitate operation,IntercoolerMultiple compressors shall be used together as far as possible.Generally, it is arranged at one end or one side of the compressor room, which should be close to both the high and low pressure compressors connected with it and the equipment room, and it should be noted that the intercooler should not block the doors and windows to affect the ventilation and lighting.
The height above the foundation ground of the intercooler shall not be less than 150 mm. The bottom foot shall be padded with 50 mm thick wood blocks with anti-corrosion treatment to prevent cold bridge. The barrel body shall be wrapped with thermal insulation layer and shall not be less than 200 mm from the wall.
The intercooler shall be set with automaticLiquid level controllerAnd ultra-high liquid level alarm.The normal liquid level can be controlled according to the liquid level height specified by the manufacturer.The alarm liquid level is controlled at 2/3 of the height of the barrel.The intercooler must be equipped with safety valve (or automatic bypass valve), pressure gauge and liquid level indicator.
Principle of reducing intake air temperature
The purpose of the intercooler is to reduce the inlet air temperature. The reason for reducing the inlet air temperature should be mentionedTurboPrinciple of.The working principle of turbocharging is to use the exhaust gas from the engine to impact the exhaust blade, and then drive the intake blade on the other side to force the compressed air to be sent to the combustion chamber. Because the temperature of the exhaust gas is usually as high as 8 or 9 degrees Celsius, the turbine body is also in an extremely high temperature state, so the temperature of the air flowing through the intake turbine end will be increased,In addition, the compressed air will also generate heat (because the compressed air molecules will squeeze and rub each other to generate heat energy when the distance is smaller). If this high temperature gas enters the cylinder without cooling, it is easy to cause the engine combustion temperature to be too high, and then it will cause the gasoline precombustion to knock, which will increase the engine temperature even more,At the same time, the volume of compressed air will also greatly reduce the oxygen content due to thermal expansion, which will reduce the pressurization efficiency, and naturally cannot produce the required power output.In addition, high temperature is also an invisible killer of the engine. If you do not try to reduce the operating temperature, once you encounter a hot environment or operate for a long time, it is easy to increase the probability of engine failure, so you need to install an intercooler to reduce the inlet air temperature.
Heat dissipation principle
The purpose of cooling the compressed air is to use many horizontal tubes to separate the compressed air, and then the outside direct cold air from the front of the car passes through the fins connected to the tube to cool the compressed air, so that the inlet air temperature is closer to the outside temperature. Therefore, if you want to increase the heat dissipation efficiency of the intercooler, just increase its area and thickness to increase the number, length and fins of the tubes,This can be achieved.
Because the longer the intercooler and the larger the area, the more likely it is to produce the problem of inlet pressure loss, which is one of the main issues discussed today
structure
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The intercooler is mainly composed of two parts, Tube and Fin, and the Core is the product of their continuous overlap.
Tube
The first part is named Tube. Its function is to provide a channel to contain compressed air to make it flow through. Therefore, the Tube must be a confined space so that the compressed air will not leak. Moreover, the shape of the Tube is also divided into three types: square, oval and long taper. The difference lies in the choice between wind resistance and cooling efficiency.
Fin
The second part is called Fin, which is commonly known as fin. It is usually located between the upper and lower layers of the tube and closely bonded with the tube. Its function is to dissipate heat, because when compressed hot air flows through the tube, it will transmit heat to the fin through the outer wall of the tube. At this time, if there is air with lower external temperature flowing through the fin, it can take away the heat,To cool the intake air temperature.
Core
The structure with the above two parts continuously overlapping until 10~20 floors is called Core, and this part is the main body of the intercooler.In addition, in order to allow the compressed gas from the turbine to have a buffer and pressure storage space before entering the core, and to increase the air flow rate after leaving the core, a part called Tank is usually installed on both sides of the core. Its appearance is like a funnel, and a circular inlet and outlet are also set on it to facilitate the connection of silicone pipes. The intercooler is composed of the above four parts.
classification
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Intercooler for ammonia
The intercooler of the ammonia refrigeration system should be designed as a two-stage compression refrigeration system, which requires the use of an intercooler, when producing a lower evaporation temperature, because the condensing water temperature is high in summer and the compressor exceeds the maximum pressure difference or compression ratio.The most widely used cycle in China is the cycle of one throttling and complete cooling
The high-temperature gas discharged from the low-pressure cylinder enters the air inlet pipe from the top, and the air inlet pipe extends straight into the lower half of the cylinder body, sinks in the ammonia liquid, and the gas is discharged. The baffle plate is welded to prevent direct impact on the bottom of the cylinder, so as not to flush the accumulated oil at the bottom.High temperature gas is cooled in ammonia liquid. At the same time, ammonia gas is separated from ammonia liquid and oil mist due to expansion of cross section, reduction of flow rate, change of flow direction, resistance and washing effect of ammonia liquid.
The ammonia gas washed by ammonia liquid flows upward in reverse direction, and there are still ammonia liquid and oil drops in it. When it passes through the perforated umbrella baffle, it is separated to avoid being brought into the high-pressure cylinder and then sucked away by the high-pressure stage.
The ammonia liquid with high pressure and normal temperature passes through the cooling coil in the intercooler barrel to release heat to the liquid ammonia and is cooled to achieve supercooling. Generally, the degree of supercooling is within 5 ℃, and then it flows to the evaporator in the liquid supply station.
The liquid supplied by the intercooler, the ammonia liquid used for washing, enters the intercooler in two ways, one is from the lower side of the intercooler, the other is from the air inlet pipe at the top of the intercooler. At this time, the liquid is mixed with the low-pressure exhaust gas to enter.
The liquid supply volume of the intercooler shall keep the liquid level at a certain height.In addition, the intercooler is also connected withLevel indicatorOil drain valve, liquid drain valve, i.e. ammonia outlet, safety valve and pressure gauge.The intercooler works at low temperature, so it is installed outside the cylinderThermal insulation materialThe serpentine coil shall also be installed with insulation layer after leaving the intercooler.
The following matters shall be noted about the operation of the intercooler
The gas flow rate in the intercooler is generally 0.5 0.8m/s.The flow rate of ammonia liquid in the serpentine coil is generally 0.4-0.7m/s, and the temperature of ammonia liquid at the outlet is 3-5 ℃ lower than that at the inlet.The intermediate pressure of the intercooler is generally about 0.3MPa gauge pressure, and should not exceed 0.4MPa gauge pressure.The suction superheat of the high-pressure stage, that is, the suction temperature is 2-4 ℃ higher than the intermediate temperature of the intercooler.The liquid level in the intercooler is generally controlled at about 50% of the height of the intercooler, which can be observed through the liquid level indicator. The liquid level is automatically controlled by the liquid level controller and the float valve. If the liquid level does not meet the requirements, it indicates that the automatic control is out of order. It can be temporarily usedManual control valveTo control the liquid level.If the liquid level is too high, it will cause wet stroke of HP turbine cylinder or hydraulic hammer. If the liquid level is too low, the cooling effect of LP exhaust will be greatly reduced. As a result, the superheat of HP suction will be significantly increased, affecting the normal operation of the refrigeration system.
The intercooler shall be drained regularly.
Intercooler for fluorine
The Freon refrigeration system mostly adopts a once throttling intermediate incomplete cooling cycle during two-stage compression. The high-temperature gas discharged from the low-pressure stage is mixed with the low-temperature saturated gas vaporized by the intercooler in the middle of the pipe and then sucked into the high-pressure cylinder by the high-pressure stage. Therefore, the fluorine intercooler is relatively simple. The liquid supply of the intercooler is automatically controlled by the thermal expansion valve,The pressure is generally 0.2-0.3MPa, which is regulated by the thermal expansion valve to provide an appropriate amount of wet saturated steam without causing wet stroke.
After depressurization and throttling by the expansion valve, the high-pressure liquid enters the intercooler, absorbs the heat of the serpentine coil and the wall of the intercooler, and vaporizes. It enters the pipeline connecting the low-pressure stage and the high-pressure stage through the outlet pipe, and mixes with the high-temperature gas discharged from the low-pressure stage to cool the low-pressure exhaust.The high pressure and normal temperature liquid dissipates heat through the serpentine coil, which also reduces the temperature and realizes supercooling, which is generally about 3-5 ℃.The liquid supply expansion valve sent to the evaporator is throttled and depressurized to enter the evaporator. Because the liquid has a certain degree of supercooling, the refrigeration effect is improved.
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Pressure loss
Although the large capacity intercooler has better cooling efficiency due to the extension of heat exchange time, the air flow rate will slow down and the pressure loss will occur, and the turbine hysteresis will become more serious. Why?This should start from two aspects.I believe that readers who have washed their own cars know that to make the water column in the water pipe spray farther and faster, just squeeze the water pipe head. Why?That is because the flow rate per unit time will not change with the pipe diameter under the condition of constant water pressure. Therefore, to achieve this goal, as long as the pipe diameter is reduced, the flow rate will naturally become faster. On the contrary, as soon as the pipe diameter is increased, the flow rate will become slower, which also occurs in the entire air inlet pipeline.Because when the air flows through the intercooler with larger space from the air inlet pipe with smaller capacity, the flow rate will become slower, and this problem is especially serious for small output turbine with large intercooler, which will make the turbine hysteresis more serious.
In addition, when the air enters the tube of the intercooler from the air inlet pipe, flow resistance will be generated due to the diversion conversion of the pipe diameter from coarse to fine, resulting in a certain degree of pressure loss. In addition, many intercoolers will set fins in the tube to increase the cooling efficiency (the tube is not necessarily in the middle air), which will also generate flow resistance,The problem of turbine hysteresis is relatively more obvious.It is worth mentioning that the pressure loss mentioned above does not mean the reduction of the boost value. Because the air inlet pipeline is closed, the pressure relief action of the exhaust pressure relief valve must reach the boost value set by the owner before proceeding. Therefore, the constant pressure value will not decrease, but will extend the time of arrival (because part of the pressure is consumed) and affect the boost response,This is also the biggest impact of pressure loss.Since the installation of intercooler will make the turbine lag more obvious, but it is necessary to install it. Therefore, how to take into account the cooling efficiency and pressure maintenance has become the primary topic of modifying the intercooler.The dilemma of retrofitting intercoolers: an intercooler that emphasizes performance-based cooling must have a good heat dissipation capacity, and the reduction of pressure loss must also be taken into account. However, the suppression of pressure loss and the improvement of cooling efficiency are completely opposite technically. For example, if an intercooler with the same size is designed completely based on heat dissipation,The tube inside needs to be made thinner and the number of fins increased, which will increase the air resistance;However, if we start with maintaining the pressure level, we have to thicken the tube and reduce the fins. In contrast, the efficiency of heat exchange is poor, so the modification of the intercooler is not as simple as we imagined.Therefore, the cooling efficiency and the method of maintaining pressure should be taken into account, most of which will start from the tube and the fin.The first part is the Tube part, which is divided into two ways. The first is to use the style of thick pipe diameter but extremely thin pipe wall, increase the smoothness of air flow with thick pipe diameter, and improve the heat dissipation by using the characteristics of thin pipe wall.Second: In the tube with thick pipe diameter, additional fins are set inside to increase the area of heated air contacting the metal sheet, so as to improve the heat transmission, and the natural heat dissipation efficiency can also be increased. However, this design is mostly used in the intercoolers of competitive cars or high supercharged vehicles, so as not to cause too much hysteresis.Next is the fin part. The fin of the general intercooler, as shown in Figure 2, is usually straight without any opening, and the fin length is as long as the width of the intercooler is. However, since the fin plays a major role in the heat dissipation function in the whole intercooler, the heat exchange power can be increased as long as the area of its contact with cold air is increased,Therefore, the fins of many intercoolers were later changed to various designs in Figure 3, among which wavy or commonly known as louver design fins are the most popular.However, as far as the heat dissipation efficiency is concerned, the so-called overlapping heat dissipation fins in Figure 4 are the best, but the wind resistance force generated is also the most obvious. Therefore, it is more common to see D1 racing cars in Japan. Because these racing cars are not fast, but they need good heat dissipation to protect the high speed engine.Carry out intercooler modification
Installation position
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Generally, it can be divided into two types: front mounted type and top mounted type,
In terms of heat dissipation, of course, the front mounted type located in the front bumper is better, but when it comes to reactivity, the top mounted type is more advantageous, which is the direct effect of pressurization brought about by its short pipeline. For example, Impreza WRCar, in order to shorten the pipeline of the front intercooler, reverses the throttle to reduce the pressure loss caused by too long pipeline,Therefore, it is not difficult to imagine the overall arrangement of the intake pipe, which is also the key point that must be paid attention to when modifying the intercooler.Therefore, when upgrading or installing the intercooler, in addition to paying attention to the size of the intercooler, the length of the pipeline should be shortened as far as possible, and it should be pulled into a straight line to reduce corners, welding points, etc. These are all ways to increase the air flow rate, because if there are too many welding points and corners, the smoothness of the air flow will be poor and pressure loss will occur.Secondly, like the intercooler principle mentioned above, if the tube of the intercooler is too thin, it will increase the resistance to impact reaction, and the temperature in the pipe wall will be higher, so it is also a good way to slightly thicken the inlet pipe diameter. As for the matching of the pipe diameter, it mainly depends on the diameter of the turbine outlet and the throttle valve.It is worth mentioning that the diameter of the inlet and outlet pipes in front of and behind the intercooler should be about 10% thicker than that in front of the inlet. The reason is that the larger outlet pipe diameter can allow the core cooling air to pass through the intercooler at a faster speed, which can help the flow rate increase positively.As for the material of intercooler, it is usually made of aluminum alloy, which is not only rich in texture to increase the beauty, but also can increase the heat dissipation effect due to the high thermal conductivity of aluminum. In addition, the advantages of lightweight are also one of the main reasons for choosing aluminum alloy material.As for the rubber connecting pipes between metal pipes, it is recommended to use three or five layers of coatingSilicone rubber productsThis silicone tube has excellent ductility, high temperature resistance, high pressure resistance and will not harden, so as small as vacuum tube, medium to water tube, and large as the entire air inlet pipeline, it is a very good substitute for the original factory. It is quite suitable for use on high heat turbine engines. In addition, the fixation of wide type double clip stainless steel beam ring can avoid tube explosion or air leakage, and is different from the black of the original factory.
