How dose a reciprocating compressor working
1. Working process of reciprocating compressor
Reciprocating compressors have cylinders, pistons and air valves. The working process of compressed gas can be divided into four processes: expansion, suction, compression and exhaust.
Example: The cylinder of a single-suction compressor. This compressor only has a suction valve and an exhaust valve in the section of the cylinder. The piston only sucks and expels air once every reciprocating.
(1) Expansion: When the piston moves to the left, the volume of the cylinder increases, the pressure drops, and the residual air originally left in the cylinder continuously expands.
(2) Inhalation: When the pressure drops slightly below the gas pressure in the intake pipe, the gas in the intake pipe pushes the intake valve open and enters the cylinder. As the piston moves to the left, the gas continues to enter the cylinder until the piston moves to the left end (also known as the left dead center).
(3) Compression: When the direction of rotation of the piston moves to the right, the volume of the cylinder gradually decreases, thus starting the process of compressing the gas. Because the suction valve has a check function, the gas in the cylinder cannot be poured back into the inlet pipe, and the gas pressure in the outlet pipe is higher than the gas pressure inside the cylinder, and the gas in the cylinder cannot run out of the cylinder from the exhaust valve. The gas in the outlet pipe can not flow into the cylinder due to the check valve of the exhaust valve. Therefore, the amount of gas in the cylinder remains constant, only because the piston continues to move to the right, reducing the gas-holding space (volume) in the cylinder, so that the pressure of the gas continues to rise.
(4) Discharge: As the piston moves to the right, when the pressure of the compressed gas rises to be slightly greater than the gas pressure in the outlet pipe, the gas in the cylinder will open the spring of the exhaust valve and enter the outlet pipe, and continuously discharge until the piston Move to the right end (also known as the right dead center). Then, the piston starts to move to the left and repeats the above actions. The continuous reciprocating movement of the piston in the cylinder causes the cylinder to reciprocate inhale and discharge gas. Each reciprocation of the piston becomes a working cycle, and the distance traveled by the piston every time it comes back or back is called a stroke.
2. Three thermal processes of compressed gas
The energy change of the gas during compression is related to the gas state (ie temperature, pressure, volume, etc.). A large amount of heat is generated when the gas is compressed, resulting in an increase in gas temperature after compression. The greater the degree of compression of the gas, the greater the degree of heating, and the higher the temperature. In addition to most of the heat generated when compressing the gas, it is left in the gas to increase the temperature of the gas, and a part of it is transferred to the cylinder to increase the temperature of the cylinder, and a small amount of heat is lost to the air through the cylinder wall.
The compression work required to compress a gas depends on the change of the gas state. Speaking of the deflation point, the amount of power consumed by the compressor is directly related to the heat generated by the removal of compressed gas. Generally speaking, the process of compressing gas has the following three types:
(1) Isothermal compression process: During the compression process, all the heat equivalent to the compression work is removed to keep the temperature of the gas in the cylinder unchanged, and this compression becomes isothermal compression. The compression work consumed in the isothermal compression process is the smallest. But this process is an ideal process, which is difficult to do in actual production.
(2) Adiabatic compression process: During the compression process, there is no slight heat exchange with the outside world, resulting in an increase in the temperature of the gas in the cylinder. This compression, which does not dissipate heat to the outside nor absorb heat from the outside, becomes adiabatic compression. This compression process consumes the most power and is also an ideal compression. Because in actual production, it is difficult to avoid heat loss in any situation.
(3) Variable compression process: In the process of compressing gas, the process is neither completely isothermal nor completely adiabatic, which becomes a variable compression process. This compression process is somewhere between an isothermal process and an adiabatic process. In the actual production, the gas compression process belongs to the variable compression process.
3. multi-level compression
The so-called multi-stage compression is to divide the cylinder of the compressor into several stages according to the required pressure, and increase the pressure step by step. And set up an intercooler after each stage of compression to cool the high temperature gas after each stage of compression. This will reduce the exhaust temperature of each stage.
4. Why multi-level compression?
Use a single-stage compressor to press the gas to a very high pressure, compression ratio (compression ratio refers to the ratio of the absolute pressure of the compressor exhaust and intake air. Example: at sea level, the absolute pressure of the intake air is 0.1 MPa, the exhaust The pressure is 0.8MPa absolute pressure. The compression ratio:
R = --------- = --------- = 8
Advantages of multi-level compression:
• Save compression work;
• Lower exhaust temperature;
• Improve the volume factor;
• For piston compressors, reduce the thrust of the gas against the piston.
It will inevitably increase, and the temperature of the compressed gas will also rise very high. The gas pressure increase ratio (pressure increase ratio: the ratio of the gas pressure after compression to the gas pressure before compression becomes the pressure increase ratio, referred to as the pressure ratio.) The greater, the higher the gas temperature rise. When the pressure ratio exceeds a certain value, the end temperature after gas compression will exceed the flash point (200-240 ° C) of ordinary compressor lubricating oil, and the lubricating oil will be burned to carbon slag, causing lubrication difficulties. In addition, the reciprocating compressor needs to carry the residual volume of the cylinder during the suction process (the so-called clearance volume refers to the space between the piston and the cylinder when the piston is at the end of the exhaust and the piston is at the dead position, and the air valve When the high-pressure gas in the passageway between the cylinder and the cylinder expands to a pressure slightly lower than the intake pressure, it starts to inhale. After expansion, the high-pressure gas occupies a part of the cylinder volume, which reduces the volume of the cylinder suction gas.
Obviously, if the pressure ratio is higher, the residual gas pressure in the clearance is also higher, the larger the volume occupied by the expansion of the residual gas, the production capacity of the press is significantly reduced. At the same time, the length, thickness and diameter of the compressor parts must be increased accordingly. Otherwise, it cannot adapt to the load it bears. As a result, not only does the cost of the press increase, but also the difficulty in manufacturing the machine parts.
Therefore, in order to achieve a higher final pressure, a multi-stage compressor must be used. The power consumed by a multi-stage compressor is much less than that of a single stage. The more stages, the more power is saved. At the same time, the more stages, the lower the temperature after the gas is compressed, and the larger the other volume that the cylinder can suck. But the number of stages of the compressor should not be too much, because every time the number of stages increases, there must be an additional set of cylinders, valves, piston rods, connecting rods and other mechanical parts, which complicates the structure of the compressor and greatly increases the equipment cost. According to the current situation in China, the compression ratio of each stage of a general compressor does not exceed 3 to 5.
5. Production capacity of compressor (displacement)
The gas discharged by the compressor per unit time, converted to the volume of gas in the initial suction state, is called the production capacity of the compressor, and also becomes the displacement of the compressor. The unit is cubic meters / hour or cubic meters / minute.
6. What are the main factors that affect the improvement of compressor production capacity?
(1) Clearance: When the clearance is large, the high-pressure gas in the clearance expands during inhalation and takes up part of the volume, resulting in a reduction in the amount of air sucked in and a reduction in the production capacity of the compressor. Of course, too small clearance is also unfavorable, because in this way the piston in the cylinder is easy to collide with the cylinder end cover and damage the machine. Therefore, the cylinder clearance of the compressor must be adjusted properly.
(2) Leakage loss: The production capacity of the compressor has a great relationship with the airtightness of the piston ring, suction and discharge valves and cylinder packing. The piston ring is sleeved on the piston. Its function is to seal the gap between the piston and the cylinder to prevent the compressed gas from channeling to the other side of the piston team. Therefore, when installing the piston ring, it should be able to expand and contract freely, that is, to form a good seal without causing too much friction between the piston and the cylinder. If the installation of the piston ring is not good or the friction with the cylinder causes wear and cannot be completely sealed, part of the compressed high-pressure gas will not be discharged through the discharge valve and leak from the looseness of the piston ring to the other side of the piston. In this way, due to the reduction of the amount of gas extruded, the production capacity of the compressor is also reduced. In actual production, it is often the case that the output of the piston ring wears down and the output is reduced.
During the operation of the compressor, the cylinder packing often rubs against the piston rod and wears out, or due to poor installation quality, air leakage will occur. Therefore, the leakage of cylinder packings is often encountered in actual production.
(3) Resistance of suction air valve: The suction air valve of the compressor should have the ability to resist the gas pressure to a certain extent, and only open when the pressure of the gas in the cylinder is slightly higher than the gas pressure in the inlet pipe. If the resistance of the suction valve is greater than the usual resistance, the opening speed will be slow, the amount of air entering the cylinder will be reduced, and the compressor's production capacity will also be reduced.
(4) The temperature of the suction gas: the volume of the compressor cylinder is constant, but if the temperature of the suction gas is high, the density of the gas in the suction cylinder will decrease, and the mass of the gas in the suction cylinder will decrease per unit time, resulting in The production capacity of the compressor is reduced. Compressor production capacity in summer is always lower than in winter, which is why. In addition, although the temperature of the gas in the inlet pipe is not high, if the cooling of the cylinder is not good, the temperature of the gas entering the cylinder chamber is too high, it will also expand the volume of the gas, reduce the density, and reduce the capacity of the compressor small.
7. Why must the compressor cylinder have a clearance?
• (1) When compressing gas, some water vapor in the gas may condense. Because water is incompressible, if there is no clearance in the cylinder, the compressor will inevitably be damaged. Therefore, a clearance must be left in the compressor cylinder.
• (2) The existence of clearance and the gas remaining in the clearance volume can act as an air cushion, and will not cause damage to the piston and cylinder head cover by collision. At the same time, in order to assemble and adjust, a certain clearance must be left between the cylinder end cover and the piston at the dead center.
• (3) The compressor is equipped with an air valve, and there will be inexhaustible residual air between the air valve and the cylinder and the air passage of the valve seat itself. These residual air can reduce the impact of the gas on the outlet valve, and at the same time It also slows the impact of the valve plate on the valve seat and lift limiter (bonnet).
• (4) Due to the thermal expansion of the metal, the piston rod and connecting rod will expand and elongate as the temperature rises during operation. Leaving a clearance in the cylinder can bring many benefits to the assembly, operation and safe use of the compressor. However, if the clearance is too large, it will not only have no benefits, but will have a bad impact on the compressor's work. Therefore, under normal circumstances, the clearance volume of the remaining compressor cylinder is about 3 to 8% of the volume of the working part of the cylinder, and for the compressor cylinder with a higher pressure and a smaller diameter, the remaining clearance volume is usually 5-12%.
8. Why is there an intercooler between the compressor stages?
After compression at all levels, due to the increase in temperature, the lubricating oil of the cylinder will reduce the viscosity, and at the same time, it will decompose the gelatinous substance and accumulate in the valve plate and other important parts, preventing the valve plate from operating normally. If the temperature is higher than the flash point of the lubricating oil, there is a potential danger of causing an explosion. Sometimes the compressed gas is hydrocarbon gas (such as petroleum gas, etc.). The physical properties of the gas will change at high temperatures, such as polymerization. Generally, the discharge temperature of the compressor should be 30-50 ℃ lower than the flash point of the lubricating oil. When compressing air, the exhaust temperature should be limited to below 160 ~ 180 ℃, petroleum gas, ethylene, acetylene gas, etc. should be limited to below 100 ℃, so there must be an intercooler.
In multi-stage compressors, the pressure of each stage is relatively low, and the interstage cooler, the exhaust gas of each stage is cooled to close to the temperature before the first stage of suction (but the cooling in the cylinder liner is not achieved) Therefore, the temperature of the gas will not be too high when the compression of each stage of the cylinder ends.
9. Why the diameter of the first-stage cylinder must be larger than that of the second-stage cylinder? What is the relationship between them?
Because after the first stage compression, the pressure of the gas increases and the volume decreases, when the gas enters the second stage cylinder, the gas volume is not as large as the first, so the diameter of the second stage cylinder is smaller than that of the first stage. In addition, if there is intermediate pumping or steam condensation, the size of the next-stage cylinder must be smaller than that of the previous-stage cylinder.
10. What are the components of the valve? What does each part do? How does the valve lift affect the compressor? How to adjust? What is the effect of the inconsistent spring force of the valve?
The air valve is composed of a valve seat, a lifting limiter, a valve plate and a spring, and they are fastened together with bolts.
The valve seat is the basis of the air valve and the main body; the lift limiter is used to control the lift of the valve plate, and several concentric bosses on the lift limiter are used for guiding. The valve plate is the key part of the air valve. It closes the inlet and outlet valves to ensure that the compressor suction air and discharge air work according to the design requirements. Its quality is related to the performance of the compressor; the spring plays a rebound quickly with the auxiliary valve , And the role of maintaining a seal.
The size of the valve lift has a direct impact on the compressor. The lift is large, and the valve plate is easy to impact, which affects the life of the valve; the lift is small, the cross-sectional area of the gas passage is small, the gas passed is large, the exhaust volume is small, and the production efficiency is low. When adjusting the lift of the valve plate, the valve valve lift limiter can be turned and processed for the gas valve without the adjustment device. For the gas valve with the adjustment device, the thickness of the spacer in the valve valve can be adjusted.
When the elastic force of the spring is inconsistent, the valve plate will be skewed and stuck.
11. What is the difference between suction valve and exhaust valve? What should be paid attention to when installing the air valve? What will happen if the suction and exhaust valves are reversed?
The valve seat of the intake valve is outside the cylinder, while the valve seat of the exhaust valve is inside the cylinder, and other parts are assembled according to the position of the valve seat. Use a screwdriver to check during the judgment. For the suction valve, the screwdriver can open the valve plate from the outside of the valve; for the exhaust valve, the screwdriver can open the valve plate from the inside of the valve
When installing the air valve, first determine the position of the exhaust valve and the intake valve. If the suction and exhaust valves are reversed, the gas cannot be sucked in.
12、 Lubrication category and method of compressor lubrication?
The lubrication function of the compressor is mainly to reduce the wear of friction parts and the friction work consumed. In addition, it can cool the friction surface of the moving mechanism, the sealing piston and the stuffing box, so as to improve the reliability of the piston and the stuffing box. So the lubricating oil of compressor is very important.
The lubrication of compressor can be divided into cylinder lubrication system and moving mechanism lubrication system.
The lubricating oil used for lubricating cylinder should have high viscosity, which can play a good role in lubricating and sealing between piston ring and cylinder. Secondly, it is also required to have a higher flash point and higher stability, so that the oil is not easy to volatilize, and it is not easy to oxidize and cause carbon deposition (carbide formed after oxidation of lubricating oil). Once the carbon deposition is burned, it will cause explosion, in addition, the carbon deposition will aggravate the wear of the cylinder valve, so the formation of carbon deposition in the cylinder is extremely adverse to the operation of the compressor. Therefore, the cylinder lubricating oil is lubricated by special compressor lubricating oil.
The consumption of air compressor cylinder lubricating oil is strictly limited. Too much oil is uneconomical and will contaminate the pipes and accessories, which will promote the formation of carbon deposits. For low-pressure and medium-pressure compressors, the average consumption of lubricating oil on the surface of horizontal compressor is 1g / minute for every 400m2, and that of vertical compressor is 1g / minute for every 500m2. Since there are coolers and oil separators behind the high-pressure compressor, the consumption of lubricating oil will increase. The average consumption of lubricating oil on the lubricating surface of every 200 ㎡ cylinder is 1 gram per minute, while the consumption of lubricating oil on the piston rod surface in every 100 ㎡ stuffing box is 3 grams per minute. When the new compressor is in trial operation (running in), the oil consumption is twice of the quota.
The lubricating oil quantity (circulation quantity) of the moving mechanism depends on whether there is a lubricating oil cooler or not. When there is a cooler, the lubricating oil quantity is 0.075 kg / min · kW, and when there is no cooler, it is 0.15 kg / min · kW.
The consumption of lubricating oil shall be determined according to the actual situation. The above data is only for reference.
There are generally two lubrication methods for compressor cylinder:
(1) Splash method: use the rotary mechanism (such as crankshaft) to throw the lubricating oil in the crankcase to the cylinder wall to supply the cylinder lubricating oil. This method is only applicable to the single-stage compressor without crosshead, but the oil supply can not be adjusted, especially when the oil scraper ring piston ring does not fit well, it will cause the excess lubricating oil and be taken away by the gas.
（2）Forced lubrication method (pressure lubrication): the lubricating oil in the cylinder and the metal packing part is pressurized and forced into the cylinder by the oiler. The commonly used oiler is a single plug vacuum oil filling type. Compared with the valve oil distribution multi plunger pump and slide valve oil distribution multi plunger pump previously used, this kind of oiler is simple in structure and advanced in technology, and can handle faults without stopping the machine. A small oil pump is arranged in the oiler, and each oil pump bears a lubricating point.
There are generally two lubrication methods for the moving mechanism of compressor:
(1) Splash method: swing the lubricating oil in the crankcase into oil drops. When oil drops into the oil hole on the bearing (PAD), it can flow to the friction surface.
(2) Pressure lubrication method: circulating lubrication with gear oil pump. In this method, the lubricating oil flows back to the oil tank through the following elements: oil tank, oil pump, filter, cooler, lubricating points of the moving mechanism. The circulation system is also equipped with a bypass valve and a pressure gauge for regulating the lubricating oil pressure.
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