Working principle of piston compressor
1. What is the compressor working process?
reciprocating compressors have cylinders, pistons and 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 type of compressor only has an intake valve and an exhaust valve in one section of the cylinder. The piston only inhales and exhausts once every time the piston reciprocates.
(1) Expansion: When the piston moves to the left, the volume of the cylinder increases, the pressure drops, and the remaining air remaining in the cylinder continues to expand.
(2) Inhalation: When the pressure drops to a little less than the pressure of the gas in the intake pipe, the gas in the intake pipe will open the intake valve and enter 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 called the left dead center).
(3) Compression: When the piston moves to the right in the direction of rotation, the volume of the cylinder gradually decreases, thus starting the process of compressing the gas. Due to the non-reverse function of the suction valve, 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 escape from the exhaust valve to the outside of the cylinder. The gas in the outlet pipe cannot flow into the cylinder due to the non-reverse effect of the exhaust valve. Therefore, the amount of gas in the cylinder remains constant, only because the piston continues to move to the right, the gas holding space (volume) in the cylinder is reduced, and 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 slightly greater than the gas pressure in the outlet pipe, the gas in the cylinder pushes the spring of the exhaust valve into the outlet pipe and continues to be discharged until the piston Move to the right end (also called right dead center). Then, the piston starts to move from the right to the left, repeating the above action. The piston keeps reciprocating movement in the cylinder, so that the cylinder reciprocates inhaling and expelling gas. Each reciprocation of the piston becomes a working cycle, and the distance traveled by the piston each time is called the stroke.
2. What are the three thermal processes of compressed gas?
The energy change of a gas in the compression process is related to the state of the gas (ie temperature, pressure, volume, etc.). A large amount of heat is generated when the gas is compressed, which causes the temperature of the gas to increase after compression. The greater the degree of gas compression, the greater the degree of heating, and the higher the temperature rise. In addition to most of the heat generated when the gas is compressed, it is left in the gas to increase the temperature of the gas, and part of it is transferred to the cylinder, which increases the temperature of the cylinder, and a small part of the 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: In the compression process, all the heat equivalent to the compression work is removed to keep the temperature of the gas in the cylinder unchanged. This compression becomes isothermal compression. The compression work consumed in the isothermal compression process is the smallest. But this process is an ideal process, and it is very difficult to do in actual production.
(2) Adiabatic compression process: During the compression process, there is no heat exchange with the outside world, resulting in an increase in the temperature of the gas in the cylinder. This compression that does not dissipate heat to the outside nor absorb heat from the outside is called 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 under any circumstances.
(3) Variable compression process: In the process of compressing gas, a process that is neither isothermal nor completely adiabatic becomes a variable compression process. This compression process is between the isothermal process and the adiabatic process. The compression process of gas in actual production is a variable compression process.
3. What is multi-stage compression?
The so-called multi-stage compression, that is, according to the required pressure, the cylinder of the compressor is divided into several stages, and the pressure is increased step by step. And set up an intercooler after each stage of compression to cool the high temperature gas after each stage of compression. In this way, the exhaust temperature of each stage can be reduced.
4. Why multi-stage compression?
Use a single-stage compressor to press the gas to a very high pressure, and the compression ratio (compression ratio refers to the absolute pressure ratio of the compressor exhaust and intake. For example: the absolute pressure of the intake is 0.1 MPa at sea level, and the exhaust The pressure is an absolute pressure of 0.8MPa. Then the compression ratio:
R=--------- =--------- =8
The advantages of multi-stage compression:
(1), save compression work;
(2), reduce the exhaust temperature;
(3), increase the volume factor; 
(4) For a piston compressor, reduce the thrust of the gas on the piston. It will inevitably increase, and the temperature of the compressed gas will also rise very high. 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 larger the gas temperature rise, the higher the gas temperature rise. When the pressure ratio exceeds a certain value, the gas The final temperature after compression will exceed the flash point of ordinary compressor lubricants (200-240℃), and the lubricant will be burned into carbon slag, causing difficulty in lubrication. In addition, reciprocating compressors need to carry residues during the suction process The clearance volume of the cylinder (the so-called clearance volume refers to the space between the piston and the cylinder and the passage space connecting the valve and the cylinder when the compressor is at the end of exhaust and the piston is at the dead center) expands to pressure It starts to inhale when it is slightly lower than the intake pressure. The high-pressure gas occupies a part of the cylinder volume after expansion, which reduces the volume of the cylinder suction gas. Obviously, if the pressure ratio is higher, the remaining gas pressure in the clearance is also higher. The larger the volume occupied by the remaining air after expansion, the production capacity of the compressor will be significantly reduced. At the same time, the length, thickness and diameter of the compressor parts must be increased accordingly. Otherwise, it will not be able to adapt to the load it bears. As a result, it not only increases the cost of the compressor, but also increases the difficulty in the manufacture of mechanical parts. Therefore, in order to achieve a higher final pressure, a multi-stage compressor must be used. The power consumed by the multi-stage compressor is simpler. The number of stages is greatly reduced. The more stages, the more power is saved. At the same time, the more stages, the lower the temperature of the gas after compression, and the larger the other volume that the cylinder can suck in. But the number of compressor stages is also It should not be too many, because every time the number of stages increases by one level, an additional set of parts such as cylinders, valves, piston rods and connecting rods is necessary, which complicates the structure of the compressor and greatly increases equipment costs. According to the current situation in China Look, the compression ratio of each stage of the general compressor does not exceed 3~5.