Let’s discuss what the compression ratio of a reciprocating compressor is

Thread Content

This post was last edited by chenziang12 on 2011-7-18 at 11:35. Following the suggestion of a classmate who studies mechanics and whose name starts with ‘L’, I came to the mechanics forum to discuss with everyone what exactly the compression ratio of compressors is. There are a few questions: 1. Does the inlet pressure of the compressor decrease, while the outlet pressure remains unchanged or also decreases? 2. Why does the outlet temperature rise when the inlet pressure decreases? 3. What are the definitions of the compression ratio and effective compression ratio of a compressor? Can changing the clearance volume change the effective compression ratio? In response to the above questions, I have written down student l’s answer as well as my own views for everyone’s discussion: 1. Student l: The exhaust pressure remains unchanged, because the book states that \"when a compressor is operating, the level of its exhaust pressure is not determined by the compressor itself, but rather by the gas pressure within the compressor’s exhaust system (back pressure)\". If it is pressed, the system pressure remains unchanged, and the exhaust pressure certainly stays the same as well. My opinion: The first part of that sentence is incorrect; the exhaust pressure is closely related to the compressor itself. For example, if the valves are damaged, it’s impossible for the outlet pressure to increase ； For example, if there is a leak in the cylinder, the outlet pressure cannot be increased either. The second half of the sentence is correct, but the exhaust pressure is determined not only by the pressure being compressed, but also by the inlet pressure, the opening degree of the circuit valves, and the equipment itself. Student L believes that it is determined by the back pressure, but simple examples can refute this: when starting to drive, the back pressure is 0, so is the back pressure also 0 when the compressor first starts? In other words, no matter how it’s adjusted, the compressor discharge pressure remains 0? In other words, we want to increase the system pressure, but since the back pressure is already there, no matter how I make adjustments, the discharge pressure remains unchanged? Or put another way, if the inlet pressure is gradually reduced, and according to student L the discharge pressure remains unchanged, then if I keep reducing it until it reaches 0 and the inlet valve is fully closed (ignoring any potential damage to the equipment), will the discharge pressure still remain unchanged? My answer: The inlet pressure decreases, and the outlet pressure certainly decreases as well. There are two scenarios to consider: Scenario 1: The circuit is completely closed. It’s simple: the decrease in outlet pressure occurs in direct proportion to the effective compression ratio. Case 2: The circuit is turned on. This is a bit more complicated, but it can be said that the export pressure will certainly decrease as well. To use an analogy, if the inlet pressure in both situations drops to a certain value, then the discharge pressure of the latter will be lower than that of the former, because the compressor in the latter has lower efficiency and some of its energy is used for backflow. 2. Student L: Not bad; the inlet pressure is low and the air intake volume is small. . But there is another accompanying issue: with lower import pressure, more energy is required to bring the pressure per unit volume to the same level. . Just as it’s not certain whether more effort is required to move 100 kilograms from the first floor to the 20th floor or to move 99 kilograms from the -1st floor to the 20th floor (one increase and one decrease). . Therefore, it cannot be said that simply because the intake pressure is low or the amount of intake air is small, the required work will be less; this requires calculation. . . . My view: When the circuit is completely closed, the work done by the compressor in terms of static pressure energy decreases, and the excess energy is converted into heat, so the outlet temperature rises. An analysis similar to this is conducted when the circuit is turned on but not operating. Therefore, we need to open up the loop and convert some of the excess energy into kinetic energy within that loop, which will naturally result in a lower outlet temperature. 3. My view: Compression ratio: outlet pressure/inlet pressure. Effective compression ratio: the ratio of the effective cylinder volume to the effective clearance volume. The effective compression ratio is a parameter that is fixed at the time the device leaves the factory; it represents the maximum value that the compression ratio can reach. (Except in special cases, such as when the back pressure is higher than the exhaust pressure and the circuit is closed; in such situations the compressor does not operate. In these cases the compression ratio is higher than the effective compression ratio, but it has no meaning.) We can change the effective compression ratio by adjusting the clearance volume (also known as the compression volume). Under normal conditions, the clearance volume is approximately 3-8% of the working volume of the cylinder, while in compressors with higher pressures and smaller diameters, the clearance volume is usually 5-12%. Neither being too large nor too small is good, so there are certain limits to adjusting the clearance volume. Appropriately reducing the clearance volume can increase the effective compression ratio and the efficiency of the compressor, thereby achieving an increase in production. The methods include adjusting the piston stroke and adjusting the end cover thickness. This task requires disassembling the cylinder when the vehicle needs to be parked. Student L: Adjusting the piston indeed allows for changing the compression volume, but the compressed gas is not necessarily forced out only when it reaches the dead center; it is pushed out as soon as the pressure inside the cylinder exceeds the pressure outside. Adjusting the stroke then is useless! The exhaust pressure will not increase either. Hehe~ Everyone is welcome to comment on the views of the two of us; feel free to share your thoughts~