Discussion on variable operating conditions of multi-stage reciprocating compressors
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This post was last edited by lein on 2017-10-26 at 20:00. Reciprocating compressors perform volumetric compression; the outlet pressure is determined by the back pressure, with mechanical suitability being a basic requirement, of course. For operating conditions with a high pressure ratio, multi-stage compression is often employed for efficiency and temperature reasons. In design, the equal pressure ratio principle is often used for calculations and re-calculations. In practice, the pressure ratio of the low-pressure stage is usually slightly higher, while that of the high-pressure stage is lower. The pressure ratio, or the inlet and outlet pressure values for each stage, affects factors such as load balance and the maintenance of the reverse angle. We know that the actual inter-stage pressure ratio is influenced by factors such as cylinder diameter, clearance, humidity, temperature, etc. I understand that for the balance of the inter-stage pressure ratio, one assumption is that the amount entering each stage is equal to the amount leaving it. But so far, it seems that there is no (or I haven’t found one) complete theoretical microscopic description of the compression process and the pressure changes between stages, especially under variable operating conditions such as when the inlet pressure gradually decreases while the outlet pressure remains constant ； Interstage pumping condition ； Inter-stage pressurization conditions, and so on. I have a question: what is the microscopic process that occurs within the compressor stages, from the moment the crankshaft starts moving until the pressures in each stage gradually reach equilibrium? I think it’s the pressure at each stage that is built up gradually from the lowest level by applying pressure forward, until a certain equilibrium value is reached; but how is this equilibrium point determined? And how to control it? Is there a theoretical calculation method, or is it based on some principle? All designers and operators are welcome to discuss. Since most selections are made using software calculations, and the detailed underlying processes aren’t necessarily clear, I think it’s still meaningful to discuss this.
The pressure in a reciprocating machine is determined by the back pressure; it builds up stage by stage, starting from the last stage and moving towards the first stage, until equilibrium is reached. In fact, every time I check it after turning it on, the pressure increases step by step, from the first stage to the final stage.
Let me share my understanding: since it is a positive-displacement compressor, there is a corresponding relationship between the volume ratio and the pressure ratio. In simple terms, once the compression ratio is determined, the volume of each cylinder stage is also fixed ； Conversely, once the volume is determined, even if the total system pressure ratio changes, the reallocation of the pressure ratios at each stage is still based on the actual volume. Follow the script and talk about the design process. Once the total pressure ratio is known, it is necessary to determine the number of stages and the distribution of the pressure ratios. In addition to isobaric ratios, there are also isodynamic design methods. The influence of the condensation coefficient, that is, the discharge of condensate between stages, was also taken into account in the design; therefore, the gas volume per stage is not exactly the same. Then, based on the volumetric flow rate at the inlet, and taking into account factors such as the pressure ratio coefficient, volume coefficient, temperature coefficient, leakage coefficient, and water separation coefficient, the volume of each cylinder stage is calculated. The cylinder dimensions must be rounded, and after rounding, the final actual pressure ratio is determined. A deviation of the operating conditions from the design assumptions will inevitably lead to a redistribution of the pressure ratio. Of course, in actual operation it takes place within certain limits, so the compressor remains suitable. If it really isn’t suitable, there are various methods to adjust the flow rate (i.e., by adjusting the pressure ratio). I’m not sure either about the microscopic processes mentioned by the original poster. From startup to the establishment of pressure, it should be a process driven by the final-stage thrust reversal (determined by backpressure). As for the establishment and control of balance, as long as constraints such as exhaust temperature permit it, it is of course an automatic balancing process. When it exceeds the acceptable limits, manual intervention is necessary; this is why alarms, shutdown mechanisms, and status indicators are provided. For example, if the intake temperature rises (indicating that the previous stage of cooling is no longer effective), or if the exhaust pressure increases, measures must be taken; if it gets any worse, the system will shut down immediately. The reason why the equilibrium point is established automatically becomes clear once one understands the design process.
Then why does the pressure increase gradually from stage one to the final stage during startup? Every time I turn the machine on, I observe that the pressure gauge for the first stage rises, then the one for the second stage starts to rise as well, and finally the one for the third stage rises. I can’t figure out what’s causing this issue
I cannot confirm your question; what you described is the process of initiating pressure generation. The possible reasons are as follows for your reference: Assume the inlet absolute pressure is 1 bar, the volume of the first-stage cylinder is 3 L, the pressure ratio for the first stage is 3, and the volume of the second stage is 1 L. At startup, each stage only needs to overcome the resistance of valves and pipes in order to exhaust gas to the next stage. Thus, the compression ratio at stage 1 is very low; the volume of gas discharged from 3L may be compressed to only 2.5L (under normal conditions it is compressed to 1L, and the volume ratio can be simply considered as the compression ratio). Stage 2 can absorb only 1L, leaving 1.5L of gas in the storage tank of stage 1. The second cycle needs to compress another 3L of gas in, and the volume of the buffer tank after stage 1 is limited; therefore, this gas must be compressed first, which means the pressure after stage 1 has to increase first. In the future, each level will follow the same pattern, with the voltage gradually increasing.
During startup, can it be understood that the previous stage has not yet reached its maximum compression ratio, which is equivalent to an intake state; the subsequent stages are only involved in suction and discharge. More importantly, the back pressure in the piping system has not yet increased, especially in multi-stage compressors with configurations such as two-in-one or three-in-one