Reciprocating compressor issues
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As shown in the figure, the line with a control valve is the compressor return line. I would like to ask whether the pressures at points A and B are equal when the return line at the compressor outlet is open? Also, if the pressure of the gas medium in the compressor is gradually increasing (not due to the return line being open), then if the return line of the compressor is fully opened, will the pressures at the inlet and outlet of the compressor eventually become equal? And since the pressure of the gas medium keeps rising, will the compressor eventually stop doing work with equal pressures at its inlet and outlet?
The diameter of the return line and the control valve serve to throttle and reduce pressure, so the pressures will not be the same. The diameter of the return line should be much smaller than that of the main line, right?
This post was last edited by LQ198619 on 2017-3-5 at 08:37. The return line at the outlet of the centrifugal compressor serves to supplement the inlet flow when there is a low flow rate at the compressor inlet and thus prevent surge. Additionally, the system pressure can also be utilized; for example, in catalytic reactions, the flow rate can be controlled through the anti-surge valve of the compressor in order to regulate the reaction pressure. Therefore, the return flow from the outlet to the inlet does have an impact on the inlet pressure. However, I don’t think the pressures on the two sides will be equal as you mentioned, and moreover, actual operating conditions do not permit the anti-surge valve to be kept fully open. It’s just my humble opinion (and also, the poster mentioned a reciprocating compressor, but the diagram shows a centrifugal compressor). Reciprocating compressors do have an outlet that leads back to the inlet, but this is used during startup and shutdown! (I haven’t carried out specific operations for this.) A reciprocating compressor doesn’t require any outlet flow compensation when it is operating normally; it won’t experience surging either, and some models even have energy-saving systems that can adjust the compressor’s load!
It is smaller than the main caliber, but not by much
The return line at the outlet of a reciprocating compressor is sometimes used to regulate the outlet pressure of the reciprocator. Haha, what I said was a bit off, but in reality it is indeed a reciprocating compressor
Return line: used to adjust the pressure difference between the inlet and outlet; the pressures at the two points shown in the diagram are roughly equal
The pressure difference between A and B is more or less the same; a reciprocating compressor will keep increasing pressure as long as there is enough gas and power available (which poses a significant risk in terms of pressure issues), and it won’t experience the situation you described where the pressure at the inlet and outlet is the same
Adjustment is applied when pressure and flow rates are such that the power is slightly higher than the load, in order to meet the subsequent operating conditions; when the values are too high, the load is reduced
If it is reciprocating and equipped with a load regulator, the outlet pressure and inlet pressure are almost equal when no load is applied； If there is no load regulator, then the outlet pressure and the inlet pressure are also equal, after all the circuit is a pipe with a changing diameter
As I understand it, the first question is whether the AB values are equal; if the medium keeps flowing in the backward direction, there must be a certain pressure difference. If the backward path is closed and all flow goes through the anti-surge system, then the pressures should be similar. Another issue is that if the inlet pressure increases, with the compressor operating under the same load, more medium will be compressed, and thus the outlet pressure will also increase as well – it’s not possible for the pressures at the inlet and outlet to be equal. This is just my personal understanding
Theoretically, they can be approximately equal; certainly the outlet flow will be greater than the inlet flow, otherwise how could the gas from the outlet return to the inlet? When starting or stopping, the one at position B at the back will close first.