Anti-surge flow rate of centrifugal compressors

How does the anti-surge flow rate of a centrifugal compressor change when the molecular weight (compared to the designed molecular weight) increases, while the inlet pressure decreases and the compression ratio drops? According to the polytropic head formula for compressors, at a constant speed and constant outlet pressure, as the molecular weight increases and the compression ratio decreases, the anti-surge flow rate of the centrifugal compressor increases; however, at this point the operating point of the centrifugal compressor is far away from the surge zone. For example, in a hydrogenation cycle hydrogen compressor, hydrogen is normally used as the working fluid, while nitrogen is used during startup. How does the anti-surge flow rate change in these two situations? These two cannot be compared; under normal operation, the speed and compression ratio of the H2 compressor are different from those of the N2 compressor at startup. The N2 operating condition cannot achieve the speed and pressure ratio of the H2 operating condition. If forced to compare, the anti-surge flow rate at N2 will be lower than that under H2 conditions.

Shouldn’t the compression ratio increase as the inlet pressure decreases?

How does the anti-surge flow rate of a centrifugal compressor change when the molecular weight (compared to the designed molecular weight) increases, while the inlet pressure decreases and the compression ratio drops? Is the reason for the decrease in pressure ratio due to an increase in molecular weight a power-related issue or a problem with the process system? If the pressure ratio decreases significantly, the anti-surge flow rate generally decreases; the conditions for surge occur at low flow rates and high outlet pressures.

First of all, I have to say that I don’t understand this{:1_90:}. But I have a question: under the condition of constant pressure and with the medium remaining unchanged, as the compression ratio decreases, the inlet pressure should definitely increase, right? Doesn’t this contradict what you said?

I didn’t mention anything about inlet pressure. What I mean is that, at constant pressure and constant speed, if the compression ratio decreases or the molecular weight increases, or both occur, the anti-surge flow rate of the centrifugal compressor increases, but the operating point is then far away from the surge zone. If the inlet pressure decreases, at constant pressure and constant speed, the compression ratio increases. The anti-surge flow rate of the centrifugal compressor decreases, and at this point the operating condition is very close to the surge zone, or surge may already have occurred.