Relationship between the on/off inlet valve and outlet pressure
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The book explains very clearly the relationship between the opening and closing of the fan outlet valve and the outlet pressure. However, the relationship between the inlet valve of the switch fan and the fan’s outlet pressure is rarely discussed. Is it still the case that the smaller the valve opening, the higher the outlet pressure? I’m not sure which fellow sailor is familiar with this; please give me some advice. Thank you!
The smaller the inlet valve of the fan, the lower the flow rate of the fan, and due to the performance characteristics of the fan, the pressure difference between the inlet and outlet increases. However, due to the flow restriction at the inlet, there is actually a loss of pressure behind the inlet valve; in other words, the pressure at the inlet of the fan impeller decreases. So as the inlet valve is closed, the pressure difference between the inlet and outlet of the fan increases, but the pressure at the fan’s inlet decreases; it is not certain whether the pressure at the fan’s outlet will increase or decrease. But in extreme situations, such as when it is fully closed or nearly closed, the outlet pressure is definitely very low due to the negative pressure at the inlet and the backflow losses of the fan, among other factors. These are my personal opinions; please correct me if I’m wrong.
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May I understand that the compression ratio of the fan remains essentially constant: when the inlet valve is closed, the inlet pressure decreases; and since the compression ratio stays the same, the outlet pressure also decreases?
For blade-type fans, the pressure ratio does not change much, so your assumption should be correct.