Actual air flow in the impeller is not uniform in each section but is fast at the backside of the blades and is slow at the front side of the blades.
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Note : "front" and "back" are the words concerning rotating direction.
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So, the pressure at the front side of blades is higher than those at the back sides.
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When average air speed in the impeller is much higher, that is, the air volume through the impeller is much larger, the air flow is normal with small reverse flow area.
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When average air speed become slow, that is, the air volume through the impeller become smaller, the reverse flow areas at the blade front side develop and at certain air volume the reverse flow area reaches to eye (inlet) portion of the impeller causing shut down of normal air flow.
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When the normal air flow has vanished, the reverse flow diminishes and also disappear. And again the normal flow begins. This alternation is continued while the air volume through the impeller is kept below certain point.
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This abrupt alternation of the normal and the reverse air flow in the impeller is called surging. The compressor operation becomes unstable in surging. Discharge pressure fluctuates, air temperature to and from the impeller rises higher and the rotor shaft vibration become bigger.
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This phenomenon is inherent in centrifugal compressor operation when it is operated at small air volume.
Actual air flow in the impeller is not uniform in each section but is fast at the backside of the blades and is slow at the front side of the blades.---------------------------Note : "front" and "back" are the words concerning rotating direction.------------------------------So, the pressure at the front side of blades is higher than those at the back sides.-----------------------When average air speed in the impeller is much higher, that is, the air volume through the impeller is much larger, the air flow is normal with small reverse flow area.---------------------When average air speed become slow, that is, the air volume through the impeller become smaller, the reverse flow areas at the blade front side develop and at certain air volume the reverse flow area reaches to eye (inlet) portion of the impeller causing shut down of normal air flow.-------------------When the normal air flow has vanished, the reverse flow diminishes and also disappear. And again the normal flow begins. This alternation is continued while the air volume through the impeller is kept below certain point.--------------------This abrupt alternation of the normal and the reverse air flow in the impeller is called surging. The compressor operation becomes unstable in surging. Discharge pressure fluctuates, air temperature to and from the impeller rises higher and the rotor shaft vibration become bigger.---------------This phenomenon is inherent in centrifugal compressor operation when it is operated at small air volume.
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