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I have been studying a coal-fired power plant, and the power plant cycle has an induced draft fan that is used to create negative pressure to help get the flue gas travel to the stack. I am confused on how the induced draft fan achieves negative pressure? Does a negative pressure mean that the inlet pressure at the fan is negative? And if fans have pressure rise across, then how is a negative pressure maintained?

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    $\begingroup$ There is no such thing as negative pressure. There is only negative differential pressure. $\endgroup$
    – CuriousOne
    Jul 22 '16 at 2:44
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Usually in engineering lingo, negative pressure means below-atmospheric pressure. If flue gases are at atmospheric pressure, then to make them flow towards the fan, the fan must create a negative pressure region at its inlet. Subsequently there is rise in pressure across the fan, going from inlet side to outlet side. Power supplied to fan is expended in doing work on flue gases to make it move from low pressure region (inlet) to high pressure region (outlet), from where it is expelled to atmosphere.

In general, for a fan to suck out air, it must create lower pressure at its inlet compared to pressure at which air exists (on the inlet side).

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