Which statement is true about static pressure relative to a fan?

Which statement is true about static pressure relative to a fan?

• A. Upstream from the fan is negative and downstream is positive
• B. Upstream is positive, downstream is negative
• C. Both are zero
• D. Both are positive

Answer: (A)

The main idea here is how a fan changes static pressure around it. A running fan adds energy to the moving air, creating a pressure difference from the inlet to the outlet. Air is drawn toward the fan, which lowers the static pressure at the inlet relative to ambient (a suction effect). As air passes through the fan and exits, the fan raises the static pressure, so the discharge side sits above ambient. Relative to ambient, the upstream side is negative and the downstream side is positive, providing the driving force to push air through the system. The other scenarios would imply no pressure difference or a reversal of the expected pressure rise, which isn’t how a fan behaves.

The main idea here is how a fan changes static pressure around it. A running fan adds energy to the moving air, creating a pressure difference from the inlet to the outlet. Air is drawn toward the fan, which lowers the static pressure at the inlet relative to ambient (a suction effect). As air passes through the fan and exits, the fan raises the static pressure, so the discharge side sits above ambient. Relative to ambient, the upstream side is negative and the downstream side is positive, providing the driving force to push air through the system. The other scenarios would imply no pressure difference or a reversal of the expected pressure rise, which isn’t how a fan behaves.