Why does static pressure decrease as CFM increases on blower fan charts?

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Discussion Overview

The discussion revolves around the behavior of static pressure in blower fan charts, specifically why static pressure decreases as cubic feet per minute (CFM) increases. The scope includes theoretical understanding and practical implications related to fan performance and ductwork systems.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions why static pressure drops with increased CFM, suggesting an expectation of rising pressure differences with higher airflow.
  • Another participant explains that while the pressure required to move air through ductwork increases with airflow, fans behave differently, indicating that greater resistance leads to lower airflow.
  • A participant seeks clarification on whether fan charts indicate the CFM a fan can achieve against a specific resistance, such as that from duct walls, measured in pressure units like Pa or inches of water.
  • A later reply confirms that fan curves alone do not provide complete performance data, emphasizing the need to match the fan curve with system resistance to determine actual performance.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between static pressure and CFM, with some clarifying the nature of fan performance and resistance, but no consensus is reached on the initial question regarding static pressure behavior.

Contextual Notes

The discussion highlights assumptions about fan performance and the influence of ductwork resistance, but does not resolve the underlying complexities of these interactions.

hali24953
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Hi, Looking at blower fan charts, why does the static pressure drop with increased CFM, what makes sense to me should be a pressure difference rise with increasing CFM.
 
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Welcome to PF!

The pressure required to blow air through a ductwork system rises with increasing airflow. But fans aren't ductwork. For a fan, the greater the resistance, the lower their airflow.
 
Thanks!
Does that mean that fan charts tell what CFM flow will the fan be able to give at a given resistance to its inflow?
The resistance coming from a pipe wall for example, a fan at the outlet will be able to suck the air out at a given CFM overcoming the wall resistance given in terms of Pa/inches of water ?
 
hali24953 said:
Does that mean that fan charts tell what CFM flow will the fan be able to give at a given resistance to its inflow?
The resistance coming from a pipe wall for example, a fan at the outlet will be able to suck the air out at a given CFM overcoming the wall resistance given in terms of Pa/inches of water ?
Yes, the fan curve alone won't tell you the performance of the fan, just the performance range. To find the actual performance you need to match the fan curve to the system resistance.
 
oooooooooo thanks!
 

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