How do stacked fans affect airflow and pressure?

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Discussion Overview

The discussion revolves around the effects of stacking two fans in series on airflow and pressure. Participants explore the theoretical implications of fan operation in different conditions, including external resistance to airflow and the characteristics of pressure and volume in the system.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that stacking two fans will ideally double the pressure but not change the volume of airflow.
  • Another participant counters that the outcome is highly dependent on external resistance to airflow, noting that two fans in series would not double the volume of airflow.
  • It is mentioned that airflow through each fan will remain the same, but they will operate at different pressures, leading to slightly different pressure-volume (PV) characteristics.
  • A participant questions the meaning of PV, leading to clarification that it refers to the relationship between pressure and volumetric flow rate, as indicated by the fan curve.
  • There is speculation that while an increase in pressure might occur, it could also lead to an increase in flow, resulting in less pressure generated by each fan.
  • One participant expresses a belief that a small percentage increase in flow and a large percentage increase in pressure is realistic, but acknowledges the need for more details about the system's resistance to provide a definitive answer.
  • It is noted that if the fans are stacked in free air, there would be no change in performance, whereas in a duct, interactions between pressure and flow would yield different results.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views regarding the effects of stacking fans, particularly concerning airflow and pressure dynamics in different conditions.

Contextual Notes

Limitations include the dependence on external resistance to airflow, the need for more specific details about the system, and the unresolved nature of how pressure and flow interact in various configurations.

KrisOhn
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I have two fans that I want to stack in series. They both have stationary fans on the downwind side of the blades to create optimal conditions for the next fan, they have been designed to stack. Am I correct in assuming that the two fans will only (ideally) double the pressure of the air, and not change the volume?
 
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The answer is highly dependent on the external resistance to airflow.
Those fans are designed to move a large volume of air with very little pressure rise. Two fans in series would not double volume of airflow.

The airflow through each fan will be the same. But the fans are operating in air at different pressures so they will have slightly different PV characteristics. The pressure across each fan will be similar but not identical.

Ideally, if two fans doubled the pressure, then the airflow would have to remain the same.
But an increase in pressure would probably increase the flow, so they would generate less pressure each.
Two fans could not quite double the pressure of one fan.
 
Baluncore said:
The answer is highly dependent on the external resistance to airflow.

There is effectively 0 external resistance to airflow. The air is flowing cleanly through a pipe.

The airflow through each fan will be the same. But the fans are operating in air at different pressures so they will have slightly different PV characteristics. The pressure across each fan will be similar but not identical.
What do you mean by PV?

Ideally, if two fans doubled the pressure, then the airflow would have to remain the same.
But an increase in pressure would probably increase the flow, so they would generate less pressure each.
Two fans could not quite double the pressure of one fan.

So realistically I'm looking at a small percentage increase in flow and a large percentage increase in pressure?
 
KrisOhn said:
There is effectively 0 external resistance to airflow. The air is flowing cleanly through a pipe.
You said there was pressure: pressure is the resistance to the flow (or if you prefer, a consequence of the resistance).
What do you mean by PV?
Pressure vs volumetric flow rate. He's referring to the fan curve. Random example:
rpm_curves.gif

So realistically I'm looking at a small percentage increase in flow and a large percentage increase in pressure?
Probably, but we can't really answer without more details about the system's resistance or lack thereof. If you stack the two fans on top of each other in free air, you get no change in anything. If you have them in a duct, you will get some interaction between pressure and flow that results in a new airflow and pressure.
 

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