How do stacked fans affect airflow and pressure?

[KrisOhn]

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?

 

[Baluncore]

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.

 

[KrisOhn]

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?

 

[russ_watters]

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:

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.