How does temperature not affect fan airflow?

Please correct me if I am wrong : fans are devices moving a constant volume of air, given a constant rotation speed. Though, flowrate depends of the total resistance of a circuit and the ability of the fan to overcome it. This resistance depends of the circuit iself, and also the air velocity.

Now, since temperature affects pressure, how come that air temperature does not affect the volume flowrate?

The other thing I would like to understand is: take a system with 2 branches, each one with its own fan, joining each other on exhaust. With mass conservation law, I can write that :

total mass flowrate = branch 1 mass flowrate + branch 2 mass flowrate

But, if volume flowrates are also constant, even if branch 1 air temperature is 50°C, and branch 2 air temp is 250°C, does it mean that I can write:

total volume flowrate = branch 1 volume flowrate + branch 2 volume flowrate ?

• Have you considered density differences of the air? Also, have you considered the effect of water vapor at different temperatures? Using mass flow rates is difficult because of the effect of water vapor on the mass of the vapor air mixture. Using volume flow rates overcomes this.
– Fred
Mar 16, 2022 at 17:07
• temperature affects pressure ... it affects air density Mar 16, 2022 at 19:46
• anything that affect the density of the air will affect the flow rate. So both ambient temperature and pressure, plus the flow rate itself. Mar 16, 2022 at 23:45
• a fan at a set rpm can produce various effective flow rates- just change properties of the fluid. Even if at tge surface of a blade, the velocity of fluid matches that of the blade, the velocity profile (velocity as a function of position relative to the rotor) differs due to fluid properties (which do change with temperature). Furthermore you mention resistance which does not matter if already given a constant rpm- power needed to exactly maintain rpm is given, otherwise rpm changes and that easily affects flow rate.
– Abel
Mar 17, 2022 at 3:52