- #1
MikeMass
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- TL;DR Summary
- Are Two Fans Better Than One?
A while back, you guys helped me out with a project on my '69 Camaro. It was all about the Bernoulli principle and airflow taking the path of least resistance. I'm happy to say that the lower closeout panel I built (see pix below) seems to have done the trick as the car I no longer having any sort of overheating issues. I did install a more powerful single 19" fan to replace the two 12" ones and that put it over the top. And that's the basis for my new post... CFM.
Lets have some fun OK? Say you have a radiator core of roughly 23 X 17 which means 391 square inches of surface with a core depth of around 2.75". Using my Cold Case BBC radiator as the general example (actual core is 23" X 16.85"). Hard to calculate the actual cooling surface because its not only there size, plus the 1.250" flattened internal tubes, but then you have 16 fins per inch on the tubes and you'd have to measure the fin sizes as well.
But generally speaking a single 17" OD fan that's rated at 1750 CFM at zero static pressure (no shroud or radiator) is probably going to pull a greater percentage of air within its diameter when mounted to a shroud/radiator... right? That 17" OD equates out to 227 square inches. That means that about 2/3rd of the core/shroud is getting the most pull. I'd guess that the other 1/3rd is progressively getting less pull the further away from the fan blades it is.
So if we switch to a dual 12" setup with the same size radiator core and shroud, the total square inches of the two fans is almost exactly what one 17" fan is... 226 square inches. So while the square inches are similar, the circumference of the dual 12" fan setup is about 76 inches while the single 17" is only about 53 inches. Maybe that means something?
In the close quarters of the radiator/shroud/fan assembly I still have this nagging thought that two 1750 CFM fans will pull more air through the radiator than the single 1750 CFM fan because they would have shorter distances from the fan blades to the edges of the core and therefore promote better cooling. But everything is relative, and if the 17" fan had a more powerful motor and was rated at 4000 CFM it would obviously be the winner. There's so many variables in this theoretical discussion that its hard to conclude anything LOL. So here's a crude diagrams showing the comparison.
As my enlightened gear-head friends here in the Physics Forum explained to me a while back, air will follow the path of least resistance, and if helped along by one or two puller fans in the "controlled airspace", a greater volume of air will move through that controlled airspace. The controlled air space in this case being the finned openings in the radiator core followed immediately by the fan shroud, followed immediately by the puller fan(s), followed by the void of the open (underneath) engine bay.
Another question was raised about stacking one puller on top of another which may not be practical (space) or productive. My layman's understanding about fans is that air doesn't always flow exactly straight back from an angled fan blade, so turbulence might show its ugly face if one tried stacking two fans. There are helicopters and conventional aircraft with counter-rotating blades but I don't know if the distance between blades is fractions of an inch or several inches and whether this would work in an automotive environment.
This thread has my head spinning LOL. Which setup (single 17" OD 1750 CFM vs dual 12" OD 1750 CFM) will actually move a greater volume of air through the radiator core? Thanks in advance for your thoughts!
Mike
(Retired Industrial Designer)
Lets have some fun OK? Say you have a radiator core of roughly 23 X 17 which means 391 square inches of surface with a core depth of around 2.75". Using my Cold Case BBC radiator as the general example (actual core is 23" X 16.85"). Hard to calculate the actual cooling surface because its not only there size, plus the 1.250" flattened internal tubes, but then you have 16 fins per inch on the tubes and you'd have to measure the fin sizes as well.
But generally speaking a single 17" OD fan that's rated at 1750 CFM at zero static pressure (no shroud or radiator) is probably going to pull a greater percentage of air within its diameter when mounted to a shroud/radiator... right? That 17" OD equates out to 227 square inches. That means that about 2/3rd of the core/shroud is getting the most pull. I'd guess that the other 1/3rd is progressively getting less pull the further away from the fan blades it is.
So if we switch to a dual 12" setup with the same size radiator core and shroud, the total square inches of the two fans is almost exactly what one 17" fan is... 226 square inches. So while the square inches are similar, the circumference of the dual 12" fan setup is about 76 inches while the single 17" is only about 53 inches. Maybe that means something?
In the close quarters of the radiator/shroud/fan assembly I still have this nagging thought that two 1750 CFM fans will pull more air through the radiator than the single 1750 CFM fan because they would have shorter distances from the fan blades to the edges of the core and therefore promote better cooling. But everything is relative, and if the 17" fan had a more powerful motor and was rated at 4000 CFM it would obviously be the winner. There's so many variables in this theoretical discussion that its hard to conclude anything LOL. So here's a crude diagrams showing the comparison.
As my enlightened gear-head friends here in the Physics Forum explained to me a while back, air will follow the path of least resistance, and if helped along by one or two puller fans in the "controlled airspace", a greater volume of air will move through that controlled airspace. The controlled air space in this case being the finned openings in the radiator core followed immediately by the fan shroud, followed immediately by the puller fan(s), followed by the void of the open (underneath) engine bay.
Another question was raised about stacking one puller on top of another which may not be practical (space) or productive. My layman's understanding about fans is that air doesn't always flow exactly straight back from an angled fan blade, so turbulence might show its ugly face if one tried stacking two fans. There are helicopters and conventional aircraft with counter-rotating blades but I don't know if the distance between blades is fractions of an inch or several inches and whether this would work in an automotive environment.
This thread has my head spinning LOL. Which setup (single 17" OD 1750 CFM vs dual 12" OD 1750 CFM) will actually move a greater volume of air through the radiator core? Thanks in advance for your thoughts!
Mike
(Retired Industrial Designer)