Why Static Pressure & Max Flow Specs Are Poor Measures of Fan Performance

Posted: February 18, 2013 in Fans

This is just a quick graphing demonstration of why fan box specs using static pressure and max air flow are generally poor tools in comparing fan performance.  Very much like pumps, fans have a pressure vs flow rate relationship that is not at all linear.  Some are flat some are curved, some have big bumps and dips and it’s the area between these two points that ultimately matters, not what is at the ends.

For example, let’s compare these two fans using box specs:

Cougar Vortex PWM

http://www.cougar-world.com/fileadmin/downloads/COUGAR%20VORTEX%20PWM.pdf

Max Airflow = 70.5CFM

Static Pressure = 2.2 mm H20

Gentle Typhoon AP-15

http://www.nidecamerica.com/fanpdfs/d1225c.pdf

Max Airflow = 58CFM

Static Pressure = .08inwg = 2.03mm H20

Using these specs alone you would assume the Cougar would produce more air flow on a radiator.  Both max air flow and max pressure are greater, you can’t go wrong with that can you?

Well, luckily both of these fans do provide P-Q curves(Most do NOT), so I spent some time cropping out their P-Q curves, converting units and overlaying them.  First here are their native P-Q curves in different units of coarse making a comparison difficult.

CougarVsGT15part2

So I brought both into Excel and plotted them out the best I could, here is how that looks:

CougarVsGT15part4

In addition noise box specs are “Open Air” with no restriction and no adverse “Real World” affects when mounted to a radiator.  Since most fans out there don’t bother publishing P-Q curves, you essentially don’t know what there real world performance will be and the noise specs don’t mean much on a radiator either.

So, just goes to show you…don’t rely too much on fan specs, they are not very good measures of real world performance.  Just like pumps, it’s the pressure somewhere between these two points that matters in the real world and that isn’t static pressure and it isn’t max air flow, it can only be represented by a full P-Q curve.

And my intent here is not to pick on any one fan. I would like to try some cougar fans out myself considering they are supposedly producing this curve at only 1500RPM, but the curves were available so I wanted to see for myself how the PQ curves compared.  Sure enough it produces a very contradicting conclusion vs. looking at only static pressure and max air flow.

Hope this muddy’s the waters for the box spec discussion out there…:)

Martin

Comments
  1. Guy Cotnoir says:

    Okay, I just don’t understand here. As I see it CFM is the amount of air moved, the equivalent of speed for a car if you will. While static pressure is the fan’s ability to push the air through restriction, or the equivalent of a car’s engine torque, or a car’s ability to pull a bigger trailer…if you excuse the poor parallel.

    So how can it be that a slower fan will have more static pressure?
    Am I missing something here?

    • Martinm210 says:

      Generally fans with more blades and larger blade coverage tend to be more pressure oriented than fans with large gaps and fewer blades. All this post talks about is trying to convey what static pressure and max air flow are. Max air flow happens when there is no restriction at all. The fan is sitting in open air and pressure is zero. Static pressure is the opposite. They are the two extremes of the pressure vs flow response curve.

      In reality the fans operate somewhere between these two points. What is interesting is the curve shape itself. Many fans have a relatively linear curve, but not all. Fans with a bump or humped curve can produce a net higher flow that what you would assume. This just means the two spec we often use for comparison are an incomplete picture and should be relied upon 100%. The only way to truely compare actual performance would be to compare the entire curve.

      In a nutshell, it is more complicated than static pressure and max air flow.

  2. Michael says:

    [quote]This just means the two spec we often use for comparison are an incomplete picture and should be relied upon 100%.[quote]
    I think you mean ” … should NOT be relied upon … “. FYI: in all my builds I make certain I have positive filtered-air pressure. I never, ever, use exhaust fans, I never go without filters over fan intakes. And I close off whatever potential exhaust-points I need to in order to direct overall airflow. I ensure that all main components have fan-air blowing directly on them, including if possible, the waterblock. The intake fans wind up being mounted on a perforated case panel. I only use 7-14mm of shroud between a case panel and the fan to quiet blade noise, and between a push fan / radiator in [superstitious] hope the spacing will allow more air to flow over the portion of the fins lined up with the fan hub. Superstitious hope means: no hard data. I too have found no generic results.
    You have done a mind-bending amount of work. Thank you.

  3. Michael says:

    I thought I should add: I always use bottom-mounted fans on HDDs, and use resistors to slow them way down. This keeps them cooler than hoping for enough airflow from case intake fans, and is quieter. Same with all fans: I slow them down and use them precisely where needed. I have maybe 10 hdds spinning for over 7 years always cool to the touch! Sort of like I do with my speakers: with speakers in each room, sometimes more than one set, I can keep the volume down and hear all over without waking up the neighbors or needing headphones. Using several small and slow fans keeps things cooled by direct airflow. Barely adds to noise, and only slight increase in electrical power usage.
    I hope my comments are somehow useful.

  4. Rita Fernandes says:

    Hey martin! interesting post. Indeed the operating point and the real system resistance is maybe more important than the maximum air flow provided by the fan.

    I am writing my thesis about electrical fans for the buses engine cooling system. do you think you can help with some questions? thanks!