How to decode fan specs?

[GetMore]

I am trying to choose a fan assembly, and right now I'm down to two choices that look like they'll fit.
One has 13" fans with 11 blades apiece, turns at 1680 rpm, and draws 50A with 265W motors. Claimed cfm is 4000. The fan blades are curved.
The other has 13.5" fans with 8 blades apiece, turns 2100 rpm, and draws 28A. Claimed cfm is 4600. The fan blades are straight.



Offhand I'd say the 4600 cfm unit pulling 28A is the way to go, but it seems too good to be true. I couldn't get charts from the companies, so I don't know what pressure drop these numbers are at, and what the flow is with some restriction.

How do you determine the best fan?

[Adam@Autoloc]

have you checked out zirgo fans? they have high cfm rating with low draw..

www.zirgo.com

[GetMore]

They really don't give any REAL specs, either. I'm looking for real world specs.
Besides, I want a twin fan unit, and they just sell the fans. Though, I suppose I could make a shroud myself and then just mount a couple fans to it.

[KUL FIR CHICK N]

Your best clue to decode the marketing hype on fans is amp draw. If they're drawing more amps, they're doing more work (moving air). Of course it's more complex than that, but you're unlikely to have one fan with near twice the efficiency of the other.

Spal's published #'s are very accurate, many others are not even close. Also, fans are rated with a "fan curve" which shows flow vs. restriction. Without a fan curve, your best bet is amp draw. That is, until retailers start inflating amp #'s too.

[GetMore]

inflating amp #'s too
I like that!

I had emailed two companies, asking for info, and one did respond, and the other did not.
One told me they have some (most? I don't remember now) of the info, but don't have it in a format they can send out. They did say that their ratings are more real world than most other company's. It all sounded great, but could have been a bunch of BS, which of course is why I turned to you guys. I wanted some actual end user info.

Your comment about amp draw relating to airflow was one of those things that I did think about. What I was wondering about from those specs was if the power requirement was going to be the same when the fans were installed, or once they had some resistance would they start drawing a lot more power?

I was leaning toward the unit with the higher draw, since their numbers just seemed more honest, or realistic.

[GetMore]

What do you thing of Derale and Flex-A-Lite?

[tonykim]

What the fan companies publish is airflow vs speed typically with the fan in it's shroud mounted in a tunnel without backside restriction. And even if they do publish flow values it's only with an assumption of what the downstream restriction is (front profile of the engine and exit restriction of the engine compartment). Overall I agree that power consumption would be a reasonable estimate of actual airflow. It also is related to the efficiency of the motors and the efficiency of the blade design).

[thoroco]

What the fan companies publish is airflow vs speed typically with the fan in it's shroud mounted in a tunnel without backside restriction. And even if they do publish flow values it's only with an assumption of what the downstream restriction is (front profile of the engine and exit restriction of the engine compartment). Overall I agree that power consumption would be a reasonable estimate of actual airflow. It also is related to the efficiency of the motors and the efficiency of the blade design).

I'd have to agree with this and add that the back side of the fan is very important. The Mark VIII and Taurus back sides are very open and the motor support structure is angles with the air flow greatly increasing the CFM. Look at a Zirgo and its has a lot of motor support structure on the back side so they can reduce the depth of the fan and that reduces the the CFM and increases amp draw. To get the same CFM as a MarkVIII or even a Taurus you need the $300 Zirgo Ultra series.