I’ve been asked my opinion on the Vortex and CAN FAN in-line fans; so I did some basic research.
I can’t recommend the Vortex line simply because the manufacturer does not provide the static pressure curves for their fan lines. This is very important because you need to know how a given fan will perform in your exhaust system. A given fan might have a CFM output of 650 CFM at zero inches of static pressure (free air), but what is its output at a half inch or more of static pressure? It could very well be far less, and in that case, the fan is not meeting your design criteria.
The CAN FAN brand (by C F Group, Inc. http://www.canfilters.com/fan_metal_home.html ) has a number of in-line fans, and the static pressure information is available on their website. One thing I have noticed is that the distributors of the CAN FAN often mis-represent the CFM of the fans. Here’s an example, taken from Botanical.com ( http://www.botanical.com/hydro/air/canfan.html ) Botanical.com lists the 8″ standard with a free air CFM of 493, while the manufacturers website lists it as 483. The 8″ High output is listed by Botanical.com as 737 CFM, and the manufacturer lists it as 722. You need to be very careful when purchasing fans based on the sellers description — always go back to the manufacturer and get their numbers when designing your system.
I have been reading a lot of the threads written on LE where people are purchasing their fans from a given seller and purchasing a specific fan based on a person’s recommendation, not on the design requirements of their ventilation system. This worries me a lot — it tells me that a lot of people are just mounting a fan without bothering to actually sit down with pen, paper and calculator to check their system requirements. While you may get lucky and get close to the fan you need by using a fan that someone else is using, please remember that everyones ventilation system is different — different lengths of ducting, different size of hoods etc. Unless the ventilation system is exactly the same across the board, you should not purchase a fan from a seller just because your best on-line friend is using the exact same fan.
Chaotic Glass by Mike Aurelius 
Thank you, Mike. I am one of the ones who purchased a Vortex 8″ 747 CFM fan, which is the correct CFM based on the face opening of my hood at 6 square feet.
Is there a way to test static pressure? My system seems to work quite well – it passes all the smoke tests, and the flame (and presumably the fumes) are getting sucked towards the ducting. So are the inevitable bugs that make their way into that part of the garage. I have torched in a studio with poor ventilation, so I know how it feels to have the fumes surrounding you. I don’t sense that at all when my fan is on.
Not really — those kinds of tests are done with very expensive professional equipment. I can calculate what your static pressure is, based on the diameter of the ducting, the total length of the ducting and how many bends you have, but there is no way to determine what your actual CFM is without the SP curves for that particular fan model.
You can e-mail me at the address found here: https://mikeaurelius.wordpress.com/questions/ with your design information and I can calculate your system SP for you with the above information.