Vent hood fan + exhaust fan?

[Mercury]

I have my shop set up in a small trailer with a 200 CFM range hood (18" x 30") installed flush with the wall. I want to get up to around 700 CFM and would rather not mess with my range hood installation since it works pretty well as is...just insufficient CFM. I'm considering installing a fan right in the wall in front of the torch...something in the range of 500-600 CFM. This would bring the total CFM up to the correct range for my setup and vent the combustion fumes from the hood and the heavier than air particulate from the lower fan.

Any opinions on how effective this strategy is and also very importantly, any suggestions for a low-noise fan to use in this position?

[MikeAurelius]

Nope. Won't work. The two fans will fight each other. Stacking fans is not additive. The max you will get is 500-600 CFM, not 700-800 CFM, plus you will burn out the smaller fan in the hood.

The best thing to do is to rip out the fan in the hood and install a proper sized fan either on the hood or at the exhaust point on the installation (inside or outside the wall).

[Mercury]

I understand why stacking two fans inline won't work. But is this really the case with separate fans in two locations? I'm concerned about heavier than air particulate and would really like to utilize a fan directly in front of the torch to exhaust all of this immediately. But of course you always need a hood system for the gases that rise with the hot air too.

[MikeAurelius]

Yes. This is true any time you have two (or more) fans in a system.

The rule is one system, one fan.

You are much better served re-designing your system with one single fan ducted outside.

[Mercury]

Yes. This is true any time you have two (or more) fans in a system.

The rule is one system, one fan.

You are much better served re-designing your system with one single fan ducted outside.


Can you tell me why this is? It doesn't make intuitive sense to me and I like to know why I'm doing things. I never was very good at following rules unless I knew why they existed.

[Dale M.]

Stronger fan will over power weaker fan and and actually reverse air flow through weaker fan, and use it as a make up air source instaed of additional exhaust port.

Dale