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inkool 2017-11-10 11:09am

Ventilation setup critique

I am in the processes of building a studio in my house and I have found the information on this site invaluable in helping me plan my ventilation setup. Thanks!!!

I would like to get some informed opinions on my proposed setup before I start building in case I am missing something. (sorry in advance for the wall of text).

I plan on running my torch on natural gas and oxygen concentrator.

My ventilation will be a 30" x 22" hood with a 900 CFM max fan. The duct coming out is 7" and will run around 12" to 18" before going outside with one 90 degree elbow. Based on what I have read that setup should work and I most likely will not have to run the fan at full blast.

My make up air will be coming in from around 15 feet away with two or three 90 degree elbows. I plan on using 10" ducting ending in two 4" x 10" register vents under the exhaust hood on my work bench. The 10" may not fit so my second idea is to get two 8" ducts pulling from different locations.

Please let em know if I am missing anything or overdoing it.

Thanks Again!

Speedslug 2017-12-03 10:18am

I don't see that you are missing anything.

In my humble opinion you might be right at the -"just enough"- edge so keep a close eye on any symptoms of eye irritation, stuffiness or over tiredness.
Especially if the winds outside are strong.

I know I spent way too many years as the "Safety Training" guy in my days in the Navy but unless you are buying a jet engine to use as an exhaust fan I don't think it is possible to -over do it-.

aommaster 2017-12-03 8:17pm

In addition to what Speedslug said, I'd like to also bring your attention to another aspect of fan sizing.

Exhaust fans are specified with two parameters, the volumetric flow rate of air moved (the CFM) and the maximum developed pressure, or suction head. The head is normally provided in units of water column inches (you'll see things like 0.3 inches water column) or Pascals (around 75 pascals).

You'll have to make sure that the fan is able to develop the necessary pressure in order to overcome pressure drop as a result of ducting and elbows. From your description, it looks like there are no elbows or any other constrictions in your duct that can cause a pressure drop, which means the pressure drop in your duct is very minimal. Note here that a pressure drop still exists, but it's not a significant amount, and any small fan will be able to overcome that drop.

However, if you do decide to modify the design of your ducting, the pressure drop is something you should also consider.

Speedslug 2017-12-04 3:08am

I will only add that manufacturers of bath fans and kitchen fans often greatly over state the capacity of their equipment to move air.

A 500 cfm bath fan often only moves 90 cfm in the real world and kitchen fans listed as 900 cfm come in closer to 300 when tested by someone other than the maker.

These things are only moving cooking fumes and bathroom steam so no one is going to get poisoned if they stretch the truth a lot.

Industrial fans have a much better chance of actually expressing the true capacity of the purchased equipment.

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