Same old, same old…

More fallacy from Dennis Brady on LE:

I mean 2 – 45 deg elbows connected together to produce the 90 deg bend. The wider arc causes less air flow resistance. Even better would be a wide arc curve (these are available for metal ducting but are pretty pricey).

For at least 40 years, the plumbing code for drains and vents stated specifically that in any run of drain or vent, one only 90 was permitted but an unlimited number of 45’s was allowed. If the experts insist that makes a critical difference for venting plumbing drains, it’s reasonable to assume it’ll make a significant difference for venting torching fumes.

I guess Dennis has never looked under his sink. A standard plumbing P trap consists of at least 3, count them, 3 90 degree bends to form the drain trap. 90 degree bends are in every single plumbing run made. They are not built from multiple 45 degree bends, but cast PVC 90 degree bends. Walk down the aisle in your local home improvement store and you will see 90 degree bends by the hundreds. If your basement is unfinished, look at the plumbing — you will see 90 degree bends practically everywhere.

And as far as “reasonable to assume”…well, if he can get it wrong with 90 degree bends in plumbing, it is “reasonable to assume” he’s wrong for moving air as well.

Compare it to a road. A corner slows flow down more then a curve. The wider the curve, the less it slows down. On a really wide curve, you can travel the same speed as a straight run. When you install your ducting, think of it like building a road where you want traffic to keep moving steadily without having anywhere it has to slow down.

Now we are comparing driving a car on a road to moving air in a duct…will the silliness never end? Plumbing was at least a fair stretch, but a road???

There has been extensive discussion on the needs for a high capacity fan. There are factors that will DEFINITELY reduce the size of fan needed:

1. The closer the hood is to the torch, the less fan capacity is needed.
2. The more enclosed the work area is, the less fan capacity is needed.
3. The lower the ducted air flow obstruction, the less fan capacity is needed.
4. The less other air movement, the less fan capacity is needed.

The 125 rule is a guideline only. It is NOT a commandment from God and it is NOT a legal requirement.

I’ve covered this before —

1. It is the size of the hood, not how close it is to the torch.
2. This is somewhat true, in that a 5 sided box (workstation enclosure) needs only 100 CFM per square foot of opening versus an overhead hood which requires 125 CFM per square foot of opening.
3. Air flow is based on the size of the hood, and the resultant drop due to static pressure from length of run and number of bends. Once the drop is taken into account, you oversize your fan to meet the requirements of the system design.
4. Huh?? What other air movement? All we are concerned about here is the movement of air to exhaust the workstation, not the entire house. If Dennis means any airconditioning or air movement through the house, he’s way off base.

The 125 Rule is a code item, which means it IS law. In the United States.

Once again, if Dennis is going to advise people living in the US, he needs to get up to speed on the code.

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