Ventilation question

From the ISGB forum:

Hi , right now I’ve been using the system Vince described in the Drew Fritts book. I had been using a minor and everything worked well.
Recently purchased an Lucio torch from Carlisle. I noticed I needed to upgrade my ventilation system. So I like to start from scratch, build a new hood etc. My room is 8 x 24.
I have also read an interesting article, and would like to get your thought on it please. Here is a link.
http://andreaguarino.com/ventilation/

I took a look at Andrea Guarino’s website and the link on her ventilation system. If you are a regular reader of my blog, you know my feelings on open duct ventilation (I don’t believe they work and don’t recommend them).

I found quite a few ventilation fallacies being perpetuated in the design description. Here’s a few:

Note: There is some turbulence caused by the ribs of the flex tubing however an adequate fan like the one recommended can overcome the small losses caused by this. An artist needs to be able to use flex tubing when curving around obstacles. Also, it is important to have a system that can be cheaply and quickly assembled rather than worry about small turbulence losses due to the ribs of the tubing.

“Some turbulence” is right. When calculating static pressure, part of the calculation is loss due to friction and turbulence from the relative roughness of the material being used in the ducting. The “standard” material used in ducting is galvanized metal, which has an average roughness of 0.0005 inches per foot. Corrugated ducting, such as shown in the website referenced above, has an average surface roughness of approximately 0.25 inches per foot, or 500 TIMES more roughness. This translates directly to 500 times more turbulence, which results in a huge increase in static pressure.

As an example, the fan shown is 289 CFM. Let’s assume a 6 foot duct run. If normal galvanized ducting is used, the static pressure including friction loss from the duct is 0.06 inches. For corrigated ducting, the static pressure including friction loss from the duct is 2.60 inches. Many fans cannot handle such high static pressures.

Now, scroll down a bit and look at the picture of the Mini-CC flame. showing the flame (poorly adjusted flame, mind you), being sucked down into the duct. I’d like to know how any glassworker can work in a flame where all the heat is being sucked away from the torch so fast. This is another reason why I don’t like duct-type ventilation systems. You need far more air movement per duct area in order to trap all the fumes than you would need for a conventional overhead hood. When the flame starts to be affected by the air movement, there is a serious design flaw that needs to be remidied. Even the next picture down, with the rod warmer attachement over the duct face shows similar flame distortion from air movement. The author notes that the flame distortion shows “the power of the system”, I’d counter that with “the flame distortion shows that you are moving too much air and your glass work will chill faster than it needs to.” This isn’t about P-O-W-E-R, it is about exhausting bad air, but only the bad air. A properly designed overhead hood or workstation enclosure is not going to distort the torch flame at all.

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