Constructive Criticism

It seems Dennis Brady is now styling himself as an “expert” in ventilation.

The article is fairly well written, but contains some factual errors as well as some mis-conceptions about ventilation. It’s fairly obvious that Dennis has only a bare minimum of understanding about the concepts and requirements of air movement. You also need to keep in mind that Dennis has only Canadian “experts” to call on, not American experts. Canadian standards and US standards are quite different. Take everything he says with a grain of salt.

You don’t need a fan so powerful it can vacuum the tools off your work table. You just need to control where the air moves. Air that is moving slowly in a controlled direction produces better exhaust then a high speed exhaust. A large slow moving fan is more effective at moving air then a small fast moving one that can create air turbulence. To control the air flow requires ensuring the air flows only in from the replacement air intake and out the exhaust exit. Any other places air comes in (like an open window or door) will interfere with that controlled air flow and reduce the effectiveness of your exhaust system. Sealing all air entry to your work area, other than the replacement air intake, might seem excessive – but it will help control air flow.

True and false. Slowly moving air does not properly exhaust torch fumes. This is a common misconception by those who seek to justify using small (or for that matter large) slow moving fans. They point to the fans used in a greenhouse (for example) as justification for their statement, totally disregarding the fact that greenhouses do not contain fumes from torches or molten glass.

Stating that fresh air coming in from a door or window interferes with “controlled” air flow is a total mis-statement. Fresh air is fresh air, no matter where it comes from. An open window or door WILL reduce the amount of fresh air coming through a ducted fresh air system, and only in that particular case should you keep other windows and doors closed.

Your exhaust system will work better if you work in a booth than in an open space. If you have an overhead hood, install partition walls from the hood down to your work table to create a 3 sided booth. This will force the air to vent up and out and prevent it from escaping left or right.

True, but it is actually a 4 sided box with the hood being the top.

What size fan you need will depend on how close you place your exhaust hood to where you work. A 200 cfm fan will be adequate if your hood is placed less then a foot away but an overhead hood several feet away would require more then 400 cfm in an enclosed booth and 800 cfm in an open work area. You might be tempted to just install the biggest fan you think you might need, but the larger the fan you use, the more replacement air you must provide. 

False. The fan size is solely based on the side of either the hood or the “face” opening of your enclosure. I have yet to see a 200 CFM fan work properly for ventilating a glass workstation.

A rough guideline is to assume your fan should draw 125 times the surface area of your hood or work area. For example – for a 24″ x 30″ exhaust hood installed in a 3 sided closed booth above your work table, you would calculate having a 5 sq ft work area. 5 times 125 = 625. To effectively vent fumes, you would need a fan capable of drawing 625 cubic feet per minute. If you didn’t have closed sides, you would need a larger capacity fan. If you had the exhaust hood mounted immediately beside or over the torch work area, you could use a smaller capacity fan. It is NOT sufficient to consider only the hood size in calculating fan size. If you use a 2 square foot hood over a 5 square foot work area, you still must exhaust from 5 square feet.

False. 125 CFM per square foot for overhead hoods and 100 CFM per square foot for workstation enclosures. If your workstation enclosure has a face (the opening that faces you) of 36″ wide by 36″ high, that is 9 square feet 9 x 100 = 900 CFM. If the hood measures 24″ x 36″, that’s 6 square feet (not 5), and you would need 6 x 125 = 750 CFM. The only measurement you need to make is either the opening of the workstation enclosure or the opening of the hood. Nothing else matters.

You can use a burning incense stick, lit candle, or lit cigarette to test to see if your exhaust system is removing all fumes. Move it to all the places on your work table you
might be working. Watch the fumes and see if they are all being pulled out the exhaust. Is there some place where the fumes do not totally exhaust?

True for as far as it goes. Smoke flow is not the end-all be-all of a good ventilation system. I have seen plenty of ventilation systems that pass the smoke flow test but fail to completely remove NOX and other odors which lead to breathing problems for a large number of people.

You need a fan that actually draws air as opposed to a duct fan that is designed only to boost air that has already been pushed into the duct. Squirrel cage type fans (as used for furnace blowers) or centrifugal in-line fans are preferred. In-line fans are more affected by the heat from torches and should be installed at least 3 feet from the flame. The longer the duct line, the more restriction is created. Booster fans are inexpensive and, if you have to have a relatively long exhaust ducting, it’s a very good idea to install one to help boost air flow along. They are especially helpful in helping reduce air flow restrictions created by elbows.

Both true and false. Booster fans should never be installed in a glassworking ventilation system. A properly designed system where the total pressure of the duct run is taken into account will not need a booster.

It’s essential you have a fresh air intake that brings in fresh air to replace the air being exhausted. The fresh air intake should be the same size as the exhaust and should be at least 10 feet away from the exhaust to ensure it isn’t interfering with the exhaust flow.
DO NOT have more replacement air then exhaust air. This will create air turbulence that reduces the effectiveness of your exhaust system.

I prefer to use a fresh air duct that is one size larger than the exhaust ducting. For example, if the exhaust ducting is 8″, I use 10″ for the fresh air. The replacement fresh air should be passive, that is unpowered – meaning it does not have a fan. You can always have more fresh air supplied than exhaust air. In fact, in many cases, this helps prevent some types of ventilation problems caused by not enough fresh air. It never reduces the effectiveness of an exhaust system to have too much fresh air.

Bends and curves in your ducts will slow down the flow of air traveling through the ducts. The more gentle the bends and curves, the less they’ll restrict flow. If you must install a 90° bend, you create less restriction if instead of using a 90° elbow, you use two 45° elbows connected together to create the bend. It’ll work even better if you use flexible ducting on a long gentle bend. Avoid using corrugated ducting. It creates turbulence that restricts flow.

True and false. Bends are bends. They have a fixed amount of restriction. A 45 degree bend has exactly half the amount of restriction that a 90 degree bend has. And when you add the possibility of some turbulence from the joint, putting two 45 degree bends together to form a 90 degree bend may actually increase the amount of restriction. Never use flexible ducting — it has the similar turbulence problems that corrigated ducting has. All ducting must be smooth sided.

There are times when high speed air movement is needed and other times when it creates problems. If possible, install a fan with variable speed controls.

False. Remember that your fan size in CFM is based on the size of the opening of either the workstation enclosure or the overhead hood. Reducing the speed of the motor decreases the air flow and therefore increases the risk of contaminates not being exhausted by the system.


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