Altitude and air temperature make a substantial difference in the performance of any ventilation system. The calculations I have provided in the Ventilation Primer, Part Two, are based on sea level and 70 degrees F.
Overhead hoods have a problem: they are non-functional over most of the width of the hood. The only part of the hood that actually functions as designed is an area about 1-2” wide surrounding the suction duct. This is fine for small torches while making small beads, but what happens when you upgrade your torch and start working with larger pieces of glass?
Insertion of large pieces of glass into the flame plume deviates it and the open duct in your hood is no longer functional. And once you start working with powders, enamels and/or fuming, the only draw area is directly in line with the duct opening. How can the standard overhead hood be made to work more efficiently for the lampworker? Continue reading
Ok. We’ve talked about the basics, now let’s take a look at some basic design issues.
Exhaust System Design
So, where do we start? Well, let’s talk first about a couple of important numbers and calculations that have to be made first.
CFM: Cubic Feet per Minute. The amount of air that a ventilation system can move. It is based on how much air a given fan can move against a given amount of pressure.
Velocity: The speed the air moves inside the duct. It is measured in Feet per Minute.
Velocity Pressure: The pressure created by trying to force air at a given Velocity through a given duct size.
SP: Static Pressure. The total pressure against which the fan moves air. SP increases as the size of the duct decreases, with the addition of bends, and with any amount of turbulence. As SP increases, the efficiency of the fan to move air goes down, or, to state it differently, the higher the SP, the lower the CFM from design.
Loss Factor: A multiplier, usually fractional, that is the amount of friction induced by ducts. This is number is a constant for specific duct types and is usually presented in a look up chart form. The chart we will be using in all these calculations can be found in here: https://mikeaurelius.files.wordpress.com/2007/12/table1.pdf .
Each one of these numbers or calculations factors into the design of an exhaust system.
I will present several different designs to show how each affects the total design.
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. Continue reading
The ever delightful Dennis Brady never fails to entertain. His latest on using a window AC unit as a source of fresh air from LE:
Air is air. A specially dedicated incoming air supply is a lovely thing but is NOT essential. Whether or not air coming in through a heater or AC is sufficient replacement depends on the volume and consistency of that incoming air. If there’s enough volume to replace air being exhausted, and it comes in fairly steadily, it will work perfectly. Continue reading
As I’ve mentioned in the past, the folks who hang out at Wet Canvas seem to be living in their own little world, isolated from the rest of the glass world, at least as far as safety issues are concerned.
The thread starts with a very good question, especially at this time of year:
Every year I have the same problem in the heat of summer. Obviously I want my exhaust to work well, but the window unit in my studio can not cool the replacement air fast enough. I thought I remembered a thread about this years ago, but I can’t find it? Opposite problem in the dead of winter with heating. Any ideas/solutions?
And immediately goes south: Continue reading