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.
Greetings, fellow glassworkers!
This is the first in what will be a series of spotlight articles covering a wide variety of safety and technical topics for the glass studio. Throughout this document and those to follow, the issues discussed will adhere as closely as possible to meet and/or exceed any existing national (United States) codes (be it building codes, mechanical codes, electrical codes, recommended practice, etc.). Occasionally, a method of doing something will be pointed out as being outside the codes and if your studio is following a method like that described, you would be well advised to change your method to follow the codes.
The things that will be discussed in this series of articles will not be cheap, inexpensive quick fixes. Doing it right the first time is expensive. Nevertheless, it is better to do it right the first time than have an accident or, heaven forbid, a death. The books and videos never tell us what the real cost of glassworking is and that’s a shame. Our craft is a continuously evolving monster with many heads. There are so many directions that the glassworker can turn in their personal discovery of glass art that keeping up with safety and technology can sometimes take a back seat to the artists’ passion for their craft. The best example of this is our knowledge about ventilation.
In the latest entry of incompetence, Dale writes on LE:
In theory 2- 45° turns will have a larger radius turn and be less restrictive to air flow than a single 90° turn….
The FACT, Dale, is that a 45 bend has EXACTLY half the pressure resistance of a 90 degree bend. So two 45 degree bends EQUALS the pressure resistance of a single 90 degree bend. And because of the additional joint that could cause some turbulence, joining two 45 degree bends to form a 90 degree bend, there is a chance that the actual pressure resistance would be slightly higher than using a single 90 degree bend. Continue reading
Can anyone give me an estimate of the temperatures one would expect in an 8 inch exhaust line from a minor torch. I have a baffled 4 sq ft hood about 2 feet above my work table and about 8 feet of pipe to the outside opening. My in line fan is not quite enough at 500 cft/min and I need a bigger inline but don’t want to melt a new one. An outside mounted squirrel cage is an option but not really what I need because of covenant restrictions. I can have a grill propane tank but no outside fans or a/c units -go figure.
Well, first of all, your hood is seriously underpowered. Standard overhead hoods should have at least 125 CFM per square foot, and at 16 square feet, you hood needs at least 2,000 CFM to properly exhaust the fumes captured inside. Continue reading
Early this morning, I came across this post on one of the glassworking forums.
… My hubby went to a furnace expert and asked about squirrel cage fans from a furnace and using them for an exhaust hood. That we wanted to use 8″ duct. They pretty much told him he couldn’t, we’d need 10″ or larger to do get anywhere near 1200 cfm. I was frustrated because hubby said it sounded like we couldn’t do it. I said that if science labs can have a fume hood with a strong draft there was no reason why we couldn’t. And since I worked in a lab once, I know how strong those fume hoods can be. Continue reading