The Basics of Ventilation, Part Four: Temperature and Altitude adjustments

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.

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The Basics of Ventilation, Part Three: Overhead hood design

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

The Basics of Ventilation, Part Two: Doing the numbers

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:  .

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.

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The Basics of Ventilation, Part One: Overview


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.

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Economic issues have required changes

As you may have noticed, we are no longer selling cerium oxide. For about the past six to nine months, the world-wide supplies of cerium have dropped significantly. This is due to export restrictions placed by China on raw materials. They have also placed rather large tariffs on all outbound shipments of some of the more “precious” raw materials. The combination of factors has caused our cost to skyrocket by 750%. Needless to say, we no longer have excess polish that we can afford to sell.

About the same time (roughly six to nine months ago) Schott in Germany shut down their welding glass (float glass) manufacturing processing plant. This was due (they claimed) to low demand for the product. Schott was the last large-scale manufacturer of welding sheet glass in America/Europe. The last of the Schott glass is gone from our supplier, and they are currently trying to get glass from China. This has lead to a shortage of sheet glass in shade 4, 5 and 6, which has also hit us.

We are starting to work with the Chinese glass, but it has some drawbacks, mainly an apparent color difference. Schott welding glass is grey-green in appearance. Chinese welding is yellow-green in appearance. Therefore, we will no longer be able to replace single lens units in any of the AGW-250, 286 or 325 filters, they must be sold in pairs so they color match. This will *NOT* be covered under our warranty.

More on amperage ratings for kilns

I’ve received some feedback from folks wanting more information about my previous post “The rule of 80% and kilns“.

First, here is a direct link to the NEC (National Electrical Code): (scroll to near the bottom of the page and click on the link that says “View the 2008 edition of this document”)

The important sections are 210.21 and 210.23. In part, they say:

Table 210.21(B)(2) Maximum Cord-and-Plug-Connected Load to Receptacle

Circuit Rating …………………… Receptacle Rating ………………. Max Load (Amperage)
15 or 20 Amps ………………………. 15 Amps ……………………………… 12 Amps
20 Amps ………………………………. 20 Amps ……………………………… 16 Amps

210.23 Permissible Loads

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place

The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit amperage rating.

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