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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A copycat?

I’ve been accused of being a copycat:

IT was adopted on ISGB forum first by Vince, than Mike copycatted it. It’s the specification for industrial class A fume hoods…..

Ummm…no, sorry Dale, but you are incorrect. Both Vince and I use the ACGIH (American Conference of Governmental and Industrial Hygenists ) recommendations for ventilation systems from their “Industrial Ventilation: A manual of Recommended Practice”. There is no “copycatting” going on here, just the usage of an internationally recognized organizations recommended practices. It is unfortunate that Dale chose to use the wording he did as it denigrates the work I do on behalf of lampworkers around the world. Continue reading

Chemical exposure, decoding the limits

When talking about exposure to hazardous chemicals, the safety industry has developed some acronyms that make it difficult to understand what the actual exposure limitations are.

ppm: parts per million

OSHA: Occupational Safety and Health Administration

NIOSH: National Institute for Occupational Safety and Health

ACGIH: American Conference of Governmental Industrial Hygenists

TWA: Time Weighted Average. The averaged exposure to a chemical over certain period of time (usually 8 hours for OSHA).

PEL: Permissible Exposure Limit. Usually provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over an 8 hour TWA.

STEL: Short Term Exposure Limit. Provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over a short term time period such as 15 minutes. This is also a TWA (time weighted average).

REL: Recommended Exposure Limit. This is purely a NIOSH term that takes into account OSHA data as well as “real life”, and sets a limit at which health and life may be affected for exposures beyond the REL.