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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More on in-line fans

I’ve been asked my opinion on the Vortex and CAN FAN in-line fans; so I did some basic research.

I can’t recommend the Vortex line simply because the manufacturer does not provide the static pressure curves for their fan lines. This is very important because you need to know how a given fan will perform in your exhaust system. A given fan might have a CFM output of 650 CFM at zero inches of static pressure (free air), but what is its output at a half inch or more of static pressure? It could very well be far less, and in that case, the fan is not meeting your design criteria. Continue reading

On in-line fans…

In the past, I’ve been an outspoken critic of in-line fans for a lot of reasons, many of them annotated elsewhere in this blog. Lately, I’ve heard of a company that manufactures in-line fans that are far beyond anything else available in this style of fan.

The company in question is Fantech. They are located in Sarasota Florida and provide a variety of fans that can easily be used by glassworkers almost anywhere. Continue reading