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.