It’s obvious that moving air too quickly through ducts can be a problem. Faster air means more turbulence, more resistance, and more noise. But I run into a lot of people who think that low velocity also can be a problem in ducts. Just recently I heard someone talking about how low velocity causes “rolling air” in the ducts. I don’t know what he meant by that (turbulence, perhaps?), but is low velocity really something we should worry about? And if so, when?
I’ve written about the relationship between velocity and duct size before, so let’s take a quick look back at this important principle. The diagram below shows equal volumes of air in two parts of a duct system—one smaller, one larger. What can we say about the air flow in the two places?
One thing we know is that if we have an air flow rate through one part of a duct and no air leaks out or is diverted through another duct, the rate at other parts of the duct must be the same. In other words, if we have 100 cubic feet per minute (cfm) of air flow in the larger piece of duct below, then the air must still flowing at 100 cfm when it gets into the smaller duct. That stems from the Law of Conservation of Mass and the good assumption that air flow in ducts happens at constant density.
The continuity equation relates the velocity of air in a duct to the cross-sectional area. Photo: Energy VanguardIf the air flow rate is constant, it’s straightforward to show that the quantity A x v is constant, too. That’s what we call the continuity equation. The simple way of stating this principle is: