Adapted from Indoor Air Quality from the Indoor Environment Technician's Library series. Visit www.lamabooks.com.

AIR LOSSES IN A DUCT

FRICTION LOSS
When water flows down a stream, friction is created as the water rubs along the banks. As a result of this friction, water along the bank flows slower than the water in the center of the stream.

In much the same way, air rubbing against the sides of duct causes friction. The airflow is slower along the sides of the duct. The friction causes some loss of pressure in the duct and is known as friction loss. The more duct there is, the more friction loss there is.

DYNAMIC LOSS
Water flowing in a stream always has some disturbance to flow. For example, a rock in the stream makes the water swirl. A sharp bend in the stream causes swirls and eddies.

Air moving in a duct also has disturbances. Dampers, coils, elbows, and offsets make the air swirl and eddy and disturb the smooth flow of air. Loss of pressure due to air disturbance is called dynamic loss. The more fittings there are, the more dynamic loss there is.

Adapted from Airflow in Ducts by Leo Meyer, from the Indoor Environment Technician's Library series. Visit www.lamabooks.com.

Publication date: 01/23/2006