Leo's Tech Help: Ventilation And Air Losses In Duct
VENTILATIONBe sure to provide the amount of ventilation air set by codes. Insufficient ventilation can create health problems.
Adapted from Indoor Air Quality from the Indoor Environment Technician's Library series. Visit www.lamabooks.com.
AIR LOSSES IN A DUCTThe longer and more complex a duct system is, the less air it can deliver. This is because the duct creates resistance to airflow. Resistance to air movement is caused by friction losses and dynamic losses.
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.
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