Flow of air is caused by a pressure differential between two points. Flow will originate from an area of high energy or pressure and proceed to area of lower energy or pressure.
Duct air moves according to three fundamental laws of physics,
- Conservation of mass,
- Conservation of energy
- Conservation of momentum.
Conservation of mass:
Mass is neither created nor destroyed. From this principle it follows that the amount of air mass coming into a junction in a ductwork system is equal to the amount of air mass leaving the junction, or the sum of air masses at each junction is equal to zero.
In most cases the air in a duct is assumed to be incompressible, an assumption that overlooks the change of air density that occurs as a result of pressure loss and flow in the ductwork. In ductwork, the law of conservation of mass means a duct size can be recalculated for a new air velocity using the simple equation:
Conservation of energy:
Energy cannot disappear, it is only converted from one form to another. This is the basis of one of the main expression of aerodynamics, the Bernoulli equation. Bernoulli’s equation in its simple form shows that, for an elemental flow stream, the difference in total pressures between any two points in a duct is equal to the pressure loss between these points,
Conservation of momentum:
Newton’s law states that a body will maintain its state of rest or uniform motion unless compelled by another force to change that state. This law is useful to explain flow behavior in a duct system.
