Fluid mechanics, a fundamental area of physics, has to do with the behavior of fluids, both at rest and in motion. It deals with properties of fluids, such as density and viscosity, and relates to other aspects of physics, such as thermodynamics and heat transfer, which add the issues of energy to the functions of the basic fluid flow.

For this briefreminder paragraph, remember:

The static pressure at a point in a fluid system is directly proportional to the density of the fluid and to the height of the fluid column.
Static pressure is exerted equally in all directions.

The velocity pressure of a flowing fluid is proportional to the square of the fluid velocity; i.e., doubling the velocity quadruples the velocity pressure.

The friction loss of a fluid flowing in a conduit is proportional to the square of the velocity.

The pumping power required to move a fluid is proportional to the fluid density and viscosity, as well as the volume of fluid handled and the pressure against which the fluid is pumped.

Since the friction loss is proportional to the square of the flow, the pumping power in a defined system is proportional overall to the cube of the flow rate.

For HVAC purposes, air is considered to be an incompressible fluid.

For incompressible fluids, the amount of fluid in a closed system is constant. Any outflows must be offset by equivalent inflows, or there must be a change in the amount of fluid held in the system.

This is the Law of Conservation of Mass and allows us to account for fluid in a process just as we count money in the bank.

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