Explain the relationship between duct velocity, static pressure, and total pressure in an air distribution system.

In an air distribution system, the relationship between duct velocity, static pressure, and total pressure is fundamental to understanding airflow dynamics. Total pressure represents the sum of static pressure and velocity pressure. Static pressure is the potential energy of the air, representing the force exerted by the air on the duct walls. It is the pressure that would be measured by a gauge that is not affected by the moving air. Velocity pressure, on the other hand, is the kinetic energy of the air, representing the pressure due to the air's movement. Duct velocity refers to the speed at which air is moving through the duct. As duct velocity increases, the velocity pressure also increases because more of the air's energy is in the form of motion. However, since total pressure remains relatively constant in a closed system (ignoring friction losses), an increase in velocity pressure results in a corresponding decrease in static pressure. This is because the total energy of the air (total pressure) is being redistributed, with more energy allocated to motion (velocity pressure) and less to potential (static pressure). Conversely, if duct velocity decreases, velocity pressure decreases, and static pressure increases. This principle is expressed by the equation: Total Pressure = Static Pressure + Velocity Pressure. This relationship is critical for designing and balancing air distribution systems. Engineers use these pressure measurements to determine airflow rates, identify areas of excessive pressure drop, and adjust dampers to ensure proper air distribution throughout a building. For instance, constricting a duct (reducing its size) increases air velocity at that point, reducing static pressure, but the total pressure loss is related to friction and turbulence introduced by the constriction.