A fan is a rotating device used to create a pressure difference. That differential results in the movement of air. The movement of air is measured in cubic feet per minute or CFM. It is the measure of the air volume over a period of time.
Three types of pressure readings are used for evaluating airflow in HVAC systems - static pressure, total pressure, and velocity pressure. Static pressure is typically sued for fan selection. Total pressure is to find velocity pressure. Velocity pressure is used by a field technician or a system balancer to measure CFM in an installation. These pressures are related through the equation: Static pressure plus velocity pressure equals total pressure. These terms are typically measured in inches of water gauge or water column.
Static pressure is exactly that, static – the air does not move. It is the resistance to air movement, equal in all directions, similar to the pressure inside a balloon. Static pressure can be positive or negative. Static pressure can be measured by using a manometer, which is a U – shaped tube filled with water. One end of the tube is placed perpendicular to the airstream while the other end is open to the atmospheric air. The vertical difference between the two columns equals static pressure, measured in inches of water gauge.
To measure total pressure, a tube is placed in the duct with its opening facing in to the airflow. The resulting pressure is the force of the moving air and is typically greater than the static pressure.
Velocity pressure is defined as total pressure minus static pressure. To meausre veocity pressure, use a pilot tube. A pilot tube is a device which connects one end of the manometer to a total pressure tap and the other end to a static pressure tap. The resulting reading would be the total pressure minus the static pressure which is velocity pressure.