Static, velocity and total pressure at a duct section
Static pressure acts in all directions and can be above or below the chosen reference pressure. In an extract duct upstream of a fan it may be below room or atmospheric pressure. A negative reading is a pressure difference, not a negative amount of air.
Velocity pressure, also called dynamic pressure, is ½ρv², where ρ is air density in kg/m³ and v is velocity in m/s. Total pressure at the same section is static pressure plus velocity pressure. Use measurements referenced consistently and taken in an appropriate flow region.
A wall tapping and a correctly aligned pressure probe do not necessarily measure the same quantity. The technician’s method, probe orientation and reference connections belong in the measurement record.
A worked pressure example
For an illustrative duct section, take air density as 1.2 kg/m³ and velocity as 10 m/s. Velocity pressure is 0.5 × 1.2 × 10² = 60 Pa. If static pressure at that section is −200 Pa relative to the chosen reference, total pressure is −140 Pa relative to that same reference.
| Quantity | Value |
|---|---|
| Static pressure | −200 Pa |
| Velocity pressure | 60 Pa |
| Total pressure | −140 Pa |
The example shows why a static reading cannot simply be labelled total pressure. It also shows why the sign must be retained. These values illustrate the calculation; they are not a specified laboratory duty.
Fan static vs total pressure
Fan total pressure is the total-pressure rise from fan inlet to fan outlet. Fan static pressure is conventionally fan total pressure minus the fan outlet velocity pressure. That is different from casually subtracting two duct-wall static readings.
The inlet and outlet areas, velocities and test arrangement affect the relationship. Before comparing a selection sheet with a duct calculation, check whether it quotes fan static pressure, fan total pressure or an external static pressure defined for a particular assembly.
The right question is: does this curve show the same pressure quantity as the specified duty? Our fan curve guide helps you locate the duty point once that basis is clear.
Build the fan pressure calculation from the system
List the resistance of straight duct, bends, transitions, branches and connected equipment at the design airflow. Include the inlet and discharge conditions used by the designer. Pressure loss through a component is assessed at its own flow and geometry, not taken from an unrelated nominal size.
Keep the airflow schedule with the calculation. A branch carrying one cupboard’s airflow and a main carrying several branches have different duties. Changing a duct size alters velocity and the associated losses; equal cross-sectional area also does not guarantee equal friction between different shapes.
For the layout inputs, see ductwork design and our comparison of round and rectangular duct.
Avoid double counting and misleading comparisons
Check whether a quoted loss already includes the entry, exit or transition being added elsewhere. Keep velocity-pressure terms and friction losses distinguishable. A larger pressure number on a quotation is not automatically a stronger or more suitable fan.
Use the same air-density assumptions, airflow units and pressure basis across the schedule and curves. Record any conversion, such as L/s to m³/s, so another reviewer can reproduce the calculation. The motor power should then be assessed at the selected duty, not at an unrelated maximum-flow point.
Finally, allow for the real installation. Poor approach flow can reduce performance beyond a simple fitting-loss calculation. Fan system effect explains why the inlet and outlet arrangement needs review.
Questions about static pressure vs total pressure
Is static pressure always positive?
No. Static pressure can be negative relative to the chosen reference, as it often is on the suction side of an extract fan. Record the reference and retain the sign.
Can I compare a static-pressure duty with a total-pressure curve?
Only after the pressure quantities have been put on a consistent basis. The fan inlet and outlet conditions are part of that conversion.