Static pressure (Pi)
Static pressure is the pressure exerted on the duct wall by the irregular motion of air molecules. Measured from absolute vacuum it is absolute static pressure; from atmospheric pressure it is relative static pressure. In air conditioning, static pressure means relative. It is the potential energy per unit volume of gas, a force that compresses gas and presses the wall. Absolute static pressure in a duct can be positive (above atmospheric) or negative (below).
Dynamic pressure (Pb)
Dynamic pressure is the pressure generated by moving air; any flow in the duct has some. It is the kinetic energy per unit volume, a force that changes the gas velocity. It acts only in the flow direction and is always positive.
Total pressure (Pq)
Total pressure is the algebraic sum of static and dynamic: Pq = Pi + Pb. It is the total energy per unit volume. From atmospheric reference it can be positive or negative.
External residual pressure
This term usually comes from the catalogue. The external residual pressure is the total pressure the unit can supply after its own pressure loss. Whether it means external total or external static pressure, it is best read as external total pressure; it is shown as static pressure because dynamic pressure is taken as zero in testing. So the term is not strictly standard, but it is always the total pressure after the unit's own loss.
How static, dynamic, total and residual pressure form
Static pressure comes from molecular motion and is equal at every point in the field; it exists whether the fluid moves or not. Dynamic pressure comes from fluid momentum and exists only against the flow. The fan outlet has high velocity and dynamic pressure but lower static; when an AHU outlet enters a plenum box, nearly all energy becomes static there. After resistance loss, static and dynamic convert into each other. Total pressure is their sum and shows the work capacity. We normally quote fan head as total pressure.
External residual pressure is the total pressure after the fan overcomes its own loss, i.e. the difference between inlet and outlet total pressure.
Relations
Total = static + dynamic; dynamic = 0.5 × air density × velocity²; residual = total − sum of section resistances. Example: a unit with return, primary, cooling, intermediate, heating and supply-fan sections has resistances 20 Pa, 80 Pa, 120 Pa, 20 Pa, 100 Pa, 50 Pa; internal loss 290 Pa. For 500 Pa external residual, fan total ≥790 Pa; for 1100 Pa, ≥1390 Pa. High residual is typical of clean units; higher is not always better. AHUs often put the fan last; to cut dynamic and raise static, add a silencer plenum at the outlet.