Accurate fan selection starts with the static pressure the fan must overcome, which is the sum of friction and fitting losses along the duct run.
The Basic Method
Static pressure is the resistance the fan works against, separate from the velocity pressure carried by the moving air. For a supply or exhaust system, the fan must replace all the pressure lost between the inlet and the outlet of the network, or the design airflow will not be achieved.
Duct pressure drop is the sum of friction loss along the run and the dynamic loss at every fitting, and it sets the external static pressure the fan must deliver at the design airflow. Get it wrong and the selected fan either cannot move the required volume or runs at a wasteful operating point, so the calculation is the hinge of a correct selection.
Friction loss is estimated from the air velocity and the duct size using a friction chart or Darcy-Weisbach relation, with typical design friction rates in the 0.8-1.5 Pa/m band for commercial systems. Oversizing the duct lowers velocity and friction but costs metal and space, while undersizing raises velocity, noise, and pressure drop, so the duct is a trade-off balanced against the fan energy over the plant's life.
Each elbow, branch, damper, and entrance adds a loss expressed as an equivalent length of straight duct or as a loss coefficient K multiplied by the velocity pressure. A 90-degree elbow can equal several metres of straight run, and a dirty or partially closed damper can dominate the system, which is why the fitting list must be counted, not guessed, when summing the total resistance.
The fan is then selected on the combined curve: at the required airflow it must supply total static pressure equal to the duct friction plus all fitting and terminal losses, plus any filter or coil resistance. The supplier verifies the point on the fan performance curve in its laboratory so the quoted unit meets the duty rather than a nominal free-delivery figure that ignores the real network.
Because the company's 80,000 m² production site includes its own performance testing laboratory, selection enquiries can be returned with a curve point backed by test rather than catalogue assumption, and engineers respond within 24 hours to sizing questions that hinge on an accurate pressure-drop estimate.
Friction loss comes from air dragging along duct walls. It is usually expressed as a rate in pascals per metre of straight duct and rises sharply with velocity, roughly with the square of the air speed, so small velocity cuts give large savings in both pressure and the fan power needed to overcome it.
Fitting losses come from elbows, reducers, dampers, and entries. Each is assigned a loss coefficient; the pressure drop equals that coefficient times the local velocity pressure, so a tight 90-degree elbow can cost more than a metre of straight duct and deserves careful layout to minimise its count.
The system total is the longest critical path: from the farthest supply outlet back through filters and coils to the fan, or from the farthest exhaust pickup to the fan discharge, whichever sets the highest required pressure, because the fan must satisfy the worst branch, not the average one.
Working Values
Main supply ducts commonly run at 4-8 m/s and return or exhaust at 3-6 m/s, with higher velocities reserved for spaces where duct size is constrained and noise is acceptable, such as plant rooms or short exposed runs where the extra pressure drop is tolerable.
A typical design friction rate is 0.5-1.5 Pa/m in the main run; pushing higher saves duct cost but raises fan power and noise, so the rate is a balance rather than a fixed rule and is often set by the acoustic limit of the served space.
Filter and coil resistances are taken from the component data, not estimated. A fresh bag filter may sit near 100-150 Pa while a loaded HEPA can approach 250-400 Pa, which is why fan spare margin matters and why a differential-pressure gauge should drive the replacement schedule.
A safety margin of about 10-20% is added to the calculated total so that minor layout changes or component loading do not push the fan off its selected point. The margin should be applied to the final sum, not to individual fittings, to avoid compounding an excessive allowance.
| Item | Specification | Note |
|---|---|---|
| Duct velocity (main) | 4-8 m/s | Balance noise vs size |
| Friction rate | 0.5-1.5 Pa/m | Design target |
| Bag filter | 100-150 Pa | Clean condition |
| HEPA (loaded) | 250-400 Pa | Final-stage loss |
| Safety margin | 10-20% | On total SP |
| Velocity pressure | ½ρv² | Drives fitting loss |
Selection Checklist
- Draw the longest run and list every fitting with its loss coefficient before totalling pressure.
- Keep main velocity in the 4-8 m/s band unless space forces a higher value.
- Take filter and coil pressure drops from the component datasheet, not a guess.
- Add 10-20% margin so loading and small changes stay within the fan curve.
- Match the fan's published static pressure to your calculated duty, not to catalogue peak.
- Re-check the sum if the layout changes after the fan is ordered.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.