How to Design a Centralised Extraction System

Aug 5, 2025

2 min read

Updated: Sep 4

"What Is a Centralised Filtration System?" covered when a central system is the right choice. This article covers how to design one once that decision has been made, step by step.

The Design Sequence

First, every extraction point and its required flow rate is mapped individually. If not all points run at once, a realistic diversity factor is applied. Duct routing is planned to minimise length and the number of fittings. Branch and main duct diameters are sized individually to maintain adequate transport velocity at every branch in the network, not just at the fan inlet — a common design error is overlooking that branches far from the fan can lose velocity once pressure balances across the network.

Fan Selection and Balancing

The fan is selected based on total pressure loss calculated through the network's most resistive (usually longest) path. Because as-built conditions almost always deviate somewhat from calculation, balancing dampers on the branches are recommended so the system can be adjusted after commissioning.

Planning for Growth Up Front

If additional stations are planned for the future, sizing the main trunk with a bit of extra capacity now is usually more economical; undersizing the main line to save cost today can become far more expensive to expand later.

Conclusion

Designing a central system isn't a single calculation — it's a connected sequence of decisions: mapping every point, applying diversity, checking velocity branch by branch, sizing the fan against the most resistive path, and adding balancing hardware that allows field adjustment.

Frequently Asked Questions

Can every branch be the same diameter? — No, each branch needs to be sized individually to its own flow rate.

Are balancing dampers necessary? — Yes, field conditions almost always deviate from calculation.

References

● ACGIH — Industrial Ventilation: A Manual of Recommended Practice for Design (duct design chapter).

● General local exhaust ventilation (LEV) design practice.