What are the main types of duct systems for local ventilation?

There are two main types of duct systems: a tapered main system and plenum.

The main duct of a tapered system gets larger as branches are added and flows are merged together. This tapering allows the air speed to remain almost constant throughout the duct. Due to this characteristic, the tapered duct system is the main system used in local ventilation. (Figure 1)

Figure 1

Tapered duct system

In the plenum system, the main duct stays the same size throughout the system. The speed of the air is lower than in the tapered ducts, and sometimes lower than the minimum speed that is required to transport contaminants. For this reason the plenum system is mainly used only when the separation of particulate matter (large particles or liquid droplets) is intended. Explosive dusts, however, must not be exhausted through a plenum duct system.

One advantage of a plenum system is that new branches can be added anywhere, and the addition is limited only by the total airflow and pressure available at the fan.

Figure 2

Plenum system