Some facilities may need gases not to pass through the filter at certain times, due to foreseeable or unforeseeable exceptional circumstances; for example, high or low temperatures, unstable starting conditions and particularly aggressive gases.

For these occasions, the system can be provided with a bypass pipe, directly linking an area of dirty air with the fan inlet. Usually this bypass is external and has its isolation on/off valves as sealed as possible.

Depending on the geometric configuration of the filters, and applied to large process filters, it is possible to install an “internal” bypass.