Aspirating Smoke Detector Filters

An Aspirating Smoke Detector ("ASD") is an electromechanical device and type of active smoke detection that draws air in through a network of pipes through a filter and into a sensing chamber to detect smoke. The smoke sensing chamber of an aspirating smoke detector is a nephelometer, a device that measures the reflection and refraction of particles (including smoke) in air from a light beam. The filter in an aspirating smoke detector removes large airborne particles leaving smoke and smaller particles to enter the smoke sensing chamber.

CEO, Firewize

The filter arrangement of an aspirating smoke detector varies from manufacturer to manufacturer. In an aspirating smoke detection system, there are generally two types of filtration systems employed.

  • particulate separation;
  • particulate media filtration.

There are other types of filtration that may be employed such as electrostatic precipitator filtration and catalytic filtration however these methods are not discussed in this article.

Particulate separation

VESDA LaserPlus

Particulate separation may be used in an aspirating smoke detection system to remove heavier and larger particles, helping preserve or extend the life of a down stream media filter.

Particulate separation occurs when there is a change in air flow direction and the momentum of the heavier particulates are exhausted from the aspirating smoke detector while the lighter particulates are carried through to the second stage media filter and then on to the sensing chamber to detect smoke.

Media filtration

Filter Media

A particulate media filter is a device composed of a fibrous, or porous material (the filter medium or membrane) that acts a permeable barrier to restrict the flow of particulate matter of a certain size through the filter medium. The air passing through an aspirating smoke detection system is filtered to remove large particulate matter leaving small particulate matter to pass through the filtration system and into the sensing chamber.

In an aspirating smoke detector, the particulate media filter is a disposable item that must be replaced periodically according to the required air-flow rate of the aspirating smoke detector or the duration defined by the manufacturer.  The environment in which the aspirating smoke detector is installed may also have an effect on the lifespan or duration of the filter.

In an aspirating smoke detection system, there may be multiple filters employed including inline filters designed for harsh or very contaminated environments that should be replaced as often as necessary to maintain the performance of the aspirating smoke detection system.