In coal handling plant applications twin conveyors may be required to be monitored. A single blower may purge both sensors by employing a “T” branch in the air ducting.
 

 
There are limits to the distance that any Air Purge Blower (APB) can deliver the required air to the infra-red sensor which are mainly dependant on the APB specification and duct bore size. Other factors such as ducting surface characteristic and the air temperature/pressure also have an effect.
5000 Series Air Purge units (APBs) are centrifugal blowers available in a variety of multistage configurations. The standard unit is a two stage unit Type 5040, the second most prevalent is the four stage Type 5044.
Tables A & B to the left indicate typical limitations for smooth bore PVC ducting with the worst case air parameters of minus 20°C at sea level pressure. (Warmer Air enables greater distance)
The tables take into account 2m flexible hose connections at Sensor(s), and 1m at the Blower.