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Duct Detection
There are two principle applications for sampling smoke from a duct using ASD; either as a DSD to prevent smoke spread or to provide early warning of a fire condition in an area or piece of equipment from which air is extracted.
Within these applications there are a number of issues to be considered when designing a duct detection system using ASD. The primary ones being:
- Pressure equalisation
- Maintenance accessibility (Flexible joints to allow the pipe to be removed)
- Monitoring upstream or downstream of filters, (generally on the upstream side)
Sampling pipe installed within the duct will require the sample points positioned at an angle of 30- 60º off centre, into the airflow, unless directed otherwise by the ASD manufacturer. It is recommended that each sample point shall have a maximum area coverage of 0.4m2 of the cross sectional area.
The use of a union socket is recommended to allow the selection of the final orientation during the commissioning of the system. Where possible the sampling pipe should be installed in the centre of the airflow and away from bends. It is also recommended that the sampling pipe be installed close to inspection hatches for maintenance purposes. Due to the use of High Efficiency Particle Arrestor (HEPA) filters within HVAC systems, it is recommended that monitoring be carried out before the filter, as smoke particles may also be removed by the filter.
Due to the dilution of the smoke particles from the protected area, due to high airflows, it is recommended that only Class A & B ASD’s are suitable for early warning applications.
Note: The preliminary European product Standard for DSDs (prEN 54-27) defines ASD as a type 5 DSD and requires that they are approved to EN 54-20
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