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Aspirating Smoke Detector Design - How To

Posted on April 06 2017

Follow our brief guide to designing a successful HSSD system.

Mention the words "Aspirating Smoke Detection" to even the most seasoned design / estimating engineer, and for a split second, some may question their design knowledge.

In essence, planning the layout of an aspirating smoke detection system is similar to developing a point detection scheme. In the UK, fire alarm systems are generally designed in accordance with BS5839 / EN54 and High Sensitivity Smoke Detection is no exception, everywhere a smoke detector is placed, a sampling hole should exist.

The detection area coverage of a point detector is determined using a radius of 7.5m from the centre of the detector. Most designers draw circles around the point detector to highlight the coverage and then place detectors so that the extruded area (the circle) overlaps with the adjacent detector.

 Smoke Detector CoverageFig. 1 - Smoke detector with 7.5m radius coverage to the furthest building corner.

 

Smoke Detector CoverageFig. 2 - Multiple smoke detectors with overlapping coverage.

In figure 2, it can be seen that there is no single area of the room left unprotected by a smoke detector. The smoke detector spacing above can be greatly simplified for Aspirating Detection.

Decreasing the distance between smoke detectors could result in extra detection being required incurring additional cost. Aspirating Smoke Detectors do not suffer from this consequence, adding an additional detector simply means drilling an extra sampling hole.

With the above in mind, measuring the spacing between the detectors and rounding down reveals a distance of 10m. In addition to the 10m spacing, placing a sampling point 5m from a wall provides coverage of the 7.5m radius into the corner of a room.

In summary, if sampling holes and sampling pipe are spaced 10m apart and sampling holes are placed within 5m of a wall, the design will fit perfectly within the required 7.5m radius spacing required by BS5839.

Simplified Smoke Detector SpacingFig. 3 - Detector spacing simplified.

As mentioned previously, an Aspirating Smoke Detector or HSSD system follows the same design principles for hole placement. Everywhere a point detector exists, a sampling hole should. In our example above, we can simply change the smoke detector symbols to sampling holes and connect the sampling holes together to provide a basic Aspirating Smoke Detector pipe layout.

Aspirating Smoke Detection Sampling Point SpacingFig. 4 - HSSD Pipework with sampling holes.

Once we have the basic pipe network designed, with sampling holes located at the correct spacing to ensure total coverage of the protected environment, we can choose a suitable location for the Aspirating Smoke Detector and add the remaining aspirating pipe to the design.

Full HSSD Pipe LayoutFig. 5 - Detector placement.

All that is left to do to complete the design is to choose the most suitable HSSD detector based on the pipe lengths and hole quantities on the design. In the example above it would make sense to use a detector that can support at least three sampling pipe networks. In addition to the number of pipes, the length of each pipe needs to be considered. The length of the longest pipe in our design from the detector to the end hole is 54.5m. The Vesda-E VEP detector supports up to 4 pipes each having a length of 75m; this detector would suit our design perfectly.

Once the design is complete, the next step would be to calculate it. This involves working out the correct hole sizes for the sampling points to create an evenly distributed and balanced system ... but that's for another time.

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