Beam Detectors

[Simon Morriss]

What is the recommended distance that a Beam Detector is placed away from the ceiling?

I have a proposal that due to English Heritage limitation it is necessary to place the beams pointing up at an angle.  According to the BS the width of the coverage is 7.5 meters but if the beam provides supplementary detection the width is reduces to 12.5% of the height above the likely seat of the fire.  

There is no mention (as far as I can see) of the standard distance from ceiling to beam.

Thanks

Simon

[Graeme]

You should try and site the beam in the apex or within 600mm of the ceiling for best coverage.(up to 25m in height)
Detectors sited more than 600mm from ceiling need spaced at 25% of their height that they are from floor.

[Simon Morriss]

Thanks Graeme

The issue is that the beams are at an angle with only 25% of the total length within the 600mm.  As I mentioned previousley this is due to english heritage issues and not wanting to have wires running through the roof area.

I think I'll have to get the consultane to run some calculation and see what they come up with.

Any other thoughts will be great.

Simon

[Allen Higginson]

Hi Simon.
You've lost me a bit with the angle and the 25% length thing.Could you put in idiot terms for an idiot!!!

[Simon Morriss]

Hi Buzz

I'm sure you are not an idiot.

We have a recessed ceiling which forms about 50% of the total roof area.  This recess is of interest to the heritage people and so we can not run wires into the area.  The void created is also a lot greater than 600mm.

What has been proposed to put a beam transmitter/receiver at a lower level (greater than 600mm) and fire diagonally it to a mirror in the corner of the void.  About 25% of the run of the beam will be with in the 600mm guide with the remaining 75% having a distance greater than 600mm away from the ceiling.

I hope this helps.

Simon

[Graeme]

.Never had the instance to go diagonally and by the sounds of what you describe it's not going to give full coverage of that void.

I would check with a manufacturer with your queery of sending the beam up at an angle first.

[Allen Higginson]

How much financial restraint have you got?I'd go for an aspirating system if it was accepptable and for aesthetic reasons you could use polished copper tubing.If there are suspended light fittings you could run capillary tubing down the chain from above.
As far as positioning the beam to "fire" diagonally into the corner from below  tghis cant be done due to the way the smoke plume forms from the source to the point of smoke layering.Outside the 600 mm depth then (as Graeme says) you would have to allow for more than one beam to cover the decreased smoke width.