HSSD systems and air cooler units in computer rooms

[kurnal]

Today I have come across something I have not seen before and would be interested in your views.

A computer room is protected by an HFC gas system operated by conventional detectors on double knock.  In addition am HSSD system has been installed, sampling the floor and ceiling voids- the floor appears to be used as a plenum for the main air conditioning system. Not sure about the ceiling void.

Suspended from the ceiling are conventional cooler units drawing air in at the base - about 450mm below ceiling level and blowing it out at the front of the unit across the underside of the false ceiling. 

Immediately beneath the inlet on the base of the unit a section of HSSD sampler pipe, with a hole drilled every 300mm has been installed, in the path of the air entering the cooler. The sampler pipe is about 30mm below the inlet grille and on some coolers the holes face the base of the cooler, on others the holes face the room.

Now I dont know if I have had a sheltered upbringing or whatever- but I have never seen this done before and wonder, bearing in mind the relative velocity of the air as it aproaches the cooler and the velocity of air in the sampling pipe whether it will provide any useful early warning of fire. And which way the holes should face? 

I have not yet  been able to ascertain whether or how the HSSD is interfaced to the fire alarm , the suppression system or the HVAC system.

[Davo]

Prof

I came across the same problem quite recently
I will ask our Estates guys if they can assist


davo

[kurnal]

Well theres a thing !!!.
Thanks Davo I  would be interested in what they say. I wonder if yours are interfaced to the suppression system, the main fire alarm or if they just shut down the HVAC?

[Big_Fella]

I've seen this on more than one occassion, although mainly on MOD sites within their computer suits for some strange reason.

I would be suprised if it is used as part of initiating the HFC system, as the relevent standards BS6266/BS7273 indicates that HSSD is not recommended for use to initiate the release of an extinighant agent, but only provided as supplementary detection.

I have found in the past it does sample the air quite effectively installed at this position, provided installed correctly.

[kurnal]

Thanks Big Fella.
Yes I have seen HSSD sampling the return air to the main air conditioning system but these are recirculating coolers and the velocity of the air passing into the cooler in the visinity of the pipe was substantial, and armed with my boy scout knowledge of aerodynamics I could imagine that air may be drawn out of the sampling pipe on those systems where the holes were pointing towards the cooler. Thanks for the comment on HSSD and suppression systems- I had overlooked that. Still not been able to find out the cause and effect for the HSSD.   

Been reading a couple of design guides- xtralis and airsense- where used with air conditioning both appear to indicate (through diagrams) that the sampling holes should face the approaching path of the air rather than facing the inlet. 

[Benzerari]

Quite similar to what I was looking for a while ago: http://forum.fire.org.uk/index.php?topic=3862.0 , good news

[Thebeardedyorkshireman]

Hello Prof
Ok, where do I start???
Point detectors in the room. Got to have them, but won't work due to the air flow and dilution with the A/C running ( unless you have a serious amount of smoke). This is a short version of a very long story.
HSSD as a back up is useful to draw attention to a potential fire, but can be used to stop A/C fans thus making the point detectors ( now in still air) usefull for gas discharge.
HSSD pipes can be run throughout the risk but are most usefull sitting in the return airflow to the A/C.
You could put them in the discharge but it will be turbulent air and not suitable for detection due to many factors.
In the return air, the holes should face the direction of airflow BUT at the actual point where the air hits the tube you can find unusual turbulance and pressure ( long story but just go with it)
Therefore if you rotate the tube ( holes) by 45 deg to the room side you will achieve maximum detection from a test burn.
Holes located towards the grill may sit in negative pressure due to the vortex effect but each site may be different and the only way to confirm this is to do a wire test burn. Details are in the BS.
I hope this helps.
If you need any more info, ping me an email. Next time you are in Leeds give me a call and I can take you to a site and show you how it all goes together ( and you can see my underground car parks fitted with HSSD.)
Dave.
Ps thanks for fixing my account to this site

[Allen Higginson]

This would be a common enough set up in (as TBY has said) rooms with high volume airflow that affects the detection capabilities of the point detection;the aspirating system shuts down the air con etc.
It is in a BFPSA guide/recommendation on aspirating and is seen as secondary detection.

[kurnal]

Many thanks Dave and Buzz.

Dave- could I bring along a colleague?

If there are four of these units and the holes are in completely differnet directions I guess the chances are that this was not as a result of wire test burn?

I will order the BFPSA guide- I already have the relevant BS 6266/ BS7273. It appears then  that the Alarm Industry were dissatisfied with the BS in the same way that the BASA are dissatified with BS9251 and have published additional recommendations?

[Thebeardedyorkshireman]

Prof
If the holes are all in different angular locations then I would suspect two scenarios:
A) cock up
OR
B) Air conditioning service engineer removed the pipe to gain access to the filters and put it back in the wrong place

And yes you are welcome to bring a colleague but give me some notice to arrange access
Dave