Reza,
Most subfloors in computer rooms that I know of, are not only used
for cable runs and some equipment, but also for cooling the equipment
cabinets on top of the floor by ventilation. For that purpose the
equipment cabinets in the room are placed over relatively large
venting holes in the subfloor, thus allowing cool air from under the
floor to be blown up through the cabinets and leave through grilles
in the sides or top of the cabinets. The warm air in the room is
sucked in by air-cool units and discharged underneath the subfloor,
thus recirculating the air through the room.
Because of all these holes, the space under the subfloor and in the
room above have to be considered as one continuum.
If you discharges CO2 under the subfloor, it will inevitably spread
equally above the subfloor as well and within only a few minutes the
concentration above the subfloor is the same as below. For that
reason you have to design your CO2 system for the whole space, below
+ above the subfloor.
Unless of course, the subfloor is not used for ventilation and you
are sure that there are no holes in the subfloor, not even under the
footprint of all the above floor cabinets.
I agree with al those responders on the forum that recommend not to
use CO2 for a computer room at all, because of personnel safety. If
you must use a gaseous protection system, use FM200 or an inertizing
agent and apply it to the whole room, including below the subfloor.
The emergency response people will be grateful.
Besides that, many fires start while electricians or other workers
are making modifications. In other words, when there are people
present in the room of origin.
An additional way of protection which might please your client is by
installing an early warning system with smoke aspiration detectors.
Client should organize a good emergency response plan to respond to
such early fire alarm and excersize this regularly. Provide mobile
CO2 extinguishers which enables the emergency response people to
manually suppress a starting fire in an early stage without leaving
any traces. Also, hang one of those special keys to lift the tiles of
the sub-floor near to each cluster of mobile extinguishers. The
response people will need it to investigate the space underneath the subfloor.
And beyond that, don't forget to install a wet pipe or pre-action
sprinkler system that will at least reduce the damage when other
suppression systems fail. Manual suppression might fail because the
response people do not respond or decide that they "don't see any
fire". Automatic suppression by extinguishing gas did sometimes fail
because response people (even fire fighters) opened the door to the
room too soon, thus allowing fresh air to flow in and re-ignite the
fire. If the manual response to the early warning is in due time and
adequate, the temperature in the room will never rise to a level
where it could trigger a sprinkler. If the response fails, the
owner/user might be gratefull to you that the sprinklers confined the fire.
However, that is how we recommend it. More experienced owner/users
allow to leave out the gaseous flooding systems: only early warning
with manual suppression in combination with a wet pipe or pre-action
sprinkler system.
Frans Stoop
TOS architecture & fire protection
Netherlands <[EMAIL PROTECTED]>
At 09:02 2007-08-23 -0700, you wrote:
Thanks All,
Ford, if the intent of slow flow rate for deep seated fires is
preventing over pressurization, so why NFPA-12 suggest high flow
rate for Surface fires involving flammable liquids, gases, and solids?
NFpa12 States that "Surface fires are the most common hazard
particularly adaptable to extinguishment by total flooding systems.
They are subject to prompt extinguishment when carbon dioxide is
quickly introduced into the enclosure in sufficient quantity to
overcome leakage and provide an extinguishing concentration for the
particular materials involved."
NFpa12 also States that "For deep-seated fires, the required
extinguishing concentration shall be maintained for a sufficient
period of time to allow the smoldering to be extinguished and the
material to cool to a point at which reignition will not occur when
the inert atmosphere is dissipated."
I think I explained at first in a bad manner. There are two protected area.
1- Computer room: would be protected with FM-200 system
2- Computer room subfloor with electrical equipment and cable
runs: would be protected with high pressure carbon dioxide system.
My question is about subfloor, while NFPA 12 states that "For
deep-seated fires, the design concentration shall be achieved
within 7 minutes, but the rate shall be not less than that required
to develop a concentration of 30 percent in 2 minutes." Is it a
problem if the total discharge time would be only 1 minute for this case?
Thanks,
Reza
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