ACTIVATION OF HEAT DETECTORS

[ashvin]

Hi,

I wanted to know, upon what parameters does the activation of heat
detectors depend on in CFAST?

I do realize we need to specify RTI and activation temperature... i
also specified a ceiling jet OFF for this case.

I ran the code, and saw that heat detector's take a lot more time than
smoke detectors to activate. The smoke detector is set at the same
activation temperature as the heat detector. Is there any specific
reason for this?

Ashvin

[Dimitrios]

the activation of smoke detectors may be linked via a correlation to
the activation temperature. That means that there is a relation
between the gas temperature and the smoke concentration which leads to
the smoke detection.
For this the smoke detector needs not specification of the RTI while
the heat detector temperature is computed via the RTI - value and is
not the same as the gas temperature.

[cfas...@gmail.com]

I would expect that the smoke detectors would alarm faster than a
similar heat detector. The smoke detector is just based on the local
temperature at the detection position. This may be a ceiling jet
temperature (if it's turned on) or the upper layer temperature if it
is not. At the moment, it is not based on the smoke concentration.

The heat detector is determined by the link temperature of the
detector. The RTI value determines how fast this link heats with
input from the local temperature and velocity at the detection
position. Thus, this will take longer to heat up than the equivalent
smoke detector.

You can think of the smoke detectors as a heat detector with an
infinitely fast RTI.

[ashvin]

I placed two heat detectors in a room of size 5m by 4m by 2.5m, half
way on the rear wall. They are placed 30 cm and 45 cm below the
ceiling. I set the activation temperature to 100°C and an RTI of 400.
It was found that the detector nearest to the ceiling(30cm below)
activated at 158 sec. while the other detector activated at 633 sec.

What may be the reason for such a huge jump in activation times? The
ceiling jet is turned on to Ceiling + Walls.

The fire object is a constrained Tsquare fire with a maximum heat
release rate of 2MW.

[cfas...@gmail.com]

I suspect that the ceiling jet is not involved in this case. Usually,
the ceiling jet thickness is on the order of a few centimeters. This
difference may simply be a result of the layer descending from the
ceiling. Without the actual test case, including fire and venting, it
would be difficult to diagnose further.

[Chris]

A high RTI-value means a long time till activation. So I would put
RTI=1 for a detector. The activation temperature for a detector is not
100°C, it is 60°C.
Try this.
Chris

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