Research has shown (see 1 and 2 below) that, unlike sound and light, smell does not wake people up. The opposite tends to happen, with smoke having a soporific effect on already sleeping people. Therefore in the event of a fire developing and in the absence of any other sensory signal people will simply fall into a deeper sleep as the smoke enters the room and slowly poisons them. This is of concern to everyone, of course, but particularly to parents with young children.
The National Institute of Standards and Technology (NIST), a United States federal technology agency have developed and maintain the Fire Dynamic Simulator software that allows the modelling of fire and smoke simulations using computational fluid dynamic principles. I have used this software to examine whether the presence and use of an air-conditioning unit in a bedroom has any affect on the spread of smoke;
- Does the air-conditioning slow down the entry of smoke into a bedroom?
- And if so, by how much?
In particular I was interested to determine whether, or not, the use of an air-conditioning unit provides any meaningful benefit to sleeping individuals in terms of additional time before the smoke from a fire overcomes them.
It was noted in researching this topic that whilst there has been a good body of research papers carried out on the affect of forced ventilation in commercial environments, there appears to have been little, or none, done for residential dwellings and I was unable to find anything covering the effect of air-conditioning in the sleeping environment.
The model used was of a simplified home, with two otherwise identical bedrooms.
Although a simplified model, the aim was to reflect the proportions of a real home, so the dimensions used were based upon an average of some real life samples looked at on an Australian property website. It was necessary to use a ceiling height of 3.2m as this dimension works well in the the Fire Dynamic Simulator software for computational purposes. The ‘normal’ ceiling height of the residential dwellings looked at varied, and whilst generous, it was felt that 3.2 m was not unreasonable.
Floor areas modelled:
- Main living area where the fire occurs of 69.76m2
- Two identical (apart from air-conditioning) bedrooms each of 16.77 m2
- A bathroom (door open) and communal hall between the two bedrooms
For the purposes of the simulation the ambient temperature in the house was set at 26º C, which whilst warm is not an uncommon night time temperature in the southern hemisphere during the summer months. The temperature of the simulated air-conditioning unit was set to input air into the room at a constant temperature of 20º C, some 6º C below ambient and maintain it at that level.
As it was not possible to change the ambient starting temperature in just one room, using the fire dynamic simulator software, it was deemed preferable to start the simulation running for a short period of time before the fire itself started. That way the room containing the air conditioning was allowed to get down to a lower temperature, before the fire started, thereby simulating a fire starting some time after the residents have gone to bed. This was set so the fire started at 150 seconds (2½ minutes).
We calculated the air flow one could realistically expect from an air-conditioning unit, based upon a random sample of commercially available residential units for sale, and used an average flow rate of 1.25 m/s for our air-conditioning vent.
The fire surface in the model was set to polyurethane as this is a material in common use in modern housing, which after comprehensive comparative experimentation by NIST has been shown to produce compartment fire results in the Fire Dynamic Simulator as close to a real fire as possible. Polyurethane foam is often used, in the form of flexible polyurethane foam (PUF), in furniture construction such as sofas and chairs and is also a common constituent in adhesives and paints used in homes.
It was expected that at the point when the fire started, having an air-conditioning unit in the bedroom, would result in that room having both;
- a lower temperature than the ambient temperature in the rest of the property, and
- as the door was fully closed, albeit with a gap at the bottom, there would be a natural resistance to the buoyant smoke being able to enter the room
The questions we set out to answer were whether the above expectations were born out and and if so what effect this had on the spread of smoke into that room. The assumption being that the slightly lower ambient temperature in the air-conditioned bedroom plus the positive inflow velocity would hold back the ingress of smoke, but to what extent?
Using the the Fire Dynamic Simulator software, it would appear that having an operating air conditioning unit in a bedroom (with the door closed) does indeed slow down the ingress of smoke into the bedroom and (in this simulation) by some considerable time, nearly ten minutes. If corroborated by further more extensive simulation and/or experimentation this knowledge could be valuable for fire safety in the home.
Note: The above is a much shortened excerpt of the full research conducted. If you would like the full research paper please contact me.
1. Nocturnal Olfactory Response To Smoke Odor
BY: J. L. Lynch – Irondale Fire and Rescue Service Irondale, Alabama
2. Scents will not rouse us from slumber, says new Brown University study
Kristen Cole – The News Service, 38 Brown Street / Box R, Providence RI 02912