Containing or delaying the spread of fire in a building by restricting its flammability or combustibility can help mitigate hazards and loss of life and property. Noman Qamar illustrates how compartmentation can go a long way in achieving this.

Dividing a building into discrete fire zones or fire compartmentation is regarded as the most effective way of limiting fire damage. In this regard, it is important to understand the basics of compartmentation. While constructing a building, it is important to establish correct compartmentation in order to isolate a fire at birth and to improve safety of people. Here are a few cardinal steps to ensure a safe compartmentation.

Basics of compartmentation

Definition:
A fire compartment is defined as an area of a building which is totally separated from the rest of the building by continuous construction. This area could be a single room or a series of rooms or even an entire floor area.

Objective:
The purpose of compartmentation is to subdivide a building into fire compartment(s) to isolate the fire at birth in a small volume and to limit spread of fire, smoke and heat through HVAC ductwork.

How to achieve it:
Fire dampers need to be installed in fire separating walls and floors to prevent fire and to reduce smoke and heat spreading from one fire compartment to another through HVAC ductwork.

Fire dampers

A fire damper is a device installed in an air distribution system with blades open, but designed to close automatically upon detection of heat or close through remote signal from fire alarm panel in order to interrupt the airflow and to restrict the passage of flame, smoke and heat. A fire damper can be manual or motorised.

A manual fire damper, typically also known as a curtain fire damper, closes automatically via melting of fusible link when temperature exceeds the temperature rating of fusible link.

A motorised fire damper is connected with Fire Alarm Control Panel (FACP) and Building Management system (BMS). It can be closed through remote signal from FACP in the early stage of fire, upon detection of smoke (to avoid spread of cold smoke to other areas through HVAC ductwork). It is also equipped with fusible link or Thermal Responsive Device (TRD) to close automatically on detection of heat, thus ensuring dual safety.

In case of fire …

The first priority – limit the spread  of smoke

The first priority when fire is detected is to limit the spread of smoke through the HVAC ductwork. This is because, at the start of fire, smoke develops rapidly as compared to the rise in temperature.

As this smoke is cold, a manual fire damper will not close automatically, and therefore, smoke could easily spread through the HVAC ductwork to other areas of the building.

Examples of incidents of fire

MGM Grand Hotel fire in 1980
According to the “Investigation report on the MGM Grand Hotel fire – Las Vegas, Nevada – which occurred on November 21, 1980 (Best and Demers 1982)”, available on NFPA website, “the HVAC systems operated during the fire and contributed to smoke spread through the high-rise tower. The equipment was not equipped with smoke detector arranged to shut down the systems upon sensing products of combustion. In addition, some fire dampers were arranged so that they could not close when the fusible links melted and others did not close completely”.

Consequently, “the fire was limited to the first floor, but smoke spread throughout the building. Some occupants on upper floors were exposed to smoke for hours before rescue. The death toll was 85, and the majority of the deaths were on floors far above the fire.” (Source: Design of smoke management systems, J Klote J Milke, ASHRAE Special Publications, 1992.)

Hilton New York Hotel fire in 2005
“A fire in an electrical shaft at the Hilton New York hotel forced the evacuation of the 45-storey Midtown building, sent 33 people to the hospital with smoke inhalation […]. No one was seriously injured. The fire started about 4pm […]. The smoke spread throughout the 2,017-room hotel through the ventilation system. Hotel guests from the eighth to the 33rd floors said that the hallways were thick with smoke.” (Source: http://www.iklimnet.com/hotelfires/case52.html)

Conclusion

To stop the smoke propagation through the HVAC ductwork needs to be the first priority of any compartmentation system with quick activation through a smoke detection system. Therefore, fire dampers need to be:

• Motorised with a quick or instantaneous (1s) operation via a fire alarm control panel (FACP) connected to smoke detectors
• Airtight at low as well as high temperatures to stop propagation of smoke, preferably with intumescent seal to seal gaps
• Installed in an HVAC system with fan shut-off quickly after the start of the fire

The second priority

– stop flame heat transfer

On the basis of the report, “Examination of the fire resistance requirements for ducts and dampers”, made in 2005 in the UK by Building Research Establishment (BRE), the principle causes of ignition and fire spread inside ducts are fluff, dust, food residue and grease contents inside ducts. All these can have an impact on both fire ignition and fire spread inside the HVAC ducts.

Heat transfer through fire dampers, even with blades in close position, could ignite any fluff or dust on the opposite side and, then, spread the fire throughout the building.

The main danger due to the heat transfer comes from the pyrolysis of any material and dust. For example, the pyrolysis of a piece of wood starts at 200°C, and the pyrolysis of the products (gases…) starts to burn (with flames) between 500°C and 700°C, leading to the fire propagation.

Conclusion

To prevent flames and heat transfer from one compartment to another is the second priority of any compartmentation system. Therefore, it is essential that:

• Fire damper blades are made of refractory material to avoid heat transfer to the unexposed side of the damper
• No flame should be allowed to transfer to the unexposed side through the closed damper

It is evident that limiting the spread of smoke through the HVAC ductwork and stopping flame heat transfer need to be top priorities. In this regard, it is important to establish a correct compartmentation to isolate fire at its onset.

The writer is with the Marketing Department of Aldes Middle East. He can be contacted at noman@aldesme.com