Heatwave hazards: How smarter optical beam design can combat the fire detection challenges of buildings under thermal stress

As risk managers will be aware, warnings from across the globe that the past decade has been the hottest on record ­­­due to persistent heatwaves are prompting fire safety specialists to focus on the implications for fire detection when buildings are under high levels of thermal stress.

To grasp the gravity of new threats posed by global warming, consider the scenario of protecting a building with an open volume of 100 metres length (twice the length of an Olympic-sized swimming pool), while rising temperatures can significantly affect structures by causing them to expand by day and contract at night. Hunter Seymour investigates the issue and highlights what fire professionals may need to be taking into consideration for optical beam detector design and installation.

Although such expansion and shrinkage is estimated at the design stage, consider that, theoretically, a temperature variation up to 40°C could result in a thermal expansion of 5cm over 100m of concrete wall.

Therefore, where optical beam smoke detection is the most practical technology of choice, it is of critical consequence for system designers to recognise that just 1 degree of beam misalignment due to building movement over 100m equates to approximately 1.75m movement at the opposite wall (when viewed over the length of the beam from source).

Smoke stratification impacted by solar heat

Anticipation of the ‘thermal barrier effect’ in determining effective smoke detection is a key advance made possible by optical beam detection technology.

The challenges arise from what is known as a stratification layer. The hazard of a thermal layer forming above the a smoke plume is a phenomenon of a complexity resolvable only by smart innovations, since smoke particles are heavier than air and are carried upward by the warm air around them through cooler air, which cools the plume such that warm air, trapped by the ceiling, forms a thermal cushion preventing the smoke reaching the ceiling. The temperature of this layer of air can often exceed 50°C.

This issue is compounded when solar heat generates its own thermal barrier, which can add a pre-stratification layer of heated air with a density to augment the variable stratified heat due to the effect of a developing fire.

This ‘thermal barrier’ effect can be a direct cause of fire alarm inaction. Optical Beam Smoke Detectors should therefore be used for applications in which smoke stratification is foreseen due the structural conditions.

Supplementary angled beams to probe stratified smoke

An Optical beam smoke detector may be installed at an angle from the horizontal, pointing down from the ceiling, in order to provide supplementary detection of smoke that’s stratified and failed to reach the ceiling. Specifiers should refer to BS5839 Pt1 22.5d Note 2 for guidance.

The spacing recommendations for angled optical beam smoke detectors within the three-dimensional space below a ceiling are not fully systemised as a code of practice yet, and under review. However, while it is possible that a single angled optical beam smoke detector will eventually detect a spreading layer of stratified smoke, the installation of more beams, strategically positioned through the three-dimensional space, will provide a substantially better chance of detecting stratified smoke and reduce detection times. In general terms, for every one optical beam smoke detector at the ceiling a further two or three detectors angled through the space could be considered appropriate.

One additional recommendation from FIA (Fire Industry Association) research, which has not yet been incorporated into BS5839-1, is that when installing angled beams they can be deployed to advantage in a criss-cross arrangement. This is intended to ensure that the distance to a beam at any height is not excessive.

Innovative auto-aligning tracking technology

Optical beam smoke detectors are the proven technology for economical and effective protection of large, open-plan spaces with relatively high ceilings (warehouses or manufacturing plants or retail complexes, for example), particularly if access to point smoke detectors for maintenance could present practical difficulties.

It is, however, essential that beam devices should be mounted to a solid construction. When doing so, care should be taken to evaluate any potential tendencies in the building to ‘flex’ as a result of temperature changes or imposed load, as this can cause misalignment of the optical beam and, hence, fault signals or false alarms. Even periodic increases in the volume of bulk storage within a building can cause movement and significant stresses to its core structure.

Fortunately, new developments in auto-aligning, motorised, reflective beam technology are providing solutions to avoid such misreadings due to building shrinkage.

One innovator in this field is FFE, whose motorised, reflective, auto-aligning Fireray One introduces Building Movement Tracking and Light Cancellation Technology, both designed to raise beam detection technology to a new level of performance.

Fireray One combines an infrared transmitter and receiver in the same unit and operates by projecting a well-defined beam to a reflective prism, which returns the beam to the receiver for analysis. The device is also said to support ease of installation with its Auto-Align interface, which automatically aligns the beam to its reflector “eight times faster” than previous detectors, according to the company. These Building Tracking features are augmented by FFE’s Light Cancellation Technology, which is said to prevent false alarms if the sun shines directly onto the face of the detector.

Mitigating effects of heatwaves on building safety

Worryingly, extreme weather events are predicted to become the norm. As a result, now may be the time for responsible risk managers to begin reviewing practical measures to defeat the ‘thermal barrier’ and those increasing hazards arising from building malfunctions due to exceptional heat. Urban areas are worst affected, with the Urban Heat Island (UHI) the phenomenon named to recognise the fact that cities have a distinct climatology because of the concentration of heat sources from its packed infrastructure.

A round-up of remedial actions to provoke thought and action include:

• Cool roofs: The most efficient conditions for a cool roofing surface require the roof to be both highly reflective and highly emissive to minimise the amount of light converted into heat and to maximise the amount of heat to radiate from it. An ‘albedo’ roof (or white roof) could be considered to assist in reducing overheating in the building. Dark roofs mostly reflect no more than 20% of incoming sunlight, while a new white roof reflects about 70-80% of sunlight. Simply painting a roof white can often be the most economic solution, with potential reductions of up to 20% on energy bills.
• Cool building surfaces: Severe problems develop in open volume structures where heat cannot be dissipated. Recommended solutions for climate-sensitive buildings include smart materials that coat the surfaces of such buildings to reflect heat gain.
• Shading devices: Overhang shading where the architecture features large-scale glass expanses is often a supplementary measure necessary for creating a comfortable indoor climate.
• Insulation: the addition of thermal insulation to walls and roofs substantially contributes to preventing solar gain.
• Monitoring: Low-cost portable data loggers to monitor temperature and humidity and ‘tell-tale’ gauges to measure the physical movement inside and outside buildings are essential tools to build a picture of thermal stress for decision making by risk analysts.

Note: Many of these basic solutions have been part of traditional building practices, but together with more recent technological interventions, the measures outlined hold the potential to improve and maintain thermal stability and to defeat the new threats arising from global warming.

Further reading

• New Fire Industry Association research sheds light on detection of stratified smoke
• The science behind optical beam detection