Smoke Hang Time Index

The hang time, or persistency of the smoke effect produced by all artificial smoke systems is a function of some key factors:

Boiling range and evaporation rate of the constituent chemicals

The higher the boiling range and lower the evaporation rate of the smoke chemical, the greater the smoke persistency.

Viscocity and surface tension

Generally, within chemical groups, the higher the viscosity and surface tension, the higher the boiling range.

Particle size

The bigger the particle size, the lower the ratio of surface area to volume, hence the greater persistency. However, remember, the bigger the particle size, the faster the settling velocity.

Ambient temperature

Generally, the higher the ambient temperature of the air in which the smoke is suspended, the faster the smoke will "evaporate".

Many smoke systems (largely those based on the entertainment industry requirements) produce a dense white smoke effect, but one that evaporates very quickly. Hence, although the initial burst is impressive, filling large volumes with smoke becomes problematical, as one quickly reaches the stage where the smoke produced a few minutes ago has evaporated, and smoke density no longer increases. Also, systems which quote a smoke output unqualified by smoke density or visibility are of no use when trying to make comparisons.

Concept have introduced a Hang Time Index which better allows a user to establish the relative performance of smoke systems, in particular with regard to large volume or extremely dense smoke concentrations.

Generally, oil based smokes, with their very high boiling range, will produce a smoke effect that lasts for an exceptionally long time. This makes the ViCount type system particularly suitable for smoke logging exceptionally large volumes, which would need perhaps 10 or 12 times as many high output water based systems to achieve the same performance.

For Concept’s most persistent water based smoke, (eg Colt 4 ,Spirit 900A) we have selected a water miscible pharmeuctical grade chemical with the highest boiling range in its class, and used a very high percentage of this active ingredient in the mix. This gives the “Holy Grail” in terms of water based smoke of high persistency combined with the small particle size and hence insignificant settling velocity.

Where quickly dispersing smoke is required (localised air flow tracing for example) the Colt 3 or Spirit 900B would be chosen, as the active ingredient has the lowest boiling range of the commonly used simulants.

Using the Hang Time Index in conjunction with quoted smoke output figures, (providing those figures are quantified in terms of smoke density or visibility) allows a simplistic comparison to be made of smoke systems.

This is particularly relevant with larger volumes or denser smoke concentrations, where the persistency of the smoke becomes of greater significance.

Smoke Type: Hang Time (20°C) Index
Food Quality Oil Based Smoke
Concept ViCount
3 - 4 hours 10 - 12
Persistent Water Based Smoke
Concept Colt 4 / Spirit 900A
1.25 - 1.5 hours 4 - 6
Quickly Dispersing Smoke
Concept Colt 3 / Spirit 900B/AirTrace
2 – 4 minutes 0.17
Typical "entertainment" smoke system 15- 20 min 1

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East Sussex County Council

"I was very impressed with the smoke generators provided by Concept Engineering and feel they could be confidently recommended at our group meetings to other tunnel operators who may be contemplating an emergency exercise.

The reaction of the tunnel environmental control system visibility sensors to the smoke was excellent, not only did the fans cut in but the volume of smoke was such that it immediately tripped a high level alarm which additionally sends a signal in the form of a general alarm to the Sussex Police Headquarters.  Had the smoke been generated by a real incident the alarm would have alerted the Police to the problem in the tunnel probably before a call was made from a tunnel emergency phone." 

After Operation Orpheus - Dartford Tunnel