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PPV - AIRSTREAMS PAUL GRIMWOOD |
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There is no doubt that the introduction of Positive Pressure Ventilation (PPV) during the 1980s has revolutionised tactical firefighting operations and the use of PPV in both post-fire and pre-attack situations is now seen by many as a highly effective option. The removal of smoke and toxins from a post-fire structure is a routinely straight-forward operation and the training implications associated in utilising PPV units to achieve this aim are minimal. However, the injection of PPV airflows into structures or compartments where the fire remains in a ventilation controlled state, as a method of directing smoke and heat towards a pre-determined exhaust outlet point, is a strategy that must be used with great care and coordination.
In relation to the differing styles of airflow produced by PPV units there has been much confusion and an apparent misunderstanding of their effects. The fire officer who refers to a fan’s airflow capability or performance by quoting blade sizes in inches or millimetres is clearly failing to grasp the fundamentals of air dynamics associated with PPV. The bulkier large-bladed units designed to produce a conventional style air-stream will direct a steadily advancing ‘cone’ of air towards the entry point. One of the disadvantages of this style of air-flow is that much of the air will fail to enter the structure as it spreads out around the doorway. It is a fallacy that such a stream will form a true air seal around the door, preventing any outflow of smoke, fire gases or even flames and tests by the FEU and ULH have disproved this belief. However, many firefighters still assume that this ‘air-cone’ has the ability to seal the entry point despite widespread scientific evidence to the contrary. Another disadvantage of the conventional style air-stream is that it can be badly affected by side-winds, severely compromising any potential to pressurise the structure effectively. In contrast, the turbo style air-flow is a faster moving and narrower stream of air that entrains additional air on route to the entry point. This style of air-stream is less affected by side-winds and more air will pass through the entry doorway to pressurise the structure. It is scientifically documented that turbo-bladed air-streams emitting from some smaller units are more likely to achieve reduced smoke clearance times[1] and greater compartmental temperature reductions[2] than air-flows from most of the larger and bulkier conventional units. These effects are often clear to see during practical demonstrations and comparisons of various units. Another common misconception of PPV air dynamics concerns the widespread belief that turbo airflows may create dangerous conditions at or near air entry points due to negative areas around the air-stream possibly drawing dangerous and igniting fire gas-layers back towards advancing firefighters. In this scenario the evidence suggests that conventional air-streams are just as likely to initiate ignitions of the fire gases if injected into under-ventilated compartments and any such ignition will most certainly burn back towards and possibly beyond the air-entry point, no matter what type of air-flow is in use. A recent report[3] from Lund University in Sweden demonstrated the likely effects of injecting PPV airflows into ‘apartment-sized’ under-ventilated compartments where the forced gravity current created unfavourable conditions in the gas layers for a period in excess of 40 seconds prior to a ‘safe’ environment being achieved. Both airflows from conventional and turbo designed units are generally capable of achieving effective results in a wide range of scenarios requiring smoke clearance and temperature reductions. The main advantage of the more compact turbo designs may lie in the reduction in stowage space created to house other large items such as light-weight pumps and rescue gear.
[1] University of Le Havre PPV Airflow Dynamics Research Project [2] Report of Tyne & Wear Tactical Ventilation Trials at FSC – July 1998 [3] Theoretical and Practical Aspects on Firefighting Tactics in a Backdraft Situation – D. Gojkovic & L. Bengtsson – Lund University (Sweden). |
