@ Palmpie
Water dat niet vrij beweegt is 'droog'? Is de zee 'droog' buiten de wolken?
Waarom denkt u dat zij deze quasi wetenschappelijk wartaal verzinnen?
Mystificatie en massahypnose!
Het heeft niets te maken met de natuurwetenschappelijk warmte overdracht mechanismen waar het over zou moeten gaan; buiten de 'Boemeramg' gelovigen op bedevaart naar Leeuwarden, via Delft, Roermond, of andere voor intelligentie beledigende routes.
'Debunking'is een Amerikaans begrip waar wij in Nederland helaas geen equivalent voor hebben. Wij deden het vandaag.
Met respect,
Fred Vos.
Een studie uit 2000
http://www.civil.canterbury.ac.nz/fire/pdfreports/SDavis.pdf(...)
9.5.5 Compressed Air Foam (CAF)
The NZFS is introducing compressed air foam systems (CAF) with
all new first response vehicles. At present the CAF system will be in addition to the existing highpressure delivery (HPD) hoses. The CAF system hoses are 41 mm hose with screwed couplings and will be permanently connected and stored on easily accessible reels. The branch is a 25 mm straight nozzle with an on/off control. The nozzle can be quickly interchanged with a standard combination nozzle. The introduction of CAF must be considered
one of the most significant recent advances in fire fighting.
The use of CAF has the following benefits:
• Reduction in water consumption
• Foam adheres to fuel
• Reduction in pressure drop through hose
• Maintain visibility due to reduction in steam
• Reduce flare ups
• Ability to apply from outside building
• Lower % concentration than other additives
The work done by Dunn (1998) on unshielded fires and by Gravestock (1998) on shielded fire indicates a 35 % reduction in water usage when using CAF.
These studies also indicated that reignition is less likely with CAF and this further reduces
the quantity of water used.
The figures quoted exclude the additional water used for damping down and the protection of exposures from radiation. The fire service project water between the fire and adjacent buildings, or onto the adjacent building, so as to minimise any damage
from thermal radiation to the non-involved building. This water will have no effect on the reduction of the fire size but must be taken into account for calculating water requirements.
The traditional fire-fighting procedure for a residential household fire is to advance into the building and extinguish the fire with a high-pressure delivery. The fire fighter forces the fire to vent through the windows, in that the fire fighter positions himself between the fire and the unburnt portion of the building. This procedure requires fire fighters to be skilled in the use of self-contained breathing apparatus (SCBA).
The use of CAF allows fire fighters to attack the fire through external openings and thus the need to enter the burning building is removed. This has the advantages, already discussed from the safety point of view. An added advantage for volunteer crews, who do not get a lot of experience in the use of SCBA, is the reduced need for the use of breathing apparatus. As CAF can be applied from outside the building, this obviates the necessity to work in zero visibility in SCBA. The mounting of a CAF nozzle on a tele-boom (hydraulic arm) would allow a greater reach and thus fires in multi-storey buildings can be suppressed.
The foam is produced by the injection of foam concentrate into the water stream. This occurs in the piping between the pump outlet and the manifold discharge. Compressed air is then also introduced into the plumbing. The foam reduces the surface tension of the water in the hose and hence lowers the pressure resistance of the hose to the passage of the water/foam. This means the mixture may be pumped further. The selection of the fixed bore tip will dictate the type of foam generated and the throw of the foam. A smaller tip produces wetter foam but a longer throw than a larger tip.
CAF is termed a class A foam and is used at lower concentrations than other foams. These concentrations vary between 0.1 and 1.0 %. Because the concentrate is lower, CAF consists of a higher % of water. CAF breaks down the surface tension of water, which thus allows the foam to penetrate vegetation and stick to vertical surfaces.
Because it adheres to the fuel surface it wets the fuel for a longer time and because of its bulk, brings more water into intimate contact with the fuel. The foam blanket shields the fuel from impinging radiation, which thus stops the production of more pyrozlates. The CAF blanket also excludes the oxygen from the fuel.As the name implies the foam is aspirated with compressed foam at the truck rather than the traditional mechanical aspiration that occurs at the branch. This is because the pressure of the compressed air pushes along the foam and means the hoses are
much lighter (approx. 65% lighter) and thus easier to manoeuvre. The reduction in pressure drop due to the low surface tension of the foam means longer lays of hose can be made. This is especially important when extinguishing vegetation fires, which are typically distant from where pumps can be located.
The main disadvantage of CAF is the inability to produce a dispersed wide pattern spray so as to protect fire fighters against the effects of radiation from the fire. To overcome this problem research is being conducted into prototype tips that are
capable of producing a water shield as well as foam. Preliminary tests into the effect CAF has on forensic evidence reveal that fingerprints etc remain unaffected. (Noble, 1999)
The NZFS CAF pumps are fitted with a helical screw water flow measuring. These are subject to mechanical damage and thus must be protected by upstream strainers. The electronic signal from the flow-measuring device is fed to an electronic gauge.
This gauge has in-built logic to proportion the quantity of foam injected into the water stream based on the measured water flow. These measuring devices could be replaced with magnetic devices. However these devices are more expensive but having no moving parts, cannot be damaged. The magnetic devices have the drawback of attracting ferrous material and thus must be regularly cleaned. This water flow data could be recorded by a data logger and would thus provide valuable information on the water used during a fire. This information coupled with other parameters will provide invaluable data on the operation and use of peak water flows, total water used and the time it takes to physically intervene in a fires growth.
(...)
CHAPTER 10 CONCLUSION
The extinguishment of a class A fire requires the removal of heat from the fuel surface. Only 7 to 10 % of the energy from combustion supports further production of pyrolyzates. Thus the water requirement is more related to the specific heat content of the fuel and the thermal conductivity of the fuel surface than the energy released from the combustion process. For this reason, effective fire fighting is best achieved by the application of a solid stream of water that is swept over the fuel surface. This requires emphasis in training and development of tactics, especially for large fires.
(...)
