First Strike Tech's Company Information
The ultimate purpose of a firefighting nozzle is to aid in "Fire Suppression" (extinguishment). Therefore, the basics of fire suppression must be understood and applied in any evaluation of fire apparatus or equipment.
A brief overview of some of the principals involved in fire suppression and the role of firefighting nozzles is provided. A more thorough review and understanding should be obtained with a review of the current issue of the National Fire Protection Association’s National Fire Protection Handbook (NFPA).
According to the NFPA’s National Fire Protection Handbook, Fire Suppression or "Extinguishment can be accomplished by cooling either the gaseous combustion zone or the solid or liquid combustible. In the latter case the cooling prevents the production of combustible vapors. This is probably the primary mode of action when a wood fire is extinguished by applying water."
"As an alternative to removing heat from the combustion zone to slow the reactions, it is also possible to reduce the temperature of the flame by modifying of the air, which supplies the oxygen. Air contains 21% by volume of oxygen, the remainder being almost entirely the inert gas nitrogen. The nitrogen, which is drawn into the flame along with the oxygen, absorbs heat, with the result that the flame temperature is much lower than it would be in a fire burning in pure oxygen. If additional nitrogen or some other chemically unreactive gas, such as steam, carbon dioxide, or a mixture of combustion products, were to be added to the air entering the flame, the heat absorbed by these inert molecules would cause the flame temperature to be even lower."
"The flame temperature is so important because the rate of a key combustion reaction (H+O2®OH+O) is very sensitive to temperature. A small decrease in flame temperature causes a disproportionately large decrease in the rate of this reaction, according to Arrhenies' Law." (NFPA Seventeenth Edition page 1-73)
Furthermore, according to NFPA the Application Rate & Type Stream determine the speed of suppression with water. The rate of application is commonly referred to as gallons per minute (gpm). The type of stream refers to heat absorption. The two common ways of increasing a stream’s heat absorption is by breaking the stream into smaller droplets and increasing the number of droplets by increasing your gpm. "To be effective, the water droplet must be formed near the base of the fire, or be large enough to have sufficient energy to reach the seat of the fire despite air resistance, the force of gravity, and the fire thermal column. When droplets are too small they can be deflected by the fire plume, or be evaporated before they reach the base of the fire." Also, an aspirated water droplet stream used in a heavy smoke environment will improve visibility by the attachment of carbon particles in the smoke to the water droplets thus removing them from the air.
There are two well known nozzles in the fire service, the smooth bore or solid stream and the combination nozzle, which may be a basic spray, constant gallonage, automatic or a constant/select gallonage as is recognized by the NFPA 1964 standard. One other nozzle, although not subject to the 1964 standard, is the Vindicator nozzle which operates in a different manner providing the advantages of the smooth bore for high application rates and an unusually effective type stream for rapid heat absorption similar to that of a combination nozzle. It also has the advantage of further reducing the temperature of the flame by modifying the air supply or oxygen supply.
According to NFPA we know that solid streams are useful where extreme range is desired, where it is necessary to penetrate soft material, or when thermal degradation of spray nozzle streams prevents proper penetration.
The advantages of the smooth bore nozzle are high flow rates, less nozzle reaction, more mobility, and less steam generation. Solid stream nozzles have a lower surface-area-to-volume ratio, do not have as good heat-transfer characteristics as spray nozzles and , consequently, are not as effective in absorbing heat. Some compensation of this disadvantage is made by breaking up the stream by deflecting it off a solid surface such as the bulkheads or overheads. It’s the high flows that help make up for its lack of heat absorption, thus making it a good choice as an attack nozzle. One problem to be aware of is what happens when it’s not operated at its recommended pressure (50 psi NP for hand lines & 80 psi NP for Master streams). If the nozzle is under-pressurized, the result is an inadequate gpm flow and a stream that loses its effective reach. If it’s over-pressurized the stream breaks up as soon as it leaves the tip. The end result is that you may not be delivering enough gpm to the fire because of loss from wind, gravity, and evaporation from the thermal column.
According to NFPA we know that spray or combination nozzles offer a higher surface-area-to-volume ratio and have better heat-transfer characteristics than solid stream nozzles and, consequently, are more effective in absorbing heat. However, this advantage is significantly reduced or lost when we switch the unit from a wide spray or fog pattern to a more narrow or straight stream pattern. Another advantage of a spray pattern is that it is a poor conductor of electricity, unlike the solid stream. The combination nozzle can typically handle being over-pressurized, yet flows may drop dramatically when they are under-pressurized. A combination nozzle is more effective than a smooth bore on fires involving flammable liquids and it can provide protection for firefighters in limited situations where a heat shield is needed. An example would be when the fire team is making an approach to secure a valve. Large volumes of air can be moved, which is useful in ventilation. It should be kept in mind that this same ability may spread the fire, create a dangerous thermal inversion, and cause steam burns to fire fighters and/or victims if not executed properly.
The primary disadvantage of spray nozzles is that they typically give up a large amount of Application Rate (gpm). Please note that it is agreed that a spray nozzle can flow as much or more than a solid stream nozzle in certain situations. However, to do so you end up with an unmanageable fire stream which can limit mobility and endanger personnel on the hose.
...and there's Vindicator Nozzles!
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