Understanding the Relationship Between Nozzle Size and Fire Extinguishing Power

[eosphoro]

my teacher on fire estinguising subject explained me that with the same pump if you reduce the size of the nozle you increase reaction on the fire stinguisher

so does reaction depend on cinetic energy sent away?

but if by halving the nozle you get double force of reaction with the same pump, can't you half the nozle infinitally with ideal materials and fluid, and double the force of reaction infinitally

then what would avoid you from using this force of reaction to feed the pump since this force in theory can be infinite by making the nozle infinitesimal (this is what i understand from my teachers explanation)

 

[krishnaprasad]

I like your thinking.. The theory of halving nozzle size and doubling pressure or force of reaction works only for sizes bigger than some limit say 2cm in diameter.
Apart from pressure from inside, the liquid/gas flowing in the air will be expering friction-type forces on the walls of the nozzle

When we reduce the diameter of nozzle, these frictional forces will be huge and they will overcome the pressure. and will stop the liquid coming out.

You can try this will a syringe/injection. Without the needle, water comes fast, with needle it takes some time because the needle is so thin that it have more friction-force against liquid coming out. (these forces are also called vanderwall forces and property is called viscosity)

 

[sir-pinski]

The relationship between nozzle size and fire extinguishing power is a complex one that involves a variety of factors. The size of the nozzle does play a significant role in the effectiveness of a fire extinguisher, but it is not the only factor that determines the extinguishing power.

As your teacher explained, reducing the size of the nozzle can increase the reaction on the fire extinguisher. This is because a smaller nozzle creates a higher velocity of water or agent being discharged, which can penetrate deeper into the fire and smother it more effectively. However, this is not the only factor at play.

The reaction on the fire extinguisher also depends on the kinetic energy of the water or agent being discharged. This kinetic energy is a combination of the velocity of the discharge and the mass of the water or agent. So, while a smaller nozzle may increase the velocity, if the mass of the water or agent is too low, it may not have enough kinetic energy to effectively extinguish the fire.

Furthermore, there are limits to how small a nozzle can be made and still maintain its effectiveness. As you mentioned, in theory, a smaller nozzle could produce an infinite force of reaction. However, in practice, there are limitations to the materials and fluid being used. If the nozzle becomes too small, it may clog or become damaged, rendering it ineffective.

In addition, there are safety considerations to take into account. Using an excessively small nozzle could create a high-pressure discharge that could be dangerous for the user and those in the surrounding area. It is important to have a balance between nozzle size and pressure to ensure both effectiveness and safety.

In conclusion, while reducing the size of the nozzle can increase the reaction on the fire extinguisher, it is not the only factor that determines the extinguishing power. There are limitations to how small a nozzle can be made and other factors, such as mass and safety, must also be considered. It is important to follow manufacturer's guidelines and use the appropriate nozzle size for the specific fire extinguisher.