Fire and the fire triangle
Fire needs three things: heat, fuel (wood, paper, oil) and oxygen (from air). This is the fire triangle. If you remove any one side, the fire stops. Water cools (removes heat). Foam and CO2 cut off oxygen. Closing a gas valve removes fuel.
Which buildings need fire protection?
Laws list buildings that must have fire equipment. These are places where many people gather or where fire spreads fast: schools, hospitals, hotels, malls, cinemas, factories, high-rise flats and warehouses. The bigger the building, the more people, or the more dangerous the goods inside, the more equipment it needs.
Fire equipment has three groups:
- Warning: smoke or heat detectors, manual call points, alarm bells.
- Extinguishing: portable extinguishers, indoor and outdoor hydrants, sprinklers, foam and gas systems.
- Escape: exit signs, emergency lights, stairs, escape ladders.
Types of extinguishing systems
- Portable extinguisher: for small fires. Water type for paper and wood; CO2 or dry-powder for oil and electrical fires.
- Hydrant: a hose connected to a pressured water pipe. Used by people or firefighters.
- Sprinkler: heads on the ceiling. A glass bulb with liquid inside bursts at about 68°C, so the sprinkler opens by itself, only over the fire.
- Foam: covers burning oil so air cannot reach it.
- Gas systems (CO2 or inert gas): for server rooms and machine rooms where water would harm things.
Never use water on an electrical or oil fire.
Designing a sprinkler or hydrant system
Designers do four things:
- Decide the flow needed: number of heads working together × flow of one head (about 80 L/min per sprinkler head is a common design value).
- Find the tank size: tank volume = flow × time the water must last (for example 20 minutes).
- Find the pressure needed at the highest head. Every 10 m of height needs about 0.1 MPa (1 bar) of pressure, plus losses by pipe friction.
- Choose the pump, pipe sizes and where to place the heads, usually 2 to 4 m apart.
Try it: the fire-drill walk
Walk through your school or home. Count how many things you can find: exit signs, extinguishers, alarm buttons. Predict first: how many will you find? Then check. Draw the shortest escape path from your classroom to outside.
Key formulas and definitions
- Fire triangle: heat + fuel + oxygen
- Flow needed (L/min) = number of heads working × flow per head
- Tank volume (L) = flow (L/min) × time (min)
- Pressure for height: about 0.1 MPa per 10 m of water
- Sprinkler bulb bursts at about 68°C
Worked examples
1. Name the three sides of the fire triangle.
Heat, fuel and oxygen.
2. A sprinkler head gives 80 L/min. Six heads open together. What is the flow?
6 × 80 = 480 L/min.
3. The flow is 480 L/min and water must last 20 minutes. What tank volume is needed?
480 × 20 = 9,600 L (about 9.6 m³).
4. The highest sprinkler is 30 m above the pump. What pressure is needed just to lift water there?
0.1 MPa per 10 m, so 3 × 0.1 = 0.3 MPa, plus extra for pipe friction.
5. Which extinguisher is right for a burning computer?
CO₂ or dry powder. Water is dangerous with electricity and can damage the computer.
6. Why does only the sprinkler above the fire open, not all?
Each head has its own heat-sensitive bulb. Only the bulb that gets to 68°C bursts.
Common mistakes
- Using water on an oil or electrical fire.
- Thinking all sprinklers open together. Only the heated one opens.
- Using the lift to escape in a fire. Use the stairs.
- Forgetting to count the time the water must last when sizing the tank.