Boilers
A boiler is a closed steel vessel that heats water into steam. Fuel (coal, oil, gas or wood) burns in a furnace; hot gases pass around or through the water. Steam goes out by a pipe to run engines and turbines, to heat and to clean or cook.
- Fire-tube boiler: hot gases flow inside tubes, water is outside. Simple, low pressure.
- Water-tube boiler: water flows in tubes, hot gases outside. Makes high-pressure steam fast; used in power plants and big ships.
Fittings for safety: pressure gauge (shows pressure), safety valve (opens when pressure is too high), water-level gauge (water must never run too low, or the metal overheats), feed pump (adds water), stop valve (opens or shuts the steam).
Freezing and refrigeration equipment
Refrigeration moves heat from a cold place to a warmer place. This needs work, done by the compressor. A special liquid-gas, the refrigerant, goes round a closed loop with four main parts:
- Compressor: squeezes the gas, so it gets hot and at high pressure.
- Condenser: hot gas gives its heat to the outside air or water and turns into a liquid.
- Expansion valve: a narrow opening; pressure drops and the liquid becomes very cold.
- Evaporator: the cold liquid takes heat from the food space and boils into gas. The gas goes back to the compressor.
Used in fridges, freezers, cold stores and fish hold cooling. A freezer just runs at a lower temperature (about −18 °C) than a fridge (about 4 °C). A thermostat switches the compressor on and off.
Air-conditioning equipment
An air conditioner uses the same four-part loop, but its job is to control the air in a room: temperature, humidity (moisture), cleanness and air flow. Air from the room is blown over the cold evaporator; it gives up heat and water vapour (which drips out as water), and cool dry air returns to the room. The condenser unit sits outside and blows the heat away.
Other parts: fan or blower, air filter, thermostat and ducts. A heat pump is an air conditioner that can run backwards to heat the room. Ships and fish-processing plants use big air-conditioning and cooling plants too.
Key formulas and definitions
- Heat taken in at evaporator + work by compressor = heat given out at condenser
- Coefficient of performance COP = heat removed / work done
- Boiler efficiency = heat in steam / heat from fuel × 100%
- Pressure: 1 bar ≈ atmospheric pressure (100 kPa)
Worked examples
1. A fridge removes 600 J of heat from the food while the compressor does 200 J of work. Find its COP and the heat given out at the back.
COP = 600 / 200 = 3. Heat out = 600 + 200 = 800 J.
2. A boiler burns fuel giving 5000 kJ of heat. The steam gets 4000 kJ. Find the efficiency.
Efficiency = 4000 / 5000 × 100 = 80%.
3. An air conditioner takes 3.0 kW of heat from a room and uses 1.0 kW of electric power. How much heat goes out of the condenser each second?
Heat out = 3.0 + 1.0 = 4.0 kW, so 4000 J every second.
4. Why is the safety valve set below the pressure the boiler wall can bear?
So that the valve opens and lets steam out before the wall is stressed to its limit. This stops an explosion.
Common mistakes
- Thinking a fridge makes cold. It only moves heat out; the kitchen gets slightly warmer.
- Mixing up the parts: the condenser is hot and gives out heat; the evaporator is cold and takes heat in.
- Believing a boiler can run with low water. This overheats and weakens the metal.
- Leaving out the safety valve. It is the key safety fitting.