We all know what happens when water boils in a kettle. Addition of heat turns the water into steam. Water boils at a temperature of 100°C. We say that it changes phase from a liquid to a gas. Notice that we said 'addition of heat'. That heat has to be at 100°C or more to boil the water, but other substances boil at lower temperatures. This will come in handy as we shall see when we take a look at the effect of pressure on boiling point. Condensation
Condensation is the reverse of boiling. The steam gives up its heat and turns back to water. When we mix steam with air, which is what happens when the kettle boils, the cool air tends to condense the steam, and very small droplets of water form. This is what we see and commonly call 'steam'. Pure steam is a colourless gas.
What are the other changes of phase that can take place? Test your knowledge!Sensible and Latent Heat
What happens when we add heat? Think of ice. Starting when the ice is very cold, it gradually warms up, then it starts melting. While it's melting, it stays at zero degrees C. When it has all melted, the water (which was the ice) starts warming up until it reaches boiling point. Then it boils into steam. While it's boiling, it stays at 100 degrees C. Adding even more heat, raises the temperature of the steam. When heat is being added to raise the temperature, it's called Sensible Heat. When the heat is being added to melt ice or boil the water it's called Latent Heat. Sensible? Ice was one of the very first "Refrigerants". Ice was stored and because a large amount of heat is absorbed in the melting process it lasted a long time. The latent heat of evaporation of fluids is also large and this is why evaporation is used in the cooling process.
Pressure and Boiling Point
When we use a pressure cooker, the water boils at a temperature of more than 100°C. The increase in pressure raises the boiling point. This makes the food cook faster. If we boil a kettle on a mountain top it boils at a lower temperature because the pressure is lower. Cooking is much slower.
We use the terms Vaporization or Evaporation instead of boiling, because this is a bit more technical and refers to boiling at a particular pressure. We say the evaporating temperature of a fluid at a certain pressure is xx °C. In the camping gas stove propane is stored under pressure as a liquid. When the valve is opened the liquid starts to boil (sorry, evaporate!). Where does the heat come from to boil the propane? It comes from the surrounding air, and because the propane is evaporating at a low temperature, the outside of the can becomes cold. The liquid is drawing in latent heat to enable it to boil. The can cools down the surrounding air. That's good - we've made the first step to building our fridge!
Choosing a Refrigerant
Each fluid has its own distinct boiling temperature - pressure characteristic. This is called the Saturation Pressure Temperature relationship, or P-T data for short. The normal boiling point is atmospheric pressure, which is approximately 1 bar(a).
R134a boils at -26°C
R22 boils at -40.8°C
R404A boils at -46°C
The curves show the evaporating or condensing temperature of each of these fluids together with carbon dioxide, over the range of temperatures of interest to refrigeration engineers. Normally it is necessary to choose a fluid which evaporates between, say, -50 and +10 °C and which condenses at 40 to 70 °C. The pressure for both processes should be greater than atmospheric - but not too high. Generally the higher the saturation pressure for a given temperature, the better the thermal capacity of the fluid. So less fluid is required to do the job. Unfortunately carbon dioxide will not condense at pressures above 70bar because this is the critical pressure. This can be dealt with, but its beyond the scope of this page. Phase Change - Test your Knowledge
To play the game, click on the name of the process, then on the blank space in the chart where you think it fits............