What is Boiling

The first step to understanding how the air conditioning refrigeration cycle works is to look at some fundamental natural phenomena we experience daily.

Let’s start with boiling. What happens when you heat a saucepan filled with water? We can try this experiment if we have a thermometer. Water boils at 100°C, so that we will need a thermometer up to 120°C.

Safely put your thermometer into the saucepan, and ignite the stove to start heating the water. What do you observe happening to the water temperature?

Latent Heat of Vapourisation

The heating of water caused the water temperature to rise. What if you stop the heating? The temperature will stop rising. After some time, the water temperature will drop as the water cools.

We can easily explain this natural phenomenon that we take for granted all the time. As we supply heat to the water, its temperature rises, and soon after we stop the heating, the hot water rejects heat to its surrounding. Hence the water temperature falls.

Now, if you tum the burner back on, the temperature of the water starts to rise again. But let’s see, if we leave the burner on, what will happen to the temperature of the water? Will it rise indefinitely?

Once it reaches 100°C, the water temperature stabilises and will increase no further, although the burner still supplies heat.

At sea level, we know that water starts to boil at 100°C. Even if we continue to heat the water, its temperature stabilises at 100°C.

At 100°C, we will see steam evolving above the saucepan. The water level will reduce as we continue heating. Why?

The water in the saucepan experienced a change of phase from liquid to vapour. This change of state is a fundamental physical phenomenon.

The drop in water level drops when water vapourises.

It is critical to note that the water temperature remained at 100°C and will not increase even with more heat added.

Let’s represent this experiment graphically to help us understand this phenomenon better.

Say we have water at 20°C at equilibrium with the surrounding air, also at 20°C when we started the experiment. As long as we do not add heat to the water, its temperature will remain at 20°C — mark 20°C as the starting point on the graph.

Next, we start the heating. The water temperature will increase steadily. The water remained ‘calm’ in the saucepan. At time t1, the water temperature is 50°C, and the surface remains relatively flat.

If we continue to heat the water, its temperature rises steadily.

We start to see more and more ripples and breaking of the water surface tension as the water temperature approaches 100°C.

At t2, when the water temperature reaches 100°Cthe water is boiling, and you will observe more and larger bubbles. Plot this on the graph.

What will be the water temperature as we continue to heat the water? The water temperature remains stable at 100°C!

At t3, water remains at 100°C, but a lot of steam evolves from the saucepan, which causes the water level to fall. This phenomenon is the vapourisation of water.

The horizontal step remains constant at 100°C after the initial temperature rises from 20°C.

Additional heat added beyond t2 does not increase the water’s temperature. Instead, heat energy added transforms liquid water into water vapour.

When a liquid evaporates, it will absorb heat. As a result, its temperature will remain constant. The heat absorbed is called the latent heat of vapourisation.

These experiments demonstrate that when water changes from liquid to vapour, it absorbs heat without changing its temperature.

At normal atmospheric pressure, water vaporisation always takes place at 100°C.

Latent Heat of Fusion

Let’s do another experiment, this time with ice cubes and cold water. Again, take the homogenous water and ice temperature in the saucepan.

The thermometer will read 0°C. This temperature is the melting temperature of ice. Now start to heat the ice/ water mixture.

What do you think will happen?

Despite adding heat, the water temperature remains stable at 0°C, but the ice melts and changes its state to liquid. This phenomenon is called the fusion of ice.

As we add heat, the water temperature will rise as soon as there is no ice left.

These experiments allow us to observe the fundamental phenomena critical to understanding air conditioning systems’ refrigeration cycle.

Water changes from the solid state to the liquid state at a constant temperature of 0°C by absorbing heat. This heat is known as the Latent Heat of Fusion.

When we add heat, and there is no increase in temperature, but it causes a change in its physical state, we refer to it as latent heat.

On the other hand, when the temperature changes by adding heat, for example, when we heat water from 20° to 40°C, we say it is due to sensible heat. So, in a way, we can sense the temperature change!

We must always remember that evaporation needs energy (heat). And more generally, liquid can not evaporate, nor does ice melt without adding heat.

The liquid absorbs enormous amounts of energy as it evaporates. For example, our experiment shows that liquid water requires 5.4 times more energy to change into vapour at a constant temperature of 100°C than to raise its temperature from 0°C to 100°C.

Of course, once it reaches 100°C, the temperature stabilises; this is the beginning of a second step, the one we have already seen.

This experiment has allowed us to observe some fundamental phenomena.

Water changes from solid to liquid at a constant temperature (0°C) by absorbing heat. This heat is known as the Latent Heat of Fusion.

Water changes from the liquid state to the Vapour State at a constant temperature (100°C) by absorbing heat. This heat is known as the Latent Heat of Vaporisation.

Latent heat is the heat needed to cause the change in the physical state of a body at a constant temperature.

We must always remember that evaporation needs energy (heat). And more generally, liquid can not evaporate, nor does ice melt without adding heat.

What heat is required to transform 1 kg of pure ice at -50°C into 1 kg water vapour at 150°C?

The unit of heat (energy) is in kilojoules (kJ). These values are only for background, don’t worry if they mean little now. Just note that the vapourisation step lasts seven times longer than the fusion step with the same heat source and the same amount of water.

The liquid absorbs enormous amounts of energy as it evaporates. For example, our experiment shows that liquid requires 5.4 times more energy in vapour at a constant temperature of 100°C than raising its temperature from 0°C to 100°C.

These are the laws of natural physics and are the basis of the refrigeration cycle.