Three phase changes: the driving power behind a cooling machine
– by Jeroen Fijan, 1 November 2018
We are in the middle of autumn and approaching that time of year when temperatures dip below freezing at night while staying well above zero during daytime. Puddles of water in the street are frozen solid in the morning but melt again once the sun comes up.
This melting and freezing is called a phase change. The term refers to any shift of a substance from gas to liquid, from liquid to solid, and vice versa. This natural process – especially the shift between liquid and gas – is very important in HVAC technology: in fact, it can be said to be the driving power behind a cooling machine. Let me tell you more about this physical process and how it’s deployed inside a cooling machine.
There are three states of matter: solid, liquid and gas. A block of ice will melt if the temperature is high enough and eventually a liquid will evaporate when it is cooking. If we lower the temperature, things will go the other way around: a gas will condense and turn into liquid again and when it gets cold enough a liquid will freeze into ice.
Figure 1: The three phases of matter
It’s all a matter of heat
Changes in phase are caused by temperature shifts. Physicists distinguish between two sorts of heat, sensible and latent heat.
Sensible heat is the heat you can feel, like when you walk out of an air-conditioned room into the street on a summer afternoon. Latent heat is a form of heat which does not involve changes to ambient temperature. The only thing that is changing is the phase of a matter.
Let’s take the example of boiling water for a cup of tea. At 100 °C, water can still be a liquid, but it can also be vapour. The difference between 100-degree liquid water and 100-degree water vapour is that latent heat is greatly increased in the latter.
Figure 2: Solid to liquid to vapor
The cooling machine
A cooling machine contains a coolant that is pumped around. This coolant is a substance that changes in phase as it cycles around in the cooling machine: from liquid to gas and into liquid again.
As you can see in the diagram above, evaporating costs a lot of energy in the form of latent heat. A cooling machine uses this to its advantage. To evaporate, it draws heat from the surrounding environment. While the coolant is not changing temperature – it is only changing phase – the ambient temperature starts to drop as the heat is drawn from it.
The evaporator is placed inside an environment – or a system – that needs to be cooled, like the inside of a fridge or a heat exchanger. A cooling machine works as a cycle process. Heat is drawn out of the system at low pressure and eventually channelled inside the condenser at high pressure.
Figure 3: The cycle of a cooling machine
Another important property in phase changing is pressure. In the average kitchen, water evaporates at 100 °C. Higher up, like on a mountain top, say, water evaporates at a lower temperature, 80 °C, because the pressure is lower. Adjusting the pressure makes it possible to manipulate the cooling process.
There are many different cooling machines with various coolants, pressures and evaporation temperatures. It’s smart to pick the right cooler for a given job – but it’s even better to build one.
A custom-made solution
Every project is unique. That’s why each situation calls for a bespoke solution. It is our philosophy to create custom-made solutions. That is why we engineer our cooling machines ourselves, enabling us to tweak them so that they supply the best possible performance.
Jeroen Fijan | R&D Manager
Jeroen Fijan has been working at Heinen & Hopman since 2001. He started as a draughtsman and, over the years, worked his way up to the top of the R&D division. Sustainability is a top priority in the quest to improve H&H’s products and processes.