Mechanical Cooling

33 Phase Changes

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We are relatively familiar with the three main forms of matter: solids, liquids and gasses. Since cooling systems make use of the changing properties of a refrigerant as it goes from liquid to gas form, it is useful to understand the relationships between temperature and heat energy in a system.

Solids are defined as having a low heat-energy level, meaning the atoms are locked in place and the object has a fixed volume and shape. For example a block of ice is water in its solid form.

As ice melts it becomes the liquid water that we are all familiar with. Its atoms are free to flow about, and it has a fixed volume, but will spread out to fill whatever container it is in, meaning its shape is not fixed.

As water is heated, it eventually boils and becomes a gas. In this state, its atoms are free to move about, and it has no fixed shape or volume. This is its highest energy level.

The relationship between the temperature of a medium versus the amount of heat energy added or removed from the environment is shown below.

A graph showing the phase changes that a medium goes through as the temperature and heat energy increases or decreases.
The relationship between temperature and thermal energy of a system. Endothermic is heat energy absorbed from the environment. Exothermic is heat energy released to the environment.

It is important to note the flat portions of the graph labelled “Phase Change” are horizontal with respect to the temperature axis. This means that a significant amount of heat energy must be either added or removed from the surrounding environment in order to change a material from a solid to a liquid, or a liquid to a gas. This heat energy is sometimes referred to as latent heat.

Only once the material has completely changed its phase will the temperature continue to rise. Once the materials temperature starts to rise, the energy which is now added (or removed) from the system is called sensible heat, because a person could sense, or feel the difference in temperature.

Endothermic reactions such as melting or boiling take energy from the surrounding environment and use it to change the phase of the material in the cooling system.

Exothermic reactions such as freezing or condensing release energy from the system back into the environment.

Direct Expansion cooling systems use the boiling and condensing properties of a refrigerant to rapidly remove thermal energy from an area where it is undesirable, and dissipate it elsewhere.



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