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  5. 1-3. Cooling principles

Precision air conditioning equipmentPrecision air conditioners Technical Information

PAU Series

1-3. Cooling principles

This section explains the principle of cooling, the laws of heat transfer, and the three states of matter.

 

It is necessary to warm and cool air in order to realize the air conditioning that is described in “1-2. What is air conditioning?”.
We warm things up and cool things down every day in our lives, so it feels like the temperature can be changed very easily, but what is the principle behind this?
The principle of cooling is explained here, which also leads to the structure of air conditioners and chillers.

(1) What is “cooling”?

There are many tools and devices around us to cool things down. For example, if you look around the house, you will see things such as handheld fans, electric fans, room air conditioners, refrigerators, and so on.
These tools and devices have something in common with respect to the fundamental principle of cooling things, even though they may have different mechanisms.
When you fan yourself in a hot room, the wind generated removes the hot air around your body and the heat from your body due to the evaporation of your sweat, making you feel cool.

You can therefore understand that cooling is equivalent to the removal of heat.
You can also feel that when liquid (sweat in this example) evaporates, heat is also removed (this is known as vaporized heat, which will be described later). This vaporization heat is also used when splashing water at dusk in summer.

The principle is easy to understand intuitively when it comes to the handheld fan and the electric fan, but the room air conditioner and refrigerator also share the same mechanism in principle.

Room air conditioners and refrigerators make use of the properties of the refrigerant to create an environment with a temperature that is significantly lower than the ambient temperature.
 

(2) Heat transfer (heat exchanger)

When two objects with different temperatures touch, heat moves from the “hot” object to the “cold” object. At this point, heat is seen to be taken away from the “hot” object to cool it. Conversely, heat is seen to be received by the “cold” object, leading to it being heated or a rise in its temperature. This is known as heat exchange, and the basic principle is that heat always moves from a hot object to a cold one.

The three laws below are related to the heat exchange rate and quantity.

1. Contact surface area is wide

2. Heat transfer is smoother between materials that can transfer heat better

As the term implies, heat is transferred more quickly between materials which transfer heat well (high heat conductivity).
For this reason, the heat exchangers of air conditioners are made of materials which have a good thermal conductivity such as copper, aluminum etc.
However, materials that do not transfer heat easily are used as insulation materials e.g. glass wool etc.

3. Large temperature difference between heat exchange materials

The higher the temperature difference between two substances, the greater the movement of heat. As heat exchange continues, the transfer of heat decreases, and eventually the temperature difference disappears (equilibrium state).

Based on these laws of heat exchange, air conditioners such as room air conditioners often use fin and tube heat exchangers that come into contact efficiently with aluminum fins having a large surface area, forcing air to flow between the gaps in the aluminium fins in close contact with the copper tubes in which a cold refrigerant flows.


Fan and tube heat exchanger

Heat exchangers include liquid-to-gas heat exchangers such as fin and tube heat exchangers, plate heat exchangers and shell-and-tube heat exchangers that exchange heat from liquid to liquid and so on.

(3) Three states of matter and heat

What happens when water is heated? Of course, the temperature will rise. However, we know that there is a temperature at which the temperature will not rise further no matter how much you heat the water.
Water that rises to a temperature of 100 °C will not rise beyond this temperature. Instead, it boils and evaporates into water vapor. Similarly, even when heat is applied to ice, the temperature does not rise beyond 0 °C until the ice melts completely to become water.
On the other hand, the same is true when heat is taken away, and temperature changes do not occur until all water vapor at 100 °C turns into water, and all water at 0 °C turns into ice.
The heat used to change only the temperature of water without a change in the state is called sensible heat, and the heat used to change the state without a change in the temperature is called latent heat.

Comparing the magnitude of the latent heat with the sensible heat of water, assuming the amount of heat required to increase the temperature of water from 0 to 100 °C is 100, the latent heat of solidification and melting (change in state from ice to water and vice versa) will be 80, while the latent heat of evaporation and condensation (water to steam and vice versa) will be 540.

Compared to the rest, you will find that the latent heat of evaporation and condensation (water to steam and vice versa) is exceptionally large. In other words, when a substance evaporates, a large amount of heat is taken away from the surroundings, and when it condenses, a large amount of heat is released to the surroundings.
Refrigeration cycles used for air conditioning and other applications make good use of this principle to achieve indoor cooling. We’ll talk about how this works in the following sections.

 

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