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  6. 1-1. Why use water for cooling?
Temperature control equipment

Chiller Technical Information 1-1. Why use water for cooling?

PCU Series

1-1. Why use water for cooling?

This section explains the reasons why water is excellent as a cooling medium with relevant phenomena.

There are many things around us that use water for cooling.
When we get burned, we cool the affected area with running water. Air conditioning in buildings and large commercial facilities also use water as a medium.

 

Figure 1: Cooling a burned hand with running water

Figure 2: Building air conditioning equipment installed outdoor

 

Water is also used for cooling in a lot of other situations.

Water as an excellent cooling medium

In addition to being a substance that is easy to secure for use as a cooling medium in all kinds of situations, the properties of water also make it an excellent medium for cooling.

  1. Chemically stable

    Water is a substance that is less at risk of decomposing into different substances due to heating under room temperature or severely corroding other substances.

  2. Water has a large specific heat

    Specific heat is the amount of heat required to increase the temperature of 1 g of a substance by 1 °C (K).
    KJ/(kg/°C) is used as a unit.
    Specific heat represents the “degree of difficulty to change the temperature of a substance”. The higher the specific heat of a substance is, the more difficult it will be to heat or cool it.
    High specific heat = a lot of heat is required to raise the temperature of the water
    For example, this means that if you splash water into a fire, you can take a lot of heat away from burning objects until the water rises in temperature and boils.
    As you can see in the table below, when you compare air and water, water has a specific heat value of more than four times that of air.
    Let’s try applying this to cooling. For example, assuming 1 kg of water is required to lower the temperature of 1 kg of water by 1 °C, to cool the water by the same amount using air,
    you will need
    1 kg × 4.18/1.01 = 4.14 kg. If this is converted to volume, the ratio of water to air becomes
    1 L of water to 3436L (3.5 m3) of air.
    It can be said that water is more efficient in cooling than air due to the fact that only 1 L of water is required to achieve the same cooling effect as 3.5 m3 of air.
    The specific heat of water is also high when compared to other liquids, and it is known as the material with the highest specific heat among liquid substances at room temperature.

     

    Name of substance Specific heat [kJ/kg °C]/20 °C
    Air 1.01
    Water 4.18
    Ethanol 2.42
    Mineral oil 1.85
    Ethylene glycol 2.38

    Water is often used in cooling applications due to such properties.

  3. High evaporation heat

    Evaporation heat is the heat required to turn a liquid into a gas. Vaporization heat is a type of latent heat which is also known as evaporation latent heat. As you can see in the table below, the evaporation heat of water is known to be exceptionally high compared to other materials.

     

    Name of substance Boiling point [°C] Evaporation heat [kJ/kg]
    Water 100 2250
    Ethanol 80 393
    Mercury 357 285
    Liquid oxygen -183 213
    Liquid nitrogen -196 199

    The greater the evaporation heat, the more difficult to evaporate a liquid because the substance requires a lot of heat to evaporate. Conversely, the higher the evaporation heat, the greater the cooling capacity because the substance takes away a lot of heat from the surroundings when it evaporates. This principle is also used to cool off when water is splashed on the veranda in the summer evening.

     


    Figure 3: Splashing water

     

    In addition, cooling facilities using this principle in commerce and industry have a cooling tower. A cooling tower is a cooling system used to cool refrigerators and industrial water used in building air conditioning and local air conditioning equipment. The remaining cooling water is cooled by letting the cooling water come into direct contact with the outside air to evaporate part of the cooling water. At room temperature, water has an evaporation latent heat of about 2,250 kJ/kg and a specific heat of 4.2 kJ/kg °C, so the temperature of the remaining water will be reduced by about 6 °C due to the evaporation of 1% of cooling water.

Related column - Why is Summer in Kyoto So Hot?

Speaking of events in Kyoto in summer, the Gion Festival and the Gozan no Okuribi (Daimonjitaki) are well-known. It is crowded with many tourists every year, but Kyoto is also famous for its hot summer and harsh winter. In August 2017, the average temperature was 28.7 °C. Situated at about the same latitude, the temperature in Chosi, Chiba Prefecture is 3.1 °C lower at 25.6 °C, while the average temperature in January is 4.8 °C in Kyoto, which is 2.2 °C lower than the 7.0 °C in Choshi (based on Meteorological Agency 2017 (monthly) air temperature). But why does such a difference appear?
The properties of water are also relevant here.
In addition to its density, water also has a specific heat capacity that is very large compared to other substances, i.e. the amount of heat required to raise the temperature of 1 g of the substance by 1 °C.
For example, at 4.18 kJ/kg °C, the specific heat capacity of water is approximately 1.7 times the specific heat capacity of ethanol which is 2.42 kJ/kg °C, and about 9 times the specific heat capacity of iron which is 0.46 kJ/kg °C.
Water is often said to be hard to warm and cool, but this is because the specific heat capacity of water is large, requiring a large amount of heat to warm it. On the other hand, once water is heated, the heat can be stored to a certain extent.
Due to this large specific heat capacity, water (or an aqueous solution of antifreeze) is used to cool the engine in a water-cooled engine.
The reason why a person's body temperature does not rise rapidly in the hot summer, and why it does not go up immediately when exercising is due to the fact that more than 60 percent of the body is made up of water.
Moreover, at 2,257 kJ/kg, the vaporization heat of water is nearly three times higher compared to that of ethanol which is 838 kJ/kg, allowing a person to adjust its body temperature by sweating as the sweat will take away a large part of this vaporization heat when it evaporates.
In addition, when ice melts, a large melting heat of 334 kJ/kg is taken away from the surroundings. Conversely, the same amount of condensation heat is released when water freezes. The reason why water is cooled when ice is put into water is because melting heat is taken away when the ice melts. This value is equivalent to 80 times the specific heat capacity of the same amount of water.
In early spring, water is sprinkled onto tea fields at night to keep the tea buds at around 0 °C due to the condensation heat given out when water sticking to the tea freezes, so as to prevent them from being harmed by frost when the temperature drops below the freezing point.
For land surrounded by sea on three sides, such as Choshi in the Chiba Prefecture, a large amount of water in the sea mitigates the change in air temperature, so the difference in temperature between winter and summer is relatively small.
However, in an inland basin without any seas like Kyoto, the weather becomes very hot because heat does not escape to the sea etc. once the land is warmed up by solar heat in summer, and becomes cold in winter because there is no supply of heat from the sea.
Despite the cities being located at the same latitude, the air temperature throughout the year is affected because the water-related circumstances are very different.

Source: “Chapter 1 Properties and Role of Water 1-2 Why Is Summer in Kyoto So Hot?” (Ministry of Education, Culture, Sports, Science and Technology)
(http://www.mext.go.jp/b_menu/shingi/gijyutu/gijyutu0/shiryo/attach/1331537.htm)

 

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