2. Product information
  3. Precision air conditioning equipment Precision air conditioners PAU Series
  4. Precision air conditioners Technical Information
  5. 1-1. About air

Precision air conditioning equipmentPrecision air conditioners Technical Information

PAU Series

1-1. About air

This section provides a description of air, the air condition and how it is represented.

(1)About air

Oxygen is essential to our lives. This oxygen is taken from the “air” that exists naturally around us. We call the gases in the layer surrounding the Earth the atmosphere, and we generally refer to the gases around us, including water vapor, as air.

(2) Components of air

Air consists mainly of the following components.

1. Nitrogen (N2)

Nitrogen is a colorless, tasteless, odorless, and non-toxic gas.
It is the most common component in air, accounting for about 78%.
As it is an inert and inexpensive gas, it is used as an inert gas in production sites.


2. Oxygen (O2)

Oxygen is a colorless and odorless gas. It is slightly bluish in color when in the liquid and solid states. About 21% of air is composed of oxygen.
It reacts with other elements to produce an oxidized compound.
Oxygen is inhaled by organisms and consumed by combustion, but is produced by plants through photosynthesis.

3. Argon (Ar)

Argon is a rare gas element that is a colorless, odorless inert gas that hardly causes any chemical reactions.
It is the third most abundant component in air, constituting about 1%.
It is widely used as an inert gas for welding and silicon manufacturing etc.

4. Carbon dioxide (CO2)

Carbon dioxide is a colorless, odorless gas.
At atmospheric pressure, it is not liquid but at -79 °C, it solidifies to become dry ice.
It is generated during breathing or combustion. As a greenhouse gas that causes global warming, carbon dioxide emissions have become an issue, and people are looking to reduce CO2 emissions globally, especially in developed countries.


(3) Dry air and moist air

Air can be broadly categorized as follows.

○Dry air

Air contains a variety of components, but in theory air that does not contain any water vapor is called dry air.

○Moist air

The air around us contains moisture in addition to oxygen, nitrogen, argon and carbon dioxide.
Moisture in the air exists not as water droplets (liquid) but as water vapor (gas).
This air containing water vapor is called moist air.
The state in which no more water vapor can be contained in air is called saturated air (= saturated state), and the humidity at the saturated state is 100%. When the air touches something cold in this state, the air is cooled and can no longer stay in the air as water vapor, condensing into drops of water. This process is known as condensation.

(4) How air temperature and humidity are expressed and their types

○How temperature is expressed and its types

1. Dry bulb temperature [°C DB] (DB… Dry Bulb)
This is what we normally refer to as the normal temperature. It can be measured with a stem thermometer containing mercury or alcohol.

2. Wet bulb temperature [°C WB] (WB… Wet Bulb)
The temperature measured by a wet bulb of a rod thermometer covered with gauze, etc. that has been dampened with water. The temperature displayed is lower than the dry bulb temperature because heat is taken away when the moisture evaporates. Influenced by humidity, the lower the humidity, the less water vapor there is in the air and the easier for it to evaporate, so the wet bulb temperature is lower.

3. Dew point temperature [°C DP] (DP… Dew Point)
The temperature at which water vapor in the air begins to condense when the temperature of the air is lowered.

○How humidity is expressed and its types

1. Relative humidity [%RH] (RH… Relative Humidity)
Relative humidity is what we generally use in daily life, and it is also used when describing the weather etc.
It is defined as follows.
“The ratio of the water vapor partial pressure in moist air to the water vapor partial pressure in saturated air at the same temperature”

2. Absolute humidity [kg/kg (DA)](DA… Dry Air)
Absolute humidity refers to the weight of the water vapor relative to 1 kg of dry air in wet air.
An absolute humidity of about 0.01 kg/kg (DA) is said to be comfortable as a space for people to live and reside in.

(5) How to express air pressure and its types

The air (atmosphere) around us exists far above us in the sky. Even if the specific gravity (mass per constant volume) is very small, a weight is applied on the ground when the air accumulates far above the sky, and this pressure is known as atmospheric pressure.
An international standard based on the average atmospheric pressure above sea level exists, and this is defined as follows.
101325 Pa = 1013.25 hPa = 0.101325 MPa
. This is called standard atmospheric pressure.
There are two ways to express pressure - absolute pressure and gauge pressure. Generally, gauge pressure is often used in air conditioning.

1. Absolute pressure [MPa] (kgf/cm2 abs)
A state in which no gases exist at all is known as a perfect vacuum, and the absolute pressure is the pressure using this absolute vacuum as a reference.
It is mainly used for thermodynamic discussions.

2. Gauge pressure [MPa (G)] (kgf/cm2 G)
The pressure using standard atmospheric pressure as a reference (0) is known as the gauge pressure.
The value measured by a pressure gauge in the atmosphere is the gauge pressure. In other words, even when the pressure measured by a pressure gauge in the atmosphere is 0 MPa (G), a pressure of 1 atm (0.1013 MPa) is acting on the pressure gauge.
To summarize the relationship,
the absolute pressure = gauge pressure + atmospheric pressure (0.1MPa).

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