I. The composition of air
Air is composed of a mixture of several gases, the main components of which are nitrogen and oxygen, and other gases account for very little. In addition, air often contains a certain amount of water vapor. We call the air without steam dry air. The composition of dry air under standard condition (I .e. temperature T = 0 degree, pressure millimeter height) is shown in the table.
Composition of dry air
Composition nitrogen oxygen argon carbon dioxide other gases
Volume percentage % 78.03 20.93 0.932 0.03 0.078
Quality percentage % 75.5 23.1 1.28 0.045 0.075
II. The nature of air
The nature of air depends on the content of each part that makes up the air. There are mainly nitrogen and oxygen in the air, and their contents are basically the same. Therefore, under normal circumstances, the air can be regarded as a single gas. When the gas of each component of the air is ideal gas, then the Mixed Air also applies to the relevant laws of ideal gas.
For the mixed gas of ideal gas, the molecular movement of each component gas does not interfere with each other, so the pressure displayed by the mixed gas on the container wall should be equal to the sum of the partial pressure of each component gas. Let p be the pressure of the mixed gas, p1, p2, p3... ,
Pn means that each component is separated separately, and the pressure and temperature both keep the pressure and temperature of the original mixed gas. At this time, the volume that each component gas should have is called the volume of the component gas. Let v be the total volume of mixed gas, v1, v2 ,... , Vn is the gas volume of each component, then v = v1 + v2 +... VN
As for the mixed gas of ideal gas, according to Dalton's law, a very important conclusion can be drawn. Under the condition of constant temperature, the following relation can be written:
p iv=p vi
pi =(vi/ v) p=yip
Or
yi= vi/ v =pi/ p2
In the formula, yi -- the volume percentage of each component gas is the partial pressure of this part in the mixed gas. When the total pressure of the mixed gas is constant, the partial pressure of the composing gas is proportional to its volume, or in other words, the volume of each composing gas is equal to the ratio of its partial pressure to the total pressure.
III. Wet air
The air in the atmosphere more or less contains water vapor. This mixture of air and water vapor is wet air. Because the partial pressure of water vapor in the air is very small, it can be treated as ideal mixed gas.
When the wet air passes through the processes of compression, flood drying and so on, the dry air volume remains unchanged, and the amount of water vapor in it may also change. Therefore, when calculating the thermal properties of the wet air, use 1 kg dry air as the unit quantity.
Thermal properties and related definitions of wet air
(1) saturated wet air: refers to the wet air in which the water vapor in the air is saturated;
(2) absolute humidity: refers to 1 m3The amount of steam containing water in wet air, in numerical value, is equal to the water vapor partial pressure PH2The water vapor density under the temperature of O and wet air;
(3) P relative humidity Φrefers to the ratio of the actual amount of water vapor contained in the wet air and the amount of water vapor contained in the saturated wet air at the same temperature, namely
Φ=R/RS=PH2O/PS
(4) dew point: when the wet air cools down to a certain temperature under constant pressure, the moisture begins to elude from the wet air. This temperature is called dew point, the numerical value is equal to the saturation temperature under the partial pressure of water vapor in wet air.
