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# Humidity of constant temperature and humidity air

by：Sure     2021-08-18
Humidity is a physical quantity that expresses the content of water vapor in humid air. Generally, there are three ways to express it: 1. Absolute humidity: The mass of water vapor contained in a unit volume of air is called the absolute humidity z of humid air. z u003d mq /V kg/m3 where: mq mass of water vapor, kg; V volume occupied by water vapor, that is, the volume of humid air, m3. It can be seen from the formula that absolute humidity is humidity. It is a physical quantity that expresses the content of water vapor in humid air. There are generally three ways to express it: 1. Absolute humidity: the mass of water vapor per unit volume of air, called the absolute humidity of humid air z . z u003d mq /V kg/m3 where: mq　—mass of water vapor, kg; V—volume occupied by water vapor, that is, the volume of humid air, m3. It can be seen from the formula that the absolute humidity is the density of water vapor at that temperature and water vapor pressure. Since the volume changes with the temperature, even if mq does not change, z will change with the temperature, so it is very inconvenient in the calculation. 2. Moisture content: Moist air is composed of dry air and water vapor. The amount of water vapor coexisting with 1kg of dry air in humid air is called the moisture content d. du003dmq/mg kg/kg dry air can be regarded as ideal gas under normal temperature and pressure, so both dry air and water vapor follow the ideal gas state equation: pqVqu003dmqRqTqpgVgu003dmgRgTg. The same temperature and volume of the air, and at the same time it is known that Rqu003d461 J/(kg.K) Rgu003d287 J/(kg.K) After finishing, we can get: du003d0.622 pq/pg kg/kg dry air u003d 622pq/pg g/kg of dry air. It can be seen that when the atmospheric pressure is constant, the partial pressure of water vapor depends only on the moisture content d. Using moisture content to express air humidity can conveniently express the content of water vapor in the air, and the change in moisture content can be calculated when the air is humidified and dehumidified. 3. Relative humidity: The so-called relative humidity is the ratio of the partial pressure of water vapor in the air to the partial pressure of saturated water vapor at the same temperature. RHu003dPq/Pq,b x100% where it can be seen that the relative humidity reflects the degree to which the water vapor content in the moist air is close to saturation. Therefore, the lower the relative humidity, the drier the air; the higher the relative humidity, the more humid the air.
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