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Compressed air- This is air under pressure exceeding atmospheric pressure.

Compressed air is a unique energy carrier along with electricity, natural gas and water. In industrial conditions, compressed air is mainly used to drive pneumatically driven devices and mechanisms (pneumatic drive).

In everyday, everyday life, we practically do not notice the Air around us. However, throughout the history of mankind, people have used the unique properties of air. The invention of the sail and the forge, the windmill and the hot air balloon were the first steps in the use of air as an energy carrier.

With the invention of the compressor, the era of the industrial use of compressed air came. And the question: " what is Air, and what properties does it possess? " - has become far from idle.

When starting to design a new pneumatic system or modernizing an existing one, it will be useful to remember and about some properties of air, terms and units of measurement.

Air is a mixture of gases, mainly nitrogen and oxygen.

The average relative molar mass is -28.98. 10 -3 kg / mol

* Air composition may vary. Typically, in industrial areas, air contains

This is the mass (weight) of 1 cubic meter of a mixture of air and water vapor at a certain temperature and relative humidity. Specific volume is the volume of air and water vapor per kg of dry air.

Moisture and heat content

To determine the heat consumption for moisture, you need to know the value heat content of humid air... This value is understood as the air and water vapor contained in the mixture. It is numerically equal to the sum:

Air density is a physical quantity that characterizes the specific gravity of air under natural conditions or the mass of gas in the Earth's atmosphere per unit volume. The value of air density is a function of the height of the measurements being made, of its humidity and temperature.

The air density standard is a value equal to 1.29 kg / m3, which is calculated as the ratio of its molar mass (29 g / mol) to the molar volume, the same for all gases (22.413996 dm3), corresponding to the density of dry air at 0 ° C (273.15 ° K) and a pressure of 760 mm Hg (101325 Pa) at sea level (that is, under normal conditions).

Not so long ago, information about the density of air was obtained indirectly through observations of auroras, the propagation of radio waves, and meteors. Since the advent of artificial satellites of the Earth, the air density began to be calculated thanks to the data obtained from their deceleration.

Another method is to observe the spreading of artificial clouds of sodium vapor generated by meteorological rockets. In Europe, the air density at the Earth's surface is 1.258 kg / m3, at an altitude of five km - 0.735, at an altitude of twenty km - 0.087, at an altitude of forty km - 0.004 kg / m3.

There are two types of air density: mass and weight (specific gravity).

The weight density determines the weight of 1 m3 of air and is calculated by the formula γ = G / V, where γ is the weight density, kgf / m3; G is the weight of air, measured in kgf; V is the volume of air, measured in m3. Determined that 1 m3 of air at standard conditions(barometric pressure 760 mm Hg, t = 15 ° С) weighs 1,225 kgf based on this, the weight density (specific gravity) of 1 m3 of air is γ = 1.225 kgf / m3.

It should be taken into account that air weight is a variable quantity and varies depending on various conditions, such as latitude and the force of inertia that occurs when the Earth rotates around its axis. At the poles, the weight of air is 5% more than in the equator zone.

The mass density of air is the mass of 1 m3 of air, denoted by the Greek letter ρ. As you know, body weight is a constant value. The unit of mass is considered to be the mass of weights made of iridist platinum, which is kept in the International Chamber of Weights and Measures in Paris.

The mass density of air ρ is calculated using the following formula: ρ = m / v. Here m is the mass of air, measured in kg × s2 / m; ρ is its mass density, measured in kgf × s2 / m4.

Mass and weight density of air are dependent on: ρ = γ / g, where g is the coefficient of acceleration of gravity, equal to 9.8 m / s². Whence it follows that the mass density of air under standard conditions is 0.1250 kg × s2 / m4.

As barometric pressure and temperature change, air density changes. Based on the Boyle-Mariotte law, the higher the pressure, the greater the density of the air. However, with a decrease in pressure with height, the density of air also decreases, which introduces its own corrections, as a result of which the law of pressure change along the vertical becomes more complicated.

The equation that expresses this law of pressure change with height in the atmosphere at rest is called the basic equation of statics.

It says that with an increase in altitude, the pressure changes downward and when rising to the same height, the decrease in pressure is the greater, the greater the force of gravity and the density of the air.

Air density changes play an important role in this equation. As a result, we can say that the higher you go, the less pressure will drop when you rise to the same height. The density of air depends on temperature as follows: in warm air, the pressure decreases less intensely than in cold air, therefore, at the same height in warm air mass, the pressure is higher than in cold air.

With changing values ​​of temperature and pressure, the mass density of air is calculated by the formula: ρ = 0.0473xV / T. Here B is the barometric pressure, measured in mm Hg, T is the air temperature, measured in Kelvin.

How do you choose, according to what characteristics, parameters?

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Also, the density is determined by the humidity of the air. The presence of water pores leads to a decrease in air density, which is explained by the low molar mass of water (18 g / mol) against the background of the molar mass of dry air (29 g / mol). Moist air can be considered as a mixture of ideal gases, in each of which the combination of densities allows you to obtain the required density value for their mixture.

This kind of interpretation allows the determination of density values ​​with an error level of less than 0.2% in the temperature range from -10 ° C to 50 ° C. The density of air allows you to get the value of its moisture content, which is calculated by dividing the density of water vapor (in grams), which is contained in the air, by the density of dry air in kilograms.

The basic equation of statics does not allow solving constantly arising practical problems in the real conditions of a changing atmosphere. Therefore, it is solved under various simplified assumptions that correspond to actual real conditions, due to the advancement of a number of particular assumptions.

The basic equation of statics makes it possible to obtain the value of the vertical pressure gradient, which expresses the change in pressure during ascent or descent per unit of height, i.e., the change in pressure per unit of vertical distance.

Instead of a vertical gradient, the opposite value is often used - a baric step in meters per millibar (sometimes an outdated version of the term "pressure gradient" is still found - a barometric gradient).

Low air density results in little movement resistance. Many terrestrial animals, in the course of evolution, used the ecological benefits of this property of the air environment, due to which they acquired the ability to fly. 75% of all species of land animals are capable of active flight. For the most part these are insects and birds, but there are mammals and reptiles.

Air Density Determination Video