Compressed air is air under pressure greater than atmospheric pressure.

Compressed air is a unique energy carrier along with electricity, natural gas and water. In industrial settings, 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 human history, people have used unique properties air. The invention of the sail and the forge, the windmill and hot air balloon became the first steps in using air as an energy carrier.

With the invention of the compressor, the era of industrial use of compressed air began. And the question: “ What is Air and what properties does it have? - became far from idle.

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

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

Air composition
Element*	Designation	By volume, %	By weight, %
Oxygen
Carbon dioxide	CO2
	CH 4
	H2O

Average relative molar mass-28.98. 10 -3 kg/mol

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

DEFINITION

Atmospheric air is a mixture of many gases. Air has a complex composition. Its main components can be divided into three groups: constant, variable and random. The former include oxygen (the oxygen content in the air is about 21% by volume), nitrogen (about 86%) and the so-called inert gases (about 1%).

Content components practically does not depend on where globe a sample of dry air was taken. The second group includes carbon dioxide (0.02 - 0.04%) and water vapor (up to 3%). The content of random components depends on local conditions: near metallurgical plants, noticeable amounts of sulfur dioxide are often mixed into the air, in places where organic residues decompose - ammonia, etc. In addition to various gases, the air always contains more or less dust.

Air density is a value equal to the mass of gas in the Earth's atmosphere divided by a unit volume. It depends on pressure, temperature and humidity. There is a standard value for air density - 1.225 kg/m 3, corresponding to the density of dry air at a temperature of 15 o C and a pressure of 101330 Pa.

Knowing from experience the mass of a liter of air at normal conditions(1.293 g), we can calculate the molecular weight that air would have if it were an individual gas. Since a gram molecule of any gas occupies a volume of 22.4 liters under normal conditions, the average molecular weight of air is equal to

22.4 × 1.293 = 29.

This number - 29 - should be remembered: knowing it, it is easy to calculate the density of any gas relative to air.

Density of liquid air

When sufficiently cooled, the air turns into a liquid state. Liquid air can be stored for quite a long time in vessels with double walls, from the space between which the air is pumped out to reduce heat transfer. Similar vessels are used, for example, in thermoses.

Liquid air that evaporates freely under normal conditions has a temperature of about (-190 o C). Its composition is not constant, since nitrogen evaporates more easily than oxygen. As the nitrogen is removed, the color of the liquid air changes from bluish to pale blue (the color of liquid oxygen).

In liquid air they easily transform into solid state ethyl alcohol, diethyl ether and many gases. If, for example, carbon dioxide is passed through liquid air, it turns into white flakes similar in appearance. appearance to the snow. Mercury immersed in liquid air becomes hard and malleable.

Many substances cooled by liquid air dramatically change their properties. Thus, chink and tin become so brittle that they easily turn into powder, a lead bell makes a clear ringing sound, and a frozen rubber ball shatters if dropped on the floor.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise	Determine how many times heavier than air is hydrogen sulfide H 2 S.
Solution	The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure is called the relative density of the first gas to the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular weight air" is used conditionally, since air is a mixture of gases. D air (H 2 S) = M r (H 2 S) / M r (air); D air (H 2 S) = 34 / 29 = 1.17. M r (H 2 S) = 2 × A r (H) + A r (S) = 2 × 1 + 32 = 2 + 32 = 34.
Answer	Hydrogen sulfide H 2 S is 1.17 times heavier than air.

Many may be surprised by the fact that air has a certain non-zero weight. Exact value This weight is not so easy to determine, since it is greatly influenced by factors such as chemical composition, humidity, temperature and pressure. Let's take a closer look at the question of how much air weighs.

What is air

Before answering the question of how much air weighs, it is necessary to understand what this substance is. Air is a gaseous shell that exists around our planet, and which is a homogeneous mixture of various gases. Air contains the following gases:

• nitrogen (78.08%);
• oxygen (20.94%);
• argon (0.93%);
• water vapor (0.40%);
• carbon dioxide (0.035%).

In addition to the gases listed above, the air also contains minimum quantities neon (0.0018%), helium (0.0005%), methane (0.00017%), krypton (0.00014%), hydrogen (0.00005%), ammonia (0.0003%).

It is interesting to note that these components can be separated by condensing air, that is, turning it into a liquid state by increasing pressure and decreasing temperature. Since each component of air has its own condensation temperature, in this way it is possible to isolate all components from the air, which is used in practice.

Air weight and factors that affect it

What prevents you from answering exactly the question of how much a cubic meter of air weighs? Of course, there are a number of factors that can greatly influence this weight.

Firstly, this is the chemical composition. Above are the data for the composition clean air However, at present this air in many places on the planet is highly polluted, and accordingly its composition will be different. Thus, near large cities the air contains more carbon dioxide, ammonia, methane than in rural air.

Secondly, humidity, that is, the amount of water vapor contained in the atmosphere. The more humid the air, the less it weighs, other things being equal.

Thirdly, temperature. This is one of important factors, the lower its value, the higher the air density, and, accordingly, the greater its weight.

Fourthly, atmospheric pressure, which directly reflects the number of air molecules in a certain volume, that is, its weight.

To understand how the combination of these factors affects the weight of air, let's give a simple example: the mass of one meter of cubic dry air at a temperature of 25 ° C, located near the surface of the earth, is 1.205 kg, if we consider a similar volume of air near the surface of the sea at a temperature of 0 ° C, then its mass will already be equal to 1.293 kg, that is, it will increase by 7.3%.

Change in air density with altitude

As altitude increases, air pressure drops, and its density and weight decrease accordingly. Atmospheric air at pressures observed on Earth can, to a first approximation, be considered an ideal gas. This means that air pressure and density are mathematically related to each other through the equation of state ideal gas: P = ρ*R*T/M, where P is pressure, ρ is density, T is temperature in Kelvin, M is the molar mass of air, R is the universal gas constant.

From the above formula, you can obtain a formula for the dependence of air density on height, taking into account that the pressure varies according to the law P = P 0 +ρ*g*h, where P 0 is the pressure at the surface of the earth, g is the acceleration of gravity, h is the height . Substituting this formula for pressure into the previous expression and expressing the density, we obtain: ρ(h) = P 0 *M/(R*T(h)+g(h)*M*h). Using this expression, you can determine the density of air at any altitude. Accordingly, the weight of air (it would be more correct to say mass) is determined by the formula m(h) = ρ(h)*V, where V is the given volume.

In the expression for the dependence of density on height, it can be noted that temperature and gravitational acceleration also depend on height. The last dependence can be neglected if we are talking about heights of no more than 1-2 km. As for temperature, its dependence on height is well described by the following empirical expression: T(h) = T 0 -0.65*h, where T 0 is the air temperature near the earth's surface.

In order not to constantly calculate the density for each altitude, below we provide a table of the dependence of the main characteristics of air on altitude (up to 10 km).

Which air is the heaviest

By considering the main factors that determine the answer to the question of how much air weighs, you can understand which air will be the heaviest. In short, cold air always weighs more than warm air, since the density of the latter is lower, and dry air weighs more than humid air. The last statement is easy to understand, since it is 29 g/mol, and the molar mass of a water molecule is 18 g/mol, that is, 1.6 times less.

Determination of air weight under given conditions

Now let's solve a specific problem. Let's answer the question of how much air weighs, occupying a volume of 150 liters, at a temperature of 288 K. Let's take into account that 1 liter is a thousandth of a cubic meter, that is, 1 liter = 0.001 m 3. As for the temperature of 288 K, it corresponds to 15 ° C, that is, it is typical for many areas of our planet. Next you need to determine the air density. You can do this in two ways:

1. Calculate using the above formula for an altitude of 0 meters above sea level. In this case, the value obtained is ρ = 1.227 kg/m 3
2. Look at the table above, which was built based on T 0 = 288.15 K. The table contains the value ρ = 1.225 kg/m 3.

Thus, we have two numbers that agree well with each other. The slight difference is due to an error of 0.15 K in determining the temperature, and also to the fact that air is still not an ideal gas, but a real gas. Therefore, for further calculations, we will take the average of the two obtained values, that is, ρ = 1.226 kg/m 3.

Now, using the formula for the relationship between mass, density and volume, we get: m = ρ*V = 1.226 kg/m 3 * 0.150 m 3 = 0.1839 kg or 183.9 grams.

You can also answer how much a liter of air weighs when given conditions: m = 1.226 kg/m 3 * 0.001 m 3 = 0.001226 kg or approximately 1.2 grams.

Why don't we feel the air pressing on us?

How much does 1 m3 of air weigh? A little more than 1 kilogram. The entire atmospheric table of our planet puts pressure on a person with its weight of 200 kg! This is a fairly large mass of air that could cause a lot of trouble to a person. Why don't we feel it? This is explained by two reasons: firstly, there is also internal pressure within the person himself, which counteracts the external atmospheric pressure, secondly, air, being a gas, exerts pressure in all directions equally, that is, pressures in all directions balance each other.

Physics at every step Perelman Yakov Isidorovich

How much does the air in the room weigh?

Can you say at least approximately how much weight the air contained in your room represents? A few grams or a few kilograms? Are you able to lift such a load with one finger, or would you barely be able to hold it on your shoulders?

Now, perhaps, there are no longer people who think, as the ancients believed, that air weighs nothing at all. But even now many people will not be able to say how much a certain volume of air weighs.

Remember that a liter mug of air of the same density as it has nearby earth's surface with normal room temperature, weighs about 1.2 g. Since a cubic meter contains 1 thousand liters, a cubic meter of air weighs a thousand times more than 1.2 g, namely 1.2 kg. Now it is not difficult to answer the question posed earlier. To do this, you just need to find out how many cubic meters are in your room, and then the weight of the air contained in it will be determined.

Let the room have an area of ​​10 m2 and a height of 4 m. In such a room there are 40 cubic meters of air, which weighs forty times 1.2 kg. This will be 48 kg.

So, even in such a small room, the air weighs a little less than you. You would be able to carry such a load on your shoulders with difficulty. And the air of a room twice as spacious, loaded onto your back, could crush you.

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Density And specific volume of moist air are variable quantities depending on temperature and air environment. These values ​​​​need to be known when selecting fans for, when solving problems related to the movement of the drying agent through air ducts, when determining the power of fan electric motors.

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 1 kg of dry air.

Moisture and heat content

The mass in grams per unit mass (1 kg) of dry air in their total volume is called air moisture content. It is obtained by dividing the density of water vapor contained in the air, expressed in grams, by the density of dry air in kilograms.

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

heat content of the dry part of the air heated to the temperature of the drying process

heat content of water vapor in air at 0°C

heat content of this steam heated to the temperature of the drying process

Heat content of moist air expressed in kilocalories per 1 kg of dry air or in joules. Kilocalorie is a technical unit of heat expended on heat 1 kg of water per 1°C (at a temperature of 14.5 to 15.5°C). In the SI system