Heating of the atmosphere (air temperature).

The atmosphere receives more heat from the underlying earth's surface than directly from the Sun. Heat is transferred to the atmosphere through molecular thermal conductivity,convection, release of specific heat of vaporization at condensation water vapor in the atmosphere. Therefore, the temperature in the troposphere usually decreases with height. But if a surface gives off more heat to the air than it receives in the same time, it cools, and the air above it cools as well. In this case, the air temperature, on the contrary, increases with height. This situation is called temperature inversion . It can be observed in the summer at night, in the winter - above the snow surface. Temperature inversions are common in polar regions. The reason for the inversion, in addition to cooling the surface, may be the displacement of warm air by cold air flowing under it or the flow of cold air to the bottom of intermountain basins.

In the calm troposphere, the temperature decreases with height on average by 0.6° per 100 m. When dry air rises, this figure increases and can reach 1° per 100 m, and when humid air rises, it decreases. This is explained by the fact that rising air expands and energy (heat) is expended on this, and when moist air rises, condensation of water vapor occurs, accompanied by the release of heat.

Decrease in temperature of rising air - main cause of cloud formation . The descending air, coming under high pressure, is compressed, and its temperature rises.

Temperature air changes periodically throughout the day and throughout the year.

IN its daily course There is one maximum (after noon) and one minimum (before sunrise). From the equator to the poles, the daily amplitudes of temperature fluctuations decrease. But at the same time, they are always larger over land than over the ocean.

IN annual progress temperature air at the equator - two maximums (after the equinoxes) and two minimums (after the solstices). In tropical, temperate and polar latitudes there is one maximum and one minimum. The amplitudes of annual air temperature fluctuations increase with increasing latitude. At the equator they are less than daily: 1-2°C over the ocean and up to 5°C over land. In tropical latitudes - over the ocean - 5°C, over land - up to 15°C. In temperate latitudes from 10-15°C over the ocean to 60°C or more over land. In polar latitudes, negative temperatures predominate, with annual fluctuations reaching 30-40°C.

The correct daily and annual variation of air temperature, determined by changes in the height of the Sun above the horizon and the length of the day, is complicated by non-periodic changes caused by movements of air masses having different temperatures. General pattern of temperature distribution in the lower troposphere-its decrease in the direction from the equator to the poles.

If average annual air temperature depended only on latitude, its distribution in the Northern and Southern Hemispheres would be the same. In reality, its distribution is significantly influenced by differences in the nature of the underlying surface and heat transfer from low latitudes to high.

Due to heat transfer, the air temperature at the equator is lower and at the poles higher than it would be without this process. The Southern Hemisphere is colder than the Northern Hemisphere mainly due to the ice and snow covered land near the South Pole. The average air temperature in the lower two-meter layer for the entire Earth is +14°C, which corresponds to the average annual air temperature at 40°N.

DEPENDENCE OF AIR TEMPERATURE ON GEOGRAPHICAL LATITUDE

The distribution of air temperature near the earth's surface is shown using isotherms - lines connecting places with the same temperature. Isotherms do not coincide with parallels. They bend, moving from the continent to the ocean and vice versa.

Atmospheric pressure

Air has mass and weight, so it exerts pressure on the surface in contact with it. The pressure exerted by air on the earth's surface and all objects located on it is called atmospheric pressure . It is equal to the weight of the overlying air column and depends on the air temperature: the higher the temperature, the lower the pressure.

The atmospheric pressure on the underlying surface averages 1.033 g per 1 cm 2 (more than 10 t per m 2 ). Pressure is measured in millimeters of mercury, millibars (1 mb = 0.75 mm Hg) and hectopascals (1 hPa = 1 mb). Pressure decreases with altitude: In the lower layer of the troposphere to an altitude of 1 km it decreases by 1 mm Hg. Art. for every 10 m. The higher it is, the slower the pressure decreases. Normal pressure at ocean level is 760 mm. RT. Art.

The general distribution of pressure on the Earth's surface is zonal:

	Season	Over the mainland	Over the ocean
At equatorial latitudes
At tropical latitudes
	Low	High
At moderate latitudes		High	Low
	Low
At polar latitudes

Thus, both in winter and summer, and over the continents and over the ocean, zones of high and low pressure alternate. The pressure distribution is clearly visible on the isobar maps of January and July. Isobars - lines connecting places with the same pressure. The closer they are to each other, the faster the pressure changes with distance. The amount of pressure change per unit distance (100 km) is called pressure gradient .

The change in pressure is explained by the movement of air. It increases where there is more air, and decreases where air leaves. The main reason for air movement is its heating and cooling from the underlying surface. Heated from the surface, the air expands and rushes upward. Having reached a height at which its density is greater than the density of the surrounding air, it spreads out to the sides. Therefore, the pressure on the warm surface decreases (equatorial latitudes, mainland tropical latitudes in summer). But at the same time it increases in neighboring areas, although the temperature there has not changed (tropical latitudes in winter).

Above a cold surface, air cools and becomes denser, pressing against the surface ( polar latitudes, mainland temperate latitudes in winter). At the top, its density decreases, and air comes here from the outside. The amount of it above the cold surface increases, the pressure on it increases. At the same time, where the air has left, the pressure decreases without changing the temperature. Heating and cooling of air from the surface is accompanied by its redistribution and pressure changes.

At equatorial latitudes pressure always reduced. This is explained by the fact that the air heated from the surface rises and moves towards tropical latitudes, creating increased pressure there.

Above a cold surface in the Arctic and Antarctica pressure increased. It is created by air coming from temperate latitudes to replace the condensed cold air. The outflow of air to the polar latitudes is the reason for the decrease in pressure in temperate latitudes.

As a result, belts of low (equatorial and temperate) and high pressure (tropical and polar) are formed. Depending on the season, they shift somewhat towards the summer hemisphere (“following the Sun”).

Polar high pressure areas expand in winter and contract in summer, but persist throughout the year. Belts low blood pressure persist throughout the year near the equator and in temperate latitudes of the Southern Hemisphere.

In winter, in the temperate latitudes of the Northern Hemisphere, the pressure over the continents increases greatly and the low pressure belt “breaks”. Closed areas of low pressure persist only over the oceans - Icelandic And Aleutian lows. On the contrary, winter ice forms over the continents. highs :Asian (Siberian) And North American. In summer, in the temperate latitudes of the Northern Hemisphere, the low pressure belt is restored.

A huge area of ​​low pressure centered in tropical latitudes forms over Asia in the summer - Asian low. In tropical latitudes, the continents are always slightly warmer than the oceans, and the pressure above them is lower. That's why there are over the oceans subtropical highs :North Atlantic (Azores), North Pacific, South Atlantic, South Pacific And South Indian.

Thus, due to different heating and cooling of the continental and water surfaces (the continental surface heats up faster and cools down faster), the presence of warm and cold currents and other reasons on Earth, in addition to atmospheric pressure belts, closed areas of low and high pressure can arise.

The main source of heat that warms the earth's surface and atmosphere is the sun. Other sources - the moon, stars, the heated interior of the Earth - supply such a small amount of heat that they can be neglected.

The sun emits colossal energy into space in the form of heat, light, ultraviolet and other rays. The totality of radiant energy from the Sun is called solar radiation. The earth receives an insignificant fraction of this energy - one two-billionth part, which, however, is sufficient not only to support life, but also to carry out exogenous processes in the lithosphere, physical and chemical phenomena in the hydrosphere and atmosphere.

There are direct, diffuse and total radiation.

In clear, cloudless weather, the Earth's surface is heated mainly by direct radiation, which we feel as warm or hot sun rays.

Passing through the atmosphere, the sun's rays are reflected from air molecules, water droplets, dust particles, deviate from a straight path and are scattered. The cloudier the weather, the denser the cloud cover and the more radiation is scattered into the atmosphere. When the air is very dusty, for example during dust storms or in industrial centers, dispersion reduces radiation by 40–45%.

The importance of scattered radiation in the life of the Earth is very great. Thanks to it, objects in the shadows are illuminated. It also determines the color of the sky.

The intensity of radiation depends on the angle of incidence of sunlight on the earth's surface. When the sun is high above the horizon, its rays travel a shorter distance through the atmosphere, therefore scatter less and heat the Earth's surface more. For this reason, in sunny weather, the morning and evening are always cooler than at noon.

The distribution of radiation on the Earth's surface is greatly influenced by its sphericity and the inclination of the Earth's axis to the orbital plane. In equatorial and tropical latitudes, the sun is high above the horizon throughout the year; in mid-latitudes, its height varies depending on the time of year, and in the Arctic and Antarctic it never rises high above the horizon. As a result, in tropical latitudes, the sun's rays are scattered less, and there are more of them per unit area of ​​the earth's surface than in middle or high latitudes. For this reason, the amount of radiation depends on the latitude of the place: the further from the equator, the less it reaches the earth's surface.

The supply of radiant energy is associated with the annual and daily movement of the Earth. Thus, in middle and high latitudes its amount depends on the time of year. At the North Pole, for example, in summer the sun does not set beyond the horizon for 186 days, i.e. 6 months, and the amount of incoming radiation is even greater than at the equator. However, the sun's rays have a small angle of incidence, and most of the radiation is scattered in the atmosphere. As a result, the Earth's surface warms up slightly.

In winter, the sun in the Arctic is below the horizon, and direct radiation does not reach the Earth's surface.

The amount of incoming solar radiation is also affected by the topography of the earth's surface. On the slopes of mountains, hills, ravines, etc., facing the sun, the angle of incidence of the sun's rays increases and they heat up more.

The combination of all these factors leads to the fact that there is no place on the earth’s surface where the radiation intensity is constant.

Heating of land and water occurs differently. The surface of the land heats and cools quickly. Water heats up slowly, but retains heat longer. This is explained by the fact that the heat capacity of water is greater than the heat capacity of the rocks that make up the land.

On land, the sun's rays heat only the surface layer, but in clear water the heat penetrates to a considerable depth, as a result of which heating occurs more slowly. Evaporation also affects its speed, since it requires a lot of heat. Water cools slowly, mainly because the volume of heated water is many times greater than the volume of heated land; Moreover, when it cools, the upper, cooled layers of water sink to the bottom, as denser and heavier, and warm water rises from the depths of the reservoir to replace them.

The water uses the accumulated heat more evenly. As a result, the sea is on average warmer than land, and fluctuations in water temperature are never as extreme as fluctuations in land temperature.

Air temperature

The sun's rays, passing through transparent bodies, heat them very weakly. For this reason, direct sunlight almost does not heat the air of the atmosphere, but heats the surface of the Earth, from which heat is transferred to the adjacent layers of air. As the air heats up, it becomes lighter and rises, where it mixes with colder air, in turn heating it.

As the air rises, it cools. At an altitude of 10 km, the temperature constantly remains at around 40–45 °C.

A decrease in air temperature with height is general pattern. However, an increase in temperature is often observed as one rises upward. This phenomenon is called temperature inversion, i.e. a rearrangement of temperatures.

Inversions occur either when the earth's surface and surrounding air rapidly cool, or, conversely, when heavy cold air flows down mountain slopes into valleys. There this air stagnates and displaces warmer air up the slopes.

During the day, the air temperature does not remain constant, but continuously changes. During the day, the Earth's surface heats up and heats the adjacent layer of air. At night, the Earth radiates heat, cools, and the air cools. The lowest temperatures are observed not at night, but before sunrise, when the earth's surface has already given up all the heat. Similar to this most high temperatures air is set not at noon, but around 3 p.m.

At the equator, the daily variation of temperatures is uniform; day and night they are almost the same. The diurnal amplitudes are very small in the seas and near sea coasts. But in deserts during the day the surface of the earth often heats up to 50–60 °C, and at night it often cools down to 0 °C. Thus, daily amplitudes here exceed 50–60 °C.

In temperate latitudes greatest number solar radiation reaches the Earth on the days of the summer solstices, i.e. June 22 in the Northern Hemisphere and December 21 in the Southern. However, the hottest month is not June (December), but July (January), since on the day of the solstice a huge amount of radiation is spent on heating the earth's surface. In July (January) radiation decreases, but this decrease is compensated by the strongly heated earth's surface.

Likewise, the coldest month is not June (December), but July (January).

At sea, due to the fact that the water cools and warms up more slowly, the temperature shift is even greater. Here, the hottest month is August, and the coldest month is February in the Northern Hemisphere and, accordingly, the hottest month is February and the coldest month is August in the Southern Hemisphere.

The annual temperature range largely depends on the latitude of the place. Thus, at the equator the amplitude remains almost constant throughout the year and amounts to 22–23 °C. The highest annual amplitudes are characteristic of territories located in mid-latitudes in the interior of continents.

Any area is also characterized by absolute and average temperatures. Absolute temperatures are determined through long-term observations at weather stations. Thus, the hottest (+58 °C) place on Earth is in the Libyan Desert; the coldest (-89.2 °C) is in Antarctica at the Vostok station. In the Northern Hemisphere, the lowest temperature (-70.2 °C) was recorded in the village of Oymyakon in Eastern Siberia.

Average temperatures are determined as the arithmetic mean of several thermometer indicators. So, to determine the average daily temperature, measurements are made at 1; 7; 13 and 19 hours, i.e. 4 times a day. From the obtained figures, the arithmetic mean is found, which will be the average daily temperature of the given area. Then find the monthly averages and average annual temperatures as the arithmetic mean of daily and monthly averages.

On the map you can mark points with the same temperature values ​​and draw lines connecting them. These lines are called isotherms. The most indicative isotherms are January and July, i.e. the coldest and the coldest warm month per year. Isotherms can be used to determine how heat is distributed on Earth. In this case, clearly expressed patterns can be traced.

1. The highest temperatures are not observed at the equator, but in tropical and subtropical deserts, where direct radiation predominates.

2. In both hemispheres, temperatures decrease from tropical latitudes to the poles.

3. Due to the predominance of the sea over land, the course of isotherms in the Southern Hemisphere is smoother, and the temperature amplitudes between the hottest and coldest months are smaller than in the Northern Hemisphere.

The location of the isotherms allows us to identify 7 thermal zones:

1 hot, located between the annual isotherms of 20 °C in the Northern and Southern Hemispheres;

2 moderate, contained between the isotherms of 20 and 10 °C of the warmest months, i.e. June and January;

2 cold months located between the isotherms of 10 and 0 °C are also the warmest months;

2 areas of permanent frost, in which the temperature of the warmest month is below 0 °C.

The boundaries of the light zones passing through the tropics and polar circles do not coincide with the boundaries of the thermal zones.



— devices used for heating air in supply ventilation systems, air conditioning systems, air heating, as well as in drying installations.

According to the type of coolant, heaters can be fire, water, steam and electric .

The most widespread at present are water and steam heaters, which are divided into smooth-tube and finned; the latter, in turn, are divided into lamellar and spiral-wound.

There are single-pass and multi-pass heaters. In single-pass ones, the coolant moves through the tubes in one direction, and in multi-pass ones it changes the direction of movement several times due to the presence of partitions in the collector covers (Fig. XII.1).

The heaters come in two models: medium (C) and large (B).

The heat consumption for heating the air is determined by the formulas:

Where Q"— heat consumption for heating air, kJ/h (kcal/h); Q- the same, W; 0.278 — conversion factor kJ/h to W; G— mass amount of heated air, kg/h, equal to Lp [here L— volumetric amount of heated air, m 3 / h; p - air density (at temperature t K), kg/m 3 ]; With— specific heat capacity of air equal to 1 kJ/(kg-K); tk is the air temperature after the air heater, °C; t n— air temperature before the heater, °C.

For air heaters of the first heating stage, the temperature tn is equal to the outside air temperature.

The outside air temperature is assumed to be equal to the calculated ventilation temperature (climate parameters of category A) when designing general ventilation designed to combat excess moisture, heat and gases, the maximum permissible concentration of which is more than 100 mg/m3. When designing general ventilation intended to combat gases whose maximum permissible concentration is less than 100 mg/m3, as well as when designing supply ventilation to compensate for air removed through local suction, process hoods or pneumatic transport systems, the outside air temperature is assumed to be equal to the calculated outside temperature tn for heating design (climate parameters of category B).

Supply air should be supplied to a room without excess heat at a temperature equal temperature indoor air tB for a given room. If there is excess heat, supply air is supplied at a reduced temperature (by 5-8° C). It is not recommended to supply supply air with a temperature below 10° C to the room even in the presence of significant heat generation due to the possibility of colds occurring. The exception is the use of special anemostats.

The required heating surface area of ​​the air heaters Fк m2 is determined by the formula:

Where Q— heat consumption for heating air, W (kcal/h); TO— heat transfer coefficient of the heater, W/(m 2 -K) [kcal/(h-m 2 -°C)]; t avg.T. — average temperature coolant, 0 C; t av. - average temperature of heated air passing through the heater, °C, equal to (t n + t k)/2.

If the coolant is steam, then the average coolant temperature tav.T. equal to the saturation temperature at the corresponding vapor pressure.

For water temperature tav.T. is defined as the arithmetic mean of the hot and return water temperatures:

A safety factor of 1.1-1.2 takes into account heat loss for air cooling in air ducts.

The heat transfer coefficient K of air heaters depends on the type of coolant, the mass velocity of air movement vp through the air heater, the geometric dimensions and design features of the air heaters, and the speed of water movement through the heater tubes.

By mass velocity we mean the mass of air, kg, passing in 1 s through 1 m2 of the open cross-section of the heater. Mass velocity vp, kg/(cm2), is determined by the formula

The model, brand and number of air heaters are selected based on the open cross-sectional area fL and heating surface FK. After selecting heaters, the mass air velocity of the air is specified based on the actual open cross-sectional area of ​​the heater fD of a given model:

where A, A 1, n, n 1 and T— coefficients and exponents depending on the design of the heater

The speed of water movement in the heater tubes ω, m/s, is determined by the formula:

where Q" is the heat consumption for heating the air, kJ/h (kcal/h); pv is the density of water equal to 1000 kg/m3, sv is the specific heat capacity of water equal to 4.19 kJ/(kg-K); fTP — open cross-sectional area for coolant passage, m2, tg - temperature hot water in the supply line, °C; t 0 — return water temperature, 0C.

The heat transfer of heaters is affected by the piping scheme. With a parallel pipeline connection scheme, only part of the coolant passes through a separate heater, and with a sequential scheme, the entire coolant flow passes through each heater.

The resistance of heaters to air passage p, Pa, is expressed by the following formula:

where B and z are the coefficient and exponent, which depend on the design of the heater.

The resistance of successive heaters is:

where m is the number of heaters located in series. The calculation ends with checking the thermal performance (heat transfer) of air heaters using the formula

where QK is the heat transfer of heaters, W (kcal/h); QK - the same, kJ/h, 3.6 - conversion factor of W to kJ/h FK - heating surface area of ​​heaters, m2, adopted as a result of calculating heaters of this type; K - heat transfer coefficient of air heaters, W/(m2-K) [kcal/(h-m2-°C)]; tav.v - average temperature of heated air passing through the heater, °C; tav. T - average coolant temperature, °C.

When selecting air heaters, the margin for the calculated heating surface area is taken within the range of 15 - 20%, for resistance to air passage - 10% and for resistance to water movement - 20%.

The basic physical properties of air are considered: air density, its dynamic and kinematic viscosity, specific heat capacity, thermal conductivity, thermal diffusivity, Prandtl number and entropy. The properties of air are given in tables depending on temperature at normal atmospheric pressure.

Air density depending on temperature

A detailed table of dry air density values ​​at various temperatures and normal atmospheric pressure is presented. What is the density of air? The density of air can be determined analytically by dividing its mass by the volume it occupies. under given conditions (pressure, temperature and humidity). You can also calculate its density using the formula of the ideal gas equation of state. To do this, you need to know the absolute pressure and temperature of the air, as well as its gas constant and molar volume. This equation allows you to calculate the dry density of air.

On practice, to find out what the density of air is at different temperatures, it is convenient to use ready-made tables. For example, the table below shows the density of atmospheric air depending on its temperature. Air density in the table is expressed in kilograms per cubic meter and is given in the temperature range from minus 50 to 1200 degrees Celsius at normal atmospheric pressure (101325 Pa).

Air density depending on temperature - table

t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3	t, °С	ρ, kg/m 3
-50	1,584	20	1,205	150	0,835	600	0,404
-45	1,549	30	1,165	160	0,815	650	0,383
-40	1,515	40	1,128	170	0,797	700	0,362
-35	1,484	50	1,093	180	0,779	750	0,346
-30	1,453	60	1,06	190	0,763	800	0,329
-25	1,424	70	1,029	200	0,746	850	0,315
-20	1,395	80	1	250	0,674	900	0,301
-15	1,369	90	0,972	300	0,615	950	0,289
-10	1,342	100	0,946	350	0,566	1000	0,277
-5	1,318	110	0,922	400	0,524	1050	0,267
0	1,293	120	0,898	450	0,49	1100	0,257
10	1,247	130	0,876	500	0,456	1150	0,248
15	1,226	140	0,854	550	0,43	1200	0,239

At 25°C, air has a density of 1.185 kg/m3. When heated, the air density decreases - the air expands (its specific volume increases). With increasing temperature, for example, to 1200°C, a very low air density is achieved, equal to 0.239 kg/m 3, which is 5 times less than its value at room temperature. IN general case, reduction when heated allows a process such as natural convection to take place and is used, for example, in aeronautics.

If we compare the density of air relative to , then air is three orders of magnitude lighter - at a temperature of 4°C, the density of water is 1000 kg/m3, and the density of air is 1.27 kg/m3. It is also necessary to note the value of air density under normal conditions. Normal conditions for gases are those at which their temperature is 0°C and the pressure is equal to normal atmospheric pressure. Thus, according to the table, air density under normal conditions (at NL) is 1.293 kg/m 3.

Dynamic and kinematic viscosity of air at different temperatures

When performing thermal calculations, it is necessary to know the value of air viscosity (viscosity coefficient) at different temperatures. This value is required to calculate the Reynolds, Grashof, and Rayleigh numbers, the values ​​of which determine the flow regime of this gas. The table shows the values ​​of the dynamic coefficients μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.

The viscosity coefficient of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is equal to 15.06 10 -6 m 2 /s at a temperature of 20°C, and with an increase in temperature to 1200°C, the viscosity of air becomes equal to 233.7 10 -6 m 2 /s, that is, it increases 15.5 times! The dynamic viscosity of air at a temperature of 20°C is 18.1·10 -6 Pa·s.

When air is heated, the values ​​of both kinematic and dynamic viscosity increase. These two quantities are related to each other through the air density, the value of which decreases when this gas is heated. An increase in the kinematic and dynamic viscosity of air (as well as other gases) when heated is associated with a more intense vibration of air molecules around their equilibrium state (according to MKT).

Dynamic and kinematic viscosity of air at different temperatures - table

t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s	t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s	t, °С	μ·10 6 , Pa·s	ν·10 6, m 2 /s
-50	14,6	9,23	70	20,6	20,02	350	31,4	55,46
-45	14,9	9,64	80	21,1	21,09	400	33	63,09
-40	15,2	10,04	90	21,5	22,1	450	34,6	69,28
-35	15,5	10,42	100	21,9	23,13	500	36,2	79,38
-30	15,7	10,8	110	22,4	24,3	550	37,7	88,14
-25	16	11,21	120	22,8	25,45	600	39,1	96,89
-20	16,2	11,61	130	23,3	26,63	650	40,5	106,15
-15	16,5	12,02	140	23,7	27,8	700	41,8	115,4
-10	16,7	12,43	150	24,1	28,95	750	43,1	125,1
-5	17	12,86	160	24,5	30,09	800	44,3	134,8
0	17,2	13,28	170	24,9	31,29	850	45,5	145
10	17,6	14,16	180	25,3	32,49	900	46,7	155,1
15	17,9	14,61	190	25,7	33,67	950	47,9	166,1
20	18,1	15,06	200	26	34,85	1000	49	177,1
30	18,6	16	225	26,7	37,73	1050	50,1	188,2
40	19,1	16,96	250	27,4	40,61	1100	51,2	199,3
50	19,6	17,95	300	29,7	48,33	1150	52,4	216,5
60	20,1	18,97	325	30,6	51,9	1200	53,5	233,7

Note: Be careful! Air viscosity is given to the power of 10 6 .

Specific heat capacity of air at temperatures from -50 to 1200°C

A table of the specific heat capacity of air at various temperatures is presented. The heat capacity in the table is given at constant pressure (isobaric heat capacity of air) in the temperature range from minus 50 to 1200°C for air in a dry state. What is the specific heat capacity of air? The specific heat capacity determines the amount of heat that must be supplied to one kilogram of air at constant pressure to increase its temperature by 1 degree. For example, at 20°C, to heat 1 kg of this gas by 1°C in an isobaric process, 1005 J of heat is required.

The specific heat capacity of air increases with increasing temperature. However, the dependence of the mass heat capacity of air on temperature is not linear. In the range from -50 to 120°C, its value practically does not change - under these conditions, the average heat capacity of air is 1010 J/(kg deg). According to the table, it can be seen that temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much less than its viscosity. Thus, when heated from 0 to 1200°C, the heat capacity of air increases only 1.2 times - from 1005 to 1210 J/(kg deg).

It should be noted that the heat capacity of humid air is higher than that of dry air. If we compare air, it is obvious that water has a higher value and the water content in air leads to an increase in specific heat capacity.

Specific heat capacity of air at different temperatures - table

t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)	t, °С	C p , J/(kg deg)
-50	1013	20	1005	150	1015	600	1114
-45	1013	30	1005	160	1017	650	1125
-40	1013	40	1005	170	1020	700	1135
-35	1013	50	1005	180	1022	750	1146
-30	1013	60	1005	190	1024	800	1156
-25	1011	70	1009	200	1026	850	1164
-20	1009	80	1009	250	1037	900	1172
-15	1009	90	1009	300	1047	950	1179
-10	1009	100	1009	350	1058	1000	1185
-5	1007	110	1009	400	1068	1050	1191
0	1005	120	1009	450	1081	1100	1197
10	1005	130	1011	500	1093	1150	1204
15	1005	140	1013	550	1104	1200	1210

Thermal conductivity, thermal diffusivity, Prandtl number of air

The table presents such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. Thermophysical properties of air are given in the range from -50 to 1200°C for dry air. According to the table, it can be seen that the indicated properties of air depend significantly on temperature and the temperature dependence of the considered properties of this gas is different.

When designing an air heating system, ready-made heating units are used.

To correctly select the necessary equipment, it is enough to know: the required power of the heater, which will subsequently be installed in the supply ventilation heating system, the temperature of the air at its outlet from the heater unit and the coolant flow rate.

To simplify the calculations, we present to your attention an online calculator for calculating the basic data for the correct selection of a heater.

1. Thermal power of the heater kW. In the fields of the calculator you should enter the initial data on the volume of air passing through the heater, data on the temperature of the air entering the air inlet, and the required temperature of the air flow at the outlet of the heater.
2. Outlet air temperature. In the appropriate fields you should enter the initial data on the volume of heated air, the temperature of the air flow at the entrance to the installation and the thermal power of the heater obtained during the first calculation.
3. Coolant flow. To do this, you should enter the initial data into the fields of the online calculator: the thermal power of the installation obtained during the first calculation, the temperature of the coolant supplied to the inlet of the heater, and the temperature value at the outlet of the device.

Calculation of heater power