Heating of atmospheric air. §33

- 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 ribbed ones; the latter, in turn, are divided into lamellar and spiral-wound.

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

Heaters perform two models: medium (C) and large (B).

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

Where Q"— heat consumption for air heating, kJ/h (kcal/h); Q- the same, W; 0.278 is the conversion factor from 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 is the air density (at a temperature tK), kg / m 3]; With- specific heat capacity of air, equal to 1 kJ / (kg-K); t k - air temperature after the heater, ° С; t n— air temperature before the air heater, °C.

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

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 MPC of which is more than 100 mg / m3. When designing general ventilation designed to combat gases whose MPC is less than 100 mg / m3, as well as when designing supply ventilation to compensate for air removed through local exhausts, 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 category B).

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

The required surface area for heating heaters Fк m2, is determined by the formula:

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

If the coolant is steam, then the average temperature of the coolant tav.T. is 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:

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

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

The mass velocity is understood as the mass of air, kg, passing through 1 m2 of the living section of the air heater in 1 s. Mass velocity vp, kg/(cm2), is determined by the formula

According to the area of ​​​​the open section fЖ and the heating surface FK, the model, brand and number of heaters are selected. After choosing the heaters, the mass air velocity is specified according to the actual area of ​​​​the open section of the heater fD of this 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 air, kJ / h (kcal / h); rw 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 area for coolant passage, m2, tg — temperature hot water in the supply line, ° С; t 0 - return water temperature, 0С.

The heat transfer of heaters is affected by the scheme of tying them with pipelines. With a parallel scheme for connecting pipelines, only part of the coolant passes through a separate heater, and with a sequential scheme, the entire flow of the coolant passes through each heater.

The resistance of heaters to the passage of air 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 the heaters located in series is equal to:

where m is the number of successively located heaters. The calculation ends with a check of the heat output (heat transfer) of the heaters according to the formula

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

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

The main physical properties air: 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 the temperature at normal atmospheric pressure.

Air density versus 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 analytically determined by dividing its mass by the volume it occupies. under given conditions (pressure, temperature and humidity). It is also possible to calculate its density using the ideal gas equation of state formula. For this you need to know absolute pressure and air temperature, as well as its gas constant and molar volume. This equation allows you to calculate the density of air in a dry state.

On practice, to find out what is the density of air at different temperatures, it is convenient to use ready-made tables. For example, the given table of atmospheric air density values ​​depending on its temperature. The 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).

At 25°C, air has a density of 1.185 kg/m 3 . When heated, the density of air decreases - the air expands (its specific volume increases). With an increase in temperature, for example, up 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, the decrease in heating allows a process such as natural convection to take place and is used, for example, in aeronautics.

If we compare the density of air with respect to, then air is lighter by three orders of magnitude - at a temperature of 4 ° C, the density of water is 1000 kg / m 3, and the density of air is 1.27 kg / m 3. It is also necessary to note the value of air density at normal conditions. Normal conditions for gases are those under 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 NU) 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, Rayleigh numbers, the values ​​of which determine the flow regime of this gas. The table shows the values ​​of the coefficients of dynamic μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.

The viscosity of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is 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 the 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 interconnected through the value of 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) during heating is associated with a more intense vibration of air molecules around them. equilibrium state(according to MKT).

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

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

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 dry air. What is the specific heat capacity of air? The value of 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 as its temperature rises. 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 the temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much weaker than its viscosity. So, 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 humid air higher than dry. If we compare air, it is obvious that water has a higher value and the water content in the air leads to an increase in specific heat.

Thermal conductivity, thermal diffusivity, Prandtl number of air

The table shows such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. The 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 these properties of air significantly depend on temperature and temperature dependence considered properties of this gas is different.

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, the 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 the surface gives off more heat to the air than it receives in the same time, it cools, and the air above it also cools from it. In this case, the air temperature rises with altitude. Such a position is called temperature inversion . It can be observed in summer at night, in winter - above the snowy surface. Temperature inversion is 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 a calm troposphere, the temperature decreases with height by an average of 0.6 ° for every 100 m. When dry air rises, this indicator increases and can reach 1 ° per 100 m, and when moist air rises, it decreases. This is explained by the fact that the rising air expands and energy (heat) is expended on this, and when moist air rises, water vapor condenses, accompanied by heat release.

Lowering the temperature of the rising air - the main reason for the formation of clouds . The descending air, falling under great pressure, is compressed, and its temperature rises.

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

IN its daily course there is one maximum (in the afternoon) 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 greater over land than over the ocean.

IN annual course temperature air at the equator - two maxima (after the equinoxes) and two minima (after the solstices). In tropical, temperate and polar latitudes - one maximum and one minimum. The amplitudes of annual fluctuations in air temperature 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 the polar latitudes, the negative temperature prevails, its annual fluctuations reach 30-40°C.

The correct daily and annual course of air temperature, due to changes in the height of the Sun above the horizon and the length of the day, is complicated by non-periodic changes caused by the movement of air masses that have different temperature. General pattern of temperature distribution in the lower layer of the 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, however, its distribution is significantly affected by differences in the nature of the underlying surface and the transfer of heat from low latitudes to high latitudes.

As a result of heat transfer, the air temperature at the equator is lower, and at the poles it is higher than it would be without this process. The southern hemisphere is colder than the northern hemisphere mainly due to the land covered with ice and snow near South Pole. average temperature air in the lower two-meter layer for the whole Earth +14°C, which corresponds to the average annual temperature air at 40°N

DEPENDENCE OF AIR TEMPERATURE ON GEOGRAPHICAL LATITUDE

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

atmospheric pressure

Air has mass and weight, and therefore exerts pressure on the surface in contact with it. The pressure exerted by air on the earth's surface and all objects 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 pressure of the atmosphere on the underlying surface is on average 1.033 g per 1 cm 2 (more than 10 tons per m 2 ). Pressure is measured in millimeters of mercury, millibars (1 mb = 0.75 mm Hg) and hectopascals (1 hPa = 1 mb). With altitude, the pressure decreases: In the lower layer of the troposphere, up to a height of 1 km, it decreases by 1 mm Hg. Art. for every 10 m. The higher, the slower the pressure decreases. normal pressure at ocean level - 760 mm. Rt. Art.

The general distribution of pressure on the Earth's surface has a zonal character:

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

The change in pressure is explained by the movement of air. It rises where there is more air, and decreases where the air leaves. main reason air movement - its heating and cooling from the underlying surface. As the air warms up from the surface, it expands and rushes up. Having reached a height at which its density is greater than the density of the surrounding air, it spreads 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 did not change (tropical latitudes in winter).

Above the cold surface, the air cools and condenses, clinging to the surface (polar latitudes, the continental part of temperate latitudes in winter). At the top, its density decreases, and air comes here from the side. Its amount 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 change.

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

Above the cold surface in the Arctic and Antarctica pressure elevated. It is created by air coming from temperate latitudes to the place of 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 regions high pressure they expand in winter, shrink in summer, but exist all year round. Belts of low 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 rises strongly and the low pressure belt “breaks”. Closed areas of low pressure persist only over the oceans - Icelandic And Aleutian lows. Over the continents, on the contrary, winter 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 with a center in tropical latitudes forms over Asia in the summer - Asian Low. In tropical latitudes, the continents are always warmer than the oceans, and the pressure over them is lower. Therefore, over the oceans there are 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 faster), the presence of warm and cold currents and other reasons on Earth, except for belts atmospheric pressure closed areas of low and high pressure can occur.

Change in flue gas recirculation . Gas recirculation is widely used to expand the range of superheated steam temperature control and allows maintaining the superheated steam temperature even at low loads of the boiler unit. IN Lately Flue gas recirculation is also gaining ground as a method of reducing NO x formation. Flue gas recirculation is also used in air flow in front of the burners, which is more effective in terms of suppressing the formation of N0 x .

The introduction of relatively cold recirculated gases into the lower part of the furnace leads to a decrease in the heat absorption of the radiant heating surfaces and to an increase in the temperature of the gases at the furnace outlet and in the convective gas ducts, including the temperature of the flue gases. An increase in the total flow of flue gases in the section of the gas path before the selection of gases for recirculation contributes to an increase in the heat transfer coefficients and heat absorption of convective heating surfaces.

Rice. 2.29. Changes in steam temperature (curve 1), hot air temperature (curve 2) and flue gas losses (curve 3) depending on the share of flue gas recirculation r.

On fig. 2.29 shows the characteristics of the TP-230-2 boiler unit with a change in the proportion of gas recirculation to the lower part of the furnace. Here the share of recycling

where V rc is the volume of gases taken off for recirculation; V r - the volume of gases at the point of selection for recirculation without taking into account V rc. As can be seen, an increase in the share of recirculation by every 10% leads to an increase in the flue gas temperature by 3–4°C, Vr - by 0.2%, steam temperature - by 15 ° C, and the nature of the dependence is almost linear. These ratios are not unambiguous for all boiler units. Their value depends on the temperature of the recirculated gases (the place of gas intake) and the method of introducing them. The discharge of recirculated gases into the upper part of the furnace does not affect the operation of the furnace, but leads to a significant decrease in the temperature of the gases in the area of ​​the superheater and, as a result, to a decrease in the temperature of the superheated steam, although the volume of combustion products increases. The discharge of gases into the upper part of the furnace can be used to protect the superheater from the effects of unacceptably high gas temperatures and reduce superheater slagging.

Of course, the use of gas recirculation leads to a decrease not only in efficiency. gross, but also efficiency net of the boiler unit, as it causes an increase in electricity consumption for own needs.

Rice. 2.30. Dependence of heat losses with mechanical underburning on the temperature of hot air.

Hot air temperature change. The change in hot air temperature is the result of a change in the operating mode of the air heater due to the influence of factors such as changes in temperature difference, heat transfer coefficient, gas or air flow. Increasing the temperature of the hot air increases, albeit slightly, the level of heat release in the furnace. The hot air temperature has a significant effect on the characteristics of boiler units operating on fuel with a low volatile output. A decrease in ^ r.v in this case worsens the conditions for fuel ignition, the mode of drying and grinding of the fuel, leads to a decrease in the temperature of the air mixture at the inlet to the burners, which can cause an increase in losses with mechanical underburning (see Fig. 2.30).

. Changing the air preheating temperature. Air preheating in front of the air heater is used to increase the temperature of the wall of its heating surfaces in order to reduce the corrosive effect of flue gases on them, especially when burning high-sulfur fuels. According to PTE, when burning sulphurous fuel oil, the air temperature in front of tubular air heaters must not be lower than 110 ° C, and in front of regenerative ones - not lower than 70 ° C.

Pre-heating of air can be carried out by recirculating hot air to the inlet of blast fans, however, in this case, the efficiency of the boiler unit decreases due to an increase in electricity consumption for the blast and an increase in the temperature of the flue gases. Therefore, it is advisable to heat the air above 50°C in heaters operating on selective steam or hot water.

Air preheating entails a decrease in the heat absorption of the air heater due to a decrease in temperature difference, the temperature of the flue gases and the heat loss increase. Air preheating also requires additional energy costs for air supply to the air heater. Depending on the level and method of air preheating, for every 10° C of air preheating, the efficiency gross changes by about 0.15-0.25%, and the temperature of the flue gases - by 3-4.5 ° C.

Since the share of heat taken for air preheating in relation to the heat output of boiler units is quite large (2-3.5%), the choice of the optimal air heating scheme has great importance.

Cold air

Rice. 2.31. Scheme of two-stage air heating in heaters with network water and selective steam:

1 - network heaters; 2 - the first stage of air heating with network water of the heating system; 3 - the second stage of air heating pzrom; 4 - pump for supplying return network water to heaters; 5 - network water for air heating (scheme for summer period); 6 - network water for air heating (scheme for the winter period).

They pass through the transparent atmosphere without heating it, they reach the earth's surface, heat it, and the air subsequently heats up from it.

The degree of surface heating, and hence the air, depends primarily on the latitude of the area.

But at each specific point, it (t o) will also be determined by a number of factors, among which the main ones are:

A: height above sea level;

B: underlying surface;

B: distance from the coasts of oceans and seas.

A - Since the air is heated from the earth's surface, the lower the absolute heights of the area, the higher the air temperature (at the same latitude). In conditions of air unsaturated with water vapor, a regularity is observed: for every 100 meters of altitude, the temperature (t o) decreases by 0.6 o C.

B - Qualitative characteristics of the surface.

B 1 - surfaces different in color and structure absorb and reflect the sun's rays in different ways. The maximum reflectivity is typical for snow and ice, the minimum for dark-colored soils and rocks.

Illumination of the Earth by the sun's rays on the days of the solstices and equinoxes.

B 2 - different surfaces have different heat capacity and heat transfer. So the water mass of the World Ocean, which occupies 2/3 of the Earth's surface, due to the high heat capacity, heats up very slowly and cools very slowly. The land quickly heats up and quickly cools, i.e., in order to heat up to the same t about 1 m 2 of land and 1 m 2 of water surface, it is necessary to spend a different amount of energy.

B - from the coasts to the interior of the continents, the amount of water vapor in the air decreases. The more transparent the atmosphere, the less sunlight is scattered in it, and all the sun's rays reach the Earth's surface. In the presence of a large number water vapor in the air, water droplets reflect, scatter, absorb the sun's rays and not all of them reach the surface of the planet, while heating it decreases.

Most high temperatures air recorded in areas tropical deserts. IN central regions Sahara for almost 4 months, t about air in the shade is more than 40 o C. At the same time, at the equator, where the angle of incidence of the sun's rays is the largest, the temperature does not exceed +26 o C.

On the other hand, the Earth, as a heated body, radiates energy into space mainly in the long-wave infrared spectrum. If the earth's surface is wrapped in a "blanket" of clouds, then not all infrared rays leave the planet, since the clouds delay them, reflecting back to the earth's surface.

With a clear sky, when there is little water vapor in the atmosphere, the infrared rays emitted by the planet freely go into space, while the earth's surface cools down, which cools down and thereby reduces the air temperature.

Literature

1. Zubashchenko E.M. Regional Physiography. Climates of the Earth: teaching aid. Part 1. / E.M. Zubashchenko, V.I. Shmykov, A.Ya. Nemykin, N.V. Polyakov. - Voronezh: VGPU, 2007. - 183 p.