Air heating systems. Heat spent on heating the air per cycle Depends on heating the air

Preliminary calculation of the nozzle heating surface.

Q in = V in * (i in // - i in /) * τ = 232231.443 * (2160-111.3) * 0.7 = 333.04 * 10 6 kJ / cycle.

Average logarithmic temperature difference per cycle.

Combustion products (smoke) velocity = 2.1 m / s. Then the air speed under normal conditions is:

6.538 m / s

Average temperatures of air and smoke for the period.

935 o C

680 o C

Average temperature of the top of the nozzle in the smoke and air periods

Cycle average temperature of the top of the nozzle

Average temperature of the bottom of the nozzle in the smoke and air periods:

Cycle average temperature of the bottom of the nozzle

Determine the value of the heat transfer coefficients for the top and bottom of the nozzle. For a nozzle of the accepted type with a value of 2240 18000 the value of heat transfer by convection is determined from the expression Nu = 0.0346 * Re 0.8

The actual smoke speed is determined by the formula W d = W to * (1 + βt d). The actual air speed at temperature t in and air pressure p in = 0.355 MN / m 2 (absolute) is determined by the formula

Where 0.1013-MN / m 2 is the pressure under normal conditions.

The value of the kinematic viscosity ν and the coefficient of thermal conductivity λ for the combustion products are selected from the tables. In this case, we take into account that the value of λ depends very little on pressure, and at a pressure of 0.355 MN / m 2, the values ​​of λ can be used at a pressure of 0.1013 MN / m 2. The kinematic viscosity of gases is inversely proportional to the pressure; that value of ν at a pressure of 0.1013 MN / m 2 is divided by the ratio.

Effective beam length for block nozzle

= 0.0284 m

For a given packing m 2 / m 3; ν = 0.7 m 3 / m 3; m 2 / m 2.

The calculations are summarized in table 3.1.

Table 3.1 - Determination of heat transfer coefficients for the top and bottom of the nozzle.

Name, value and units of measurement of dimensions	Calculation formula	Advance paynemt	Refined calculation
top	bottom	top	Bottom
smoke	air	smoke	air	air	air
Average air and smoke temperatures over the period 0 С	According to the text	1277,5		592,5		1026,7	355,56
Coefficient of thermal conductivity of combustion products and air l 10 2 W / (mgrad)	According to the text	13,405	8,101	7,444	5,15	8,18	5,19
Kinematic viscosity of combustion products and air g 10 6 m 2 / s	Application	236,5	52,6	92,079	18,12	53,19	18,28
Defining channel diameter d, m		0,031	0,031	0,031	0,031	0,031	0,031
Actual speed of smoke and air W m / s	According to the text	11,927	8,768	6,65	4,257	8,712	4,213
Re
Nu	According to the text	12,425	32,334	16,576	42,549	31,88	41,91
Heat transfer coefficient by convection a to W / m2 * deg		53,73	84,5	39,804	70,69	84,15	70,226
		0,027	-	0,045	-	-	-
		1,005	-	1,055	-	-	-
Radiant heat transfer coefficient a p W / m 2 * deg		13,56	-	5,042	-	-	-
a W / m 2 * deg		67,29	84,5	44,846	70,69	84,15	70,226

The heat capacity and thermal conductivity of the brick l of the packing are calculated by the formulas:

С, kJ / (kg * deg) l, W / (mgrad)

Dinas 0.875 + 38.5 * 10 -5 * t 1.58 + 38.4 * 10 -5 t

Fireclay 0.869 + 41.9 * 10 -5 * t 1.04 + 15.1 * 10 -5 t

The equivalent half-thickness of a brick is determined by the formula

mm

Table 3.2 - Physical quantities of the material and the heat accumulation coefficient for the upper and lower half of the regenerative packing

Sizes	Calculation formula	Advance paynemt	Refined calculation
		top	bottom	top	Bottom
dinas	fireclay	dinas	fireclay
Average temperature, 0 С	According to the text	1143,75	471,25	1152,1	474,03
Bulk density, r kg / m 3	According to the text
Thermal conductivity coefficient l W / (mgrad)	According to the text	2,019	1,111	2,022	1,111
Heat capacity С, kJ / (kg * deg)	According to the text	1,315	1,066	1,318	1,067
Thermal diffusivity coefficient a, m 2 / hour		0,0027	0,0018	0,0027	0,0018
F 0 S		21,704	14,59	21,68	14,58
Heat accumulation coefficient h to		0,942	0,916	0,942	0,916

As is obvious from the table, the value of h k>, i.e. bricks are used in thermal ratio for their entire thickness. Accordingly, to the above, we take the value of the thermal hysteresis coefficient for the top of the nozzle x = 2.3, for the bottom x = 5.1.

Then the total heat transfer coefficient is calculated by the formula:

for the top of the nozzle

58.025 kJ / (m 2 cycle * deg)

for the bottom of the nozzle

60.454 kJ / (m 2 cycle * deg)

Average for the nozzle as a whole

59.239 kJ / (m 2 cycle * deg)

Heating surface of the nozzle

22093.13 m 2

Nozzle volume

= 579.87 m 3

Horizontal cross-sectional area of ​​the nozzle in the clear

= 9.866 m 2

Remember

• What device is used to measure the air temperature? What types of rotation of the Earth do you know? Why is there a change of day and night on Earth?

How the earth's surface and atmosphere heats up. The sun emits a tremendous amount of energy. However, the atmosphere lets only half of the sun's rays reach the earth's surface. Some of them are reflected, some are absorbed by clouds, gases and dust particles (Fig. 83).

Rice. 83. Consumption solar energy coming to Earth

Passing through the sun's rays, the atmosphere from them hardly heats up. The earth's surface heats up, and itself becomes a source of heat. It is from her that heats up atmospheric air... Therefore, near the earth's surface, the air in the troposphere is warmer than at altitude. When climbing upwards for every kilometer, the air temperature drops by 6 "C. High in the mountains, due to low temperatures, the accumulated snow does not melt even in summer. The temperature in the troposphere changes not only with altitude, but also during certain periods of time: days, years.

Differences in air heating during the day and year. During the day, the sun's rays illuminate the earth's surface and warm it up, and the air heats up from it. At night, the flow of solar energy stops, and the surface, together with the air, gradually cools down.

The sun rises highest above the horizon at noon. At this time, the most solar energy comes in. However, the highest temperature is observed 2-3 hours after noon, since it takes time to transfer heat from the Earth's surface to the troposphere. The coldest temperature occurs before sunrise.

The air temperature also changes according to the seasons of the year. You already know that the Earth moves around the Sun in its orbit and the Earth's axis is constantly tilted to the orbital plane. Because of this, during the year in the same area, the sun's rays fall on the surface in different ways.

When the angle of incidence of the rays is more vertical, the surface receives more solar energy, the air temperature rises and summer begins (Fig. 84).

Rice. 84. The fall of the sun's rays on the earth's surface at noon on June 22 and December 22

When the sun's rays are tilted more, the surface heats up slightly. The air temperature at this time drops, and winter comes. The warmest month in the Northern Hemisphere is July, while the coldest month is January. In the Southern Hemisphere - on the contrary: the most cold month of the year - July, and the warmest - January.

From the figure, determine how the angle of incidence of the sun's rays on June 22 and December 22 at parallels 23.5 ° N differs. NS. and y. NS.; at parallels 66.5 ° N NS. and y. NS.

Consider why the warmest and coldest months are not June and December, when the sun's rays have the greatest and smallest angles of incidence on the earth's surface.

Rice. 85. Average annual air temperatures of the Earth

Indicators of temperature changes. To identify the general patterns of temperature change, use the indicator of average temperatures: average daily, average monthly, average annual (Fig. 85). For example, to calculate the average daily temperature during the day, the temperature is measured several times, these indicators are summed up and the resulting sum is divided by the number of measurements.

Define:

• average daily temperature in terms of four measurements per day: -8 ° С, -4 ° С, + 3 ° С, + 1 ° С;
• the average annual temperature of Moscow, using the data in the table.

Table 4

When determining the change in temperature, its highest and lowest values ​​are usually noted.

The difference between the highest and lowest readings is called the temperature range.

The amplitude can be determined for a day (daily amplitude), month, year. For example, if the highest temperature per day is + 20 ° C, and the lowest is + 8 ° C, then the daily amplitude will be 12 ° C (Fig. 86).

Rice. 86. Daily range of temperatures

Determine how many degrees the annual amplitude in Krasnoyarsk is greater than in St. Petersburg, if average temperature July in Krasnoyarsk + 19 ° С, and in January -17 ° С; in St. Petersburg + 18 ° С and -8 ° С, respectively.

On maps, the distribution of average temperatures is reflected using isotherms.

Isotherms are lines connecting points with the same average air temperature over a certain period of time.

Usually shows isotherms of the warmest and coldest months of the year, i.e. July and January.

Questions and tasks

1. How does the air in the atmosphere heat up?
2. How does the air temperature change during the day?
3. What determines the difference in the heating of the Earth's surface during the year?

- devices used for heating air in supply ventilation systems, air conditioning systems, air heating, as well as in drying plants.

By the type of coolant, air heaters can be fire, water, steam and electric .

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

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

Heaters are of 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 quantity of heated air, kg / h, equal to Lp [here L- volumetric amount of heated air, m 3 / h; p - air density (at a temperature t K), kg / m 3]; with- specific heat capacity of air, equal to 1 kJ / (kg-K); t to - air temperature after the heater, ° С; t n- air temperature before the heater, ° С.

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

The outside air temperature is taken equal to the calculated ventilation (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 designed 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 taken to be equal to the calculated outside temperature. temperature tn for heating design (climate parameters of category B).

Supply air with a temperature equal to the internal air temperature tВ for the given room should be supplied to the room without heat surpluses. In the presence of heat surpluses, the supply air is supplied with 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 generation due to the possibility of colds... The exception is the cases of using special anemostats.

The required area of ​​the heating surface 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 mean T.- average temperature of the coolant, 0 С; t av. - the average temperature of the heated air passing through the heater, ° С, equal to (t n + t k) / 2.

If steam serves as the heat carrier, then the average temperature of the heat carrier tav.T. is equal to the saturation temperature at the corresponding vapor pressure.

For water, the 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 cooling the air in the air ducts.

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

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

The model, brand and number of heaters are selected based on the area of ​​the free cross-section fL and the heating surface FK. After choosing the air heaters, the mass air velocity is specified according to the actual area of ​​the air flow area of ​​the air 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 tubes of the air heater ω, m / s, is determined by the formula:

where Q "is the heat consumption for heating the air, kJ / h (kcal / h); pw 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 is free cross-sectional area for the passage of the coolant, m2, tg - temperature of hot water in the supply line, ° С; t 0 - temperature of return water, ° С.

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

The resistance of air 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 air heater.

The resistance of sequentially located heaters is equal to:

where m is the number of sequentially located heaters. The calculation ends by checking the heat output (heat transfer) of the heaters according to the formula

where QK - heat transfer from 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, taken as a result of calculating heaters of this type; K - heat transfer coefficient of heaters, W / (m2-K) [kcal / (h-m2- ° C)]; tср.в - average temperature of heated air passing through the heater, ° С; tcr. T is the average temperature of the coolant, ° С.

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

All life processes on Earth are caused by thermal energy. The main source from which the Earth receives thermal energy, is the Sun. It emits energy in the form of various rays - electromagnetic waves. The radiation of the Sun in the form of electromagnetic waves propagating at a speed of 300,000 km / s is called, which consists of rays of various lengths, carrying light and heat to the Earth.

Radiation can be direct and diffuse. Without the atmosphere, the earth's surface would receive only direct radiation. Therefore, radiation coming directly from the Sun in the form of direct sunlight and in a cloudless sky is called direct. It carries the greatest amount of heat and light. But, passing through the atmosphere, the sun's rays are partially scattered, deviate from the direct path as a result of reflection from air molecules, water droplets, dust particles and pass into rays going in all directions. Such radiation is called diffuse. Therefore, there is light also in those places where direct sunlight (direct radiation) does not penetrate (forest canopy, shady side of rocks, mountains, buildings, etc.). Scattered radiation also determines the color of the sky. All solar radiation reaching the earth's surface, i.e. direct and scattered, called total. The earth's surface, absorbing solar radiation, heats up and itself becomes a source of heat radiation into the atmosphere. It is called terrestrial radiation, or terrestrial radiation, and is largely retained by the lower atmosphere. The radiation of the Sun absorbed by the earth's surface is spent on heating water, soil, air, evaporation and radiation into the atmosphere. Terrestrial, and does not determine the temperature regime of the troposphere, i.e. the sun's rays passing through everything do not heat it up. The largest amount of heat is received and heated to the highest temperatures by the lower layers of the atmosphere, directly adjacent to the heat source - the earth's surface. Heating decreases with distance from the earth's surface. That is why in the troposphere with height it decreases on average 0.6 ° С for every 100 m of rise. This is a general pattern for the troposphere. There are times when the overlying air layers are warmer than the underlying ones. This phenomenon is called temperature inversion.

The heating of the earth's surface differs significantly not only in height. The amount of total solar radiation directly depends on the angle of incidence of the sun's rays. The closer this value is to 90 °, the more solar energy is received by the earth's surface.

In turn, the angle of incidence of sunlight on a certain point on the earth's surface is determined by its latitude. The strength of direct solar radiation depends on the length of the path that the sun's rays travel through the atmosphere. When the Sun is at its zenith (near the equator), its rays fall vertically on the earth's surface, i.e. overcome the atmosphere by the shortest route (at 90 °) and intensively give their energy to a small area. With distance from the equatorial zone to the south or north, the length of the path of the sun's rays increases, i.e. the angle of their incidence on the earth's surface decreases. More and more rays begin to slide along the Earth and approach the tangent line in the region of the poles. In this case, the same energy beam is scattered over a large area, and the amount of reflected energy increases. Thus, where the sun's rays fall on the earth's surface at an angle of 90 °, it is constantly high, and as it moves towards the poles it gets colder and colder. It is at the poles, where the sun's rays fall at an angle of 180 ° (i.e., tangentially), that there is the least heat.

Such an uneven distribution of heat on the Earth, depending on the latitude of the place, makes it possible to distinguish five heat zones: one hot, two and two cold.

The conditions for heating water and land by solar radiation are very different. The heat capacity of water is twice that of land. This means that with the same amount of heat, land heats up twice as fast as water, and when it cools, the opposite happens. In addition, water evaporates when heated, which consumes a considerable amount of heat. On land, heat is concentrated only in its upper layer; only a small part of it is transferred to the depth. In water, the rays immediately heat up a significant thickness, which is facilitated by vertical mixing of the water. As a result, water accumulates heat much more than land, retains it longer and uses it more evenly than land. It heats up more slowly and cools more slowly.

The land surface is heterogeneous. Its heating largely depends on the physical properties of soils and, ice, exposure (the angle of inclination of land areas in relation to the incident sun rays) of the slopes. The peculiarities of the underlying surface determine the different nature of the change in air temperatures during the day and year. Most low temperatures air during the day on land are noted shortly before sunrise (no influx of solar radiation and strong terrestrial radiation at night). The highest are in the afternoon (14-15 hours). During the year in the Northern Hemisphere, the most high temperatures air on land are observed in July, and the lowest in January. Above the water surface, the daily maximum of air temperature is shifted and is noted at 15-16 hours, and at least 2-3 hours after sunrise. The annual maximum (in the Northern Hemisphere) occurs in August, and the minimum in February.

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

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

But at each specific point, it (t about) 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 one latitude). In conditions of air unsaturated with water vapor, a regularity is observed: when rising for every 100 meters of height, the temperature (t o) decreases by 0.6 o C.

B - Qualitative characteristics of the surface.

B 1 - surfaces of different 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, heats up very slowly and cools very slowly because of its high heat capacity. Land heats up quickly and cools quickly, i.e., 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 different amount 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 the sun's rays are scattered in it, and all the sun's rays reach the surface of the Earth. In the presence of a large amount of water vapor in the air, water droplets reflect, scatter, absorb the sun's rays, and not all of them reach the planet's surface, while heating it decreases.

The highest air temperatures were recorded in areas of tropical deserts. In the central regions of the Sahara, for almost 4 months, the air temperature 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 greatest, 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 hold them back, 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 the air temperature decreases.

Literature

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