Physical properties of air: density, viscosity, specific heat capacity. The amount of heat during various physical processes

Changing flue gas recirculation . Gas recirculation is widely used to expand the temperature control range of superheated steam and allows maintaining the superheated steam temperature even at low boiler loads. IN lately Flue gas recirculation is also gaining ground as a method to reduce NOx formation. Recirculation of flue gases into the air stream in front of the burners is also used, which is more effective in terms of suppressing the formation of NO 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 radiation heating surfaces and to an increase in the temperature of the gases at the exit from the furnace and in the convective flues, 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 gases are taken for recirculation helps to increase the heat transfer coefficients and heat perception of convective heating surfaces.

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

In Fig. Table 2.29 shows the characteristics of the TP-230-2 boiler unit when changing the proportion of gas recirculation to the lower part of the furnace. Here is the share of recycling

where V rts is the volume of gases taken for recirculation; V r - volume of gases at the point of selection for recirculation without taking into account V rc. As can be seen, an increase in the recirculation share 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 relationships are not unique for all boilers. Their value depends on the temperature of the recirculated gases (the place where the gases are taken) and the method of their introduction. Discharge of recirculated gases into top part 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 consequence, to a decrease in the temperature of the superheated steam, although the volume of combustion products increases. 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 slagging of the superheater.

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 its own needs.

Rice. 2.30. Dependence of heat loss due to mechanical underburning on hot air temperature.

Change in hot air temperature. A change in the temperature of hot air 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 pressure, heat transfer coefficient, gas or air flow. Increasing the temperature of the hot air increases, although slightly, the level of heat release in the firebox. The temperature of hot air has a noticeable effect on the characteristics of boiler units operating on fuel with a low volatile yield. A decrease in ^ g.v in this case worsens the conditions for fuel ignition, the mode of drying and grinding of 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 due to mechanical underburning (see Fig. 2.30).

. Changing the air preheating temperature. Preheating of the air 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 the PTE, when burning sulfur fuel oil, the air temperature in front of tubular air heaters should be no lower than 110 ° C, and in front of regenerative heaters - not lower than 70 ° C.

Air preheating can be carried out by recirculating hot air to the input of blower fans, however, this reduces the efficiency of the boiler unit due to an increase in electricity consumption for blasting and an increase in the temperature of the flue gases. Therefore, it is advisable to heat air above 50°C in air heaters operating on selected steam or hot water.

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

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

Cold air

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

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

— 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 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 design 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 design one outside temperature tн for heating design (climate parameters of category B).

In a room without excess heat, supply air should be supplied with a temperature equal to the internal air temperature tB for a given room. If there is excess heat, 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 into the room even in the presence of significant heat generation due to the possibility of colds. 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 avg. - 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:

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, geometric dimensions and design features heaters, 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 velocity of air movement 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 air 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%.

When is the sun hotter - when is it higher above your head or when is it lower?

The sun is hotter when it is higher. In this case, the sun's rays fall at a right angle, or close to a right angle.

What types of rotation of the Earth do you know?

The Earth rotates around its axis and around the Sun.

Why does the cycle of day and night occur on Earth?

The change of day and night is the result of the axial rotation of the Earth.

Determine how the angle of incidence of the sun's rays differs on June 22 and December 22 at parallels 23.5° N. w. and Yu. sh.; on parallels 66.5° N. w. and Yu. w.

On June 22, the angle of incidence of the sun's rays at parallel 23.50 north latitude. 900, S. – 430. At parallel 66.50 N. – 470, 66.50 S. – sliding angle.

On December 22, the angle of incidence of the sun's rays at the parallel is 23.50 N. 430, S. – 900. At parallel 66.50 N. – sliding angle, 66.50 S. – 470.

Think about why the warmest and coldest months are not June and December, when sun rays have the largest and smallest angles of incidence at earth's surface.

Atmospheric air is heated by the earth's surface. Therefore, in June the earth's surface warms up, and the temperature reaches its maximum in July. The same happens in winter. In December the earth's surface cools down. The air cools down in January.

Define:

average daily temperature based on four measurements per day: -8°C, -4°C, +3°C, +1°C.

The average daily temperature is -20C.

average annual temperature Moscow, using table data.

The average annual temperature is 50C.

Determine the daily temperature range for the thermometer readings in Figure 110, c.

The temperature amplitude in the figure is 180C.

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

The temperature range in Krasnoyarsk is 360C.

The temperature range in St. Petersburg is 260C.

The temperature range in Krasnoyarsk is 100C greater.

Questions and tasks

1. How does atmospheric air heat up?

By transmitting the sun's rays, the atmosphere hardly heats up from them. The earth's surface heats up and itself becomes a source of heat. It is from this that the atmospheric air is heated.

2. How many degrees does the temperature in the troposphere decrease with every 100 m rise?

As you rise upward, every kilometer the air temperature drops by 6 0C. This means 0.60 for every 100 m.

3. Calculate the air temperature outside the aircraft if the flight altitude is 7 km and the temperature at the Earth’s surface is +200C.

The temperature during an ascent of 7 km will drop by 420. This means that the temperature outside the plane will be -220.

4. Is it possible to find a glacier in the mountains at an altitude of 2500 m in the summer if the temperature at the foot of the mountains is +250C?

The temperature at an altitude of 2500 m will be +100C. A glacier will not be found at an altitude of 2500 m.

5. How and why does the air temperature change during the day?

During the day, the sun's rays illuminate the earth's surface and warm it, which also heats the air. At night, the supply of solar energy stops, and the surface along with the air gradually cools down. The sun is highest above the horizon at noon. This is when the most solar energy comes in. However, the most high temperature observed 2-3 hours after noon, since it takes time to transfer heat from the Earth's surface to the troposphere. The most low temperature happens before sunrise.

6. What determines the difference in heating of the Earth’s surface throughout the year?

Over the course of a 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. When the sun's rays are more inclined, the surface heats up weakly. The air temperature drops at this time, and winter comes. Most warm month in the Northern Hemisphere it is July, and the coldest month is January. IN Southern Hemisphere- on the contrary: the most cold month of the year is July, and the warmest is January.

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

For correct selection 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 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 temperature obtained during the first calculation thermal power heater.
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