Physical properties of air: density, viscosity, specific heat capacity. Dynamic and kinematic viscosity of air at different temperatures
1. Heat consumption for supply air heating
Q t \u003d L ∙ ρ air. ∙ with air. ∙(t int. - t out.),
where:
ρ air. is the air density. The density of dry air at 15°C at sea level is 1.225 kg/m³;
with air – specific heat capacity of air equal to 1 kJ/(kg∙K)=0.24 kcal/(kg∙°С);
t int. – air temperature at the heater outlet, °С;
t out. - outdoor air temperature, °С (air temperature of the coldest five-day period with a security of 0.92 according to Building Climatology).
2. Coolant flow rate for the heater
G \u003d (3.6 ∙ Q t) / (s in ∙ (t pr -t arr)),
where:
3.6 - conversion factor W to kJ/h (to obtain flow rate in kg/h);
G - water consumption for heating the heater, kg / h;
Q t - thermal power of the heater, W;
c c - specific heat capacity of water, equal to 4.187 kJ / (kg ∙ K) \u003d 1 kcal / (kg ∙ ° С);
t pr. - coolant temperature (straight line), ° С;
t out. – heat carrier temperature (return line), °C.
3. The choice of pipe diameter for heating the heater
Water consumption for the heater , kg/h4. I-d diagram of the air heating process
The process of heating the air in the heater proceeds at d=const (at a constant moisture content).
1According to the International Energy Agency, the priority for reducing carbon dioxide emissions from cars is to improve their fuel efficiency. The task of reducing CO2 emissions by increasing the fuel efficiency of vehicles is one of the priorities for the world community, given the need for the rational use of non-renewable energy sources. To this end, international standards are constantly tightened, limiting the performance of engine start-up and operation at low and even high ambient temperatures. The article deals with the issue of fuel efficiency of internal combustion engines depending on the temperature, pressure, humidity of the ambient air. The results of a study on maintaining a constant temperature in the intake manifold of the internal combustion engine in order to save fuel and determine the optimal power of the heating element are presented.
heating element power
ambient temperature
air heating
fuel economy
optimum air temperature in the intake manifold
1. Car engines. V.M. Arkhangelsky [and others]; resp. ed. M.S. Hovah. M.: Mashinostroenie, 1977. 591 p.
2. Karnaukhov V.N., Karnaukhova I.V. Determination of the filling factor in the internal combustion engine // Transport and transport-technological systems, materials of the International Scientific and Technical Conference, Tyumen, April 16, 2014. Tyumen: Tyumen State University Publishing House, 2014.
3. Lenin I.M. Theory of automobile and tractor engines. M.: Higher school, 1976. 364 p.
4. Yutt V.E. Electrical equipment of cars. M: Publishing House Hot Line-Telecom, 2009. 440 p.
5. Yutt V.E., Ruzavin G.E. Electronic control systems for internal combustion engines and methods for their diagnosis. M.: Publishing House Hot Line-Telecom, 2007. 104 p.
Introduction
The development of electronics and microprocessor technology has led to its widespread introduction to cars. In particular, to the creation of electronic systems for automatic control of the engine, transmission, running gear and additional equipment. The use of electronic control systems (ECS) of the engine allows to reduce fuel consumption and toxicity of exhaust gases with a simultaneous increase in engine power, increase acceleration and cold start reliability. Modern ESUs combine the functions of fuel injection control and the operation of the ignition system. To implement program control, the control unit records the dependence of the injection duration (the amount of fuel supplied) on the load and engine speed. The dependence is given in the form of a table developed on the basis of comprehensive tests of an engine of a similar model. Similar tables are used to determine the ignition angle. This engine management system is used all over the world, because the selection of data from ready-made tables is a faster process than performing calculations using a computer. The values obtained from the tables are corrected by the on-board computers of the vehicles depending on the signals from the throttle position sensors, air temperature, air pressure and density. The main difference of this system, used in modern cars, is the absence of a rigid mechanical connection between the throttle valve and the accelerator pedal that controls it. Compared to traditional systems, ESU can reduce fuel consumption on various vehicles by up to 20%.
Low fuel consumption is achieved by different organization of the two main modes of operation of the internal combustion engine: low load mode and high load mode. In this case, the engine in the first mode operates with a heterogeneous mixture, a large excess of air and late fuel injection, due to which the charge is stratified from a mixture of air, fuel and the remaining exhaust gases, as a result of which it operates on a lean mixture. In high load mode, the engine starts to work on a homogeneous mixture, which leads to a decrease in emissions of harmful substances in the exhaust gases. The emission toxicity of ESA diesel engines at start-up can be reduced by various glow plugs. The ESU receives information about intake air temperature, pressure, fuel consumption and crankshaft position. The control unit processes information from the sensors and, using characteristic maps, gives the value of the fuel supply advance angle. In order to take into account the change in the density of the incoming air when its temperature changes, the flow sensor is equipped with a thermistor. But as a result of fluctuations in temperature and air pressure in the intake manifold, despite the above sensors, there is an instantaneous change in air density and, as a result, a decrease or increase in oxygen supply to the combustion chamber.
Purpose, objectives and research method
Studies were carried out at the Tyumen State Oil and Gas University in order to maintain a constant temperature in the intake manifold of the internal combustion engine KAMAZ-740, YaMZ-236 and D4FB (1.6 CRDi) of the Kia Sid, MZR2.3-L3T - Mazda CX7. At the same time, temperature fluctuations of the air mass were taken into account by temperature sensors. Ensuring normal (optimal) air temperature in the intake manifold must be carried out under all possible operating conditions: starting a cold engine, operating at low and high loads, when operating at low ambient temperatures.
In modern high-speed engines, the total value of heat transfer turns out to be insignificant and amounts to about 1% of the total amount of heat released during fuel combustion. An increase in the temperature of air heating in the intake manifold to 67 ˚С leads to a decrease in the intensity of heat transfer in engines, that is, a decrease in ΔТ and an increase in the filling factor. ηv (Fig. 1)
where ΔT is the air temperature difference in the intake manifold (˚K), Tp is the air heating temperature in the intake manifold, Tv is the air temperature in the intake manifold.
Rice. 1. Graph of the effect of air heating temperature on the filling factor (on the example of the KAMAZ-740 engine)
However, air heating above 67 ˚С does not lead to an increase in ηv due to the fact that the air density decreases. The experimental data obtained have shown that the air in naturally aspirated diesel engines during its operation has a temperature range ΔТ=23÷36˚С. Tests have confirmed that for internal combustion engines operating on liquid fuel, the difference in the value of the filling factor ηv, calculated from the conditions that the fresh charge is air or an air-fuel mixture, is insignificant and is less than 0.5%, therefore, for all types of engines, ηv is determined by air.
Changes in temperature, pressure and air humidity affect the power of any engine and fluctuate in the range Ne=10÷15% (Ne is the effective engine power).
The increase in aerodynamic air resistance in the intake manifold is explained by the following parameters:
Increased air density.
Change in air viscosity.
The nature of the air entering the combustion chamber.
Numerous studies have shown that high air temperature in the intake manifold increases fuel consumption slightly. At the same time, low temperature increases its consumption by up to 15-20%, so the studies were carried out at an outside air temperature of -40 ˚С and its heating to +70 ˚С in the intake manifold. The optimum fuel consumption is the air temperature in the intake manifold 15÷67 ˚С.
Research results and analysis
During the tests, the power of the heating element was determined to ensure that a certain temperature is maintained in the intake manifold of the internal combustion engine. At the first stage, the amount of heat required to heat the air with a mass of 1 kg at a constant temperature and air pressure is determined, for this we will take: 1. Ambient air temperature t1=-40˚C. 2. Temperature in the intake manifold t2=+70˚С.
The amount of heat required is found by the equation:
(2)
where СР is the mass heat capacity of air at constant pressure, determined according to the table and for air at a temperature from 0 to 200 ˚С.
The amount of heat for a larger mass of air is determined by the formula:
where n is the volume of air in kg required for heating when the engine is running.
When the internal combustion engine operates at a speed of more than 5000 rpm, the air consumption of passenger cars reaches 55-60 kg/h, and trucks - 100 kg/h. Then:
The heater power is determined by the formula:
where Q is the amount of heat spent on heating the air in J, N is the power of the heating element in W, τ is the time in sec.
It is necessary to determine the power of the heating element per second, so the formula will take the form:
N=1.7 kW - the power of the heating element for passenger cars and at an air flow rate of more than 100 kg / h for trucks - N=3.1 kW.
(5)
where Ttr is the temperature in the inlet pipeline, Ptr is the pressure in Pa in the inlet pipeline, T0 - , ρ0 is the air density, Rv is the universal gas constant of air.
Substituting formula (5) into formula (2), we obtain:
(6)
(7)
The heater power per second is determined by formula (4) taking into account formula (5):
(8)
The results of calculating the amount of heat required to heat the air weighing 1 kg with an average air consumption for cars more than V=55kg/h and for trucks - more than V=100kg/h are presented in Table 1.
Table 1
Table for determining the amount of heat for heating the air in the intake manifold depending on the outside air temperature
|
V>55kg/hour |
V>100kg/hour |
|||
|
Q, kJ/s |
Q, kJ/s |
|||
Based on the data in Table 1, a graph (Fig. 2) was constructed for the amount of heat Q per second spent on heating the air to the optimum temperature. The graph shows that the higher the air temperature, the less heat is needed to maintain the optimum temperature in the intake manifold, regardless of the volume of air.
Rice. 2. The amount of heat Q per second spent on heating the air to the optimum temperature
table 2
Calculation of the heating time for different volumes of air
|
Q1, kJ/s |
Q2, kJ/s |
|||
The time is determined by the formula τsec=Q/N at outdoor temperature >-40˚С, Q1 at air flow rate V>55 kg/h and Q2- V>100 kg/h
Further, according to Table 2, a graph of the time for heating the air to +70 ˚С in the ICE manifold is plotted at different heater power. The graph shows that regardless of the heating time, when the heater power is increased, the heating time for different volumes of air is equalized.
Rice. 3. Time of air heating up to +70 ˚С.
Conclusion
Based on calculations and experiments, it has been established that the most economical is the use of variable power heaters to maintain a given temperature in the intake manifold in order to obtain fuel savings of up to 25-30%.
Reviewers:
Reznik L.G., Doctor of Technical Sciences, Professor of the Department "Operation of Road Transport" FGBO UVPO "Tyumen State Oil and Gas University", Tyumen.
Merdanov Sh.M., Doctor of Technical Sciences, Professor, Head of the Department "Transport and Technological Systems" FGBO UVPO "Tyumen State Oil and Gas University", Tyumen.
Zakharov N.S., Doctor of Technical Sciences, professor, current member of the Russian Academy of Transport, head of the department "Service of cars and technological machines" FGBO UVPO "Tyumen State Oil and Gas University", Tyumen.
Bibliographic link
Karnaukhov V.N. OPTIMIZATION OF THE POWER OF THE HEATING ELEMENT TO MAINTAIN THE OPTIMUM AIR TEMPERATURE IN THE ICE INTAKE MANIFOLD // Modern Problems of Science and Education. - 2014. - No. 3.;URL: http://science-education.ru/ru/article/view?id=13575 (date of access: 01.02.2020). We bring to your attention the journals published by the publishing house "Academy of Natural History"
The main 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 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. 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 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 values of atmospheric air density 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).
| 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/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 under 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 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 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 their equilibrium state (according to the MKT).
| 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! 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 of moist 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 the air leads to an increase in specific heat.
| 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 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 the indicated properties of air depend significantly on temperature and the temperature dependence of the considered properties of this gas is different.
The flue gas temperature behind the boiler unit depends on the type of fuel burned, the feed water temperature t n v, the estimated cost of the fuel С t , its reduced humidity
where 
On the basis of technical and economic optimization, according to the condition of the efficiency of using fuel and metal of the tail heating surface, as well as other conditions, the following recommendations were obtained for choosing the value 
given in Table 2.4.
From Table. 2.4, smaller values of the optimum flue gas temperature are selected for cheap fuels, and larger values for expensive fuels.
For low pressure boilers (R ne .≤ 3.0 MPa) with tail heating surfaces, the temperature of the flue gases must not be lower than the values \u200b\u200bspecified in Table. 2.5, and its optimal value is selected on the basis of technical and economic calculations.
Table 2.4 - Optimum flue gas temperature for boilers
with a capacity of over 50 t/h (14 kg/s) when burned
low sulfur fuels
|
Feed water temperature t n in, 0 С |
Reduced fuel moisture |
||
|
|
|
|
|
Table 2.5 - Flue gas temperature for low pressure boilers
capacity less than 50 t/h (14 kg/s)
|
|
|
|
Moisture-adjusted coals and natural gas | |
|
coals with | |
|
High sulfur fuel oil | |
|
Peat and wood waste |
, 0 С
For boilers of the KE and DE types, the flue gas temperature strongly depends on t n c. At the temperature of the feed water t n in =100°C, 
, and at t n in = 80 ÷ 90 0 С it decreases to the values 
.
When burning sulfurous fuels, especially high-sulfur fuel oil, there is a danger of low-temperature corrosion of the air heater at a minimum temperature of the metal wall t st below the dew point t p of flue gases. The value of t p depends on the condensation temperature of water vapor t k at their partial pressure in flue gases P H 2 O, the reduced content of sulfur S n and ash An n in the working fuel
,
(2.3)
where 
- net calorific value of fuel, mJ/kg or mJ/m 3 .
The partial pressure of water vapor is
(2.4)
where: Р=0.1 MPa – flue gas pressure at the boiler outlet, MPa;
r H 2 O is the volume fraction of water vapor in the exhaust gases.
To completely eliminate corrosion in the absence of special protective measures, t st should be 5 - 10 ° C higher
tp ,
however, this will lead to a significant increase 
over its economic importance. Therefore, at the same time increase 
and air temperature at the inlet to the air heater 
.
Minimum wall temperature, depending on pre-selected values 
and 
determined by the formulas: for regenerative air heaters (RAH)
(2.5)
for tubular air heaters (TVP)
(2.6)
When burning solid sulphurous fuels, the air temperature at the inlet to the air heater must be 
take not lower than k, determined depending on P H 2 O.
When using high-sulphur fuel oils, an effective means of combating low-temperature corrosion is the combustion of fuel oil with small excesses of air ( 
= 1.02 ÷ 1.03). This combustion method practically eliminates completely low-temperature corrosion and is recognized as the most promising, however, it requires careful adjustment of burners and improved operation of the boiler unit.
When installing replaceable TVP cubes or replaceable cold (RVP) packing in the cold stages of the air heater, the following values of the incoming air temperature are allowed: 
in regenerative air heaters 60 - 70°С, and in tubular air heaters 80 - 90°С.
To carry out pre-heating of air up to values 
, before entering the air heater, steam heaters are usually installed, heated by selected steam from the turbine. Other methods of air heating at the inlet to the air heater and measures to combat low-temperature corrosion are also used, namely: recirculation of hot air to the fan suction, installation of air heaters with an intermediate heat carrier, gas evaporators, etc. Various types of additives are used to neutralize H 2 SO 4 vapors, both in the gas ducts of the boiler unit and in the fuel.
The air heating temperature depends on the type of fuel and the characteristics of the furnace. If high air heating is not required due to the conditions of drying or fuel combustion, it is advisable to install a single-stage air heater. In this case, the optimal air temperature of power boilers, depending on the temperature of the feed water and flue gases, is approximately determined by the formula
With a two-stage layout of the air heater, according to the formula (2.7), the air temperature behind the first stage is determined, and in the second stage of the air heater, the air is heated from this temperature to the hot air temperature adopted according to Table. 2.6.
Typically, a two-stage layout of the air heater in a "cut" with water economizer stages is used at a value of t hw > 300°C. In this case, the temperature of the gases in front of the "hot" stage of the air heater should not exceed 500°C.
Table 2.6 - Air heating temperature for boiler units
capacity over 75 t/h (21,2 kg/s)
|
Characteristics of the firebox |
Fuel grade |
"Air temperature. °С |
|
1 Furnaces with solid slag removal with a closed circuit of dust preparation |
Stone and lean coals Brown coal cutters. | |
|
2 Furnaces with liquid slag removal, incl. with horizontal cyclones and vertical pre-furnaces when drying fuel with air and supplying dust with hot air or a drying agent |
ASh, PA brown coals Hard coals and Donetsk skinny | |
|
3 When drying fuel with gases in a closed circuit of dust preparation, with solid slag removal the same with liquid slag removal |
brown coals |
300 - 350 x x 350 - 400 x x |
|
4 When drying fuel with gases in an open circuit of dust preparation with solid slag removal With liquid slag removal |
For all |
350 - 400 x x |
|
5. Chamber furnaces |
Fuel oil and natural gas |
250 – 300 x x x |
x With high-moisture peat/W p > 50%/ take 400°C;
хх Higher value at high fuel humidity;
xxx The value of t gw is checked by the formula .
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 pattern 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 amount of 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.
The highest air temperatures are recorded in areas of tropical deserts. In the central regions of the Sahara, for almost 4 months, t about air in the shade is more than 40 ° 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 ° 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
- 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. Polyakov. - Voronezh: VGPU, 2007. - 183 p.