Thermophysical characteristics and properties of gases. Heat capacity of complete combustion products in a stoichiometric volume of air Flue gas properties versus temperature
Heat of combustion. The net calorific value of dry gaseous fuel Qf varies widely from 4 to 47 MJ / m3 and depends on its composition - the ratio and quality of combustible and non-combustible
components. Lowest value Qf for blast-furnace gas, the average composition of which is about 30% combustible gases (mainly carbon monoxide CO) and about 60% non-combustible nitrogen N2. Greatest
The Qf value for associated gases, the composition of which is characterized by a high content of heavy hydrocarbons. The heat of combustion of natural gases fluctuates in a narrow range Qf = 35.5…37.5 MJ/m3.
The lower calorific value of individual gases that make up gaseous fuels is given in Table. 3.2. See Section 3 for methods for determining the calorific value of gaseous fuels.
Density. There are absolute and relative density of gases.
The absolute gas density rg, kg/m3, is the mass of gas per 1 m3 of the volume occupied by this gas. When calculating the density of an individual gas, the volume of its kilomo-la is taken equal to 22.41 m3 (as for an ideal gas).
Relative gas density Rotn is the ratio of absolute gas density under normal conditions and similar air density:
Rotn \u003d Rg / Pv \u003d Rg / 1.293, (6.1)
Where rg, pE are, respectively, the absolute density of gas and air under normal conditions, kg / m3. The relative density of gases is usually used to compare different gases with each other.
The values of the absolute and relative density of simple gases are given in Table. 6.1.
The density of the gas mixture pjM, kg/m3, is determined on the basis of the additivity rule, according to which the properties of gases are summarized, respectively, their volume fraction in the mix:
Where Xj is the volumetric content of the 7th gas in the fuel, %; (rg); - density of the j-th gas, which is part of the fuel, kg/m3; n is the number of individual gases in the fuel.
The values of the density of gaseous fuels are given in table. P.5.
Gas density p, kg/m3, depending on temperature and pressure, can be calculated by the formula
Where p0 is the gas density under normal conditions (T0 = 273 K and p0 = 101.3 kPa), kg/m3; p and T are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K.
Almost all types of gaseous fuels are lighter than air, therefore, when leaking, gas accumulates under the ceilings. For safety reasons, before starting the boiler, it is imperative to check the absence of gas in the most likely places of its accumulation.
The viscosity of gases increases with increasing temperature. The values of the dynamic viscosity coefficient p, Pa-s, can be calculated using the empirical Seser-Land equation
Table 6.1
Characteristics of the components of gas fuel (at t - O ° C chr \u003d 101.3 kPa)
|
Chemical |
Molar mass M, |
Density |
Bulk concentrates |
||
|
Name of gas |
Absolute |
Relative |
Zionic limits of ignition of gas in a mixture with air,% |
||
|
combustible gases |
|||||
|
Propylene |
|||||
|
carbon monoxide |
|||||
|
hydrogen sulfide |
|||||
|
non-flammable gases |
|||||
|
Carbon dioxide |
|||||
|
sulphur dioxide |
|||||
|
Oxygen |
|||||
|
Atmospheric air. |
|||||
|
water vapor |
Where p0 is the coefficient of dynamic viscosity of the gas under normal conditions (G0 = 273 K and p0 - 101.3 kPa), Pa-s; T is the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.
For a mixture of gases, the coefficient of dynamic viscosity can be approximately determined from the values of the viscosity of the individual components:
Where gj is the mass fraction of the j-th gas in the fuel,%; Zu - coefficient of dynamic viscosity of the j-th component, Pa-s; n is the number of individual gases in the fuel.
In practice, the coefficient of kinematic viscosity V, m2/s, is widely used, which
which is related to the dynamic viscosity p through the density p by the dependence
V = r / r. (6.6)
Taking into account (6.4) and (6.6), the coefficient of kinematic viscosity v, m2/s, depending on pressure and temperature, can be calculated by the formula
Where v0 is the coefficient of kinematic viscosity of the gas under normal conditions (Go = 273 K and p0 = 101.3 kPa), m2/s; p and G are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.
The values of the coefficients of kinematic viscosity for gaseous fuels are given in Table. P.9.
Table 6.2
Viscosity and thermal conductivity coefficients of gas fuel components
(at t \u003d 0 ° С ir \u003d 101.3 kPa)
|
Name of gas |
Viscosity factor |
Thermal conductivity coefficient N03, W/(m-K) |
Sutherland coefficient C, K |
|
|
Dynamic r-106, Pa-s |
Kinematic v-106, m2/s |
|||
|
combustible gases |
||||
|
Propylene |
||||
|
carbon monoxide |
||||
|
hydrogen sulfide |
||||
|
non-flammable gases Carbon dioxide |
||||
|
Oxygen |
||||
|
Atmospheric air |
||||
|
Water vapor at 100 °C |
Thermal conductivity. Molecular energy transfer in gases is characterized by the coefficient of thermal conductivity ‘k, W / (m-K). The thermal conductivity coefficient is inversely proportional to pressure and increases with increasing temperature. The values of the X coefficient can be calculated using the Sutherland formula
Where X,0 is the thermal conductivity of the gas under normal conditions (G0 = 273 K and Po = 101.3 kPa), W / (m-K); p and T are, respectively, the actual pressure, kPa, and the absolute temperature of the gas, K; C - coefficient depending on the type of gas, K, is taken from Table. 6.2.
The values of thermal conductivity coefficients for gaseous fuels are given in Table. P.9.
The heat capacity of gaseous fuel per 1 m3 of dry gas depends on its composition and is generally defined as
4L=0.01(CH2H2+Ccos0 +
CCH4CH4 + cCo2cOg + - + cx. X;), (6.9) - heat capacities of the constituent components of the fuel, respectively, hydrogen, carbon monoxide, methane, carbon dioxide and /-th component, kJ / (m3-K); H2, CO, CH4, CO2, ..., Xg--
The heat capacities of the combustible components of gaseous fuel are given in Table. P.6, non-combustible - in table. P.7.
Heat capacity of wet gaseous fuel
Cgtl, kJ/(m3-K), is defined as
<тл = ctrn + 0,00124cHzq йтля, (6.10) где drTn- влагосодержание газообразного топлива,
Explosiveness. A mixture of combustible gas with air in certain proportions in the presence of fire or even a spark can explode, i.e., it ignites and burns at a speed close to the speed of sound propagation. Explosive concentrations of combustible gas in air depend on the chemical composition and properties of the gas. Volume concentration ignition limits for individual combustible gases in a mixture with air are given earlier in Table. 6.1. Hydrogen (4.. .74% by volume) and carbon monoxide (12.5...74%) have the widest ignition limits. For natural gas, the average lower and upper flammability limits are 4.5 and 17% by volume, respectively; for coke - 5.6 and 31%; for domain - 35 and 74%.
Toxicity. Toxicity is understood as the ability of a gas to cause poisoning of living organisms. The degree of toxicity depends on the type of gas and its concentration. The most dangerous gas components in this respect are carbon monoxide CO and hydrogen sulfide H2S.
The toxicity of gas mixtures is mainly determined by the concentration of the most toxic of the components present in the mixture, while its harmful effect, as a rule, is markedly enhanced in the presence of other harmful gases.
The presence and concentration of harmful gases in the air can be determined by a special device - a gas analyzer.
Almost all natural gases are odorless. To detect a gas leak and take safety measures, natural gas is odorized before it enters the main, that is, it is saturated with a substance that has a pungent odor (for example, mercaptans).
Heat of combustion various kinds fuel varies widely. For fuel oil, for example, it is over 40 MJ/kg, and for blast-furnace gas and some grades of oil shale, it is about 4 MJ/kg. The composition of energy fuels also varies widely. Thus, the same qualitative characteristics, depending on the type and brand of fuel, can sharply differ quantitatively from each other.
The given characteristics of the fuel. For comparative analysis, in the role of characteristics summarizing the quality of the fuel, the given characteristics of the fuel are used, %-kg / MJ, which are generally calculated by the formula
Where хг is an indicator of the quality of working fuel, %; Q[ - specific heat of combustion (lowest), MJ/kg.
So, for example, to calculate the reduced
Humidity ash content of sulfur S „p and
Nitrogen N^p (for fuel operating condition)
Formula (7.1) takes the following form, %-kg/MJ:
TOC o "1-3" h z KP=Kl GT; (7.2)
4f=l7e[; (7.3)
snp=S’/Єї; (7.4)
^p=N7 Q[. (7.5)
As an illustrative example, the following comparison is indicative, provided that different fuels are burned in boilers of the same thermal power. So, a comparison of the reduced moisture content of coal near Moscow
Grades 2B (WЈp = 3.72% -kg / MJ) and Nazarov-
Coal 2B (W^p = 3.04%-kg/MJ) shows that in the first case, the amount of moisture introduced into the furnace of the boiler with fuel will be approximately 1.2 times greater than in the second, despite the fact that that the working humidity of coal near Moscow (W[ \u003d 31%) is less than that of
Nazarovsky coal (Wf = 39%).
conditional fuel. In the energy sector, in order to compare the efficiency of fuel use in various boiler plants, to plan the production and consumption of fuel in economic calculations, the concept of conventional fuel has been introduced. As standard fuel, such fuel is accepted, the specific calorific value (lowest) of which in working condition is equal to Qy T = 29300 kJ/kg (or
7000 kcal/kg).
For each natural fuel, there is a so-called dimensionless thermal equivalent E, which can be greater or less than unity:
When fuel carbon is burned in air according to the equation (21C + 2102 + 79N2 = 21C02 + 79N2), for each volume of CO2 in the combustion products there are 79: 21 = 3.76 volumes of N2.
Combustion of anthracite, lean coal and other fuels with a high carbon content produces combustion products that are similar in composition to carbon combustion products. When hydrogen is burned according to the equation
42H2+2102+79N2=42H20+79N2
For each volume of H20, there are 79:42 = 1.88 volumes of nitrogen.
In the combustion products of natural, liquefied and coke oven gases, liquid fuels, firewood, peat, brown coal, long-flame and gas coal and other fuels with a significant hydrogen content in the combustible mass, a large amount of water vapor is formed, sometimes exceeding the volume of CO2. The presence of moisture in the top
|
Table 36 Heat capacity, kcal/(m3. °C) |
Live, naturally, increases the content of water vapor in the combustion products.
Composition of products complete combustion the main types of fuel in the stoichiometric volume of air are given in Table. 34. From the data in this table it can be seen that the N2 content in the combustion products of all types of fuel significantly exceeds the total content of C02-f-H20, and in the carbon combustion products it is 79%.
Hydrogen combustion products contain 65% N2; combustion products of natural and liquefied gases, gasoline, fuel oil and other hydrocarbon fuels contain 70-74% N2.
Rice. 5. Volumetric heat capacity
Combustion products
4 - carbon combustion products
5 - hydrogen combustion products
The average heat capacity of the products of complete combustion that do not contain oxygen can be calculated by the formula
C \u003d 0.01 (Cc02C02 + Cso2S02 + Cn20H20 + CN2N2) kcal / (m3 - ° С), (VI. 1)
Where Сс0г, Cso2, СНа0, CNa are the volumetric heat capacities of carbon dioxide, sulfur dioxide, water vapor and nitrogen, and С02, S02, Н20 and N2 are the content of the corresponding components in the combustion products, % (vol.).
In accordance with this formula (VI. 1) takes the following form:
C \u003d 0.01. (Cc02 /? 02 + CHj0H20-bCNi! N2) kcal / (m3 "°C). (VI.2)
The average volumetric heat capacity of CO2, H20 and N2 in the temperature range from 0 to 2500 °C is given in Table. 36. Curves characterizing the change in the average volumetric heat capacity of these gases with increasing temperature are shown in fig. 5.
From the table. 16 data and curves depicted in fig. 5 shows the following:
1. The volumetric heat capacity of CO2 significantly exceeds the heat capacity of H20, which, in turn, exceeds the heat capacity of N2 over the entire temperature range from 0 to 2000 °C.
2. The heat capacity of CO2 increases with increasing temperature faster than the heat capacity of H20, and the heat capacity of H20 faster than the heat capacity of N2. However, despite this, the weighted average volumetric heat capacities of carbon and hydrogen combustion products in a stoichiometric volume of air differ little.
This situation, somewhat unexpected at first glance, is due to the fact that in the products of complete combustion of carbon in air, for each cubic meter of CO2, which has the highest volumetric heat capacity, there are 3.76 m3 of N2 with a minimum volumetric
|
Average volumetric heat capacities of carbon and hydrogen combustion products in the theoretically required amount of air, kcal/(m3-°C)
|
Heat capacity, and in the products of hydrogen combustion for every cubic meter of water vapor, the volumetric heat capacity of which is less than that of CO2, but more than that of N2, there is half the amount of nitrogen (1.88 m3).
As a result, the average volumetric heat capacities of the combustion products of carbon and hydrogen in air are equalized, as can be seen from the data in Table. 37 and comparison of curves 4 and 5 in Figs. 5. The difference in the weighted average heat capacities of the combustion products of carbon and hydrogen in air does not exceed 2%. Naturally, the heat capacities of the combustion products of fuel, which consists mainly of carbon and hydrogen, in a stoichiometric volume of air lie in a narrow region between curves 4 and 5 (shaded in Fig. 5).
Products of complete combustion of various vidoges; fuels in stoichiometric air in the temperature range from 0 to 2100 °C have the following heat capacity, kcal/(m3>°C):
Fluctuations in the heat capacity of the products of combustion of various types of fuel are relatively small. At solid fuel with a high moisture content (firewood, peat, brown coal, etc.) the heat capacity of combustion products in the same temperature range is higher than that of fuel with a low moisture content (anthracite, coal, fuel oil, natural gas, etc.) . This is due to the fact that during the combustion of fuel with a high moisture content in the combustion products, the content of water vapor increases, which has a higher heat capacity compared to diatomic gas - nitrogen.
In table. 38 shows the average volumetric heat capacities of the products of complete combustion, not diluted with air, for various temperature ranges.
Table 38
|
The value of the average heat capacities of the combustion products of fuel and air not diluted with air in the temperature range from 0 to t ° С
|
An increase in the moisture content in the fuel increases the heat capacity of the combustion products due to an increase in the content of water vapor in them in the same temperature range, compared with the heat capacity of the combustion products of fuel with a lower moisture content, and at the same time lowers the combustion temperature of the fuel due to an increase in the volume of combustion products due to water pair.
With an increase in the moisture content in the fuel, the volumetric heat capacity of the combustion products increases in a given temperature range and, at the same time, the temperature interval decreases from 0 to £max due to a decrease in the value<тах. ПОСКОЛЬКУ ТЄПЛОЄМКОСТЬ ГЭЗОВ уМвНЬ — шается с понижением температуры, теплоемкость продуктов сгорания топлива с различной влажностью в интервале температур от нуля до <тах для данного топлива претерпевает незначительные колебания (табл. 39). В соответствии с этим можно принять теплоемкость продуктов сгорания всех видов твердого топлива от 0 до tmax равной 0,405, жидкого топлива 0,401, природного, доменного и генераторного газов 0,400 ккал/(м3-°С).
This makes it possible to considerably simplify the determination of the calorimetric and calculated combustion temperatures (according to the method described in Chapter VII). The error allowed in this case usually does not exceed 1%, or 20 °.
From the consideration of curves 4 and 5 in Figs. 5 it can be seen that the ratio of the heat capacity of the products of complete combustion of carbon in a stoichiometric volume of air in the temperature range from 0 to t ° C, for example, from 0 to
|
Heat capacity of combustion products from 0 to t’mayL of various types of solid fuels with a moisture content of 0 to 40%, in a stoichiometric volume of air
|
200 and from 0 to 2100 °C are practically equal to the ratio of the heat capacities of hydrogen combustion products in the same temperature ranges. The specified ratio of heat capacities C' remains practically constant for the products of complete combustion of various types of fuel in a stoichiometric volume of air.
In table. 40 shows the ratios of the heat capacities of the products of complete combustion of fuel with a low content of ballast, which passes into gaseous combustion products (anthracite, coke, coal, liquid fuel, natural, petroleum, coke oven gases, etc.) in the temperature range from 0 to t ° С and in the temperature range from 0 to 2100 °C. Since the heat capacity of these types of fuel is close to 2100 ° C, the indicated ratio of heat capacities C' is equal to the ratio of heat capacities in the temperature range from 0 to t and from 0 to tm&x-
In table. 40 also shows the values of C', calculated for the products of combustion of fuel with a high content of ballast, which passes during the combustion of fuel into gaseous combustion products, i.e. moisture in solid fuel, nitrogen and carbon dioxide in gaseous fuel. The heat capacity of these types of fuel (wood, peat, brown coal, mixed generator, air and blast-furnace gases) is 1600-1700 °C.
Table 40
|
The ratio of the heat capacities of combustion products C 'and air K in the temperature range from 0 to t ° C to the heat capacity of combustion products from 0 to
|
As can be seen from Table. 40, the values of C' and K differ little even for fuel combustion products with different ballast content and heat output.
Humid air is a mixture of dry air and water vapor. In unsaturated air, moisture is in the state of superheated vapor, and therefore the properties of moist air can be approximately described by the laws of ideal gases.
The main characteristics of humid air are:
1. Absolute humidity g, which determines the amount of water vapor contained in 1 m 3 of moist air. Water vapor occupies the entire volume of the mixture, so the absolute humidity of the air is equal to the mass of 1 m 3 of water vapor or vapor density, kg / m 3
2. Relative humidity j is expressed by the ratio of the absolute humidity of the air to its maximum possible humidity at the same pressure and temperature, or by the ratio of the mass of water vapor contained in 1 m 3 of moist air to the mass of water vapor required to completely saturate 1 m 3 of moist air at the same pressure and temperature.
Relative humidity determines the degree of saturation of air with moisture:
, (1.2)
where is the partial pressure of water vapor corresponding to its density Pa; - pressure of saturated steam at the same temperature, Pa; - the maximum possible amount of steam in 1 m 3 of saturated moist air, kg / m 3; - vapor density at its partial pressure and temperature of moist air, kg/m 3 .
Relation (1.2) is valid only when it can be assumed that the liquid vapor is an ideal gas up to the saturation state.
The density of moist air r is the sum of the densities of water vapor and dry air at partial pressures of 1 m 3 of moist air at a temperature of humid air T, TO:
(1.3)
where is the density of dry air at its partial pressure of 1 m 3 of moist air, kg / m 3; - partial pressure of dry air, Pa; - gas constant of dry air, J/(kg×K).
Expressing and by the equation of state for air and water vapor, we obtain
, (1.5)
where is the mass flow rate of air and water vapor, kg/s.
These equalities are valid for the same volume V humid air at the same temperature. Dividing the second equality by the first, we get another expression for the moisture content
. (1.6)
Substituting here the values of gas constants for air J/(kg×K) and for water vapor J/(kg×K), we obtain the value of moisture content expressed in kilograms of water vapor per 1 kg of dry air
. (1.7)
Replacing the partial air pressure with the value , where from the previous and AT is the barometric air pressure in the same units as R, we get for moist air under barometric pressure
. (1.8)
Thus, at a given barometric pressure, the moisture content of air depends only on the partial pressure of water vapor. The maximum possible moisture content in the air, from where
. (1.9)
Since the saturation pressure increases with temperature, the maximum possible amount of moisture that can be contained in the air depends on its temperature, and the more, the higher the temperature. If equations (1.7) and (1.8) are solved for and , then we obtain
(1.10)
. (1.11)
The volume of moist air in cubic meters per 1 kg of dry air is calculated by the formula
(1.12)
Specific volume of moist air v, m 3 / kg, is determined by dividing the volume of moist air by the mass of the mixture per 1 kg of dry air:
Humid air as a heat carrier is characterized by an enthalpy (in kilojoules per 1 kg of dry air) equal to the sum of the enthalpies of dry air and water vapor
(1.14)
where is the specific heat capacity of dry air, kJ/(kg×K); t– air temperature, °C; i- enthalpy of superheated steam, kJ/kg.
The enthalpy of 1 kg of dry saturated water vapor at low pressures is determined by the empirical formula, kJ/kg:
where is a constant coefficient approximately equal to the enthalpy of steam at a temperature of 0 °C; = 1.97 kJ/(kg×K) – specific heat capacity of steam.
Substituting the values i into expression (1.14) and taking the specific heat capacity of dry air constant and equal to 1.0036 kJ / (kg × K), we find the enthalpy of moist air in kilojoules per 1 kg of dry air:
Equations similar to those discussed above are used to determine the wet gas parameters.
, (1.17)
where is the gas constant for the test gas; R- gas pressure.
Gas enthalpy, kJ/kg,
where is the specific heat capacity of the gas, kJ/(kg×K).
Absolute moisture content of gas:
. (1.19)
When calculating contact heat exchangers for air-water heat carriers, you can use the data in Table. 1.1-1.2 or calculated dependencies for determining the physicochemical parameters of air (1.24-1.34) and water (1.35). For flue gases, the data in Table 1 can be used. 1.3.
Wet gas density, kg / m 3:
, (1.20)
where is the density of dry gas at 0 ° C, kg / m 3; M g, M p are the molecular masses of gas and vapor.
Wet gas dynamic viscosity coefficient, Pa×s:
, (1.21)
where is the coefficient of dynamic viscosity of water vapor, Pa×s; - coefficient of dynamic viscosity of dry gas, Pa×s; 
- mass concentration of steam, kg/kg.
Specific heat capacity of wet gas, kJ/(kg×K):
Wet gas thermal conductivity coefficient, W/(m×K):
, (1.23)
where k is the adiabatic index; AT– coefficient (for monatomic gases AT= 2.5; for diatomic gases AT= 1.9; for triatomic gases AT = 1,72).
Table 1.1. Physical properties of dry air ( R= 0.101 MPa)
| t, °C | , kg / m 3 | , kJ/(kg×K) | , W/(m×K) | , Pa×s | , m 2 /s | Pr |
| -20 | 1,395 | 1,009 | 2,28 | 16,2 | 12,79 | 0,716 |
| -10 | 1,342 | 1,009 | 2,36 | 16,7 | 12,43 | 0,712 |
| 1,293 | 1,005 | 2,44 | 17,2 | 13,28 | 0,707 | |
| 1,247 | 1,005 | 2,51 | 17,6 | 14,16 | 0,705 | |
| 1,205 | 1,005 | 2,59 | 18,1 | 15,06 | 0,703 | |
| 1,165 | 1,005 | 2,67 | 18,6 | 16,00 | 0,701 | |
| 1,128 | 1,005 | 2,76 | 19,1 | 16,96 | 0,699 | |
| 1,093 | 1,005 | 2,83 | 19,6 | 17,95 | 0,698 | |
| 1,060 | 1,005 | 2,90 | 20,1 | 18,97 | 0,696 | |
| 1,029 | 1,009 | 2,96 | 20,6 | 20,02 | 0,694 | |
| 1,000 | 1,009 | 3,05 | 21,1 | 21,09 | 0,692 | |
| 0,972 | 1,009 | 3,13 | 21,5 | 22,10 | 0,690 | |
| 0,946 | 1,009 | 3,21 | 21,9 | 23,13 | 0,688 | |
| 0,898 | 1,009 | 3,34 | 22,8 | 25,45 | 0,686 | |
| 0,854 | 1,013 | 3,49 | 23,7 | 27,80 | 0,684 | |
| 0,815 | 1,017 | 3,64 | 24,5 | 30,09 | 0,682 | |
| 0,779 | 1,022 | 3,78 | 25,3 | 32,49 | 0,681 | |
| 0,746 | 1,026 | 3,93 | 26,0 | 34,85 | 0,680 | |
| 0,674 | 1,038 | 4,27 | 27,4 | 40,61 | 0,677 | |
| 0,615 | 1,047 | 4,60 | 29,7 | 48,33 | 0,674 | |
| 0,566 | 1,059 | 4,91 | 31,4 | 55,46 | 0,676 | |
| 0,524 | 1,068 | 5,21 | 33,6 | 63,09 | 0,678 | |
| 0,456 | 1,093 | 5,74 | 36,2 | 79,38 | 0,687 | |
| 0,404 | 1,114 | 6,22 | 39,1 | 96,89 | 0,699 | |
| 0,362 | 1,135 | 6,71 | 41,8 | 115,4 | 0,706 | |
| 0,329 | 1,156 | 7,18 | 44,3 | 134,8 | 0,713 | |
| 0,301 | 1,172 | 7,63 | 46,7 | 155,1 | 0,717 | |
| 0,277 | 1,185 | 8,07 | 49,0 | 177,1 | 0,719 | |
| 0,257 | 1,197 | 8,50 | 51,2 | 199,3 | 0,722 | |
| 0,239 | 1,210 | 9,15 | 53,5 | 233,7 | 0,724 |
The thermophysical properties of dry air can be approximated by the following equations.
Kinematic viscosity of dry air at temperatures from -20 to +140 ° C, m 2 / s:
Pa; (1.24)
and from 140 to 400 °С, m2/s:
. (1.25)
Table 1.2. Physical properties of water in a state of saturation
| t, °C | , kg / m 3 | , kJ/(kg×K) | , W/(m×K) | , m 2 /s | , N/m | Pr | |
| 999,9 | 4,212 | 55,1 | 1,789 | -0,63 | 756,4 | 13,67 | |
| 999,7 | 4,191 | 57,4 | 1,306 | 0,7 | 741,6 | 9,52 | |
| 998,2 | 4,183 | 59,9 | 1,006 | 1,82 | 726,9 | 7,02 | |
| 995,7 | 4,174 | 61,8 | 0,805 | 3,21 | 712,2 | 5,42 | |
| 992,2 | 4,174 | 63,5 | 0,659 | 3,87 | 696,5 | 4,31 | |
| 988,1 | 4,174 | 64,8 | 0,556 | 4,49 | 676,9 | 3,54 | |
| 983,2 | 4,179 | 65,9 | 0,478 | 5,11 | 662,2 | 2,98 | |
| 977,8 | 4,187 | 66,8 | 0,415 | 5,70 | 643,5 | 2,55 | |
| 971,8 | 4,195 | 67,4 | 0,365 | 6,32 | 625,9 | 2,21 | |
| 965,3 | 4,208 | 68,0 | 0,326 | 6,95 | 607,2 | 1,95 | |
| 958,4 | 4,220 | 68,3 | 0,295 | 7,52 | 588,6 | 1,75 | |
| 951,0 | 4,233 | 68,5 | 0,272 | 8,08 | 569,0 | 1,60 | |
| 943,1 | 4,250 | 68,6 | 0,252 | 8,64 | 548,4 | 1,47 | |
| 934,8 | 4,266 | 68,6 | 0,233 | 9,19 | 528,8 | 1,36 | |
| 926,1 | 4,287 | 68,5 | 0,217 | 9,72 | 507,2 | 1,26 | |
| 917,0 | 4,313 | 68,4 | 0,203 | 10,3 | 486,6 | 1,17 | |
| 907,4 | 4,346 | 68,3 | 0,191 | 10,7 | 466,0 | 1,10 | |
| 897,3 | 4,380 | 67,9 | 0,181 | 11,3 | 443,4 | 1,05 | |
| 886,9 | 4,417 | 67,4 | 0,173 | 11,9 | 422,8 | 1,00 | |
| 876,0 | 4,459 | 67,0 | 0,165 | 12,6 | 400,2 | 0,96 | |
| 863,0 | 4,505 | 66,3 | 0,158 | 13,3 | 376,7 | 0,93 |
Wet gas density, kg/m3.
2. heat carried away by exhaust gases. Let us determine the heat capacity of flue gases at tux = 8000C;
3. heat loss through masonry by thermal conductivity.
Losses through the vault
The thickness of the vault is 0.3 m, the material is fireclay. We accept that the temperature of the inner surface of the dome is equal to the temperature of the gases.
Average oven temperature:
According to this temperature, we select the coefficient of thermal conductivity of fireclay material:
Thus, the losses through the vault are:
where α is the heat transfer coefficient from the outer surface of the walls to the ambient air, equal to 71.2 kJ / (m2 * h * 0С)
Losses through walls. The masonry of the walls is made of two layers (fireclay 345 mm, diatomaceous earth 115 mm)
Wall area, m2:
methodical zone
welding zone
Tomil zone
end
Total wall area 162.73 m2
With a linear distribution of temperature over the thickness of the wall, the average temperature of fireclay will be 5500C, and diatomite 1500C.
Hence.
Total loss through masonry
4. According to practical data, heat losses with cooling water are taken equal to 10% Qx of income, that is, Qx + Qp
5. We accept unaccounted losses in the amount of 15% Q of heat input
Compose the equation for the heat balance of the furnace
The heat balance of the furnace is summarized in Table 1; 2
Table 1
table 2
| Consumption kJ/h | % |
| Heat spent on heating the metal
| 53 |
| flue gas heat | 26 |
| losses through masonry
| 1,9 |
| cooling water losses | 6,7 |
| unaccounted losses | 10,6 |
| Total: | 100 |
The specific heat consumption for heating 1 kg of metal will be
Selection and calculation of burners
We accept that burners of the "pipe in pipe" type are installed in the furnace.
There are 16 pieces in the welding zones, 4 pieces in the holding zone. total number of burners 20pcs. Determine the estimated amount of air coming to one burner.
Vв - hourly air consumption;
TV - 400 + 273 = 673 K - air heating temperature;
N is the number of burners.
The air pressure in front of the burner is assumed to be 2.0 kPa. It follows that the required air flow is provided by the DBV 225 burner.
Determine the estimated amount of gas per burner;
VG \u003d V \u003d 2667 hourly fuel consumption;
TG \u003d 50 + 273 \u003d 323 K - gas temperature;
N is the number of burners.
8. Calculation of the heat exchanger
For air heating, we design a metal loop heat exchanger from pipes with a diameter of 57/49.5 mm with a corridor arrangement of their pitch
Initial data for calculation:
Hourly fuel consumption B=2667 kJ/h;
Air consumption per 1 m3 of fuel Lα = 13.08 m3/m3;
The amount of combustion products from 1 m3 of combustible gas Vα =13.89 m3/m3;
Air heating temperature tv = 4000С;
The temperature of the flue gases from the furnace tux=8000C.
Hourly air consumption:
Hourly smoke output:
The hourly amount of smoke passing through the heat exchanger, taking into account the loss of smoke for knocking out and through the bypass damper and air leakage.
The coefficient m, taking into account the loss of smoke, we take 0.7.
The coefficient taking into account air leakage in the hogs, we will take 0.1.
Smoke temperature in front of the heat exchanger, taking into account air leakage;
where iух is the heat content of flue gases at tух=8000С
This heat content corresponds to the smoke temperature tD=7500C. (See Fig.67(3))
The thermophysical properties of gaseous combustion products necessary for calculating the dependence of various parameters on the temperature of a given gaseous medium can be established on the basis of the values given in the table. In particular, these dependences for the heat capacity are obtained in the form:
C psm = a -1/ d,
where a = 1,3615803; b = 7,0065648; c = 0,0053034712; d = 20,761095;
C psm = a + bT sm + cT 2 sm,
where a = 0,94426057; b = 0,00035133267; c = -0,0000000539.
The first dependence is preferable in terms of approximation accuracy, the second dependence can be taken to carry out calculations of lower accuracy.
Physical parameters of flue gases
(at P = 0.0981 MPa; R CO2 = 0.13; p H2O = 0.11; R N2 = 0.76)
| t, °С | γ, N m -3 | with p, W (m 2 ° С) -1 | λ 10 2, W (m K) -1 | a 10 6, m 2 s -1 | μ 10 6 , Pa s | v 10 6, m 2 s -1 | Pr |
| 12,704 | 1,04 | 2,28 | 16,89 | 15,78 | 12,20 | 0,72 | |
| 9,320 | 1,07 | 3,13 | 30,83 | 20,39 | 21,54 | 0,69 | |
| 7,338 | 1,10 | 4,01 | 48,89 | 24,50 | 32,80 | 0,67 | |
| 6,053 | 1,12 | 4,84 | 69,89 | 28,23 | 45,81 | 0,65 | |
| 5,150 | 1,15 | 5,70 | 94,28 | 31,69 | 60,38 | 0,64 | |
| 4,483 | 1,18 | 6,56 | 121,14 | 34,85 | 76,30 | 0,63 | |
| 3,973 | 1,21 | 7,42 | 150,89 | 37,87 | 93,61 | 0,62 | |
| 3,561 | 1,24 | 8,27 | 183,81 | 40,69 | 112,10 | 0,61 | |
| 3,237 | 1,26 | 9,15 | 219,69 | 43,38 | 131,80 | 0,60 | |
| 2,953 | 1,29 | 10,01 | 257,97 | 45,91 | 152,50 | 0,59 | |
| 2,698 | 1,31 | 10,90 | 303,36 | 48,36 | 174,30 | 0,58 | |
| 2,521 | 1,32 | 11,75 | 345,47 | 40,90 | 197,10 | 0,57 | |
| 2,354 | 1,34 | 12,62 | 392,42 | 52,99 | 221,00 | 0,56 |
APPENDIX 3
(reference)
Air and smoke permeability of air ducts and valves
1. To determine leaks or air leaks in relation to the ventilation ducts of anti-smoke systems, the following formulas can be used, obtained by approximating tabular data:
for class H air ducts (in the pressure range 0.2 - 1.4 kPa): ΔL = a(R - b)with, where ΔL- suctions (leaks) of air, m 3 / m 2 h; R- pressure, kPa; a = 10,752331; b = 0,0069397038; with = 0,66419906;
for class P air ducts (in the pressure range 0.2 - 5.0 kPa): where a = 0,00913545; b=-3.1647682 10 8 ; c =-1.2724412 10 9 ; d= 0,68424233.
2. For normally closed fire dampers, the numerical values of the specific characteristic of resistance to smoke and gas penetration depending on the gas temperature correspond to the data obtained during bench fire tests of various products at the experimental base of VNIIPO:
| 1. General Provisions. 2 2. Initial data. 3 3. Exhaust smoke ventilation. 4 3.1. Removal of combustion products directly from the burning room. 4 3.2. Removal of combustion products from adjacent premises. 7 4. Supply smoke ventilation. 9 4.1. Air supply to stairwells. 9 4.2. Air supply to lift shafts.. 14 4.3. Air supply to the vestibule locks.. 16 4.4. Compensating air supply. 17 5. Technical characteristics of the equipment. 17 5.1. Equipment for exhaust smoke ventilation systems. 17 5.2. Equipment for supply smoke ventilation systems. 21 6. Fire control modes. 21 References .. 22 Appendix 1. Determination of the main parameters of the fire load of premises. 22 Appendix 2. Thermophysical properties of flue gases. 24 Appendix 3. Air and smoke permeability of air ducts and valves. 25 |