State educational institution higher vocational education
Samara State Technical University»
Department of Chemical Technology and Industrial Ecology
COURSE WORK
in the discipline "Technical thermodynamics and heat engineering"
Topic: Calculation of the unit for utilizing the heat of waste gases of a technological furnace
Completed by: Student Ryabinina E.A.
ZF course III group 19
Checked by: Consultant Churkina A.Yu.
Samara 2010
Introduction
Most chemical plants generate high and low temperature thermal waste, which can be used as secondary energy resources (RER). These include exhaust gases from various boilers and process furnaces, cooled streams, cooling water and waste steam.
Thermal VER to a large extent cover the heat demand of individual industries. For example, in the nitrogen industry more than 26% of the heat demand is satisfied due to WER, in the soda industry - more than 11%.
The number of used RES depends on three factors: the temperature of the RES, their thermal power and the continuity of the output.
At present, the most widespread is the utilization of the heat of waste industrial gases, which have a high temperature potential for almost all fire-technical processes and can be used continuously in most industries. Waste gas heat is the main component of the energy balance. It is used mainly for technological, and in some cases - for energy purposes (in waste heat boilers).
However, the widespread use of high-temperature thermal RES is associated with the development of utilization methods, including the heat of incandescent slags, products, etc., new methods for utilizing waste gas heat, as well as improving the designs of existing utilization equipment.
1. Description technological scheme
In tube furnaces that do not have a convection chamber, or in radiant-convection-type furnaces, but with a relatively high initial temperature of the heated product, the temperature of the exhaust gases can be relatively high, which leads to increased heat losses, a decrease in the furnace efficiency and higher fuel consumption. Therefore, it is necessary to use the heat of the waste gases. This can be achieved either by using an air heater, which heats the air supplied to the furnace for fuel combustion, or by installing waste heat boilers, which make it possible to obtain the water vapor required for technological needs.
However, to carry out air heating, additional costs are required for the construction of an air heater, a blower, as well as additional power consumption consumed by the blower motor.
To ensure normal operation of the air heater, it is important to prevent the possibility of corrosion of its surface from the flow side. flue gas... This phenomenon is possible when the temperature of the heat exchange surface is below the dew point temperature; at the same time, part of the flue gases, directly in contact with the surface of the air heater, is significantly cooled, the water vapor contained in them partially condenses and, absorbing sulfur dioxide from the gases, forms an aggressive weak acid.
The dew point corresponds to the temperature at which the pressure of saturated water vapor is equal to the partial pressure of water vapor contained in the flue gases.
One of the most reliable methods of protection against corrosion is to preheat the air in some way (for example, in water or steam heaters) to a temperature above the dew point. Such corrosion can also occur on the surface of convection pipes if the temperature of the raw material entering the furnace is below the dew point.
The source of heat for increasing the temperature of saturated steam is the oxidation (combustion) reaction of the primary fuel. The flue gases formed during combustion give up their heat in the radiation and then convection chambers to the feed stream (water vapor). Superheated steam enters the consumer, and the combustion products leave the furnace and enter the waste heat boiler. At the outlet of the WHB, saturated water vapor is fed back to the steam superheat furnace, and the flue gases, cooling feed water, enter the air heater. From the air-reheater, the flue gases go to the KTAN, where the water flowing through the coil is heated and goes straight to the consumer, and the flue gases - into the atmosphere.
2. Calculation of the furnace
2.1 Calculation of the combustion process
Determine the net calorific value of fuel Q R n... If the fuel is an individual hydrocarbon, then the calorific value of its Q R n equal to the standard heat of combustion minus the heat of vaporization of water in the combustion products. It can also be calculated from the standard thermal effects of the formation of initial and final products based on the Hess law.
For a fuel consisting of a mixture of hydrocarbons, the heat of combustion is determined, but the additivity rule:
where Q pi n- heat of combustion i-go fuel component;
y i- concentration i-go fuel component in fractions of a unit, then:
Q R n cm = 35.84 ∙ 0.987 + 63.80 ∙ 0.0033+ 91.32 ∙ 0.0012+ 118.73 ∙ 0.0004 + 146.10 ∙ 0.0001 = 35.75 MJ / m 3.
Molar mass of fuel:
M m = Σ M i ∙ y i ,
where M i- molar mass i-go fuel component, hence:
M m = 16.042 ∙ 0.987 + 30.07 ∙ 0.0033 + 44.094 ∙ 0.0012 + 58.120 ∙ 0.0004 + 72.15 ∙ 0.0001 + 44.010 ∙ 0.001 + 28.01 ∙ 0.007 = 16.25 kg / mol.
kg / m 3,
then Q R n cm, expressed in MJ / kg, is equal to:
MJ / kg.
The calculation results are summarized in table. one:
Fuel composition Table 1
Let us determine the elemental composition of the fuel,% (mass.):
,
where n i C , n i H , n i N , n i O- the number of carbon, hydrogen, nitrogen and oxygen atoms in the molecules of individual components that make up the fuel;
Content of each fuel component, wt. %;
x i- the content of each component of the fuel, they say. %;
M i- molar mass of individual fuel components;
M m is the molar mass of the fuel.
Checking the composition :
C + H + O + N = 74.0 + 24.6 + 0.2 + 1.2 = 100% (mass).
Let us determine the theoretical amount of air required to burn 1 kg of fuel, it is determined from the stoichiometric equation of the combustion reaction and the oxygen content in the atmospheric air. If the elemental composition of the fuel is known, the theoretical amount of air L 0, kg / kg, is calculated by the formula:
In practice, to ensure the completeness of fuel combustion, an excess amount of air is introduced into the furnace, we find the actual air flow rate at α = 1.25:
L = αL 0 ,
where L- actual air consumption;
α - coefficient of excess air,
L = 1.25 ∙ 17.0 = 21.25 kg / kg.
Specific volume of air (n.a.) for combustion of 1 kg of fuel:
where ρ in= 1.293 - air density under normal conditions,
m 3 / kg.
Let's find the amount of combustion products formed during the combustion of 1 kg of fuel:
if the elemental composition of the fuel is known, then the mass composition of flue gases per 1 kg of fuel with its complete combustion can be determined on the basis of the following equations:
where m CO2 , m H2O , m N2 , m O2 is the mass of the corresponding gases, kg.
Total amount of combustion products:
m p. from = m CO2 + m H2O + m N2 + m O2,
m p. from= 2.71 + 2.21 + 16.33 + 1.00 = 22.25 kg / kg.
We check the resulting value:
where W f - specific consumption nozzle steam when burning liquid fuel, kg / kg (for gas fuel W f = 0),
Since the fuel is a gas, we neglect the moisture content in the air and neglect the amount of water vapor.
Let us find the volume of combustion products under normal conditions, formed during the combustion of 1 kg of fuel:
where m i- the mass of the corresponding gas formed during the combustion of 1 kg of fuel;
ρ i- the density of this gas under normal conditions, kg / m 3;
M i- molar mass of the given gas, kg / kmol;
22.4 - molar volume, m 3 / kmol,
m 3 / kg; 
m 3 / kg;
m 3 / kg; 
m 3 / kg.
Total volume of combustion products (n.a.) at actual air consumption:
V = V CO2 + V H2O + V N2 + V O2 ,
V = 1.38 + 2.75+ 13.06 + 0.70 = 17.89 m3 / kg.
Density of combustion products (n.a.):
kg / m 3.
Let us find the heat capacity and enthalpy of combustion products of 1 kg of fuel in the temperature range from 100 ° C (373 K) to 1500 ° C (1773 K), using the data in Table. 2.
Average specific heat capacities of gases with p, kJ / (kg ∙ K) table 2
| t, ° С |
|||||
Enthalpy of flue gases formed during combustion of 1 kg of fuel:
where with CO2 , with H2O , with N2 , with O2- average specific heat capacities at constant pressure corresponding to the lawn at temperature t, kJ / (kg K);
with t- the average heat capacity of flue gases formed during the combustion of 1 kg of fuel at a temperature t, kJ / (kg K);
at 100 ° C: kJ / (kg ∙ K);
at 200 ° C: kJ / (kg ∙ K);
at 300 ° C: kJ / (kg ∙ K);
at 400 ° C: kJ / (kg ∙ K);
at 500 ° C: kJ / (kg ∙ K);
at 600 ° C: kJ / (kg ∙ K);
at 700 ° C: kJ / (kg ∙ K);
at 800 ° C: kJ / (kg ∙ K);
at 1000 ° C: kJ / (kg ∙ K);
at 1500 ° C: kJ / (kg ∙ K);
The calculation results are summarized in table. 3.
Enthalpy of combustion products Table 3
According to the table. 3 build a dependency graph H t = f ( t ) (fig. 1) see Attachment .
2.2 Calculation heat balance furnaces, furnace efficiency and fuel consumption
Heat flow received by steam in the furnace (useful heat load):
where G- the amount of superheated water vapor per unit of time, kg / s;
H vp1 and H vp2
We take the temperature of the flue gases to be 320 ° C (593 K). Heat loss by radiation in environment will amount to 10%, with 9% of them being lost in the radiant chamber, and 1% in the convection chamber. The efficiency of the furnace is η t = 0.95.
We neglect the loss of heat from chemical underburning, as well as the amount of heat of the incoming fuel and air.
Determine the efficiency of the furnace:
where Uh- enthalpy of combustion products at the temperature of flue gases leaving the furnace, t yh; the temperature of the exhaust flue gases is usually taken at 100 - 150 ° C higher than the initial temperature of the raw material at the entrance to the furnace; q sweat- heat loss by radiation into the environment,% or fraction of Q floor ;
Fuel consumption, kg / s:
kg / s.
2.3 Calculation of the radiant chamber and the convection chamber
We set the temperature of the flue gases at the pass: t P= 750 - 850 ° С, we accept
t P= 800 ° C (1073 K). Enthalpy of combustion products at the temperature at the pass
H P= 21171.8 kJ / kg.
Heat flux received by water vapor in radiant tubes:
where N n is the enthalpy of combustion products at the temperature of flue gases in the pass, kJ / kg;
η t is the efficiency of the furnace; it is recommended to take it equal to 0.95 - 0.98;
Heat flow received by water vapor in convection pipes:
The enthalpy of water vapor at the entrance to the radiant section will be:
kJ / kg.
We take the value of the pressure loss in the convection chamber ∆ P To= 0.1 MPa, then:
P To = P - P To ,
P To= 1.2 - 0.1 = 1.1 MPa.
Water vapor entry temperature into the radiant section t To= 294 ° C, then average temperature the outer surface of the radiant tubes will be:
where Δt- the difference between the temperature of the outer surface of the radiant tubes and the temperature of the water vapor (raw material) heated in the tubes; Δt= 20 - 60 ° C;
TO.
Maximum design combustion temperature:
where t o- reduced temperature of the initial mixture of fuel and air; taken equal to the temperature of the air supplied for combustion;
THX.- specific heat capacity of combustion products at temperature t P;
° C.
At t max = 1772.8 ° C and t n = 800 ° C heat density of an absolutely black surface q s for different temperatures the outer surface of radiant tubes has the following meanings:
Θ, ° С 200 400 600
q s, W / m 2 1.50 ∙ 10 5 1.30 ∙ 10 5 0.70 ∙ 10 5
We build an auxiliary graph (Fig. 2) see Attachment, according to which we find the heat density at Θ = 527 ° C: q s= 0.95 ∙ 10 5 W / m 2.
We calculate the total heat flow introduced into the furnace:
Preliminary value for the area of an equivalent absolutely black surface:
m 2.
We take the degree of screening of the masonry Ψ = 0.45 and for α = 1.25 we find that
H s /H l = 0,73.
Equivalent flat surface:
m 2.
We accept single-row placement of pipes and a step between them:
S = 2d n= 2 ∙ 0.152 = 0.304 m. For these values, the form factor TO = 0,87.
The size of the shielded masonry surface:
m 2.
Heating surface of radiant tubes:
m 2.
We choose a BB2 oven, its parameters:
radiation chamber surface, m 2 180
convection chamber surface, m 2 180
working length of the furnace, m 9
radiation chamber width, m 1.2
execution b
flameless fuel combustion method
radiation chamber tube diameter, mm 152 × 6
diameter of convection chamber pipes, mm 114 × 6
The number of tubes in the radiation chamber:
where d n - outer diameter of pipes in the radiation chamber, m;
l floor - useful length of radiant pipes washed by the flue gas flow, m,
l floor = 9 - 0.42 = 8.2 m,
.
Heat density of the surface of radiant tubes:
W / m 2.
Determine the number of pipes of the convection chamber:
We arrange them in a checkerboard pattern of 3 in one horizontal row. Pitch between pipes S = 1.7 d n = 0.19 m.
The average temperature difference is determined by the formula:
° C.
Heat transfer coefficient in the convection chamber:
W / (m 2 ∙ K).
The heat density of the surface of convection pipes is determined by the formula:
W / m 2.
2.4 Hydraulic calculation of the furnace coil
Hydraulic calculation of the furnace coil is to determine the pressure loss of water vapor in radiant and convection pipes.
where G
ρ to V.P. - density of water vapor at average temperature and pressure in the convection chamber, kg / m 3;
dк - inner diameter of convection pipes, m;
z k is the number of flows in the convection chamber,
m / s.
ν k = 3.311 ∙ 10 -6 m 2 / s.
Reynolds criterion value:
m.
Friction pressure loss:
Pa = 14.4 kPa.
Pa = 20.2 kPa.
where Σ ζ to
- number of turns.
Total pressure loss:
2.5 Calculation of the pressure loss of water vapor in the radiation chamber
Average water vapor velocity:
where G- consumption of steam overheated in the furnace, kg / s;
ρ r vp - density of water vapor at average temperature and pressure in the convection chamber, kg / m 3;
d p is the inner diameter of convection pipes, m;
z p is the number of streams in the ventilation chamber,
m / s.
Kinematic viscosity of water vapor at average temperature and pressure in the convection chamber ν p = 8.59 ∙ 10 -6 m 2 / s.
Reynolds criterion value:
Total pipe length in a straight section:
m.
Hydraulic friction coefficient:
Friction pressure loss:
Pa = 15.1 kPa.
Loss of pressure to overcome local resistance:
Pa = 11.3 kPa,
where Σ ζ p= 0.35 - coefficient of resistance when turning by 180 ºС,
- number of turns.
Total pressure loss:
Calculations have shown that the selected furnace will provide the process of superheating of water vapor in a given mode.
3. Calculation of the waste heat boiler
Let's find the average temperature of the flue gases:
where t 1 - temperature of flue gases at the inlet,
t 2 - temperature of flue gases at the outlet, ° С;
° C (538 K).
Flue gas mass flow:
where B is the fuel consumption, kg / s;
For flue gases, the specific enthalpy is determined based on the data in Table. 3 and fig. 1 by the formula:
Enthalpies of coolants Table 4
Heat flow transmitted by flue gases:
where N 1 and H 2 - enthalpy of flue gases at the inlet and outlet temperatures of the combustion chamber, respectively, formed during the combustion of 1 kg of fuel, kJ / kg;
B - fuel consumption, kg / s;
h 1 and h 2 - specific enthalpies of flue gases, kJ / kg,
Heat flow received by water, W:
where η ku is the coefficient of heat utilization in KU; η ky = 0.97;
G n - steam capacity, kg / s;
h to VP - enthalpy of saturated water vapor at outlet temperature, kJ / kg;
h n in - entalygaya feed water, kJ / kg,
The amount of water vapor received in the KU is determined by the formula:
kg / s.
Heat flow received by water in the heating zone:
where h to in - specific enthalpy of water at evaporation temperature, kJ / kg;
Heat flow transferred by flue gases to water in the heating zone (useful heat):
where h x - specific enthalpy of flue gases at temperature t x, hence:
kJ / kg.
Enthalpy of combustion for 1 kg of fuel:
Fig. 1 flue temperature corresponding to the value H x = 5700.45 kJ / kg:
t x = 270 ° C.
Average temperature difference in the heating zone:
° C.
270 flue gases 210 Taking into account the counter-flow index:
where TO f - heat transfer coefficient;
m 2.
Average temperature difference in the evaporation zone:
° C.
320 flue gases 270 Taking into account the counter-flow index:
187 water vapor 187
Heat exchange surface area in the heating zone:
where TO f - coefficient m6transmission;
m 2.
Total heat transfer surface area:
F = F n + F u,
F= 22.6 + 80 = 102.6 m 2.
In accordance with GOST 14248-79, we select a standard vapor chamber evaporator with the following characteristics:
casing diameter, mm 1600
number of tube bundles 1
number of pipes in one bundle 362
heat exchange surface, m 2 170
cross-sectional area of one stroke
through pipes, m 2 0.055
4. Thermal balance of the air heater
Atmospheric air with temperature t ° in-x enters the apparatus, where it heats up to a temperature t x in-x due to the heat of flue gases.
Air consumption, kg / s is determined based on the required amount of fuel:
where V- fuel consumption, kg / s;
L- actual air consumption for combustion of 1 kg of fuel, kg / kg,
Flue gases, giving off their heat, are cooled from t dgZ = t dg2 before t dg4 .
=
where H 3 and H 4- enthalpy of flue gases at temperatures t dg3 and t dg4 respectively, kJ / kg,
Heat flow received by air, W:
where with in-x- average specific heat capacity of air, kJ / (kg K);
0.97 - efficiency of the air heater,
Final air temperature ( t x in-x) is determined from the heat balance equation:
TO.
5. Heat balance of KTAN
After the air heater, the flue gases enter a contact apparatus with an active nozzle (KTAN), where their temperature decreases from t dg5 = t dg4 to temperature t dg6= 60 ° C.
The removal of the heat of flue gases is carried out by two separate streams of water. One stream comes into direct contact with the flue gases, and the other exchanges heat with them through the coil wall.
Heat flux given off by flue gases, W:
where H 5 and H 6- enthalpy of flue gases at temperature t dg5 and t dg6 respectively, kJ / kg,
The amount of cooling water (total), kg / s, is determined from the heat balance equation:
where η is the efficiency of KTAN, η = 0.9,
kg / s.
Heat flow received by cooling water, W:
where G water- cooling water consumption, kg / s:
with water- specific heat capacity of water, 4.19 kJ / (kg K);
t n water and t to water- water temperature at the inlet and outlet of the KTAN, respectively,
6. Calculation of the efficiency of the heat recovery plant
When determining the value of the efficiency of the synthesized system ( η tu) the traditional approach is used.
The calculation of the efficiency of the heat recovery unit is carried out according to the formula:
7. Exergic assessment of the "furnace - waste-heat boiler" system
The exergy method of analyzing energy-technological systems allows the most objective and qualitative assessment of energy losses, which are not revealed in any way during a conventional assessment using the first law of thermodynamics. In this case, the exergy efficiency is used as an assessment criterion, which is defined as the ratio of the allocated exergy to the exergy supplied to the system:
where E sub- fuel exergy, MJ / kg;
E hole- exergy perceived by the flow of water vapor in the furnace and waste heat boiler.
In the case of gaseous fuel, the supplied exergy is the sum of the exergy of the fuel ( E sub1) and air exergy ( E sub2):
where N n and But- enthalpy of air at the temperature of the furnace entrance and the ambient temperature, respectively, kJ / kg;
That- 298 K (25 ° C);
ΔS- change in air entropy, kJ / (kg K).
In most cases, the magnitude of the exergy of air can be neglected, that is:
The allocated exergy for the system under consideration consists of the exergy perceived by water vapor in the furnace ( E otv1), and exergy perceived by water vapor in the KU ( E otv2).
For a stream of steam heated in an oven:
where G- steam consumption in the furnace, kg / s;
N VP1 and H vp2- enthalpy of water vapor at the entrance and exit from the furnace, respectively, kJ / kg;
ΔS vp- change in the entropy of water vapor, kJ / (kg K).
For the flow of water vapor received in the KU:
where G n- steam consumption in the boiler unit, kg / s;
h to vp- enthalpy of saturated water vapor at the outlet of the WHB, kJ / kg;
h n in is the enthalpy of feed water at the inlet to the CH, kJ / kg.
E hole = E hole 1 + E hole 2 ,
E hole= 1965.8 + 296.3 = 2262.1 J / kg.
Conclusion
After calculating the proposed installation (utilization of the heat of the waste gases of the technological furnace), it can be concluded that when this composition fuel, furnace productivity for steam, other indicators - the value of the efficiency of the synthesized system is high, thus - the installation is efficient; This was also shown by the exergy assessment of the “furnace - waste-heat boiler” system, however, in terms of energy costs, the installation leaves much to be desired and requires improvement.
List of used literature
1. Kharaz D .AND... Ways of using secondary energy resources in chemical industries / D. I. Kharaz, B. I. Psakhis. - M .: Chemistry, 1984 .-- 224 p.
2. Skoblo A . AND... Processes and apparatuses of the oil refining and petrochemical industry / A. I. Skoblo, I. A. Tregubova, Yu. K., Molokanov. - 2nd ed., Rev. and add. - M .: Chemistry, 1982 .-- 584 p.
3. Pavlov K .F... Examples and tasks for the course of processes and devices of chemical technology: Textbook. Manual for universities / K. F. Pavlov, P. G. Romankov, A. A. Noskov; Ed. P.G. Romankova. - 10th ed., Rev. and add. - L .: Chemistry, 1987 .-- 576 p.
Appendix
Humid air is a mixture of dry air and water vapor. In unsaturated air, moisture is in a state of superheated steam, and therefore the properties of humid 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, therefore, the absolute air humidity is equal to the mass of 1 m 3 of water vapor or vapor density, kg / m 3
2. Relative air 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 the ratio of the mass of water vapor contained in 1 m 3 of humid air to the mass of water vapor required for complete saturation of 1 m 3 of humid air at the same pressure and temperature.
Relative humidity determines the degree of moisture saturation in the air:
, (1.2)
where is the partial pressure of water vapor corresponding to its density Pa; - saturated steam pressure at the same temperature, Pa; - the maximum possible amount of steam in 1 m 3 of saturated humid air, kg / m 3; - vapor density at its partial pressure and temperature of humid air, kg / m 3.
Relation (1.2) is valid only when it can be assumed that the vapor of the liquid is an ideal gas up to the state of saturation.
The density of humid air r is the sum of the densities of water vapor and dry air at partial pressures of 1 m 3 of humid air at the temperature of humid air T, TO:
(1.3)
where is the density of dry air at its partial pressure in 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 and 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 the gas constants for air J / (kg × K) and for water vapor J / (kg × K), we obtain the value of the 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 V- barometric air pressure in the same units as R, we obtain for humid air under barometric pressure
. (1.8)
Thus, at a given barometric pressure, the moisture content of the 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 humid 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 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, ° С; i- enthalpy of superheated steam, kJ / kg.
Enthalpy of 1 kg of dry saturated water vapor at low pressures 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 humid air in kilojoules per 1 kg of dry air:
Equations similar to those discussed above are used to determine the parameters of wet gas.
, (1.17)
where is the gas constant for the test gas; R- gas pressure.
Enthalpy of gas, 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 to determine 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.
Density of wet gas, kg / m 3:
, (1.20)
where is the density of dry gas at 0 ° C, kg / m 3; M g, M p - molecular weights of gas and vapor.
Dynamic viscosity coefficient of wet gas, 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):
Thermal conductivity coefficient of wet gas, W / (m × K):
, (1.23)
where k Is the adiabatic exponent; V- coefficient (for monatomic gases V= 2.5; for diatomic gases V= 1.9; for triatomic gases V = 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 ° C, m 2 / s:
. (1.25)
Table 1.2. Physical properties of saturated water
| 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 |
Density of wet gas, kg / m 3.
2. heat carried away by flue gases. Determine the heat capacity of flue gases at tux = 8000C;
3. heat loss through the masonry by thermal conductivity.
Loss through the vault
The thickness of the vault is 0.3 m, the material is fireclay. We assume that the temperature of the inner surface of the vault is equal to the temperature of the gases.
Average oven temperature:
For this temperature, we select the coefficient of thermal conductivity of the fireclay material:
Thus, the losses through the vault are:
where α is the coefficient of heat transfer from the outer surface of the walls to the ambient air, equal to 71.2 kJ / (m2 * h * 0С)
Losses through the walls. The masonry was made in two layers (chamotte 345 mm, diatomite 115 mm)
Wall area, m2:
Methodical zone
Welding zone
Languishing zone
End
Full wall area 162.73 m2
With a linear temperature distribution over the wall thickness, the average temperature of chamotte will be 5500C, and diatomite - 1500C.
Hence.
Total losses through masonry
4. Losses of heat with cooling water, according to practical data, we take equal to 10% Qх of the arrival, that is, Qх + Qр
5. Unaccounted losses are assumed to be 15% Q of heat gain
Let's 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 |
| heat of flue gases | 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
We assume that there are pipe-in-pipe burners installed in the furnace.
In the welding zones there are 16 pieces, in the torment zone 4 pieces. the total number of burners is 20 pcs. 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 taken as 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 = B = 2667 hourly fuel consumption;
TG = 50 + 273 = 323 K - gas temperature;
N is the number of burners.
8. Calculation of the recuperator
To heat the air, we design a metal loop recuperator made of pipes with a diameter of 57 / 49.5 mm with a corridor arrangement of their pitch
Initial data for the calculation:
Hourly fuel consumption В = 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 flue gases from the furnace is tux = 8000C.
Hourly air consumption:
Hourly smoke output:
The hourly amount of smoke passing through the recuperator, taking into account the loss of smoke for knocking out and through the bypass damper and air suction.
The coefficient m, taking into account the loss of smoke, is 0.7.
The coefficient taking into account air leakage in hogs is 0.1.
Smoke temperature in front of the recuperator, taking into account air leaks;
where iux is the heat content of flue gases at tux = 8000С
This heat content corresponds to the smoke temperature tD = 7500C. (see Fig. 67 (3))
In the combustion of fuel carbon in air, according to the equation (21C + 2102 + 79N2 = 21C02 + 79N2), for each volume of CO2 in the combustion products, there is 79: 21 = 3.76 volume of N2.
The 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. In the combustion of hydrogen according to the equation
42H2 + 2102 + 79N2 = 42H20 + 79N2
For each volume of Н20, there are 79:42 = 1.88 volumes of nitrogen.
In the combustion products of natural, liquefied and coke oven gases, liquid fuel, wood, peat, brown coal, long-flame and gas coal and other types of fuel 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. ° С) |
Live, naturally, increases the content of water vapor in the combustion products.
The composition of the products of complete combustion of the main types of fuel in the stoichiometric volume of air is given in table. 34. It can be seen from the data in this table that in the combustion products of all types of fuel, the N2 content significantly exceeds the total content of C02-f-H20, and in the combustion products of carbon it is 79%.
The combustion products of hydrogen contain 65% N2, in the combustion products of natural and liquefied gases, gasoline, fuel oil and other types of hydrocarbon fuels, its content is 70-74%.
Rice. 5. Volumetric heat capacity
Combustion products
4 - products of carbon combustion
5 - products of combustion of hydrogen
The average heat capacity of complete combustion products that do not contain oxygen can be calculated using the formula
C = 0.01 (Cc02C02 + Cso2S02 + C „20H20 + CN2N2) kcal / (m3- ° C), (VI. 1)
Where Cc0g, Cso2, CHa0, CNa are the volumetric heat capacities of carbon dioxide, sulfur dioxide, water vapor and nitrogen, and CO2, S02, H20 and N2 are the content of the corresponding components in combustion products,% (vol.).
In accordance with this, formula (VI. 1) takes the following form:
C = 0.01. (Cc02 /? 02 + CHj0H20-bCNi! N2) kcal / (m3 “° C). (VI.2)
The average volumetric heat capacity of CO2, H2O 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 those given in table. 16 data and curves depicted in Fig. 5, the following is visible:
1. The volumetric heat capacity of CO2 significantly exceeds the heat capacity of H20, which, in turn, exceeds the heat capacity of N2 in 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 is faster than the heat capacity of N2. However, despite this, the weighted average volumetric heat capacities of the combustion products of carbon and hydrogen in the 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 is 3.76 m3 of N2 with a minimum volumetric
|
Average volumetric heat capacities of combustion products of carbon and hydrogen in the theoretically required amount of air, kcal / (m3- ° С)
|
The heat capacity, and in the products of hydrogen combustion, for each cubic meter of water vapor, the volumetric heat capacity of which is less than that of COg, 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 leveled, as can be seen from the data in Table. 37 and comparison of curves 4 and 5 in Fig. 5. The difference in the weighted average heat capacities of the combustion products of carbon and hydrogen in the air does not exceed 2%. Naturally, the heat capacities of the combustion products of a fuel consisting 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) ..
Complete combustion products of various types; fuels in stoichiometric air in the temperature range from 0 to 2100 ° С have the following heat capacity, kcal / (m3> ° С):
Fluctuations in heat capacity of combustion products different types fuels are comparatively small. Have 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 fuels 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 in comparison with diatomic gas - nitrogen.
Table 38 shows the average volumetric heat capacities of the products of complete combustion, not diluted with air, for different temperature ranges.
Table 38
|
Average heat capacities of fuel and air combustion products 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 combustion products due to an increase in the content of water vapor in them in the same temperature range, as compared to 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 in a given temperature range increases and, at the same time, the temperature range 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 permissible error in this case usually does not exceed 1%, or 20 °.
From consideration of curves 4 and 5 in Fig. 5 it can be seen that the ratios of the heat capacities of the products of complete combustion of carbon in a stoichiometric volume of air in the temperature range from 0 to t ° С, 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 from 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 the heat capacities C 'remains practically constant for the products of complete combustion of various types of fuel in a stoichiometric volume of air.
Table 40 shows the ratios of the heat capacities of the products of complete combustion of fuel with a low ballast content, turning into gaseous combustion products (anthracite, coke, coal, liquid fuel, natural, petroleum, coke oven gases, etc.) in the temperature range from 0 to t ° C and in the temperature range from 0 to 2100 ° C. Since the heat output of these fuels is close to 2100 ° C, the indicated ratio of the heat capacities C 'is equal to the ratio of the heat capacities in the temperature range from 0 to t and from 0 to tm & x-
Table 40 also shows the values of the C 'value calculated for the combustion products of fuel with a high ballast content, which passes into gaseous combustion products during fuel combustion, i.e. moisture in solid fuel, nitrogen and carbon dioxide in gaseous. The heating capacity of the indicated types of fuel (wood, peat, brown coal, mixed generator, air and blast furnace gases) is equal to 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 (uax
|
As you can see from the table. 40, the C 'and K values differ little even for combustion products of fuel with different ballast content and heat output.
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, the indicated 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 the approximation accuracy, the second dependence can be adopted 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, Nm -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 ventilation ducts of smoke control 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- air leaks (leaks), 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 x 10 8; c =-1.2724412 x 10 9; d = 0,68424233.
2. For fire-fighting normally closed dampers, the numerical values of the specific characteristic of resistance to smoke and gas permeation, 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 rooms adjacent to the burning room. 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 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 |
