Various types boilers have different Efficiency range from 85 to 110%. When choosing boiler equipment, many buyers are interested in how efficiency can exceed 100% and how it is calculated.
In case of electric boilers Efficiency really cannot be higher than 100%. Only boilers running on combustible fuel can have a higher coefficient.
If you remember the school chemistry course, it turns out that with the complete combustion of any fuel, what remains is CO 2 - carbon and H 2 O - water vapor containing energy. During condensation, the energy of the steam increases, that is, additional energy is generated. Based on this, the calorific value of fuel is divided into two concepts: higher and lower specific heat of combustion.
Lowest- represents the heat obtained during the combustion of fuel, when water vapor, along with the energy contained in them, enters the external environment.
Higher calorific value is heat taking into account the energy contained in water vapor.
Officially (in any regulatory documents) Efficiency, both in Russia and in Europe, calculated at the lowest specific heat combustion. But if you still use the heat contained in water vapor, and the calculations are based on the lowest specific heat of combustion, then in this case figures appear that exceed 100%.
Boilers that use the heat of condensation of water vapor are called condensation. And they have an efficiency exceeding 100%.
The difference between the lower and higher heating values of fuel combustion is about 11%. This value is the limit by which the efficiency of boilers can differ.
Main settings
Efficiency can be calculated using two parameters. In Europe, efficiency is usually calculated based on the temperature of the exhaust gases. For example, when burning a kilogram of fuel, a certain amount of kilocalories of heat is obtained, provided that the temperature of the exhaust gases and the temperature environment.
By measuring the difference between the ambient temperature and the actual temperature of the exhaust gases, it is possible to calculate the boiler efficiency from it.
Roughly speaking, the waste gases escaping into the chimney are subtracted from 100% to arrive at the actual figure.
Calculate correctly
In the USSR, and later in Russia, a fundamentally different calculation method was adopted - the so-called “ reverse balance method" It consists in the fact that heat consumption is determined by the lower calorific value. Then, a heater is placed on the pipe, and the amount of thermal energy that has gone into it is calculated, that is, the amount of energy loss. To calculate efficiency, energy losses are calculated from the total amount of heat.
This approach when determining efficiency gives more exact indicators . It was adopted as a calculation method because all the bodies of Russian boilers were very poorly thermally insulated, which is why up to 40% of the energy escaped through the walls of the boiler. According to requirements regulatory documents, in Russia it is still customary to consider efficiency using the reverse balance method. Today, this method can be successfully applied to multi-megawatt boilers operating in thermal power plants whose burners never turn off.
Advantages of modern boilers
But this technique is completely inapplicable to modern boilers, since they have a fundamentally different operating scheme. Since the burners of modern boilers operate in automatic mode: they work for 15 minutes, and then stop for 15 minutes until the generated heat is used. The higher the outside temperature, the longer the burner will “stand” and work less. Naturally, in this case we cannot talk about a reverse balance.
Another difference between modern boilers is the presence of thermal insulation. Large manufacturers produce the highest quality units, with better thermal insulation. Heat loss through the walls of such a boiler is no more than 1.5-2%. Buyers often forget about this, believing that the boiler will also heat the room by releasing heat during operation. When purchasing a modern boiler, it is worth remembering that it is not intended for heating a boiler room, and, if necessary, take care of installing heating radiators.
Modern heat preservation technologies
A good steel boiler always has higher efficiency. This is due to the fact that cast iron boilers, unlike steel ones, always have more technological limitations.
In addition, thanks to insulation, modern boilers retain heat perfectly. Even two days after it is turned off, the temperature of the boiler body drops by only 20-25 degrees.
The best examples of imported heating equipment are boiler units in which all requirements are correctly taken into account. Therefore, you should not try to “reinvent the wheel” and assemble a boiler from improvised means. After all, there is already in front of you wide choose the most modern, diverse and thought-out options for boilers that will work for a long time and properly, more than meeting all the expectations placed on them and, what is especially pleasant, saving your costs!
Our specialists will help you choose boiler and related equipment and advise on technical issues!
Contact the commercial department by phone:
The thermal efficiency of boiler equipment is indicated in the efficiency factor. Efficiency gas boiler, must be specified in the technical documentation. According to manufacturers, for some boiler models the coefficient reaches 108-109%, others operate at the level of 92-98%.
How to calculate the efficiency of a gas heating boiler
The method for calculating efficiency occurs by comparing the thermal energy expended to heat the coolant and the actual amount of all heat released during fuel combustion. In factory conditions, calculations are performed according to the formula:η = (Q1/ Qri) 100%
In the formula for calculating the efficiency of a gas-fired hot water boiler, the indicated values mean:
- Qri – total thermal energy released when burning fuel.
- Q1 – heat that was accumulated and used to heat the room.
An on-site assessment of the error in determining thermal efficiency is carried out. Another formula is used for calculations:
η=100 - (q2 + q3 + q4 + q5 + q6)
Calculations help to carry out analysis according to the features specific system heating. The abbreviations in the formula mean:
- q2 – heat loss in exhaust gases and combustion products.
- q3 – losses associated with incorrect proportions gas-air mixture, due to which gas underburning occurs.
- q4 – heat losses associated with the appearance of soot on the burners and heat exchanger, as well as mechanical underburning.
- q5 – heat loss, depending on the outside temperature.
- q6 – heat loss when cooling the furnace while cleaning it from slag. The last coefficient applies exclusively to solid fuel units and is not taken into account when calculating the efficiency of equipment running on natural gas.
The temperature of the flue gases, marked in the formula with marker q2, has the greatest impact on thermal efficiency. When the heating intensity of the outgoing degrees decreases by 10-15°C, the efficiency increases by 1-2%. In this regard, the highest efficiency is in condensing boilers belonging to the class of low-temperature heating equipment.
Which gas boiler has the highest efficiency?
Statistics and technical documentation clearly indicate that imported boilers have the highest efficiency. European manufacturers place special emphasis on the use of energy-saving technologies. A foreign gas boiler has high efficiency, since some modifications have been made to its design:- A modulating burner is used– modern boilers from leading manufacturers, equipped with smooth two-stage or fully modulated burner devices. The advantage of the burners is their automatic adaptation to the actual operating parameters of the heating system. The percentage of underburning is reduced to a minimum.
- Coolant heating– the optimal boiler is a unit that heats the coolant to a temperature of no more than 70°C, while the exhaust gases are heated to no more than 110°C, which ensures maximum heat transfer. But, with low-temperature heating of the coolant, several disadvantages are observed: insufficient traction force, increased condensation.
Heat exchangers in gas boilers with the highest efficiency, made from of stainless steel and are equipped with a special condenser unit designed to extract heat contained in the condensate. - Temperature of the supply gas and air entering the burner. Boilers closed type, connect. The air enters the combustion chamber through the outer cavity of the double-cavity pipe, preheated, which reduces the required heat input by several percent.
Burners with preliminary preparation of the gas-air mixture also heat the gas before supplying it to the burner. - Another popular modification option– installation of an exhaust gas recirculation system, when smoke does not immediately enter the combustion chamber, but passes through a broken chimney duct and enters after mixing fresh air, back to the burner device.
Maximum efficiency is achieved at the condensation temperature or “dew point”. Boilers operating in low-temperature heating conditions are called condensing boilers. They are distinguished by low gas consumption and high thermal efficiency, which is especially noticeable when connected to and.
Condensing boilers offer several European manufacturers, among which:
- Viessmann.
- Buderus.
- Vaillant.
- Baxi.
- De Dietrich.
In the technical documentation for condensing boilers, it is indicated that the efficiency of devices when connected to low temperature systems heating is 108-109%.
How to increase the efficiency of a gas heating boiler
There are all sorts of tricks to increase efficiency. The effectiveness of the methods depends on the initial design of the boiler. To begin with, use modifications that do not require changes in the operation of the boiler:- Changing the principle of coolant circulation– the building warms up faster and more evenly when a circulation pump is connected.
- Installation of room thermostats– modernization of boilers to increase efficiency using sensors that control not the heating of the coolant, but the temperature in the room, effective method increasing thermal efficiency.
- An increase in the gas utilization rate in a domestic boiler by approximately 5-7% occurs when the burner device is replaced. Installing a modulating burner helps improve the proportions of the gas-air mixture and, accordingly, reduces the percentage of underburning. The type of burner installed is directly related to the reduction of heat loss.
- Instead of a complete modification of the boiler, a partial modification of the design and adjustment of fuel consumption may be required. If you change the position of the burners and install them closer to the water circuit, you will be able to increase the efficiency by another 1-2%. The heat balance of the boiler unit will increase upward.
Efficiency decreases when the heat exchanger is dirty, due to the fact that scale, consisting of salt deposits of metals, has poor thermal conductivity. For this reason, there is a constant increase in gas consumption and subsequently, the boiler completely fails.
There is a slight increase in efficiency during combustion liquefied gas, achieved by reducing the rate of fuel supply to the burner, which leads to a reduction in underburning. But, thermal efficiency increases slightly. Therefore, natural gas continues to be the most economical of all used traditional types fuel.
The heat released during fuel combustion cannot be completely used to produce steam or hot water, some of the heat is inevitably lost, dissipating in the environment. The heat balance of a boiler unit is a specific formulation of the law of conservation of energy, which asserts the equality of the amount of heat introduced into the boiler unit and the heat expended on the production of steam or hot water, taking into account losses. In accordance with the “Standard Method”, all values included in the heat balance are calculated per 1 kg of burned fuel. The incoming part of the heat balance is called available heat :
Where Q- - lower heating value of fuel, kJ/kg; c T t T - physical heat of fuel (with t - heat capacity of fuel, / t - fuel temperature), kJ/kg; Q B - heat of air entering the furnace when heated outside the unit, kJ/kg; Q n - heat introduced into the boiler unit with steam used for atomizing fuel oil, external blowing of heating surfaces or supply under the grate during layer combustion, kJ/kg.
When using gaseous fuel, the calculation is performed relative to 1 m 3 of dry gas under normal conditions.
Physical heat of the fuel plays a significant role only when preheating the fuel outside the boiler unit. For example, fuel oil is heated before being supplied to the burners, since it has a high viscosity at low temperatures.
Air heat, kJ/ (kg fuel):
where a t is the coefficient of excess air in the furnace; V 0 H - in theory required amount air, n.m 3 /kg; from to - isobaric heat capacity of air, kJ/(n.m 3 K); / x in - temperature cold air, °C; t B - air temperature at the entrance to the furnace, °C.
Heat introduced with steam, kJDkgfuel):
Where G n - specific consumption blown steam (approximately 0.3 kg of steam per 1 kg of fuel oil is consumed for atomizing fuel oil); / p = 2750 kJ/kg - the approximate value of the enthalpy of water vapor at the temperature of the combustion products leaving the boiler unit (about 130 °C).
In approximate calculations, 0 r is taken ~Q? due to the smallness of the other components of equation (22.2).
The consumption part of the heat balance consists of useful heat (production of steam or hot water) and the amount of losses, kJDkgfuel):
where 0 2 - heat loss with gases leaving the boiler unit;
- 03 - heat loss from chemical incomplete combustion of fuel;
- 0 4 - heat loss from mechanical incomplete combustion of fuel;
- 0 5 - heat loss through the lining into the environment; 0 6 - losses with physical heat of slag removed from the boiler unit.
The heat balance equation is written as
As a percentage of available heat, equation (22.6) can be written:
The usefully used heat in a steam boiler with continuous blowing of the upper drum is determined by the equation, kJDkgfuel):
Where D- boiler steam output, kg/s; Dnp- flow rate of purge water kg/s; IN - fuel consumption, kg/s; / p, / p w, / k w - enthalpy of steam, feed and boiler water at pressure in the boiler, respectively, kJ/kg.
Heat loss with flue gases, kJ/(kg fuel):
Where s g And from to- isobaric heat capacity of combustion products and air, kJ/(n.m 3 K); g - flue gas temperature, °C; ax is the coefficient of excess air at the gas outlet from the boiler unit; K 0 G and V 0- theoretical volume of combustion products and theoretically required amount of air, n.m 3 / (kgfuel).
A vacuum is maintained in the gas ducts of the boiler unit; the volumes of gases as they move along the gas path of the boiler increase due to air suction through leaks in the boiler lining. Therefore, the actual coefficient of excess air at the outlet of the boiler unit ax is greater than the coefficient of excess air in the furnace a. It is determined by summing the coefficient of excess air in the firebox and air suction in all flues. In the practice of operating boiler plants, it is necessary to strive to reduce air suction in gas ducts as one of the most effective means combating heat loss.
Thus, the amount of loss Q 2 determined by the temperature of the exhaust gases and the value of the excess air coefficient ax. IN modern boilers the temperature of the gases behind the boiler does not fall below 110 °C. A further decrease in temperature leads to condensation of water vapor contained in gases and the formation of sulfuric acid during combustion of sulfur-containing fuel, which accelerates the corrosion of metal surfaces of the gas path. The minimum losses with flue gases are q 2 ~ 6-7%.
Losses from chemical and mechanical incomplete combustion are characteristics of combustion devices (see clause 21.1). Their value depends on the type of fuel and combustion method, as well as on the perfect organization of the combustion process. Losses from chemical incomplete combustion in modern furnaces amount to q 3 = 0.5-5%, from mechanical - q 4 = 0-13,5%.
Heat loss to the environment q 5 depend on the boiler power. The higher the power, the less relative value losses q 5 . So, with the steam output of the boiler unit D= 1 kg/s losses are 2.8%, with D= 10 kg/s q 5 ~ 1%.
Heat loss with physical heat of slag q b are small and are usually taken into account when drawing up an accurate heat balance,%:
Where a shl = 1 - a un; a un - share of ash in flue gases; with went and? shl - heat capacity and temperature of the slag; A g - ash content of the operating state of the fuel.
Efficiency factor (Efficiency) of the boiler unit is the ratio of the useful heat of combustion of 1 kg of fuel to produce steam in steam boilers or hot water in hot water boilers to the available heat.
Boiler efficiency, %:
Boiler efficiency depends significantly on the type of fuel, combustion method, flue gas temperature and power. Steam boilers, operating on liquid or gaseous fuel, have an efficiency of 90-92%. When burning solid fuel in layers, the efficiency is 70-85%. It should be noted that the efficiency of boiler units significantly depends on the quality of operation, especially on the organization of the combustion process. Operating a boiler unit with steam pressure and output less than nominal reduces efficiency. During the operation of boilers, thermal technical tests must be periodically carried out in order to determine losses and the actual efficiency of the boiler, which allows making the necessary adjustments to its operating mode.
Fuel consumption for a steam boiler (kg/s - for solid and liquid fuel; n.m 3 /s - gaseous)
Where D- steam output of the boiler unit, kg/s; / p, / p v, / k v - enthalpy of steam, feed and boiler water, respectively, kJ/kg; Q p - available heat, kJ/(kg fuel) - for solid and liquid fuels, kJ/(N.m 3) - for gaseous fuel (often taken in calculations Q p ~ Q- due to their slight differences); P - value continuous blowing, % of steam production; g| ka - efficiency of the cola unit, fraction.
Fuel consumption for hot water boiler (kg/s; n.m 3 /s):
where C in - water consumption, kg/s; /, / 2 - initial and final enthalpies of water in the boiler, kJ/kg.
Boiler efficiency gross characterizes the efficiency of using the heat entering the boiler and does not take into account the cost of electrical energy to drive blower fans, smoke exhausters, feed pumps and other equipment. When running on gas
h br k = 100 × Q 1 / Q c n. (11.1)
Energy consumption for the boiler installation’s own needs is taken into account by the boiler efficiency net
h n k = h br k – q t – q e, (11.2)
Where q t, q e– relative costs for own needs of heat and electricity, respectively. Heat consumption for own needs includes heat loss with blowing, for blowing screens, spraying fuel oil, etc.
The main ones are heat losses due to blowing
q t = G pr × (h c.v – h p.v) / (B × Q c n) .
Relative electricity consumption for own needs
q el = 100 × (N p.n /h p.n + N d.v /h d.v + N d.s /h d.s)/(B × Q c n) ,
where N p.n, N d.v, N d.s – electrical energy consumption for driving feed pumps, blower fans and smoke exhausters, respectively; h p.n, h d.v, h d.s - efficiency of feed pumps, blower fans and smoke exhausters, respectively.
11.3. Methodology for performing laboratory work
and processing of results
Balance tests in laboratory work are carried out for the stationary operating mode of the boiler when performing the following mandatory conditions:
The duration of operation of the boiler installation from lighting to the start of testing is at least 36 hours,
The duration of withstanding the test load immediately before the test is 3 hours,
Permissible load fluctuations during the break between two adjacent experiments should not exceed ±10%.
Parameter values are measured using standard instruments installed on the boiler panel. All measurements must be carried out simultaneously at least 3 times with an interval of 15-20 minutes. If the results of two experiments of the same name differ by no more than ±5%, then their arithmetic mean is taken as the measurement result. If the relative discrepancy is greater, the measurement result in the third, control experiment is used.
The results of measurements and calculations are recorded in a protocol, the form of which is given in table. 26.
Table 26
Determination of heat loss from a boiler
| Parameter name | Designation | Unit measured | Experimental results | |||
| №1 | №2 | №3 | Average | |||
| Volume flue gases | V g | m 3 /m 3 | ||||
| Average volumetric heat capacity of flue gases | C g¢ | kJ/ (m 3 K) | ||||
| Flue gas temperature | J | °C | ||||
| Heat loss with flue gases | Q 2 | MJ/m 3 | ||||
| Volume of 3-atomic gases | VRO 2 | m 3 /m 3 | ||||
| Theoretical nitrogen volume | V° N 2 | m 3 /m 3 | ||||
| Excess oxygen in flue gases | a y | --- | ||||
| Theoretical air volume | V° in | m 3 /m 3 | ||||
| Dry gas volume | V сг | m 3 /m 3 | ||||
| Volume of carbon monoxide in flue gases | CO | % | ||||
| Heat of combustion CO | Q CO | MJ/m 3 | ||||
| Volume of hydrogen in flue gases | H 2 | % | ||||
| Heat of combustion H 2 | QH 2 | MJ/m 3 | ||||
| Volume of methane in flue gases | CH 4 | % | ||||
| Heat of combustion CH 4 | Q CH 4 | MJ/m 3 | ||||
| Heat loss from chemical incomplete combustion | Q 3 | MJ/m 3 | ||||
| q 5 | % | |||||
| Heat loss from external cooling | Q 5 | MJ/m 3 |
End of table. 26
Table 27
Boiler efficiency gross and net
| Parameter name | Designation | Unit measured | Experimental results | |||
| №1 | №2 | №3 | Average | |||
| Electrical consumption energy to drive feed pumps | N p.n. | |||||
| Electrical consumption energy for driving blower fans | N d.in | |||||
| Electrical consumption energy for driving smoke exhausters | N d.s | |||||
| Efficiency of feed pumps | h Mon | |||||
| Efficiency of blower fans | h door | |||||
| Efficiency of smoke exhausters | h dm | |||||
| Relative electric consumption energy for own needs | q el | |||||
| Net boiler efficiency | h net k | % |
Analysis of laboratory results
The value of h br k obtained as a result of the work using the method of direct and reverse balances must be compared with the certified value of 92.1%.
Analyzing the effect on the boiler efficiency of the amount of heat loss with flue gases Q 2, it should be noted that an increase in efficiency can be achieved by reducing the temperature of the flue gases and reducing the excess air in the boiler. At the same time, a decrease in gas temperature to the dew point temperature will lead to condensation of water vapor and low temperature corrosion heating surfaces. A decrease in the excess air coefficient in the furnace can lead to underburning of fuel and an increase in Q 3 losses. Therefore, the temperature and excess air must not be lower than certain values.
Then it is necessary to analyze the effect on the efficiency of the boiler's operation of its load, with an increase in which losses with flue gases increase and losses Q 3 and Q 5 decrease.
The laboratory report should make a conclusion about the level of efficiency of the boiler.
- Based on what indicators of boiler operation can a conclusion be made about the efficiency of its operation?
- What is the heat balance of a boiler? By what methods can it be compiled?
- What is meant by gross and net boiler efficiency?
- What heat losses increase during boiler operation?
- How can you increase q 2?
- What parameters have a significant impact on the boiler efficiency?
Keywords: boiler heat balance, boiler gross and net efficiency, corrosion of heating surfaces, excess air coefficient, boiler load, heat loss, exhaust gases, chemical incomplete combustion of fuel, boiler operating efficiency.
CONCLUSION
In the process of performing a laboratory workshop on the course of boiler plants and steam generators, students become familiar with methods for determining the calorific value of liquid fuel, humidity, volatile yield and ash content of solid fuel, the design of the DE-10-14GM steam boiler and experimentally investigate the thermal processes occurring in it.
Future specialists study methods for testing boiler equipment and gain the necessary practical skills needed in determining the thermal characteristics of the furnace, drawing up the heat balance of the boiler, measuring its efficiency, as well as drawing up the salt balance of the boiler and determining the amount of optimal blowdown.
Bibliography
1. Khlebnikov V.A. Boiler plant equipment testing:
Laboratory workshop. - Yoshkar-Ola: MarSTU, 2005.
2. Sidelkovsky L.N., Yurenev V.N. Boiler installations industrial enterprises: Textbook for universities. – M.: Energoatomizdat, 1988.
3. Trembovlya V.I., Finger E.D., Avdeeva A.A. Thermal testing of boiler installations. - M.: Energoatomizdat, 1991.
4. Aleksandrov A.A., Grigoriev B.A. Tables of thermophysical properties of water and water vapor: Handbook. Rec. State standard reference data service. GSSSD R-776-98. – M.: Publishing house MPEI, 1999.
5. Lipov Yu.M., Tretyakov Yu.M. Boiler installations and steam generators. – Moscow-Izhevsk: Research Center “Regular and Chaotic Dynamics”, 2005.
6. Lipov Yu.M., Samoilov Yu.F., Tretyakov Yu.M., Smirnov O.K. Testing of equipment in the boiler department of the MPEI CHPP. Laboratory workshop: Tutorial on the course “Boiler installations and steam generators”. – M.: Publishing house MPEI, 2000.
7. Roddatis K.F., Poltaretsky A.N. Handbook of low-capacity boiler installations/Ed. K.F. Roddatis. – M.: Energoatomizdat, 1989.
8. Yankelevich V.I. Adjustment of gas-oil industrial boiler houses. – M.: Energoatomizdat, 1988.
9. Laboratory works in the courses “Heat generating processes and installations”, “Boiler installations of industrial enterprises” / Comp. L.M. Lyubimova, L.N. Sidelkovsky, D.L. Slavin, B.A. Sokolov and others / Ed. L.N. Sidelkovsky. – M.: Publishing house MPEI, 1998.
10. Thermal calculation of boiler units (Normative method)/Ed. N.V. Kuznetsova. – M.: Energia, 1973.
11. SNiP 2.04.14-88. Boiler installations/Gosstroy of Russia. – M.: CITP Gosstroy of Russia, 1988.
Educational edition
KHLEBNIKOV Valery Alekseevich
BOILER UNITS
AND STEAM GENERATORS
Laboratory workshop
Editor A.S. Emelyanova
Computer set V.V.Khlebnikov
Computer layout V.V.Khlebnikov
Signed for publication on 02/16/08. Format 60x84/16.
Offset paper. Offset printing.
Conditional p.l. 4.4. Uch.ed.l. 3.5. Circulation 80 copies.
Order No. 3793. S – 32
Mari State Technical University
424000 Yoshkar-Ola, pl. Lenina, 3
Editorial and Publishing Center
Mari State technical university
424006 Yoshkar-Ola, st. Panfilova, 17
In 2020, it is planned to produce 1720-1820 million Gcal.
A milligram equivalent is the amount of a substance in milligrams that is numerically equal to the ratio of its molecular weight to the valency in a given compound.
General equation of heat balance of a boiler unit
The ratio connecting the heat input and consumption in a heat generator constitutes its heat balance. The goals of compiling a heat balance of a boiler unit are to determine all incoming and outgoing balance sheet items; calculation of the efficiency of the boiler unit, analysis of balance sheet expenditure items in order to establish the reasons for the deterioration of the boiler unit.
In a boiler unit, when fuel is burned, the chemical energy of the fuel is converted into thermal energy combustion products. The released heat of the fuel is used to generate useful heat contained in steam or hot water and to cover heat losses.
In accordance with the law of conservation of energy, there must be equality between the incoming and outgoing heat in the boiler unit, i.e.
For boiler installations, the heat balance is per 1 kg of solid or liquid fuel or 1 m 3 of gas under normal conditions ( 
). The items of income and consumption in the heat balance equation have dimensions MJ/m 3 for gaseous and MJ/kg for solid and liquid fuels.
The heat from fuel combustion entering the boiler unit is also called available heat, it is denoted by .In the general case entrance part The heat balance is written as:
where is the lowest calorific value of solid or liquid fuel per working mass, MJ/kg;
Lower calorific value of gaseous fuel per dry weight, MJ/m 3 ;
Physical heat of fuel;
Physical heat of air;
Heat introduced into the furnace of a boiler with steam.
Let us consider the components of the incoming part of the heat balance. In the calculations, the lowest working heat of combustion is accepted if the temperature of the combustion products leaving the boiler is higher than the condensation temperature of water vapor (usually tg = 110...120 0 C). When cooling combustion products to a temperature at which condensation of water vapor is possible on the heating surface, calculations should be performed taking into account the higher calorific value of fuel combustion
The physical heat of the fuel is equal to:
Where With T - specific heat fuel, 
for fuel oil and 
for gas;
t t – fuel temperature, 0 C.
Upon entering the boiler solid fuel has usually a low temperature approaching zero, so Q f.t. is small in importance and can be neglected.
To reduce viscosity and improve atomization, fuel oil (liquid fuel) enters the furnace heated to a temperature of 80...120 0 C, so its physical heat is taken into account when performing calculations. In this case, the heat capacity of fuel oil can be determined by the formula:
Accounting Q f.t. is carried out only when burning gaseous fuel with a low calorific value (for example, blast furnace gas) provided it is heated (up to 200...300 0 C). When burning gaseous fuels with a high calorific value (for example, natural gas) there is an increased ratio of the mass of air and gas (approximately 10 1). In this case, the fuel - gas is usually not heated.
Physical heat of air Q f.v. is taken into account only when it is heated outside the boiler due to an external source (for example, in a steam heater or in an autonomous heater when additional fuel is burned in it). In this case, the heat introduced by the air is equal to:
where is the ratio of the amount of air at the entrance to the boiler (air heater) to the theoretically necessary one;
The enthalpy of the theoretically required air heated before the air heater, 
:
,
here the temperature of the heated air in front of the air heater of the boiler unit is 0 C;
Enthalpy of theoretically required cold air, :
The heat introduced into the boiler furnace with steam during steam atomization of fuel oil is taken into account in the form of the formula:
Where G p – steam consumption, kg per 1 kg of fuel (for steam spraying of fuel oil G n = 0.3…0.35 kg/kg);
h n – steam enthalpy, MJ/kg;
2.51 is the approximate value of the enthalpy of water vapor in the combustion products leaving the boiler unit, MJ/kg.
In the absence of heating of fuel and air from external sources, the available heat will be equal to:
The consumption part of the heat balance includes usefully used heat Q floor in the boiler unit, i.e. heat expended to produce steam (or hot water), and various heat losses, i.e.
Where Q u.g. – heat loss with exhaust gases;
Q h.n. , Q m.s. – heat loss from chemical and mechanical incomplete combustion of fuel;
Q But. – heat loss from external cooling of the external enclosures of the boiler;
Q f.sh. – loss of slag with physical heat;
Q acc. – consumption (sign “+”) and supply (sign “-”) of heat associated with the unsteady thermal operating conditions of the boiler. At steady thermal state Q acc. = 0.
So the general equation for the heat balance of a boiler unit at steady state thermal mode can be written as:
If both sides of the presented equation are divided by and multiplied by 100%, we get:
Where 
components of the expenditure part of the heat balance, %.
3.1 Heat loss from flue gases
Heat loss with flue gases occurs due to the fact that the physical heat (enthalpy) of gases leaving the boiler at a temperature t u.g. , exceeds the physical heat of the air entering the boiler α u.g. and fuel With T t t. The difference between the enthalpy of exhaust gases and the heat entering the boiler with air from the environment α u.g. , represents the heat loss with exhaust gases, MJ/kg or (MJ/m 3):
.
Heat loss with flue gases usually occupies the main place among the heat losses of the boiler, amounting to 5...12% of the available heat of the fuel. These heat losses depend on the temperature, volume and composition of combustion products, which, in turn, depend on the ballast components of the fuel:
The ratio characterizing the quality of the fuel shows the relative yield of gaseous combustion products (at α = 1) per unit heat of combustion of the fuel and depends on the content of ballast components (moisture) in it W r and ash A r for solid and liquid fuels, nitrogen N 2, carbon dioxide CO 2 and oxygen ABOUT 2 for gaseous fuel). With an increase in the content of ballast components in the fuel, and, consequently, the loss of heat with exhaust gases increases accordingly.
One of the possible ways to reduce heat loss with flue gases is to reduce the coefficient of excess air in the flue gases α ug, which depends on the coefficient of air flow in the furnace and the ballast air sucked into the boiler flues, which are usually under vacuum:
Possibility of reduction α , depends on the type of fuel, the method of its combustion, the type of burners and the crushing device. At favorable conditions By mixing fuel and air, the excess air required for combustion can be reduced. When burning gaseous fuel, the excess air coefficient is taken to be 1.1, when burning fuel oil = 1.1...1.15.
Air suction through the gas path of the boiler can, in the limit, be reduced to zero. However, complete sealing of the places where pipes pass through the lining, sealing of hatches and peepholes is difficult and practically = 0.15..0.3.
Ballast air in combustion products in addition to increasing heat loss Q u.g. also leads to additional costs electricity for the smoke exhauster.
Another important factor influencing the value Q t.g., is the temperature of the flue gases t u.g. . Its reduction is achieved by installing heat-using elements (economizer, air heater) in the tail part of the boiler. The lower the temperature of the flue gases and, accordingly, the smaller the temperature difference between the gases and the heated working fluid (for example, air), the big square heating surface is required to cool combustion products.
An increase in the temperature of the flue gases leads to an increase in losses from Q u.g. and, consequently, to additional fuel costs to produce the same amount of steam or hot water. Due to this optimal temperature t u.g. is determined on the basis of technical and economic calculations when comparing the finished capital costs for the construction of a heating surface and fuel costs (Fig. 3.).
In addition, when the boiler is operating, the heating surfaces may become contaminated with soot and fuel ash. This leads to a deterioration in the heat exchange of combustion products with the heating surface. At the same time, in order to maintain a given steam output, it is necessary to increase fuel consumption. The drift of heating surfaces also leads to an increase in the resistance of the gas path of the boiler. In this regard, to ensure normal operation of the unit, systematic cleaning of its heating surfaces is required.
3.2 Heat loss from chemical incomplete combustion
Loss of heat from chemical incomplete combustion (chemical underburning) occurs when fuel is incompletely burned within the combustion chamber and flammable gaseous components appear in the combustion products - CO, H2, CH4, CmHn, etc. the afterburning of these combustible gases outside fireboxes are almost impossible due to their relatively low temperature.
The causes of chemical incomplete combustion may be:
general lack of air;
· poor mixture formation, especially at the initial stages of fuel combustion;
· low temperature in the combustion chamber, especially in the area of fuel combustion;
· insufficient residence time of fuel within the combustion chamber, during which chemical reaction combustion cannot be completed completely.
With enough for complete combustion fuel quantity of air and good mixture formation, losses depend on the volumetric density of heat release in the furnace, MW/m 3:
Where IN– fuel consumption, kg/s;
V t – volume of the firebox, m3.
 Rice. 14.9 Dependence of heat loss on chemical incompleteness of combustion q x.n, %, from the volumetric density of heat release in the furnace q v, MW/m 3 . | The nature of the dependence is presented in Fig. 4. . In the area of low values (left side of the curve), i.e. at low fuel consumption B, losses increase due to a decrease in the temperature level in the combustion chamber. An increase in the volumetric density of heat release (with an increase in fuel consumption) leads to an increase in the temperature level in the furnace and a decrease |
However, upon reaching a certain level with a further increase in fuel consumption (the right side of the curve), losses begin to increase again, which is associated with a decrease in the residence time of gases in the furnace volume and, therefore, the impossibility of completing the combustion reaction.
Optimal value, at which losses are minimal, depends on the type of fuel, the method of its combustion and the design of the furnace. For modern combustion devices, heat loss from chemical incomplete combustion is 0...2% at 
.when burning solid and liquid fuels:
when burning gaseous fuel:
When developing measures to reduce the value, it should be borne in mind that in the presence of conditions for the appearance of products of incomplete combustion, CO is formed first as the most difficult to burn component, and then H 2 and other gases. It follows from this that if there is no CO in the combustion products, then there is no H 2 in them.
Boiler unit efficiency
Efficiency factor boiler unit is the ratio of useful heat consumed to produce steam (or hot water) to the available heat of the boiler unit. However, not all the useful heat generated by the boiler unit is sent to consumers; part of the heat is spent on its own needs. Taking this into account, the efficiency of a boiler unit is distinguished by the heat generated (efficiency - gross) and by the heat released (efficiency - net).
The difference between the generated and released heat is used to determine the consumption for auxiliary needs. Not only heat is consumed for its own needs, but also electrical energy (for example, to drive a smoke exhauster, fan, feed pumps, fuel supply mechanisms), i.e. consumption for own needs includes the consumption of all types of energy spent on the production of steam or hot water.
So, gross efficiency of a boiler unit characterizes the degree of its technical perfection, and net efficiency characterizes commercial profitability.
Efficiency - gross boiler unit can be determined either by the direct balance equation or by the reverse balance equation.
According to the direct balance equation:
For example, in the production of water vapor, the useful heat used is ( see question 2) :
Then 
From the presented expression, you can obtain a formula for determining the required fuel consumption, kg/s (m 3 /s):
According to the reverse balance equation:
Determination of efficiency– gross according to the direct balance equation is carried out mainly when reporting for a separate period (ten-day, month), and according to the reverse balance equation - when testing boiler units. Calculating efficiency using reverse balance is much more accurate, since the errors in measuring heat losses are smaller than in determining fuel consumption.
Net efficiency is determined by the expression:
where is the energy consumption for own needs, %.
Thus, to improve the efficiency of boiler units, it is not enough to strive to reduce heat losses; It is also necessary to completely reduce the consumption of thermal and electrical energy for own needs, which amount on average to 3...5% of the heat available in the boiler unit. The efficiency of the boiler unit depends on its load. To build the dependence, you need to subtract sequentially from 100% all losses of the boiler unit, which depend on the load, i.e. 
