Calculation of boiler room efficiency. Thermal balance of the steam boiler
Solid fuel heating equipment is represented today by a whole group of devices. Each solid fuel boiler manufactured today by domestic and foreign manufacturing companies is a completely new, high-tech heating device. Thanks to the introduction of technical innovations into the design of heating devices and equipping with automatic control devices, it was possible to significantly increase the efficiency and optimize the operation of solid fuel boilers.
In heating devices of this type, the traditional principle of operation is used, similar to the well-known variant for us. furnace heating. The main action is due to the process of generating thermal energy released during combustion in the boiler furnace of coal, coke, firewood and other fuel resources, followed by heat transfer to the coolant.
Like other devices that provide the generation and transmission of energy, boiler equipment has its own efficiency. Let us consider in more detail what the efficiency of units operating on solid fuel is. We will try to find answers to questions related to these parameters.
What is the efficiency of heating devices
For any heating unit whose task is to heat inner space residential buildings and structures for various purposes, an important component was, is and remains the efficiency of work. The parameter that determines the efficiency of solid fuel boilers is the efficiency factor. Efficiency shows the ratio of the spent heat energy, issued by the boiler in the process of burning solid fuel, to the useful heat supplied to the entire heating system.
This ratio is expressed as a percentage. The better the boiler works, the higher the interest. Among modern solid fuel boilers there are models with high efficiency, high-tech, efficient and economical units.
For reference: as a rough example, consider the heat generated by sitting near a fire. Released when burning wood thermal energy is able to heat the space and objects limited around the fire. Most of the heat from a burning fire (up to 50-60%) goes into the atmosphere, giving no benefit other than aesthetic content, while neighboring objects and air receive a limited amount of kilocalories. The efficiency of the fire is minimal.
The efficiency of heating technology strongly depends on what type of fuel is used and what design features devices. 
For example: when burning coal, firewood or pellets, different amounts of thermal energy are released. In many ways, the efficiency depends on the technology of fuel combustion in the combustion chamber and the type of heating system. In other words, each type of heating devices (traditional solid fuel boilers, units long burning, pellet boilers and devices operating by pyrolysis) has its own technological design features that affect the efficiency parameters.
The operating conditions and the quality of ventilation also affect the efficiency of the boilers. Poor ventilation causes a shortage of air, which is necessary for the high intensity of the combustion process of the fuel mass. The condition of the chimney affects not only the level of comfort during indoor areas, but also the efficiency of heating equipment, the performance of the entire heating system.
The accompanying documentation for the heating boiler must have the equipment efficiency declared by the manufacturer. Compliance with the real indicators of the declared information is achieved due to correct installation device, strapping and subsequent operation.
Rules for the operation of boiler devices, compliance with which affects the value of efficiency
Any kind heating unit has its own optimal load parameters, which should be as useful as possible from a technological and economic point of view. The process of operation of solid fuel boilers is designed in such a way that most of the time the equipment works in the optimal mode. To ensure such work allows compliance with the rules for the operation of solid fuel heating equipment. In this case, you must adhere to and follow the following points:
- it is necessary to observe acceptable modes of blowing and operation of the hood;
- constant control over the intensity of combustion and completeness of fuel combustion;
- control the amount of carryover and failure;
- assessment of the state of surfaces heated during fuel combustion;
- regular cleaning of the boiler.
The items listed are necessary minimum, which must be adhered to during the operation of boiler equipment during the heating season. Compliance with simple and understandable rules will allow you to obtain the efficiency of an autonomous boiler declared in the characteristics,.
We can say that every little thing, every element of the design of the heating device affects the value of the efficiency. A properly designed chimney and ventilation system provide optimal air flow into the combustion chamber, which significantly affects the quality of combustion of the fuel product. The work of ventilation is estimated by the value of the coefficient of excess air. An excessive increase in the volume of incoming air leads to excessive fuel consumption. Heat escapes more intensively through the pipe along with combustion products. With a decrease in the coefficient, the operation of the boilers deteriorates significantly, and there is a high probability of the occurrence of oxygen-limited zones in the furnace. In such a situation, in the furnace begins to form and accumulate in large quantities soot.
The intensity and quality of combustion in solid fuel boilers require constant monitoring. The loading of the combustion chamber must be carried out evenly, avoiding focal fires.
On a note: coal or firewood is evenly distributed over the grate or grate. Combustion should take place over the entire surface of the layer. Evenly distributed fuel dries quickly and burns over the entire surface, ensuring complete burnout of the solid components of the fuel mass to volatile combustion products. If you correctly put the fuel into the furnace, the flame when the boilers work will be bright yellow, straw-colored.
During combustion, it is important to prevent failures of the fuel resource, otherwise you will have to face significant mechanical losses (underburning) of the fuel. If you do not control the position of the fuel in the furnace, large fragments of coal or firewood that have fallen into the ash box can lead to unauthorized ignition of the remains of the fuel mass products.
Soot and tar accumulated on the surface of the heat exchanger reduce the degree of heating of the heat exchanger. As a result of all these violations of operating conditions, the useful amount of thermal energy required for the normal operation of the heating system decreases. As a result, we can talk about a sharp decrease in the efficiency of heating boilers.
Factors on which the efficiency of boilers depends
Boilers with a high efficiency value are currently represented by the following heating equipment:
- units operating on coal and other solid fossil fuels;
- pellet boilers;
- pyrolysis devices.
The efficiency of heating devices, in the furnace of which anthracite, coal and peat briquettes are used, averages 70-80%. Significantly higher efficiency of pellet devices - up to 85%. Loaded with granules, heating boilers of this type are highly efficient, giving out a huge amount of thermal energy during the combustion of fuel.
On a note: one load is enough for the device to operate in optimal modes up to 12-14 hours.
The absolute leader among solid fuel heating equipment is a pyrolysis boiler. These appliances use wood or wood waste. The efficiency of such equipment today is 85% or more. The units also belong to highly efficient long-burning devices, but subject to necessary condition— fuel moisture should not exceed 20%.
Important for the value of the efficiency is the type of material from which the heater is made. Today on the market are models of solid fuel boilers made of steel and cast iron.
For reference: The first is steel products. To reduce the market value of the unit, manufacturing companies use the main structural elements made of steel. For example, the heat exchanger is made of high-strength heat-resistant black steel 2-5 mm thick. The heating tubular elements used to heat the main circuit are made in the same way.
The thicker the steel used in the construction, the higher the heat transfer characteristics of the equipment. Accordingly, the efficiency increases.
In steel devices, an increase in efficiency is achieved by installing special internal partitions in the form of pipes - main flow stages and smoke dividers. Measures are forced and partial, allowing to slightly increase the efficiency of the main device. Among the models of steel solid fuel boilers, it is rare to find devices with an efficiency above 75%. The service life of such products is 10-15 years.
Foreign companies, in order to increase the efficiency of steel heating boilers, use the bottom combustion process in their models, with 2 or 3 traction flows. The design of the products provides for the installation of tubular heating elements to improve heat transfer. Such equipment has an efficiency in the range of 75-80%, and can last longer, 1.5 times.
Unlike steel units, cast-iron solid propellant apparatuses are more efficient.
The design of cast iron units uses heat exchangers made of cast iron alloy of a special grade, which has a high heat transfer. Such boilers are most often used for open heating heating systems. Products are additionally equipped with grates, thanks to which an intensive selection of thermal energy is carried out directly from the burning fuel placed on the grates.
The efficiency of such heating devices is 80%. Consideration should be given to the long service life of cast iron boilers. The service life of such equipment is 30-40 years.
How to increase the efficiency of solid fuel heating equipment
Today, many consumers, having a solid fuel boiler at their disposal, are trying to find the most convenient and practical way to increase the efficiency of heating equipment. Technological parameters of heating devices, set by the manufacturer, lose their nominal values over time, therefore, to improve the efficiency of boiler equipment, they are looking for various ways and funds.
Consider one of the most spectacular options, the installation of an additional heat exchanger. The task of the new equipment is to remove thermal energy from volatile combustion products.
In the video you can see how to make your own economizer (heat exchanger)
To do this, we first need to know what the temperature of the smoke at the outlet. You can change it with a multimeter, which is placed directly in the middle of the chimney. Data on how much additional heat can be obtained from volatile combustion products is necessary to calculate the area of an additional heat exchanger. We do the following:
- we send firewood of a certain amount to the firebox;
- we detect how long it takes to burn a certain amount of firewood.
For example: firewood, in the amount of 14.2 kg. burn for 3.5 hours. The smoke temperature at the outlet of the boiler is 460 0 C.
In 1 hour, we burned down: 14.2 / 3.5 \u003d 4.05 kg. firewood.
To calculate the amount of smoke, we use the generally accepted value - 1 kg. firewood = 5.7 kg. flue gases. Next, we multiply the amount of firewood burned in one hour by the amount of smoke obtained by burning 1 kg. firewood. As a result: 4.05 x 5.7 = 23.08 kg. volatile combustion products. This figure will become the starting point for subsequent calculations of the amount of thermal energy that can be used additionally to heat the second heat exchanger.
Knowing the value of the heat capacity of volatile hot gases, as 1.1 kJ / kg., We make a further calculation of the heat flow power if we want to reduce the temperature of the smoke from 460 0 C to 160 degrees.
Q \u003d 23.08 x 1.1 (460-160) \u003d 8124 kJ of thermal energy.
As a result, we obtain the exact value of the additional power provided by volatile combustion products: q = 8124/3600 = 2.25 kW, a large figure that can have a significant impact on improving the efficiency of heating equipment. Knowing how much energy is wasted, the desire to equip the boiler with an additional heat exchanger is fully justified. Due to the influx of additional thermal energy to work on heating the coolant, not only the efficiency of the entire heating system increases, but the efficiency of the heating unit itself increases.
conclusions
Despite the abundance of models of modern heating equipment, solid fuel boilers continue to be one of the most efficient and affordable types of heating equipment. Compared to electric boilers, which have an efficiency of up to 90%, solid fuel units have a high economic effect. The increase in efficiency on new models allowed this type of boiler equipment to come close to electric and gas boilers.
Modern solid fuel vehicles are able not only to operate for a long time, using affordable natural fuel resources, but also have high performance characteristics.
The thermal efficiency of boiler equipment is indicated in the efficiency factor. The efficiency of a gas boiler must be prescribed in the technical documentation. According to manufacturers, for some models of boilers, the coefficient reaches 108-109%, while others operate at the level of 92-98%.
How to calculate the efficiency of a gas heating boiler
The method of calculating the efficiency occurs by comparing the heat energy spent to heat the coolant and the actual amount of all heat released during fuel combustion. In the factory, calculations are performed according to the formula:η = (Q1/Qri) 100%
In the formula for calculating the efficiency of a gas-fired boiler, the indicated values mean:
- Qri is the total amount of thermal energy released during fuel combustion.
- Q1 is the heat that was accumulated and used to heat the room.
An assessment of the error in determining the thermal efficiency is carried out on site. For calculations, another formula is used:
η=100 - (q2 + q3 + q4 + q5 + q6)
Calculations help to analyze, according to the characteristics of a particular heating system. Abbreviations in the formula mean:
- q2 - heat losses in exhaust gases and combustion products.
- q3 - losses associated with incorrect proportions of the 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 during the cooling of the furnace during its cleaning from slag. The last coefficient refers exclusively to solid fuel units and is not taken into account when calculating the efficiency of equipment running on natural gas.
The flue gas temperature, marked in the formula with the q2 marker, has the strongest effect on the heat efficiency. With a decrease in the intensity of heating of outgoing degrees by 10-15 ° C, the efficiency increases by 1-2%. In this regard, the most high efficiency 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 a high efficiency, since some modifications have been made in its device:- Modulating burner used– modern boilers from leading manufacturers, equipped with smoothly two-stage or fully modulating burners. The advantage of the burners is the automatic adjustment to the actual operating parameters of the heating system. The percentage of underburning is reduced to a minimum.
- Heat carrier 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, there are several disadvantages: insufficient traction force, increased condensate formation.
Heat exchangers in gas boilers with the highest efficiency, are made of stainless steel and are equipped with a special condenser unit designed to extract heat from the condensate. - The temperature of the supply gas and air entering the burner. Boilers of closed type, connected. Air enters the combustion chamber through the outer cavity of the two-cavity pipe, preheated, which reduces the required heat consumption by several percent.
Burners with preliminary preparation of a gas-air mixture also heat the gas before it is fed to the burner. - Another popular modification- installation of an exhaust gas recirculation system, when the smoke does not immediately enter the combustion chamber, but passes through a broken chimney channel and enters after mixing fresh air, back to the burner.
Maximum efficiency is achieved at the dew point or dew point temperature. Boilers operating under conditions of low-temperature heating 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 are offered by several European manufacturers, including:
- 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 heating systems is 108-109%.
How to increase the efficiency of a gas 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, they 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 the 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 increase in thermal efficiency.
- An increase in the gas utilization factor in a domestic boiler, by approximately 5-7%, occurs when the burner is replaced. The installation of a modulating burner helps to 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 redesign and adjustment of the fuel flow may be required. If you change the position of the burners and install them closer to the water circuit, it will be possible to increase the efficiency by another 1-2%. The heat balance of the boiler unit will increase upwards.
The efficiency decreases when the heat exchanger is dirty, due to the fact that scale, consisting of metal salt deposits, 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 the combustion of liquefied gas, achieved by reducing the rate of fuel supply to the burner, which leads to a decrease in underburning. But, the thermal efficiency increases slightly. That's why, natural gas continues to be the most economical of all conventional fuels in use.
The heat released during the combustion of fuel cannot be fully used to produce steam or hot water, part 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 states the equality of the amount of heat introduced into the boiler unit and the heat spent on the production of steam or hot water, taking into account losses. In accordance with the "Normative Method" all quantities included in the heat balance are calculated per 1 kg of burned fuel. The input part of the heat balance is called available heat :
where Q-- lower calorific value of fuel, kJ/kg; c T t T - physical heat of the fuel (с t is the heat capacity of the fuel, / t is the temperature of the fuel), kJ/kg; Q B is the heat of the air entering the furnace when it is heated outside the unit, kJ/kg; Qn - heat introduced into the boiler unit with steam used for fuel oil atomization, external blowing of heating surfaces or feeding under the grate during layered combustion, kJ/kg.
When using gaseous fuels, the calculation is based on 1 m3 of dry gas under normal conditions.
The physical heat of the fuel plays a significant role only when the fuel is preheated outside the boiler. For example, fuel oil is preheated before being fed to the burners because 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 - theoretically required amount of air, Nm 3 /kg; from to - isobaric heat capacity of air, kJ / (n.m 3 K); / x in - temperature of cold air, ° С; tB- air temperature at the furnace inlet, °С.
Heat introduced with steam, kJDkgfuel):
where Gn- specific consumption of blast steam (approximately 0.3 kg of steam per 1 kg of fuel oil is consumed for spraying fuel oil); / n \u003d 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, take 0 p ~Q? in view of the smallness of the other components of equation (22.2).
The expenditure part of the heat balance consists of the useful heat (production of steam or hot water) of the sum of losses, kJDkgfuel.):
where 0 2 - heat loss with gases leaving the boiler unit;
- 03 - heat loss from chemical incompleteness of fuel combustion;
- 0 4 - heat loss from mechanical incompleteness of fuel combustion;
- 0 5 - heat loss through brickwork to the environment; 0 6 - losses with the physical heat of the 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:
Useful heat in a steam boiler with continuous blowing of the upper drum is determined by the equation, kJDkgfuel.):
where D- boiler steam capacity, kg/s; Dnp- blowdown water consumption kg/s; AT - fuel consumption, kg/s; / p, / p v, / k v - enthalpy of steam, feed and boiler water at pressure in the boiler, respectively, kJ / kg.
Heat loss with flue gases, kJ/(kg fuel):
where from g and from to- isobaric heat capacity of combustion products and air, kJ / (n.m 3 K); d - flue gas temperature, °С; а ux - coefficient of excess air at the outlet of gases from the boiler unit; K 0 G and V0- the theoretical volume of combustion products and the theoretically required amount of air, Nm 3 / (kgfuel).
Vacuum is maintained in the gas ducts of the boiler unit, the volumes of gases during their movement 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 a yx is greater than the coefficient of excess air in the furnace a. It is determined by summing the coefficient of excess air in the furnace and air suction in all gas ducts. 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 combat heat loss.
Thus, the amount of loss Q2 is determined by the temperature of the flue gases and the value of the excess air coefficient а ux. In modern boilers, the temperature of the gases behind the boiler does not fall below 110 °C. A further decrease in temperature leads to the condensation of water vapor contained in the gases and the formation of sulfuric acid during the 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 method of combustion, as well as on the perfect organization of the combustion process. Losses from chemical incomplete combustion in modern furnaces are q 3 = 0.5-5%, from mechanical - q4 = 0-13,5%.
Heat loss to the environment q 5 depend on the power of the boiler. The higher the power, the lower the relative loss q 5 . So, at the steam capacity 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 qb are small and are usually taken into account when compiling the exact heat balance,%:
where a sl = 1 - a un; a un - share of ash in flue gases; with sl and? shl - heat capacity and temperature of the slag; And Mr. ash content of the operating state of the fuel.
Efficiency (efficiency) of the boiler unit called 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 unit efficiency, %:
The efficiency of boiler units significantly depends on the type of fuel, the method of combustion, flue gas temperature and power. steam boilers operating on liquid or gaseous fuels have an efficiency of 90-92%. With layered combustion of solid fuels, 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. The operation of the boiler unit with steam pressure and less than nominal capacity reduces efficiency. During the operation of the boilers, thermal tests should be periodically carried out in order to determine the losses and the actual efficiency of the boiler, which allows you to make the necessary adjustments to its mode of operation.
Fuel consumption for a steam boiler (kg / s - for solid and liquid fuels; Nm 3 / s - gaseous)
where D- steam capacity of the boiler unit, kg/s; / p, / p v, / k v - enthalpy of steam, feed and boiler water, respectively, kJ / kg; Qp- available heat, kJ / (kg fuel) - for solid and liquid fuels, kJ / (N.m 3) - for gaseous fuels (often taken in calculations Qp~Q- due to their slight difference). P - value continuous purge, % of steam capacity; g| ka - efficiency of the boiler unit, shares.
Fuel consumption for a hot water boiler (kg / s; Nm 3 / s):
where C in - water consumption, kg / s; /, / 2 - initial and final enthalpies of water in the boiler, kJ/kg.
Create a cozy and comfortable atmosphere in country house quite simple - you just need to properly equip the heating system. The main component of an efficient and reliable heating system is the boiler. In the article below, we will talk about how to calculate the efficiency of a boiler, what factors affect it, and how to increase the efficiency of heating equipment in a particular house.
How to choose a boiler
Of course, in order to determine how efficient this or that hot water boiler will be, it is necessary to determine its efficiency (efficiency factor). This indicator is the ratio of the heat used for space heating to the total amount of generated heat energy.
The formula for calculating efficiency looks like this:
ɳ=(Q 1 ÷Q ri),
where Q 1 - heat used efficiently;
Q ri is the total amount of released heat.
What is the relationship between boiler efficiency and load
At first glance, it may seem that the more fuel is burned, the better the boiler works. However, this is not quite true. The dependence of the boiler efficiency on the load manifests itself just the opposite. The more fuel is burned, the more heat energy is released. At the same time, the level of heat loss also increases, since strongly heated flue gases go into the chimney. Consequently, fuel is consumed inefficiently.
Similarly, the situation develops in cases where the heating boiler operates at reduced power. If it does not reach the recommended values by more than 15%, the fuel will not burn completely, and the amount of flue gases will increase. As a result, the efficiency of the boiler will drop quite a lot. That is why it is worth adhering to the recommended power levels of the boiler - they are designed to operate the equipment as efficiently as possible.
Calculation of efficiency taking into account various factors
The above formula is not entirely suitable for evaluating the efficiency of the equipment, since it is very difficult to accurately calculate the efficiency of the boiler, taking into account only two indicators. In practice, a different, more complete formula is used in the design process, since not all of the heat generated is used to heat the water in the heating circuit. A certain amount of heat is lost during the operation of the boiler.
A more accurate calculation of the boiler efficiency is made using the following formula:
ɳ=100-(q 2 + q 3 + q 4 + q 5 + q 6), in which
q 2 - heat loss with outgoing combustible gases;
q 3 - heat loss as a result of incomplete combustion of combustion products;
q 4 - heat loss due to fuel underburning and ash precipitation;
q 5 - losses caused by external cooling of the device;
q 6 - heat loss together with slag removed from the furnace.
Heat loss during the removal of combustible gases
The most significant heat losses occur as a result of the evacuation of combustible gases into the chimney (q 2). The efficiency of the boiler largely depends on the combustion temperature of the fuel. The optimum temperature difference at the cold end of the water heater is achieved when heated to 70-110 ℃.
When the flue gas temperature drops by 12-15℃, the efficiency of the hot water boiler increases by 1%. Nevertheless, in order to reduce the temperature of the outgoing combustion products, it is necessary to increase the size of the heated surfaces, and, hence, the entire structure as a whole. In addition, when carbon monoxide is cooled, the risk of low-temperature corrosion increases.
Among other things, the temperature of carbon monoxide also depends on the quality and type of fuel, as well as the heating of the air entering the furnace. The temperatures of the incoming air and the outgoing combustion products depend on the types of fuel.
To calculate the heat loss index with outgoing gases, the following formula is used:
Q 2 = (T 1 -T 3) × (A 2 ÷ (21-O 2) + B), where
T 1 is the temperature of the evacuated combustible gases at the point behind the superheater;
T 3 - the temperature of the air entering the furnace;
21 - concentration of oxygen in the air;
O 2 - the amount of oxygen in the outgoing combustion products at the control point;
A 2 and B are coefficients from a special table that depend on the type of fuel.
Chemical underburning as a source of heat loss
The q 3 indicator is used when calculating the efficiency of a gas heating boiler, for example, or in cases where fuel oil is used. For gas boilers, the value of q 3 is 0.1-0.2%. With a slight excess of air during combustion, this figure is 0.15%, and with a significant excess of air, it is not taken into account at all. However, when burning a mixture of gases of different temperatures, the value of q 3 \u003d 0.4-0.5%.
If the heating equipment runs on solid fuel, q 4 is taken into account. In particular, for anthracite coal, the value of q 4 \u003d 4-6%, semi-anthracite is characterized by 3-4% heat loss, but during combustion hard coal only 1.5-2% of heat losses are formed. With liquid slag removal of burned low-reactivity coal, the value of q4 can be considered minimal. But when removing slag in solid form, heat loss will increase to the maximum limit.
Heat loss due to external cooling
Such heat losses q5 usually do not exceed 0.5%, and as the power of the heating equipment increases, they are further reduced.
This indicator is associated with the calculation of the steam output of the boiler plant:
- Under the condition of steam production D in the range of 42-250 kg/s, the value of heat loss q5=(60÷D)×0.5÷lgD;
- If the value of the steam output D exceeds 250 kg/s, the heat loss rate is considered to be 0.2%.
The amount of heat loss from slag removal
The value of heat loss q6 is only relevant for liquid ash removal. But in those cases when solid fuel slags are removed from the combustion chamber, heat losses q6 are taken into account when calculating the efficiency of heating boilers only if they are more than 2.5Q.
How to calculate the efficiency of a solid fuel boiler
Even with a perfectly designed design and high-quality fuel, the efficiency of heating boilers cannot reach 100%. Their work is necessarily associated with certain heat losses caused both by the type of fuel burned and by a number of external factors and conditions. To understand how the calculation of the efficiency of a solid fuel boiler looks in practice, we will give an example.
For example, heat loss from the removal of slag from the fuel chamber will be:
q 6 \u003d (A sl × W l × A p) ÷ Q ri,
where A sl is the relative value of the slag removed from the furnace to the volume of fuel loaded. With proper use of the boiler, the share of combustion waste in the form of ash is 5-20%, then this value can be equal to 80-95%.
Z l - the thermodynamic potential of ash at a temperature of 600 ℃ under normal conditions is 133.8 kcal / kg.
A p is the ash content of the fuel, which is calculated for total mass fuel. AT various types fuel ash content ranges from 5% to 45%.
Q ri is the minimum amount of thermal energy that is generated in the process of fuel combustion. Depending on the type of fuel, the heat capacity varies within 2500-5400 kcal/kg.
In this case, taking into account the indicated values of heat loss q 6 will be 0.1-2.3%.
The value of q5 will depend on the power and design output of the heating boiler. The operation of modern low-power installations, which are often used to heat private houses, is usually associated with heat losses of this type in the range of 2.5-3.5%.
Heat losses associated with mechanical underburning of solid fuel q 4 largely depend on its type, as well as on the design features of the boiler. They range from 3-11%. This is worth considering if you are looking for a way to make the boiler work more efficiently.
The chemical underburning of fuel usually depends on the concentration of air in the combustible mixture. Such heat losses q 3 are usually equal to 0.5-1%.
The largest percentage of heat loss q 2 is associated with the loss of heat along with combustible gases. This indicator is influenced by the quality and type of fuel, the degree of heating of combustible gases, as well as operating conditions and the design of the heating boiler. With an optimal thermal design of 150 ℃, evacuees carbon monoxide must be heated to a temperature of 280 ℃. In this case, this value of heat loss will be equal to 9-22%.
If all the listed loss values are summarized, we get the efficiency value ɳ=100-(9+0.5+3+2.5+0.1)=84.9%.
This means that a modern boiler can only operate at 85-90% of its capacity. Everything else goes to ensure the combustion process.
Note that achieving such high values is not easy. To do this, you need to correctly approach the selection of fuel and provide optimal conditions for the equipment. Usually, manufacturers indicate what load the boiler should work with. At the same time, it is desirable that most of the time it be set to an economical level of loads.
To operate the boiler with maximum efficiency, it must be used in accordance with the following rules:
- periodic cleaning of the boiler is obligatory;
- it is important to control the intensity of combustion and completeness of fuel combustion;
- it is necessary to calculate the thrust taking into account the pressure of the supplied air;
- it is necessary to calculate the share of ash.
On the quality of solid fuel combustion in a positive way the calculation of the optimal thrust is reflected, taking into account the pressure of the air supplied to the boiler and the rate of carbon monoxide evacuation. However, as the air pressure increases, more heat is removed into the chimney along with the products of combustion. But too little pressure and restriction of air access to the fuel chamber leads to a decrease in the intensity of combustion and more severe ash formation.
If you have a heating boiler installed in your home, pay attention to our recommendations for increasing its efficiency. You can not only save on fuel, but also achieve a comfortable microclimate in the house.
Different types of boilers have different efficiency range from 85 to 110%. When choosing boiler equipment, many buyers are interested in how the efficiency can generally exceed 100% and how it is calculated.
In the case of electric boilers, the efficiency really cannot be higher than 100%. Only boilers operating on combustible fuel can have a higher coefficient.
If you recall a school chemistry course, it turns out that with the complete combustion of any fuel, CO 2 remains - 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, calorific value fuel is divided into two concepts: higher and lower specific calorific value.
Inferior- represents the heat obtained during the combustion of fuel, when water vapor, together 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. And if, nevertheless, the heat contained in the water vapor is used, and the calculations are carried out according to 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 just they have an efficiency exceeding 100%.
The difference between the lower and higher calorific value of fuel is about 11%. This value is the limit by which the efficiency of boilers can differ.
main parameters
Efficiency can be calculated in two ways. In Europe, the efficiency is usually calculated from the flue gas temperature. 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 flue gas temperature, it is possible to calculate the boiler efficiency from it.
Roughly speaking, the exhaust gases flown into the pipe are subtracted from 100%, and the actual figure is obtained.
Count right
In the USSR, and later in Russia, a fundamentally different method of calculation was adopted - the so-called " reverse balance method". It consists in the fact that the 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, that is, the amount of energy loss, is calculated. To calculate the efficiency, the energy loss is calculated from the total amount of heat.
Such an approach with determining efficiency gives more accurate readings. It was adopted as a calculation method because all Russian boiler bodies were very poorly insulated, due to which up to 40% of the energy escaped through the boiler walls. According to requirements normative documents, in Russia it is still customary to consider the efficiency using the reverse balance method. Today, this method can be successfully applied to boilers of several megawatts operating in CHP plants, where the burners never turn off.
Advantages of modern boilers
But this technique is completely inapplicable to modern boilers, since they have a fundamentally different scheme of operation. 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 temperature outside, the longer the burner will “stand” and work less. Naturally, in this case we cannot talk about the reverse balance.
Another difference between modern boilers is the presence of thermal insulation. Major manufacturers produce the highest quality units, with the best 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 due to the release of heat during operation. When purchasing a modern boiler, it is worth remembering that it is not intended for heating the boiler room, and, if necessary, take care of installing heating radiators.
Modern technologies for heat preservation
A good steel boiler is always more efficient. 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 perfectly retain heat. Even two days after it was turned off, the temperature of the boiler body drops by only 20-25 degrees.
The best samples 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 the boiler from improvised means. After all, you already have wide selection the most modern, diverse and thought out to the smallest detail options for boilers that will work for a long time and properly, more than justifying all the expectations placed on them and, which is especially nice, saving your costs!
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