Thermal balance and efficiency of the boiler unit. determination of fuel consumption. How to calculate boiler efficiency - overview of heat loss factors What is the efficiency of a boiler system
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There are 2 methods for determining efficiency:
By direct balance;
By reverse balance.
Determining boiler efficiency as the ratio of useful heat expended to the available heat of the fuel is determined by direct balance:
The boiler efficiency can also be determined by the reverse balance - through heat losses. For the steady thermal state we obtain
. (4.2)
The boiler efficiency, determined by formulas (1) or (2), does not take into account electrical energy and heat for its own needs. This boiler efficiency is called gross efficiency and is denoted by or.
If the energy consumption per unit of time for the specified auxiliary equipment is, MJ, and the specific fuel consumption for electricity generation is, kg/MJ, then the efficiency of the boiler plant taking into account the energy consumption of the auxiliary equipment (net efficiency), %,
. (4.3)
Sometimes called the energy efficiency of a boiler plant.
For boiler installations of industrial enterprises, energy costs for their own needs account for about 4% of the generated energy.
Fuel consumption is determined:
Determination of fuel consumption is associated with a large error, so the efficiency by direct balance is characterized by low accuracy. This method is used to test an existing boiler.
The reverse balance method is characterized by greater accuracy and is used in the operation and design of the boiler. In this case, Q 3 and Q 4 are determined according to recommendations and from reference books. Q 5 is determined from the graph. Q 6 is calculated (rarely taken into account), and essentially the determination by reverse balance comes down to the determination of Q 2, which depends on the temperature of the flue gases.
The gross efficiency depends on the type and power of the boiler, i.e. productivity, type of fuel burned, firebox design. The efficiency is also affected by the boiler operating mode and the cleanliness of the heating surfaces.
In the presence of mechanical underburning, part of the fuel does not burn (q 4), and therefore does not consume air, does not form combustion products and does not release heat, therefore, when calculating the boiler, the calculated fuel consumption is used
. (4.5)
Gross efficiency only takes into account heat losses.
Figure 4.1 - Change in boiler efficiency with load change
5 DETERMINATION OF HEAT LOSS IN A BOILER UNIT.
WAYS TO REDUCE HEAT LOSS
5.1 Heat loss with flue gases
The loss of heat with the exhaust gases Q.g occurs due to the fact that the physical heat (enthalpy) of the gases leaving the boiler exceeds the physical heat of the air and fuel entering the boiler.
If we neglect the small value of the enthalpy of the fuel, as well as the heat of the ash contained in the flue gases, the heat loss with the flue gases, MJ/kg, is calculated by the formula:
Q 2 = J ch.g - J c; (5.8)
where is the enthalpy of cold air at a=1;
100-q 4 – proportion of burned fuel;
a у.г – coefficient of excess air in the flue gases.
If the ambient temperature is zero (t x.v = 0), then the heat loss with the exhaust gases is equal to the enthalpy of the exhaust gases Q a.g = J a.g.
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, and is determined by the volume and composition of combustion products, which significantly depend on the ballast components of the fuel and on the temperature of the flue gases:
The ratio characterizing the quality of the fuel shows the relative yield of gaseous combustion products (at a = 1) per unit of fuel combustion heat and depends on the content of ballast components in it:
– for solid and liquid fuels: moisture W Р and ash А Р;
– for gaseous fuel: N 2, CO 2, O 2.
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 a c.g., which depends on the air flow rate in the furnace a T and the ballast air sucked into the boiler flues, which are usually under vacuum
a y.g = a T + Da. (5.10)
In boilers operating under pressure, there are no air suctions.
With a decrease in a T, the heat loss Q.g. decreases, however, due to a decrease in the amount of air supplied to the combustion chamber, another loss may occur - from the chemical incompleteness of combustion Q 3.
The optimal value of a T is selected taking into account the achievement of the minimum value q y.g + q 3.
The decrease in a T depends on the type of fuel burned and the type of combustion device. Under more favorable conditions of contact between fuel and air, the excess air a T required to achieve the most complete combustion can be reduced.
Ballast air in the combustion products, in addition to increasing heat loss Q.g., also leads to additional energy costs for the smoke exhauster.
The most important factor influencing Q a.g. is the temperature of the exhaust gases t a.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 exhaust gases and, accordingly, the lower the temperature difference Dt between the gases and the heated working fluid, the larger the surface area H is required for the same cooling of the gas. An increase in t y.g leads to an increase in losses from Q y.g and to additional fuel costs DB. In this regard, the optimal t c.g is determined on the basis of technical and economic calculations when comparing annual costs for heat-using elements and fuel for different values of t c.g.
In Fig. 4 we can highlight the temperature range (from to ), in which the calculated costs differ slightly. This gives grounds for choosing as the most appropriate temperature , at which the initial capital costs will be lower.
There are limiting factors when choosing the optimal one:
a) low-temperature corrosion of tail surfaces;
b) when 
0 C it is possible for water vapor to condense and combine with sulfur oxides;
c) the choice depends on the temperature of the feed water, the air temperature at the inlet to the air heater and other factors;
d) contamination of the heating surface. This leads to a decrease in the heat transfer coefficient and an increase.
When determining heat loss with flue gases, the reduction in gas volume is taken into account
. (5.11)
5.2 Heat loss from chemical incomplete combustion
Heat loss from chemical incomplete combustion Q 3 occurs when fuel is incompletely burned within the combustion chamber of the boiler and flammable gaseous components CO, H 2 , CH 4 , C m H n appear in the combustion products... The combustion of these combustible gases outside the furnace is practically impossible due to -due to their relatively low temperature.
Chemical incomplete combustion of fuel can result from:
– general lack of air;
– poor mixture formation;
– small size of the combustion chamber;
– low temperature in the combustion chamber;
– high temperature.
If the air quality and good mixture formation are sufficient for complete combustion of fuel, q 3 depends on the volumetric density of heat release in the furnace
The optimal ratio at which the loss of q 3 has a minimum value depends on the type of fuel, the method of its combustion and the design of the furnace. For modern combustion devices, the heat loss from q 3 is 0÷2% at q v =0.1÷0.3 MW/m 3.
To reduce heat loss from q 3 in the combustion chamber, they strive to increase the temperature level, using, in particular, heating the air, as well as improving the mixing of combustion components in every possible way.
Heating equipment that runs on solid fuel is represented today by a whole group of devices. Every solid fuel boiler produced today by domestic and foreign manufacturing companies is a completely new, high-tech heating device. Thanks to the introduction of technical innovations and automatic control devices into the design of heating devices, it was possible to significantly increase efficiency and optimize the operation of solid fuel boilers.
Heating devices of this type use a traditional principle of operation, similar to the well-known version of stove heating. The main action is due to the process of generating thermal energy released during the combustion of coal, coke, firewood and other fuel resources in the boiler furnace, followed by heat transfer to the coolant.
Like other devices that provide energy generation and transmission, boiler equipment has its own efficiency factor. 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 the interior space of residential buildings and structures for various purposes, operating efficiency was, is and remains an important component. The parameter that determines the efficiency of solid fuel boilers is the efficiency factor. Efficiency shows the ratio of the expended thermal energy produced by the boiler during the combustion of 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, one should evaluate the thermal effect obtained by sitting near a fire. The thermal energy released when burning wood can 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, providing no benefit other than aesthetic content, while neighboring objects and air receive a limited amount of kilocalories. The efficiency of a fire is minimal.
The efficiency of heating equipment strongly depends on what type of fuel is used and what are the design features of the device. 
For example: when burning coal, wood or pellets, different amounts of thermal energy are released. Efficiency largely depends on the technology of fuel combustion in the combustion chamber and the type of heating system. In other words, each type of heating device (traditional solid fuel boilers, long-burning units, pellet boilers and devices operating through pyrolysis) has its own technological design features that affect the efficiency parameters.
Operating conditions and quality of ventilation also affect the efficiency of boilers. Poor ventilation causes a lack of air necessary for the high intensity of the combustion process of the fuel mass. Not only the level of comfort in the interior, but also the efficiency of heating equipment and the performance of the entire heating system depend on the condition of the chimney.
The accompanying documentation for the heating boiler must contain the equipment efficiency declared by the manufacturer. Compliance of real indicators with the declared information is achieved through proper installation of the device, wiring and subsequent operation.
Operating rules for boiler devices, compliance with which affects the efficiency value
Any type of heating unit has its own optimal load parameters, which should be as useful as possible from a technological and economic point of view. The operation process of solid fuel boilers is designed in such a way that most of the time the equipment operates in optimal mode. This work can be ensured by following the rules of operation of heating equipment operating on solid fuel. In this case, you must adhere to and follow the following points:
- it is necessary to observe acceptable modes of blowing and exhaust operation;
- constant control over the intensity of combustion and completeness of fuel combustion;
- control the amount of entrainment and failure;
- assessment of the condition of surfaces heated during fuel combustion;
- regular boiler cleaning.
The listed points are the necessary minimum that 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 stated in the characteristics.
We can say that every little thing, every element of the design of a heating device affects the value of the efficiency factor. A properly designed chimney and ventilation system ensure optimal air flow into the combustion chamber, which significantly affects the quality of combustion of the fuel product. Ventilation performance is assessed by the excess air coefficient. An excessive increase in the volume of incoming air leads to excessive fuel consumption. Heat leaves more intensely through the pipe along with combustion products. When the coefficient decreases, the operation of boilers deteriorates significantly, and there is a high probability of oxygen-limited zones appearing in the furnace. In this situation, soot begins to form and accumulate in large quantities in the firebox.
The intensity and quality of combustion in solid fuel boilers require constant monitoring. The combustion chamber must be loaded evenly, avoiding focal fires.
On a note: coal or firewood is evenly distributed over the grates or grate. Combustion should occur 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 have correctly placed fuel in the firebox, the flame when the boilers are operating will be bright yellow, straw-colored.
During combustion, it is important to prevent failure of the fuel resource, otherwise you will have to face significant mechanical losses (underburning) of fuel. If you do not control the position of the fuel in the firebox, large fragments of coal or firewood falling into the ash box can lead to unauthorized combustion of the remaining fuel mass products.
Soot and resin accumulated on the surface of the heat exchanger reduce the degree of heating of the heat exchanger. As a result of all of the above violations of operating conditions, the useful volume 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 boiler efficiency depends
Boilers with a high efficiency value today are represented by the following heating equipment:
- units running on coal and other solid fossil fuels;
- pellet boilers;
- pyrolysis type devices.
The efficiency of heating devices that fire anthracite, coal and peat briquettes is on average 70-80%. Pellet devices have a significantly higher efficiency – up to 85%. Loaded with pellets, heating boilers of this type are highly efficient, producing a huge amount of thermal energy during fuel combustion.
On a note: one load is enough to operate the device at optimal modes for up to 12-14 hours.
The absolute leader among solid fuel heating equipment is the pyrolysis boiler. These appliances use firewood or waste wood. The efficiency of such equipment today is 85% or more. The units also belong to highly efficient long-burning devices, but subject to the necessary conditions - fuel moisture content should not exceed 20%.
An important factor for the efficiency value is the type of material from which the heating device is made. Today on the market there are models of solid fuel boilers made of steel and cast iron.
For reference: The first includes steel products. To reduce the market value of the unit, manufacturing companies use basic structural elements made of steel. For example, the heat exchanger is made of high-strength, heat-resistant black steel with a thickness of 2-5 mm. The heating tubular elements used to heat the main circuit are manufactured in the same way.
The thicker the steel used in the structure, the higher the heat transfer characteristics of the equipment. The efficiency increases accordingly.
In steel devices, an increase in efficiency is achieved through the installation of special internal partitions in the form of pipes - main flow stages and smoke dissipators. Measures are forced and partial, allowing to slightly increase the efficiency of the main device. Among the models of steel solid fuel boilers, you can rarely find devices with an efficiency above 75%. The service life of such products is 10-15 years.
In order to increase the efficiency of steel heating boilers, foreign companies use a 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 of 75-80%, and can last longer, 1.5 times.
Unlike steel units, cast iron solid fuel units are more efficient.
The design of cast iron units uses heat exchangers made of a special grade of cast iron alloy, which has high heat transfer. Such boilers are most often used for open heating heating systems. The products are additionally equipped with grate bars, thanks to which intensive extraction of thermal energy is carried out directly from the burning fuel placed on the grate bar.
The efficiency of such heating devices is 80%. The long service life of cast iron boilers should be taken into account. The service life of such equipment is 30-40 years.
How to increase the efficiency of heating equipment running on solid fuels
Today, many consumers, having at their disposal a solid fuel boiler, are trying to find the most convenient and practical way to increase the efficiency of heating equipment. The technological parameters of heating devices set by the manufacturer lose their nominal values over time, so various methods and means are being sought to increase the efficiency of boiler equipment.
Let's consider one of the most effective options, installing 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 is. You can change it using a multimeter, which is placed directly in the middle of the chimney. Data on how much additional heat can be obtained from evaporating combustion products is necessary to calculate the area of the additional heat exchanger. We do the following:
- we send a certain amount of firewood into the firebox;
- We measure how long it takes for a certain amount of firewood to burn.
For example: firewood, in the amount of 14.2 kg. burn for 3.5 hours. The smoke temperature at the boiler outlet is 460 0 C.
In 1 hour we burned: 14.2/3.5 = 4.05 kg. firewood
To calculate the amount of smoke, we use the generally accepted value of 1 kg. firewood = 5.7 kg. flue gases. Next, we multiply the amount of wood burned in one hour by the amount of smoke produced 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 additionally used 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 smoke temperature from 460 0 C to 160 degrees.
Q = 23.08 x 1.1 (460-160) = 8124 kJ 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 increasing the efficiency of heating equipment. Knowing how much energy is wasted, the desire to equip the boiler with an additional heat exchanger is completely justified. Due to the influx of additional thermal energy for heating the coolant, not only the efficiency of the entire heating system increases, but also 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 effective 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 has allowed this type of boiler equipment to come closer to electric and gas boilers.
Modern solid fuel devices are capable of not only operating for a long time using affordable natural fuel resources, but also have high performance characteristics.
The heat released during fuel combustion cannot be fully 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 - theoretically required amount of air, n.m 3 /kg; from to - isobaric heat capacity of air, kJ/(n.m 3 K); / x in - cold air temperature, °C; t B - air temperature at the entrance to the furnace, °C.
Heat introduced with steam, kJDkgfuel):
Where G n - specific consumption of blown steam (approximately 0.3 kg of steam per 1 kg of fuel oil is consumed for atomizing fuel oil); / n = 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 outlet of gases 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 of 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 gas temperature 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 perfection of the 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 lower the relative amount of loss 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, %:
The efficiency of boiler units 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 w, / k w - 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 is the value of 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.
The thermal efficiency of boiler equipment is indicated in the efficiency factor. The efficiency of a 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 is the total amount of 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 an analysis according to the characteristics of a particular heating system. The abbreviations in the formula mean:
- q2 – heat loss in exhaust gases and combustion products.
- q3 – losses associated with incorrect proportions of the gas-air mixture, which causes underburning of gas.
- 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 operating 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 modulating 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 are made of stainless steel and equipped with a special condenser unit designed to extract heat from the condensate. - Temperature of the supply gas and air entering the burner. Closed type boilers, connected. 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 feeding it to the burner. - Another popular modification option– installation of an exhaust gas recirculation system, when the smoke does not immediately enter the combustion chamber, but passes through a broken chimney duct and, after mixing fresh air, returns 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 are offered by several European manufacturers, including:
- Viessmann.
- Buderus.
- Vaillant.
- Baxi.
- De Dietrich.
The technical documentation for condensing boilers states that the efficiency of the devices when connected to low-temperature heating systems 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, an effective method of 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 the combustion of liquefied gas, achieved by reducing the rate of fuel supply to the burner, which leads to a decrease in underburning. But, thermal efficiency increases slightly. Therefore, natural gas continues to be the most economical of all traditional fuels used.
For a modern liquid fuel boiler room, the efficiency will often reach 80%, provided that the boiler room is clean and free of soot. However, the real efficiency on average (for those boiler houses that were measured) is approximately 65%. Most often, the boiler room is not so clean that it can receive heat from the flame and transfer the maximum amount of heat to the water.
The situation is much more complicated when boiler house manufacturers begin to talk about efficiency reaching 95%. It is not clear what conditions were used to determine the efficiency, and what efficiency is meant.
In the technical/economic field, at least 6 definitions are used for boiler room efficiency. Since many people do not know the conditions for determining the efficiency of a boiler room, suppliers, without fear of being accused of lying, give high efficiency. However, these high figures have nothing to do with the reality of the heat payer.
1. COMBUSTION EFFICIENCY
Combustion efficiency is the amount of fuel energy that is RELEASED during combustion.
The release of fuel energy and its conversion into heat in the hearth (stove) of the boiler room does not indicate the high efficiency of the boiler room. Combustion efficiency is provided by some boiler house manufacturers as boiler room efficiency, because 1) the figure is high (approximately 93-95%) 2) it is easy to measure combustion efficiency - you need to install the instrument in the chimneys.
The release of heat from fuel occurs in most boiler houses with high combustion efficiency.
Consequently: The release of fuel energy plus its conversion into heat in the hearth (stove) is not the same heat that is received by the boiler!! We are interested in the heat received by the boiler!!
2. BOILER ROOM efficiency
Boiler house efficiency is the amount of fuel energy that is usefully used, i.e. is transformed into another energy-carrying medium.
Another energy-carrying medium means, for example, warm water that heats a house.
Boiler house efficiency is the most used definition of efficiency in all types of combustion plants.
Boiler room efficiency is more difficult to measure than combustion efficiency, so many people are content with only measuring combustion efficiency. In fact, the boiler room efficiency is 10-15% lower than the combustion efficiency.
3. EFFICIENCY OF COMBUSTION EQUIPMENT
THE EFFICIENCY OF COMBUSTION EQUIPMENT SHOWS HOW EFFECTIVELY COMBUSTION AND HEAT RECEPTION OCCUR IN THE BOILER ROOM. Even these calculations are often presented as a result of flue gas analysis.
Often, the efficiency of furnace equipment is used as an approximate analogue of the efficiency of a boiler room, since the measurement technique in this case is easier. Using this technique, you can get an approximate figure for the efficiency of a boiler room: it is necessary to constantly analyze the composition of oxygen or CO2 in the combustion gases. Losses are subtracted, since, for example, some heat is present in the ash/slag (this is especially true for slag-forming fuels). As for liquid fuel, the efficiency of furnace equipment and the efficiency of the boiler room are approximately the same, since liquid fuel does not contain ash/slag. But if you use this concept for coal or biofuels, then the errors (errors) are much higher.
4. EFFICIENCY OF THE INSTALLATION
When calculating the efficiency of an installation, the ratio between the total amount of useful energy and the total amount of energy is determined. The total amount of energy also includes “auxiliary energy”, for example, electrical energy necessary to operate boiler room pumps, ventilation, chimneys, etc. For a liquid fuel installation, "auxiliary energy" corresponds to approximately 1% of the total fuel energy; for solid fuel installations, "auxiliary energy" equals 5% of the fuel energy.
The efficiency of the installation will thus be lower than the efficiency of the boiler house.
5. SYSTEM EFFICIENCY
Determining the efficiency of a system expands the boundaries of the system to:
Heat production with losses
- heat distribution with losses in heating mains, etc.
- heat use
According to UNICHAL (International Union of Heat Suppliers), the following typical losses in pipes when distributing hot water to apartments occur:
Sweden - 8% losses in pipes, i.e. heat is transferred to the ground and surrounding district heating pipes
Denmark - 20%
Finland - 9%
Belgium - 13%
Switzerland - 13%
West Germany - 11%
6. Annual efficiency
The efficiency per year in principle corresponds to the efficiency of the boiler house, but then the average efficiency of the boiler house is calculated for the entire year. The efficiency per year also includes periods with poor combustion levels, for example, when starting a boiler room, etc.
Efficiency per year depends on the size of the installation, service life, etc.
The above shows that different definitions for efficiency are used, so there is a high probability that an erroneous figure will be given if the concept and definition of efficiency is not clarified. Thus, there is no need to be afraid of being insensitive, since in fact, many manufacturers, with or without knowledge, provide erroneous figures.
The important figures are those that reflect the real economic side of the fuel that the consumer buys. If you lose consumer trust due to providing too high efficiency, then the emergence of big problems in the market is inevitable.
As stated, "all suppliers" (at least many) give combustion efficiency when they offer boiler room efficiency information.
You cannot use combustion efficiency when calculating the economics of the installation!!!
THE CONSUMER IS NOT BUYING FUEL, BUT A MEANS FOR PRODUCING HEAT. It is not the fuel that should be cheap, but the heat that consumers receive during winter blizzards.