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Calculation of the efficiency of boilers. What is the boiler efficiency? Gas boilers with the highest efficiency

BOILER EFFICIENCY

(Boiler efficiency) - the ratio of the amount of heat transferred to the boiler water to turn it into steam during combustion 1 kg fuel, to the value of the calorific value of the fuel, i.e. the amount of heat that is released during complete combustion 1 kg fuel. The efficiency of boilers reaches a value of the order of 0.60-0.85.

Samoilov K.I. Marine dictionary. - M.-L.: State Naval Publishing House of the NKVMF of the USSR, 1941


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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 the boiler, what factors affect it and how to increase efficiency. heating equipment in a particular home.

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 in chimney highly heated flue gases escape. 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 indicator q 3 is used when calculating the efficiency gas boiler heating, 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% of heat loss, but when coal is burned, only 1.5-2% of heat loss is 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 cases where slag is removed from the combustion chamber solid fuel, 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 given value may 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 weight 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. Operation of modern installations with low power, which very often 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 competently approach the selection of fuel and provide for equipment optimal conditions. 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 according to 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.

The quality of solid fuel combustion is positively affected by the calculation of the optimal thrust, taking into account the air pressure 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.

The coefficient of performance (COP) of a boiler unit is defined as the ratio of useful heat used to generate steam (or hot water), to the available heat (the heat supplied to the boiler unit). In practice, not all useful heat selected by the boiler unit is sent to consumers. Part of the heat is spent on own needs. Depending on this, the efficiency of the unit is distinguished by the heat released to the consumer (net efficiency).

The difference between the generated and released heat is the consumption for the boiler house's own needs. Own needs consume not only heat, but also electrical energy (for example, to drive a smoke exhauster, a fan, feed pumps, fuel supply and dust preparation mechanisms, etc.), so the consumption for own needs includes the consumption of all types of energy spent on production of steam or hot water.

The gross efficiency of a boiler unit characterizes the degree of its technical excellence, and the net efficiency - commercial profitability.

Gross efficiency of the boiler unit ŋ br, %, can be determined by the direct balance equation

ŋ br \u003d 100 (Q floor / Q p p)

or by the inverse balance equation

ŋ br \u003d 100-(q y.g + q x.n + q m.n + q n.o + q f.sh),

where Q floor useful heat used to generate steam (or hot water); Q p p- available heat of the boiler unit; q c.g +q c.n +q m.n +q n.o +q f.sh- relative heat losses by items of heat consumption.

The net efficiency according to the reverse balance equation is defined as the difference

ŋ net = ŋ br -q s.n.,

where q s.n- relative energy consumption for own needs, %.

The efficiency factor according to the direct balance equation is used mainly when reporting for a separate period (decade, month), and the efficiency factor according to the reverse balance equation is used when testing boiler units. Determining the efficiency by the inverse balance is much more accurate, since the errors in measuring heat losses are smaller than in determining fuel consumption, especially when burning solid fuels.

Thus, to improve the efficiency of boiler units, it is not enough to strive to reduce heat losses; it is also necessary to reduce in every possible way the cost of heat and electric energy for own needs. Therefore, a comparison of the efficiency of the operation of various boiler units should ultimately be carried out according to their net efficiency.

In general, the efficiency of the boiler unit varies depending on its load. To build this dependence, it is necessary to subtract from 100% successively all the losses of the boiler unit Sq sweat \u003d q y.g + q x.n + q m.n + q n.o which depend on the load.

As can be seen from Figure 1.14, the efficiency of the boiler unit at a certain load has a maximum value, i.e. the operation of the boiler at this load is the most economical.

Figure 1.14 - Dependence of the boiler efficiency on its load: q c.g, q x.n, q m.s., q n.o.,S q sweat- heat losses with exhaust gases, from chemical incomplete combustion, from mechanical incomplete combustion, from external cooling and total losses

The value is from 0.3 to 3.5% and decreases with increasing boiler power (from 3.5% for boilers with a capacity of 2 t/h to 0.3% for boilers with a capacity of more than 300 t/h).

Loss with physical heat of slag occurs because when burning solid fuel, the slag removed from the furnace has a high temperature: with solid ash removal = 600 ° C, with liquid - = 1400 - 1600 ° C.

Heat losses with physical heat of slags, %, are determined by the formula:

,

where - proportion of slag collection in the combustion chamber; - slag enthalpy, kJ/kg.

With layered combustion of fuels, as well as with chamber combustion with liquid slag removal = 1 - 2% and higher.

For chamber combustion of fuel with solid ash removal, the loss is taken into account only for multi-ash fuels at > 2.5%∙kg/MJ.

Efficiency of the boiler unit (gross and net).

The efficiency of a boiler unit is the ratio of the useful heat used to generate steam (hot water) to the available heat (the heat supplied to the boiler unit). Not all useful heat generated by the boiler is sent to consumers, part of it is spent on own needs (drive of pumps, draft devices, heat consumption for heating water outside the boiler, its deaeration, etc.). In this regard, a distinction is made between the efficiency of the unit in terms of the generated heat (gross efficiency) and the efficiency of the unit in terms of the heat released to the consumer (net efficiency).

Boiler efficiency (gross), %, can be determined by the equation direct balance

,

or equation reverse balance

.

Boiler efficiency (net), %, according to the reverse balance is determined as

where is the relative energy consumption for own needs, %.

Topic 6. Layer furnace devices for burning fuel in a dense and fluidized (fluidized) bed

Furnaces for burning fuel in a dense layer: principle of operation, scope, advantages and disadvantages. Classification of furnaces for burning fuel in a dense layer (non-mechanized, semi-mechanical, mechanical). Fuel dispensers. Mechanical furnaces with moving grates: principle of operation, scope, varieties. Layered furnace devices for fuel combustion in a fluidized bed: principle of operation, scope, advantages and disadvantages.

Layer furnace devices for burning fuel in a dense layer.

Layered furnaces designed for combustion of solid lumpy fuel (from 20 to 30 mm in size) are easy to operate and do not require a complex expensive fuel preparation system.

But since the process of fuel combustion in a dense layer is characterized by a low burning rate, inertia (and, therefore, it is difficult to automate), reduced efficiency (fuel combustion occurs with large losses from mechanical and chemical underburning) and reliability, it is economically feasible to use layer combustion for boilers with steam capacity up to 35 t/h.

Layered furnaces are used for burning anthracites, coals with moderate caking capacity (long-flame, gas, lean), brown coals with low moisture and ash content, as well as lumpy peat.

Classification of layer furnaces.

Maintenance of the furnace, in which the fuel is burned in the layer, is reduced to the following basic operations: fuel supply to the furnace; drilling (mixing) of the fuel layer in order to improve the conditions for supplying the oxidizer; removal of slag from the furnace.

Depending on the degree of mechanization of these operations, layered furnace devices can be divided into non-mechanized (all three operations are performed manually); semi-mechanical (one or two operations are mechanized); mechanical (all three operations are mechanized).

Non-mechanized layer furnaces are furnaces with manual periodic supply of fuel to a fixed grate and manual periodic removal of slag.

semi-mechanical furnace devices are distinguished by the mechanization of the process of supplying fuel to the grate using various casters, as well as the use of special slag removers and rotary or rocking grates.

The efficiency of a heating boiler is the ratio of the useful heat used to generate steam (or hot water) to the available heat of the heating boiler. Not all useful heat generated by the boiler unit is sent to consumers, part of the heat is spent on own needs. With this in mind, the efficiency of the heating boiler is distinguished by the generated heat (gross efficiency) and by the released heat (net efficiency).

According to the difference between the generated and released heat, the consumption for own needs is determined. For own needs, not only heat is consumed, but also electrical energy (for example, to drive a smoke exhauster, a 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.

As a result, the gross efficiency of a heating boiler characterizes the degree of its technical perfection, and the net efficiency - commercial efficiency. For the boiler unit gross efficiency, %:
according to the direct balance equation:

η br \u003d 100 Q floor / Q r r

where Q floor is the amount of useful heat, MJ / kg; Q p p - available heat, MJ / kg;

according to the inverse balance equation:

η br \u003d 100 - (q y.g + q x.n + q n.o)

where q c.g, q x.n, q n.o - relative heat losses with exhaust gases, from chemical incompleteness of fuel combustion, from external cooling.

Then the net efficiency of the heating boiler according to the inverse balance equation:

η net = η br - q s.n

where q s.n - energy consumption for own needs,%.

Determination of efficiency by the direct balance equation is carried out mainly when reporting for a separate period (decade, month), and by the inverse balance equation - when testing a heating boiler. Calculation of the efficiency of a heating boiler according to the inverse balance is much more accurate, since the errors in measuring heat losses are smaller than in determining fuel consumption.

Dependence of boiler efficiency η on its load (D/D nom) 100

q o.g, q x.n, q n.o - heat losses with exhaust gases, from chemical and mechanical incompleteness of combustion, from external cooling and total losses.

Thus, in order to increase the efficiency of a heating boiler, it is not enough to strive to reduce heat losses; it is also necessary to reduce in every possible way the costs of heat and electricity for own needs, which average 3 ... 5% of the heat available from the boiler unit.

The change in the efficiency of the heating boiler depends on its load. To build this dependence (Fig.), it is necessary to subtract from 100% sequentially all the losses of the boiler unit that depend on the load, i.e. q c.g., q x.n., q n.d. As can be seen from the figure, the efficiency of the heating boiler at a certain load has a maximum value. The operation of the boiler at this load is the most economical.