What is boiler efficiency and how to calculate it. How to calculate boiler efficiency - overview of heat loss factors What is called boiler efficiency
Different types of boilers have different Efficiency range from 85 to 110%. When choosing boiler equipment, many buyers are interested in how efficiency can exceed 100% and how it is calculated.
In the case of electric boilers, the efficiency really cannot be higher than 100%. Only boilers running on combustible fuel can have a higher coefficient.
If you remember the school chemistry course, it turns out that with the complete combustion of any fuel, what remains is CO 2 - carbon and H 2 O - water vapor containing energy. During condensation, the energy of the steam increases, that is, additional energy is generated. Based on this, the calorific value of fuel is divided into two concepts: higher and lower specific heat of combustion.
Lowest- represents the heat obtained during the combustion of fuel, when water vapor, along with the energy contained in them, enters the external environment.
Higher calorific value is heat taking into account the energy contained in water vapor.
Officially (in any regulatory documents) Efficiency, both in Russia and in Europe, calculated based on the lowest specific heat of combustion. But if you still use the heat contained in water vapor, and the calculations are based on the lowest specific heat of combustion, then in this case figures appear that exceed 100%.
Boilers that use the heat of condensation of water vapor are called condensation. And they have an efficiency exceeding 100%.
The difference between the lower and higher heating values of fuel combustion is about 11%. This value is the limit by which the efficiency of boilers can differ.
Main settings
Efficiency can be calculated using two parameters. In Europe, efficiency is usually calculated based on the temperature of the exhaust gases. For example, when burning a kilogram of fuel, a certain amount of kilocalories of heat is obtained, provided that the temperature of the exhaust gases and the ambient temperature are equal.
By measuring the difference between the ambient temperature and the actual temperature of the exhaust gases, it is possible to calculate the boiler efficiency from it.
Roughly speaking, the waste gases escaping into the chimney are subtracted from 100% to arrive at the actual figure.
Calculate correctly
In the USSR, and later in Russia, a fundamentally different calculation method was adopted - the so-called “ reverse balance method" It consists in the fact that heat consumption is determined by the lower calorific value. Then, a heater is placed on the pipe, and the amount of thermal energy that has gone into it is calculated, that is, the amount of energy loss. To calculate efficiency, energy losses are calculated from the total amount of heat.
This approach when determining efficiency gives more accurate indicators. It was adopted as a calculation method because all the bodies of Russian boilers were very poorly thermally insulated, which is why up to 40% of the energy escaped through the walls of the boiler. According to the requirements of regulatory documents, in Russia it is still customary to calculate efficiency using the reverse balance method. Today, this method can be successfully applied to multi-megawatt boilers operating in thermal power plants whose burners never turn off.
Advantages of modern boilers
But this technique is completely inapplicable to modern boilers, since they have a fundamentally different operating scheme. Since the burners of modern boilers operate in automatic mode: they work for 15 minutes, and then stop for 15 minutes until the generated heat is used. The higher the outside temperature, the longer the burner will “stand” and work less. Naturally, in this case we cannot talk about a reverse balance.
Another difference between modern boilers is the presence of thermal insulation. Large manufacturers produce the highest quality units, with 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 by releasing heat during operation. When purchasing a modern boiler, it is worth remembering that it is not intended for heating a boiler room, and, if necessary, take care of installing heating radiators.
Modern heat preservation technologies
A good steel boiler always has higher efficiency. This is due to the fact that cast iron boilers, unlike steel ones, always have more technological limitations.
In addition, thanks to insulation, modern boilers retain heat perfectly. Even two days after it is turned off, the temperature of the boiler body drops by only 20-25 degrees.
The best examples of imported heating equipment are boiler units in which all requirements are correctly taken into account. Therefore, you should not try to “reinvent the wheel” and assemble a boiler from improvised means. After all, you already have a wide selection of the most modern, diverse and carefully thought-out boiler options that will work for a long time and properly, more than meeting all the expectations placed on them and, what is especially pleasant, saving your costs!
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The coefficient of performance (efficiency) of a boiler unit is defined as the ratio of the useful heat used to produce steam (or hot water) to the available heat (heat entered into the boiler unit). In practice, not all useful heat selected by the boiler unit is sent to consumers. Part of the heat is spent for its own needs. Depending on this, the efficiency of the unit is distinguished by the heat supplied to the consumer (net efficiency).
The difference between the generated and released heat represents the consumption for the boiler house’s own needs. Not only heat is consumed for own needs, but also electrical energy (for example, to drive a smoke exhauster, fan, feed pumps, fuel supply and dust preparation mechanisms, etc.), therefore, 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 perfection, and the net efficiency characterizes its commercial efficiency.
Gross efficiency of the boiler unit ŋ br, %, can be determined using the direct balance equation
ŋ br = 100(Q floor /Q r r)
or according to the reverse balance equation
ŋ br = 100-(q u.g +q h.n +q m.n +q n.o +q f.sh),
Where Q floor useful heat expended to produce steam (or hot water); Q r r- heat available from the boiler unit; q u.g +q h.n +q m.n +q n.o +q f.sh- relative heat losses by heat consumption items.
Net efficiency according to the reverse balance equation is determined as the difference
ŋ net = ŋ br -q s.n,
Where q s.n.- relative energy consumption for own needs, %.
The efficiency according to the direct balance equation is used mainly when preparing reports for a separate period (decade, month), and the efficiency according to the reverse balance equation is used when testing boiler units. Determining efficiency by reverse 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 consumption of thermal and electrical energy for our own needs. Therefore, a comparison of the operating efficiency of various boiler units should ultimately be carried out based on their net efficiency.
In general, the efficiency of a boiler unit varies depending on its load. To construct this relationship, you need to subtract sequentially all losses of the boiler unit from 100%. Sq sweat = q u.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., operation of the boiler at this load is the most economical.
Figure 1.14 - Dependence of boiler efficiency on its load: q u.g, q x.n, q m.n., 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
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 obtain 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 an 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.
BOILER EFFICIENCY COEFFICIENT
(Boiler efficiency) - the ratio of the amount of heat transferred to the boiler water to convert it into steam during combustion 1 kg fuel, to 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 values 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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