Boiler efficiency as a function of temperature. Thermal balance of the steam boiler

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All about boiler efficiency

What is boiler efficiency

The efficiency of a heating boiler is the ratio of 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.

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How to calculate boiler efficiency

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

ηbr = 100 Qpol / Qrr

where Qpol - the amount of useful heat, MJ / kg; Qrr - available heat, MJ/kg;

according to the inverse balance equation:

ηbr = 100 – (q2 + q3 + q4 + q5 + q6),

where q - heat loss in%:

• q2 - with outgoing gases;
• q3 - due to chemical underburning of combustible gases (CO, H2, CH4);
• q4 - with mechanical underburning;
• q5 - from external cooling;
• q6 - with physical heat of slags.

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

ηnet = ηbr - qs.n

where qs.n - energy consumption for own needs, %.

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

How to increase the efficiency of a gas boiler with your own hands

Create right conditions operation of a gas boiler and thereby increase the efficiency can actually be done without calling a specialist, that is, with your own hands. What do I need to do?

1. Adjust the blower damper. This can be done experimentally by finding at what position the coolant temperature will be the highest. Carry out the control using a thermometer installed in the boiler body.
2. Be sure to ensure that the pipes of the heating system do not overgrow from the inside, so that scale and mud deposits do not form on them. With plastic pipes today it has become easier, their quality is known. And yet, experts recommend periodically blowing the heating system.
3. Monitor the quality of the chimney. It should not be allowed to clog and stick to the walls of soot. All this leads to a narrowing of the cross section of the outlet pipe and a decrease in the boiler draft.
4. A prerequisite is cleaning the combustion chamber. Of course, gas does not smoke much like wood or coal, but it is worth washing the firebox at least once every three years, cleaning it of soot.
5. Experts recommend reducing the draft of the chimney in the coldest time of the year. To do this, you can use a special device - a thrust limiter. It is installed on the uppermost edge of the chimney and regulates the cross section of the pipe itself.
6. Reduce chemical heat loss. There are two options here in order to achieve the optimal value: install a draft limiter (it was already mentioned above) and immediately after installing the gas boiler, properly configure the equipment. We recommend that you entrust this to a specialist.
7. You can install a turbulator. These are special plates that are installed between the firebox and the heat exchanger. They increase the area of ​​thermal energy extraction.

Boiler efficiency gross characterizes the efficiency of using the heat supplied to the boiler and does not take into account costs electrical energy to drive blowers, smoke exhausters, feed pumps and other equipment. When running on gas

h br k \u003d 100 × Q 1 / Q c n. (11.1)

Energy costs for auxiliary needs of the boiler plant are taken into account by the efficiency of the boiler net

h n k \u003d h br k - q t - q e, (11.2)

where q t, q e- relative costs for own needs of heat and electricity, respectively. Heat losses for own needs include heat losses with blowing, blowing screens, spraying fuel oil, etc.

The main among them are heat losses with blowdown.

q t \u003d G pr × (h k.v - h p.v) / (B × Q c n) .

Relative electricity consumption for own needs

q el \u003d 100 × (N p.n / h p.n + ​​N d.v / h d.v + N d.s / h d.s) / (B × Q c n) ,

where N p.n, N d.v, N d.s - the cost of electrical energy to drive feed pumps, draft fans and smoke exhausters, respectively; h p.n, h d.v, h d.s - efficiency of feed pumps, draft fans and smoke exhausters, respectively.

11.3. Methodology for performing laboratory work
and processing results

Balance tests in laboratory work are carried out for the stationary operation of the boiler, subject to the following mandatory conditions:

The duration of the boiler installation from kindling to the start of testing is at least 36 hours,

The duration of maintaining the test load immediately before the test is 3 hours,

Permissible load fluctuations in the interval between two adjacent experiments should not exceed ± 10%.

Measurement of parameter values ​​is carried out using standard instruments installed on the boiler shield. All measurements should be made simultaneously at least 3 times with an interval of 15-20 minutes. If the results of two experiments of the same name differ by no more than ±5%, then their arithmetic mean is taken as the measurement result. With a larger relative discrepancy, the measurement result in the third, control experiment is used.

The results of measurements and calculations are recorded in the protocol, the form of which is given in table. 26.

Table 26

Determination of heat losses by the boiler

The end of the table. 26

Table 27

Boiler gross and net efficiency

Analysis of laboratory work results

The value of h br k obtained as a result of the work by the method of direct and reverse balances must be compared with the passport value equal to 92.1%.

Analyzing the influence on the boiler efficiency of the amount of heat loss with flue gases Q 2 , it should be noted that an increase in efficiency can be achieved by lowering the flue gas temperature and reducing excess air in the boiler. At the same time, lowering the gas temperature to the dew point temperature will lead to water vapor condensation and low-temperature corrosion of heating surfaces. A decrease in the value of the excess air coefficient in the furnace can lead to underburning of the fuel and an increase in losses Q 3 . Therefore, the temperature and excess air must not be below certain values.

Then it is necessary to analyze the impact on the efficiency of the boiler operation of its load, with the growth of which the losses with flue gases increase and the losses Q 3 and Q 5 decrease.

The lab report should conclude on the efficiency level of the boiler.

test questions

1. According to what indicators of the boiler operation can a conclusion be made about the efficiency of its operation?
2. What is the heat balance of the boiler? By what methods can it be compiled?
3. What is meant by gross and net boiler efficiency?
4. What heat losses increase during boiler operation?
5. How can q 2 be increased?
6. What parameters have a significant impact on the boiler efficiency?

Keywords: boiler heat balance, boiler gross and net efficiency, corrosion of heating surfaces, excess air ratio, boiler load, heat loss, flue gases, chemical incompleteness of fuel combustion, boiler efficiency.

CONCLUSION

In the process of performing a laboratory workshop on the course of boiler plants and steam generators, students get acquainted with the methods for determining the calorific value of liquid fuel, humidity, volatile output and ash content solid fuel, the design of the steam boiler DE-10-14GM and experimentally investigate the thermal processes occurring in it.

Future specialists study the methods of testing boiler equipment and acquire the necessary practical skills necessary for determining the thermal characteristics of the furnace, compiling the heat balance of the boiler, measuring its efficiency, as well as compiling the salt balance of the boiler and determining the value of the optimal blowdown.

Bibliographic list

1. Khlebnikov V.A. Boiler plant equipment testing:
Laboratory practice. - Yoshkar-Ola: MarGTU, 2005.

2. Sidelkovskii L.N., Yurenev V.N. Boiler plants industrial enterprises: Textbook for universities. – M.: Energoatomizdat, 1988.

3. Trembovlya V.I., Finger E.D., Avdeeva A.A. Thermal engineering tests of boiler installations. - M.: Energoatomizdat, 1991.

4. Alexandrov A.A., Grigoriev B.A. Tables of thermophysical properties of water and steam: a Handbook. Rec. State. standard reference data service. GSSSD R-776-98. – M.: MEI Publishing House, 1999.

5. Lipov Yu.M., Tretyakov Yu.M. Boiler plants and steam generators. - Moscow-Izhevsk: Research Center "Regular and Chaotic Dynamics", 2005.

6. Lipov Yu.M., Samoilov Yu.F., Tretyakov Yu.M., Smirnov O.K. Tests of the equipment of the boiler room of the MPEI CHPP. Laboratory workshop: Tutorial on the course "Boiler installations and steam generators". – M.: MPEI Publishing House, 2000.

7. Roddatis K.F., Poltaretsky A.N. Handbook of low-capacity boiler plants / Ed. K.F.Roddatis. – M.: Energoatomizdat, 1989.

8. Yankelevich V.I. Adjustment of oil-gas industrial boiler houses. – M.: Energoatomizdat, 1988.

9. Laboratory works on the courses "Heat generating processes and installations", "Boiler installations of industrial enterprises" / Comp. L.M. Lyubimova, L.N. Sidelkovsky, D.L. Slavin, B.A. Sokolov and others / Ed. L.N. Sidelkovsky. – M.: MEI Publishing House, 1998.

10. Thermal calculation of boiler units (Normative method) / Ed. N.V. Kuznetsova. - M.: Energy, 1973.

11. SNiP 2.04.14-88. Boiler plants/Gosstroy of Russia. - M .: CITP Gosstroy of Russia, 1988.

Educational edition

KHLEBNIKOV Valery Alekseevich

BOILER INSTALLATIONS
AND STEAM GENERATORS

Laboratory workshop

Editor A.S. Emelyanova

computer set V.V. Khlebnikov

Computer layout V.V. Khlebnikov

Signed for publication on 16.02.08. Format 60x84/16.

Offset paper. Offset printing.

R.l. 4.4. Uch.ed.l. 3.5. Circulation 80 copies.

Order No. 3793. C - 32

Mari State Technical University

424000 Yoshkar-Ola, pl. Lenina, 3

Editorial and publishing center

Mari State technical university

424006 Yoshkar-Ola, st. Panfilova, 17

In 2020, it is planned to generate 1720-1820 million Gcal.

A milligram equivalent is the amount of a substance in milligrams, numerically equal to the ratio of its molecular weight to the valence in a given compound.

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. The calculation of the efficiency of a heating boiler by 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 cost of heat and electricity for own needs, which averages 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.

The coefficient of performance (COP) of a boiler unit is defined as the ratio of the 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 certain 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 fuel.

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

Reading time: 4 min

A properly selected heating system will bring not only warmth and comfort to every home, but also relieve unpleasant consequences and extra costs for repairs. hot water boiler - basis heating system at home.

Before choosing and buying, it is worth making a correct calculation of the boiler efficiency and clarifying all its parameters and factors that will affect its operation and the amount of heat generated.

What is boiler efficiency

The efficiency of steam and hot water boilers is determined by the efficiency - their heat efficiency. That is, it is the volume of generated heat for the production of a nominal volume of hot water in relation to the nominal volume of fuel burned.

Manufacturers indicate the initial capabilities of the equipment, where the efficiency of a hot water boiler can reach 110%, but more often their value adheres to parameters of 95-98%. The consumer in the future during operation can increase these indicators with the help of technical upgrades and thermal insulation.

An independent calculation of the boiler efficiency is made at the installation site and depends on many factors, including a well-built smoke exhaust system, elimination of installation errors, etc. All resources spent for the operation of the coolant (fuel, electricity) are compared with the amount of heat released by it.

How to calculate efficiency

The gross efficiency of the boiler characterizes the degree of technical equipment, the net efficiency - fuel economy.

To identify the boiler efficiency indicators, the following formula is used:

Boiler efficiency = (Q1/ Q_total)x100%, where Q1 is the accumulated heat used for heating, and Q_total is the total amount of heat released during fuel combustion.

The calculations do not affect many points, so their result is averaged. Any failure or abnormal operation of the equipment or external factors affecting heat loss will distort the result obtained by this formula.
To eliminate a larger number of distorting factors, the result is corrected with a refinement of the heat efficiency. Depending on the characteristics of a particular heating system.

Boiler efficiency=100-(Q2+Q3+Q4+Q5+Q6)

Where Q2 - heat loss in the form of smoke released through the ventilation system,
Q3 - insufficient combustion of the gas mixture with incorrectly used volumes of the gas-air mixture,
Q4 - heat loss due to contamination of the heat exchanger, as well as if gas burners are contaminated,
Q5 - heat loss due to external cold air (affects the performance of the boiler plant),
Q 6 - heat loss during cleaning of the combustion chamber.
The main factor affecting body efficiency is the outgoing waste combustion products, with a decrease in their heating within 10-12 ° C, the overall efficiency of a gas heating boiler can be increased by several percent.

For the same reason, condensing boilers have the highest efficiency, i.e. the lower the temperature of the heating equipment, the higher this value. by the most low rate due to minimal functionality and simple device.
Two options are used in determining the efficiency of gas heating boilers: reporting for a specific period of time and during initial installation tests. In the latter version, the calculation result will be more accurate, due to the clarity in calculating heat losses.

How to increase the efficiency of a gas boiler

You can create suitable conditions for increasing the efficiency by optimizing the processes yourself or with the involvement of a specialist. Initially, all parameters are incorporated in the design of the electric boiler, the effectiveness of the measures taken to increase the efficiency of the equipment will depend on these data.

To begin with, modernization is carried out without changing the structure of solid fuel boilers:

1. Room thermostats. They control the temperature in residential premises without affecting the operation of the coolant.
2. Installing a circulation pump, so you can stabilize the uniformity and speed of heating.
3. Replacement gas burner, will increase the increase in efficiency solid fuel boiler by 5-7%. The modulating burner will allow the gas-air mixture to be consumed in the correct proportions, which will eliminate incomplete combustion.
4. The location of the burners near the water circuit will add a few percent to the total efficiency. Such a partial modification will positively affect fuel consumption and increase the thermal balance of the entire system.

Regular maintenance and cleaning of the equipment will increase its efficiency. Scale in the pipes of the heating system and soot on the outer walls of the chimney, formed during operation, can take up to 5%. plastic pipes they need less maintenance, but they need to be purged periodically.

A clogged chimney narrows the passage of the smoke exhaust pipe, which leads to a decrease in draft, and this is not only a loss of heat, but also a threat to the health of people in residential premises.

Also, a heat exchanger with visible signs of pollution, which are salt deposits of metals, provokes a strong consumption of all types of energy spent on work, which reduces thermal conductivity and can damage the boiler. Cleaning the combustion chamber is mandatory and is carried out several times a year.

As an option to reduce chemical heat losses, for this, a highly qualified adjustment of the equipment system is carried out. It is better to refrain from self-tuning and entrust the matter to a specialist.
The fight against underburning is solved by increasing the rate of liquefied gas entering the burner, so the combustion process is more active, and the efficiency, accordingly, increases.

Although the increase in efficiency has practically no effect on the thermal efficiency of the boiler unit. To date, natural gas remains the most economical, equipment on this fuel is more common and economically justified than boilers on traditional solid wood fuel or coal.

Gas boilers with the highest efficiency

The best quality of boilers, which also have high efficiency rates, are of foreign origin. Energy-saving technologies that comply with EU requirements are decisive in the production of such equipment.

High performance is provided by modern modernization tools, for example, as modulating burner.

Automatic and economical, it has a wide range that allows you to adapt to the individual parameters of a particular boiler and heating system. Its combustion is carried out in a constant mode.
Also, the main advantage is their maximum heat transfer. The most optimal heating agent heating value, presented by a foreign manufacturer, is up to 70°C. Combustion products are heated no more than 110°C.
They produce a heat exchanger for boilers with the highest efficiency rates from stainless steel. Additionally, they are equipped with a unit for extracting heat from the condensate. Cons that are characteristic of low temperature heating: the traction force develops with insufficient force and the formation of excessive condensate.

The supply of already heated gas and gas-air mixture to the burner, as well as air entering the chamber through a two-cavity pipe into the furnace - reduces the total number of heat consumption for closed-type boilers by 1-2%.

A good option for upgrading the boiler unit is to install exhaust gas recirculation. With this option, the combustion products enter the burner after passing through the chimney channel with strong kinks, while being enriched with oxygen from the environment. The maximum efficiency is reached at a temperature due to which condensation forms (dew point).

Condensing boilers operating under heating conditions at low temperatures are characterized by relatively low gas consumption. This determines their thermal efficiency, especially when connected to gas-balloon installations. It also makes such a boiler economical.
List of condensing boilers of well-known and distinguished European manufacturers with best quality assembly and high level efficiency:

• Baxi.
• Buderus.
• De Dietrich.
• Vaillant.
• Viessmann.

As stated by their manufacturers in the accompanying documentation, the efficiency of these boiler units, when connected to low-temperature systems, corresponds to 107-110%.