Waste heat recovery flue gases

Flue gases leaving working space ovens have a very high temperature and therefore carry away a significant amount of heat. In open-hearth furnaces, for example, about 80% of the total heat supplied to the working space is carried away from the working space with flue gases, in heating furnaces about 60%. From the working space of the furnaces, the flue gases carry away more heat with them, the higher their temperature and the lower the heat utilization coefficient in the furnace. In this regard, it is advisable to ensure the recovery of heat from exhaust flue gases, which can be done in principle by two methods: with the return of part of the heat taken from the flue gases back to the furnace and without returning this heat to the furnace. To implement the first method, it is necessary to transfer the heat taken from the smoke to gas and air (or only air) going into the furnace. To achieve this goal, heat exchangers of recuperative and regenerative types are widely used, the use of which makes it possible to increase the efficiency of the furnace unit, increase the combustion temperature and save fuel. With the second recovery method, the heat of exhaust flue gases is used in thermal power boiler houses and turbine units, which achieves significant fuel savings.

In some cases, both described methods of waste heat recovery are used simultaneously. This is done when the temperature of the flue gases after regenerative or recuperative heat exchangers remains sufficiently high and further heat recovery in thermal power plants is advisable. For example, in open-hearth furnaces, the temperature of the flue gases after the regenerators is 750-800 °C, so they are reused in waste heat boilers.

Let us consider in more detail the issue of recycling the heat of exhaust flue gases with the return of part of their heat to the furnace.

It should be noted, first of all, that a unit of heat taken from the smoke and introduced into the furnace by air or gas (a unit of physical heat) turns out to be much more valuable than a unit of heat obtained in the furnace as a result of combustion of fuel (a unit of chemical heat), since the heat of the heated air (gas) does not entail heat loss with flue gases. The value of a unit of sensible heat is greater, the lower the fuel utilization factor and the higher the temperature of the exhaust flue gases.

For normal operation The oven should supply the required amount of heat to the workspace every hour. This amount of heat includes not only the heat of the fuel, but also the heat of heated air or gas, i.e.

It is clear that with = const an increase will reduce . In other words, utilization of heat from flue gases makes it possible to achieve fuel savings, which depends on the degree of heat utilization from flue gases

where is, respectively, the enthalpy of heated air and flue gases leaving the working space, kW, or kJ/period.

The degree of heat recovery can also be called efficiency. recuperator (regenerator), %

Knowing the degree of heat recovery, you can determine fuel economy using the following expression:

where I"d, Id are, respectively, the enthalpy of the flue gases at the combustion temperature and those leaving the furnace.

Reducing fuel consumption as a result of using the heat of exhaust flue gases usually provides a significant economic effect and is one of the ways to reduce the cost of heating metal in industrial furnaces.

In addition to saving fuel, the use of air (gas) heating is accompanied by an increase in calorimetric combustion temperature, which may be the main purpose of recovery when heating furnaces with fuel with a low calorific value.

An increase in at leads to an increase in combustion temperature. If it is necessary to provide a certain value, then an increase in the air (gas) heating temperature leads to a decrease in the value, i.e., to a decrease in the share of gas with a high calorific value in the fuel mixture.

Since heat recovery allows for significant fuel savings, it is advisable to strive for the highest possible, economically justified degree of utilization. However, it must immediately be noted that recycling cannot be complete, i.e. always. This is explained by the fact that increasing the heating surface is rational only up to certain limits, after which it already leads to a very insignificant gain in heat savings.

Heat recovery methods. The flue gases leaving the working space of the furnaces have a very high temperature and therefore carry away a significant amount of heat. In open-hearth furnaces, for example, about 80% of the total heat supplied to the working space is carried away from the working space with flue gases, in heating furnaces about 60%. From the working space of the furnaces, the flue gases carry away more heat with them, the higher their temperature and the lower the heat utilization coefficient in the furnace. In this regard, it is advisable to ensure the recovery of heat from exhaust flue gases, which can be done in principle by two methods: with the return of part of the heat taken from the flue gases back to the furnace and without returning this heat to the furnace. To implement the first method, it is necessary to transfer the heat taken from the smoke to gas and air (or only air) going into the furnace. To achieve this goal, heat exchangers of recuperative and regenerative types are widely used, the use of which makes it possible to increase the efficiency of the furnace unit, increase combustion temperature and save fuel. With the second method of utilization, the heat of exhaust flue gases is used in thermal power boiler houses and turbine plants, which achieves significant fuel savings.

In some cases, both described methods of heat recovery from flue gases are used simultaneously. This is done when the temperature of the flue gases after regenerative or recuperative heat exchangers remains sufficiently high and further heat recovery in thermal power plants is advisable. For example, in open-hearth furnaces, the temperature of the flue gases after the regenerators is 750-800 °C, so they are reused in waste heat boilers.

Let us consider in more detail the issue of recycling the heat of exhaust flue gases with the return of part of their heat to the furnace.

It should first of all be noted that a unit of heat taken from the smoke and introduced into the furnace by air or gas (a unit of physical heat) turns out to be much more valuable than units of heat obtained in the furnace as a result of combustion of fuel (a unit of chemical heat), since the heat of heated air ( gas) does not entail heat loss with flue gases. The value of a unit of sensible heat is greater, the lower the fuel utilization factor and the higher the temperature of the exhaust flue gases.

For normal operation of the furnace, the required amount of heat must be supplied to the working space every hour. This amount of heat includes not only the heat of the fuel Q x, but also the heat of heated air or gas Q F, i.e. Q Σ = Q x + Q f

It is clear that for Q Σ = const an increase in Q f will allow you to decrease Q x. In other words, utilization of heat from flue gases makes it possible to achieve fuel savings, which depends on the degree of heat utilization from flue gases

R = N in / N d

where N in and N d are, respectively, the enthalpy of heated air and flue gases escaping from the working space, kW or

kJ/period.

The degree of heat recovery can also be called the heat recovery coefficient of the recuperator (regenerator), %

efficiency p = (N in / N d) 100%.

Knowing the degree of heat recovery, you can determine fuel economy using the following expression:

where N " d and N d are, respectively, the enthalpy of the flue gases at the combustion temperature and those leaving the furnace.

Reducing fuel consumption as a result of using the heat of exhaust flue gases usually provides a significant economic effect and is one of the ways to reduce the cost of heating metal in industrial furnaces.

In addition to saving fuel, the use of air (gas) heating is accompanied by an increase in the calorimetric combustion temperature T k, which may be the main purpose of recovery when heating furnaces with fuel with a low calorific value.

Increase in Q F at leads to an increase in combustion temperature. If it is necessary to provide a certain amount T k, then an increase in the temperature of heating the air (gas) leads to a decrease in the value , i.e., to reduce the share of gas with a high calorific value in the fuel mixture.

Since heat recovery allows for significant fuel savings, it is advisable to strive for the highest possible, economically justified degree of recovery. However, it must immediately be noted that recycling cannot be complete, i.e. always R< 1. Это объясняется тем, что увеличение поверхности нагрева рационально только до определенных пределов, после которых оно уже приводит кочень незначительному выигрышу в экономии тепла.

Characteristics of heat exchange devices. As already indicated, the recovery of heat from exhaust flue gases and their return to the furnace can be carried out in heat exchange devices of regenerative and recuperative types. Regenerative heat exchangers operate in a non-stationary thermal state, while recuperative heat exchangers operate in a stationary thermal state.

Regenerative type heat exchangers have the following main disadvantages:

1) cannot provide a constant temperature for heating air or gas, which drops as the bricks of the nozzle cool, which limits the possibility of application automatic regulation ovens;

2) cessation of heat supply to the furnace when the valves are switched;

3) when heating the fuel, gas is carried out through the chimney, the value of which reaches 5-6 % full flow rate;

4) very large volume and mass of regenerators;

5) inconveniently located - ceramic regenerators are always located under the furnaces. The only exceptions are cowpers placed near blast furnaces.

However, despite very serious disadvantages, regenerative heat exchangers are sometimes still used in high-temperature furnaces (open hearth and blast furnaces, in heating wells). This is explained by the fact that regenerators can operate at very high temperature flue gases (1500-1600 °C). At this temperature, recuperators cannot yet operate stably.

The recuperative principle of heat recovery from exhaust flue gases is more progressive and perfect. Recuperators provide a constant temperature for heating air or gas and do not require any changeover devices - this ensures smoother operation of the furnace and greater opportunity for automation and control of its thermal operation. Recuperators do not carry gas into the chimney; they are smaller in volume and weight. However, recuperators also have some disadvantages, the main ones being low fire resistance (metal recuperators) and low gas density (ceramic recuperators).

General characteristics of heat exchange in recuperators. Let's consider general characteristics heat exchange in the recuperator. The recuperator is a heat exchanger operating under stationary thermal conditions, when heat is constantly transferred from cooling flue gases to heated air (gas) through the dividing wall.

Full quantity heat transferred in the recuperator is determined by the equation

Q = KΔ t av F ,

Where TO- total heat transfer coefficient from smoke to air (gas), characterizing the overall level of heat transfer in the recuperator, W/(m 2 -K);

Δ t avg- average (over the entire heating surface) temperature difference between flue gases and air (gas), K;

F- heating surface through which heat is transferred from flue gases to air (gas), m2.

Heat transfer in recuperators includes three main stages of heat transfer: a) from flue gases to the walls of recuperative elements; b) through the dividing wall; c) from the wall to the heated air or gas.

On the smoke side of the recuperator, heat from the flue gases to the wall is transferred not only by convection, but also by radiation. Therefore, the local heat transfer coefficient on the smoke side is equal to

where is the heat transfer coefficient from the flue gases to the wall

convection, W/(m 2 °C);

Heat transfer coefficient from flue gases to the wall

by radiation, W/(m 2 °C).

Heat transfer through the dividing wall depends on the thermal resistance of the wall and the condition of its surface.

On the air side of the recuperator, when heating the air, heat is transferred from the wall to the air only by convection, and when heating the gas - by convection and radiation. Thus, when air is heated, heat transfer is determined by the local convection heat transfer coefficient; if the gas is heated, then the heat transfer coefficient

All noted local heat transfer coefficients are combined into the total heat transfer coefficient

, W/(m 2 °C).

In tubular recuperators, the total heat transfer coefficient should be determined for a cylindrical wall (linear heat transfer coefficient)

, W/(m °C)

Coefficient TO called the heat transfer coefficient of the pipe. If it is necessary to attribute the amount of heat to the area of ​​the internal or external surface of the pipe, then the total heat transfer coefficients can be determined as follows:

,

Where a 1 - heat transfer coefficient per inside

pipes, W/(m 2 °C);

a 2 - the same, on the outside of the pipe, W/(m 2 °C);

r 1 and r 2 - respectively, the radii of the inner and outer

pipe surfaces, m. In metal recuperators, the value of the thermal resistance of the wall can be neglected , and then the total heat transfer coefficient can be written in the following form:

W/(m 2 °C)

All local heat transfer coefficients necessary to determine the value TO, can be obtained based on the laws of heat transfer by convection and radiation.

Since there is always a pressure difference between the air and smoke sides of the recuperator, the presence of leaks in the recuperative nozzle leads to air leakage, sometimes reaching 40-50%. Leaks sharply reduce the efficiency of recuperative installations; the more air sucked in, the lower the proportion of heat usefully used in the ceramic recuperator (see below):

Leakage, % 0 25 60

Final flue gas temperature,

°C 660 615 570

Air heating temperature, °C 895 820 770

Recuperator efficiency (without taking into account

losses), % 100 84 73.5

Air leakage affects the value of local heat transfer coefficients, and air trapped in the flue gases not only

Rice. 4. Schemes of the movement of gaseous media in recuperative heat exchangers

reduces their temperature, but also reduces the percentage of CO 2 and H 2 0, as a result of which the emissivity of gases deteriorates.

Both with an absolutely gas-tight recuperator and with a leak, the local heat transfer coefficients change along the heating surface, therefore, when calculating recuperators, the values ​​of the local heat transfer coefficients for the top and bottom are determined separately and then the total heat transfer coefficient is found using the average value.

LITERATURE

1. B.A.Arutyunov, V.I. Mitkalinny, S.B. Stark. Metallurgical heat engineering, vol. 1, M, Metallurgy, 1974, p. 672
2. V.A. Krivandin and others. Metallurgical heat engineering, M, Metallurgy, 1986, p. 591
3. V.A. Krivandin, B.L. Markov. Metallurgical furnaces, M, Metallurgy, 1977, p.463
4. V.A. Krivandin, A.V. Egorov. Thermal work and designs of ferrous metallurgy furnaces, M, Metallurgy, 1989, p.463

The flue gas condensation system allows the capture and recovery of large amounts of thermal energy contained in the wet boiler flue gas, which is usually discharged through the chimney into the atmosphere.

The heat recovery/flue gas condensation system makes it possible to increase heat supply to consumers by 6 - 35% (depending on the type of fuel burned and installation parameters) or reduce consumption natural gas by 6-35%

Main advantages:

• Fuel economy (natural gas) - the same or increased heat load of the boiler with less fuel combustion
• Reduction of emissions - CO2, NOx and SOx (when burning coal or liquid fuels)
• Obtaining condensate for the boiler make-up system

Principle of operation:

The heat recovery/flue gas condensation system can operate in two stages: with or without the use of an air humidification system supplied to the boiler burners. If necessary, a scrubber is installed before the condensation system.

In the condenser, the exhaust flue gases are cooled using return water from the heating network. When the temperature of the flue gases decreases, condensation occurs large quantity water vapor contained in the exhaust gas. The thermal energy of vapor condensation is used to heat the return heating network.

Further cooling of the gas and condensation of water vapor occurs in the humidifier. The cooling medium in the humidifier is the blast air supplied to the boiler burners. Since the blast air is heated in the humidifier, and warm condensate is injected into the air flow in front of the burners, an additional evaporation process occurs in the exhaust flue gas of the boiler.

The blown air supplied to the boiler burners contains increased amount thermal energy due to increased temperature and humidity.

This leads to an increase in the amount of energy in the exhaust flue gas entering the condenser, which in turn leads to more effective use heat from a centralized heating system.

In the flue gas condensation unit, condensate is also produced, which, depending on the composition of the flue gases, will be further purified before being fed into the boiler system.

Economic effect.

Comparison of thermal power under the following conditions:

1. No condensation
2. Flue gas condensation
3. Condensation together with humidification of the air supplied for combustion

The flue gas condensation system allows the existing boiler house to:

• Increase heat production by 6.8% or
• Reduce gas consumption by 6.8%, as well as increase revenue from the sale of CO,NO quotas
• Investment size is about 1 million euros (for a boiler house with a capacity of 20 MW)
• Payback period is 1-2 years.

Savings depending on the return temperature:

Use of flue gas heat in gas-fired industrial boiler houses

Use of flue gas heat in gas-fired industrial boiler houses

Candidate of Technical Sciences Sizov V.P., Doctor of Technical Sciences Yuzhakov A.A., Candidate of Technical Sciences Kapger I.V.,
Permavtomatika LLC, sizovperm@ mail .ru

Abstract: the price of natural gas varies significantly around the world. This depends on the country’s membership in the WTO, whether the country exports or imports its gas, gas production costs, the state of industry, political decisions, etc. The price of gas in the Russian Federation in connection with our country’s accession to the WTO will only increase and the government plans to equalize prices for natural gas both within the country and abroad. Let's roughly compare gas prices in Europe and Russia.

Russia – 3 rubles/m3.

Germany - 25 rubles/m3.

Denmark - 42 rubles/m3.

Ukraine, Belarus – 10 rubles/m3.

The prices are quite reasonable. In European countries, condensing-type boilers are widely used, their total share in the heat generation process reaches 90%. In Russia, these boilers are mainly not used due to the high cost of boilers, the low cost of gas and high-temperature centralized networks. And also by maintaining the system for limiting gas combustion in boiler houses.

Currently, the issue of more complete use of coolant energy is becoming increasingly relevant. The release of heat into the atmosphere not only creates additional pressure on the environment, but also increases the costs of boiler house owners. At the same time, modern technologies make it possible to more fully utilize the heat of flue gases and increase the efficiency of the boiler, calculated based on the lower calorific value, up to a value of 111%. Heat loss with flue gases occupies the main place among the heat losses of the boiler and amounts to 5 ¸ 12% of generated heat. In addition, the heat of condensation of water vapor that is formed during fuel combustion can be used. The amount of heat released during condensation of water vapor depends on the type of fuel and ranges from 3.8% for liquid fuels and up to 11.2% for gaseous fuels (for methane) and is defined as the difference between the higher and lower heat of combustion of the fuel (Table 1 ).

Table 1 - Values ​​of higher and lower calorific values ​​for various types fuel

It turns out that the exhaust gases contain both sensible and latent heat. Moreover, the latter can reach a value that in some cases exceeds sensible heat. Sensible heat is heat in which a change in the amount of heat supplied to a body causes a change in its temperature. Latent heat is the heat of vaporization (condensation), which does not change body temperature, but serves to change state of aggregation bodies. This statement is illustrated by a graph (Fig. 1, on which enthalpy (the amount of heat supplied) is plotted along the abscissa axis, and temperature is plotted along the ordinate axis).

Rice. 1 – Dependence of enthalpy change for water

Location on graphics A-B water is heated from a temperature of 0 °C to a temperature of 100 °C. In this case, all the heat supplied to the water is used to increase its temperature. Then the change in enthalpy is determined by formula (1)

(1)

where c is the heat capacity of water, m is the mass of the heated water, Dt – temperature difference.

Section of the B-C graph demonstrates the process of water boiling. In this case, all the heat supplied to the water is spent on converting it into steam, while the temperature remains constant - 100 ° C. Plot graphics C-D shows that all the water has turned into steam (boiled away), after which the heat is spent to increase the temperature of the steam. Then the enthalpy change for section A-C characterized by formula (2)

Where r = 2500 kJ/kg – latent heat of vaporization of water at atmospheric pressure.

The biggest difference between the highest and lowest calorific values, as can be seen from table. 1, methane, so natural gas (up to 99% methane) gives the highest profitability. From here, all further calculations and conclusions will be given for methane-based gas. Consider the combustion reaction of methane (3)

From the equation of this reaction it follows that for the oxidation of one methane molecule, two oxygen molecules are needed, i.e. For complete combustion of 1 m 3 of methane, 2 m 3 of oxygen is required. It is used as an oxidizer when burning fuel in boiler units. atmospheric air, which represents a mixture of gases. For technical calculations, the conditional composition of air is usually taken as consisting of two components: oxygen (21 vol. %) and nitrogen (79 vol. %). Taking into account the composition of the air, to carry out the combustion reaction, complete combustion of the gas will require a volume of air 100/21 = 4.76 times more than oxygen. Thus, to burn 1 m 3 of methane it will take 2 ×4.76=9.52 air. As you can see from the oxidation reaction equation, the result is carbon dioxide, water vapor (flue gases) and heat. The heat that is released during fuel combustion according to (3) is called the net calorific value of the fuel (PCI).

If you cool water vapor, then under certain conditions they will begin to condense (transition from gaseous state into liquid) and at the same time an additional amount of heat will be released (latent heat of vaporization/condensation) Fig. 2.

Rice. 2 – Heat release during condensation of water vapor

It should be borne in mind that water vapor in flue gases has slightly different properties than pure water vapor. They are in a mixture with other gases and their parameters correspond to the parameters of the mixture. Therefore, the temperature at which condensation begins is different from 100 °C. The value of this temperature depends on the composition of the flue gases, which, in turn, is a consequence of the type and composition of the fuel, as well as the excess air coefficient.
The temperature of the flue gases at which condensation of water vapor in the products of fuel combustion begins is called the dew point and looks like Fig. 3.

Rice. 3 – Dew point for methane

Consequently, for flue gases, which are a mixture of gases and water vapor, the enthalpy changes according to a slightly different law (Fig. 4).

Figure 4 – Heat release from the steam-air mixture

From the graph in Fig. 4, two important conclusions can be drawn. First, the dew point temperature is equal to the temperature to which the flue gases were cooled. Secondly, it is not necessary to go through it as in Fig. 2, the entire condensation zone, which is not only practically impossible but also unnecessary. This, in turn, provides various implementation options heat balance. In other words, almost any small volume of coolant can be used to cool flue gases.

From the above, we can conclude that when calculating the boiler efficiency based on the lower calorific value with subsequent utilization of the heat of flue gases and water vapor, the efficiency can be significantly increased (more than 100%). At first glance, this contradicts the laws of physics, but in fact there is no contradiction here. The efficiency of such systems must be calculated based on the higher calorific value, and determination of efficiency by lower calorific value it is necessary to carry out only if it is necessary to compare its efficiency with the efficiency of a conventional boiler. Only in this context does efficiency > 100% make sense. We believe that for such installations it is more correct to give two efficiencies. The problem statement can be formulated as follows. To more fully utilize the heat of combustion of flue gases, they must be cooled to a temperature below the dew point. In this case, the water vapor generated during gas combustion will condense and transfer the latent heat of vaporization to the coolant. In this case, cooling of flue gases should be carried out in heat exchangers special design, depending mainly on the temperature of the flue gases and the temperature of the cooling water. The use of water as an intermediate coolant is the most attractive, because in this case it is possible to use water with the lowest possible temperature. As a result, it is possible to obtain a water temperature at the outlet of the heat exchanger, for example, 54°C, and then use it. If the return line is used as a coolant, its temperature should be as low as possible, and this is often only possible if there are low-temperature heating systems as consumers.

Flue gases from high-power boiler units are usually discharged into a reinforced concrete or brick pipe. If special measures are not taken for the subsequent heating of partially dried flue gases, the pipe will turn into a condensation heat exchanger with all the ensuing consequences. There are two ways to solve this issue. The first way is to use a bypass, in which part of the gases, for example 80%, is passed through the heat exchanger, and the other part, in the amount of 20%, is passed through the bypass and then mixed with the partially dried gases. Thus, by heating the gases, we shift the dew point to the required temperature at which the pipe is guaranteed to operate in dry mode. The second method is to use a plate recuperator. In this case, the exhaust gases pass through the recuperator several times, thereby heating themselves.

Let's consider an example of calculating a 150 m typical pipe (Fig. 5-7), which has a three-layer structure. Calculations were performed in the software package Ansys -CFX . From the figures it is clear that the movement of gas in the pipe has a pronounced turbulent character and, as a consequence, minimum temperature on the lining may not be in the tip area, as follows from a simplified empirical method.

Rice. 7 – temperature field on the surface of the lining

It should be noted that when installing a heat exchanger in a gas path, its aerodynamic resistance will increase, but the volume and temperature of the exhaust gases will decrease. This leads to a decrease in the current of the smoke exhauster. The formation of condensate imposes special requirements on the elements of the gas path in terms of the use of corrosion-resistant materials. The amount of condensate is approximately 1000-600 kg/hour per 1 Gcal of useful heat exchanger power. The pH value of the condensate of combustion products when burning natural gas is 4.5-4.7, which corresponds to an acidic environment. In case of a small amount of condensate, it is possible to use replaceable blocks to neutralize the condensate. However, for large boiler houses it is necessary to use caustic soda dosing technology. As practice shows, small volumes of condensate can be used as make-up without any neutralization.

It should be emphasized that the main problem in the design of the systems noted above is the too large difference in enthalpy per unit volume of substances, and the resulting technical problem is the development of the heat exchange surface on the gas side. The industry of the Russian Federation mass-produces similar heat exchangers such as KSK, VNV, etc. Let's consider how developed the heat exchange surface on the gas side is on the existing structure (Fig. 8). An ordinary tube in which water (liquid) flows inside, and air (exhaust gases) flows from the outside along the fins of the radiator. The calculated heater ratio will be expressed by a certain

Rice. 8 – drawing of the heater tube.

coefficient

K =S nar /S vn, (4),

Where S nar – outer area of ​​the heat exchanger mm 2, and S vn – internal area of ​​the tube.

In geometric calculations of the structure we obtain K =15. This means that the outer area of ​​the tube is 15 times larger than the inner area. This is explained by the fact that the enthalpy of air per unit volume is many times less than the enthalpy of water per unit volume. Let's calculate how many times the enthalpy of a liter of air is less than the enthalpy of a liter of water. From

enthalpy of water: E in = 4.183 KJ/l*K.

air enthalpy: E air = 0.7864 J/l*K. (at a temperature of 130 0 C).

Hence the enthalpy of water is 5319 times greater than the enthalpy of air, and therefore K =S nar /S vn . Ideally, in such a heat exchanger, the coefficient K should be 5319, but since the outer surface in relation to the inner surface is developed 15 times, the difference in enthalpy essentially between air and water is reduced to the value K = (5319/15) = 354. Technically develop the ratio of the areas of the internal and external surfaces to obtain the ratio K =5319 very difficult or almost impossible. To solve this problem, we will try to artificially increase the enthalpy of air (exhaust gases). To do this, spray water (condensate of the same gas) from the nozzle into the exhaust gas. Let's spray it in such an amount relative to the gas that all the sprayed water will completely evaporate in the gas and the relative humidity of the gas will become 100%. The relative humidity of the gas can be calculated based on Table 2.

Table 2. Values ​​of absolute gas humidity with a relative humidity of 100% for water at various temperatures and atmospheric pressure.

From Fig. 3 it is clear that with a very high-quality burner, it is possible to achieve a dew point temperature in the exhaust gases T dew = 60 0 C. In this case, the temperature of these gases is 130 0 C. The absolute moisture content in the gas (according to Table 2) at T dew = 60 0 C will be 129,70 g/m 3 . If water is sprayed into this gas, its temperature will drop sharply, its density will increase, and its enthalpy will rise sharply. It should be noted that it makes no sense to spray water above 100% relative humidity, because... When the relative humidity threshold exceeds 100%, the sprayed water will stop evaporating into gas. Let us carry out a small calculation of the required amount of sprayed water for the following conditions: Tg – initial gas temperature equal to 120 0 C, T rise - gas dew point 60 0 C (129.70 g/m 3), required IT: Tgk - the final temperature of the gas and Mv - the mass of water sprayed in the gas (kg.)

Solution. All calculations are carried out relative to 1 m 3 of gas. The complexity of the calculations is determined by the fact that as a result of atomization, both the density of the gas and its heat capacity, volume, etc. change. In addition, it is assumed that evaporation occurs in an absolutely dry gas, and the energy for heating water is not taken into account.

Let's calculate the amount of energy given by the gas to water during the evaporation of water

where: c – heat capacity of gas (1 KJ/kg.K), m – gas mass (1 kg/m 3)

Let's calculate the amount of energy given up by water during evaporation into gas

Where: r – latent energy of vaporization (2500 KJ/kg), m – mass of evaporated water

As a result of substitution we get the function

(5)

It should be taken into account that it is impossible to spray more water than indicated in Table 2, and the gas already contains evaporated water. Through selection and calculations we obtained the value m = 22 g, Tgk = 65 0 C. Let's calculate the actual enthalpy of the resulting gas, taking into account that its relative humidity is 100% and when it is cooled, both latent and sensible energy will be released. Then according to we obtain the sum of two enthalpies. Enthalpy of gas and enthalpy of condensed water.

E voz = Eg + Evod

Eg we find from reference literature 1.1 (KJ/m 3 *K)

EvodWe calculate relative to the table. 2. Our gas, cooling from 65 0 C to 64 0 C, releases 6.58 grams of water. The enthalpy of condensation is Evod=2500 J/g or in our case Evod=16.45 KJ/m 3

Let's sum up the enthalpy of condensed water and the enthalpy of gas.

E voz =17.55 (J/l*K)

As we can see by spraying water, we were able to increase the enthalpy of the gas by 22.3 times. If before spraying water the gas enthalpy was E air = 0.7864 J/l*K. (at a temperature of 130 0 C). Then after sputtering the enthalpy is Evoz = 17.55 (J/l*K). This means that to obtain the same thermal energy on the same standard heat exchanger type KSK, VNV, the heat exchanger area can be reduced by 22.3 times. The recalculated coefficient K (the value was equal to 5319) becomes equal to 16. And with this coefficient, the heat exchanger acquires quite feasible dimensions.

Another important issue when creating such systems is the analysis of the spraying process, i.e. what diameter of a drop is needed when water evaporates in gas. If the droplet is small enough (for example, 5 μM), then the lifetime of this droplet in the gas before complete evaporation is quite short. And if the droplet has a size of, for example, 600 µM, then naturally it remains in the gas much longer before complete evaporation. The solution to this physical problem is quite complicated by the fact that the evaporation process occurs with constantly changing characteristics: temperature, humidity, droplet diameter, etc. For this process, the solution is presented in, and the formula for calculating the time of complete evaporation ( ) drops look like

(6)

Where: ρ and - liquid density (1 kg/dm 3), r – energy of vaporization (2500 kJ/kg), λ g – thermal conductivity of gas (0.026 J/m 2 K), d 2 – drop diameter (m), Δ t – average temperature difference between gas and water (K).

Then, according to (6), the lifetime of a droplet with a diameter of 100 μM. (1*10 -4 m) is τ = 2*10 -3 hours or 1.8 seconds, and the lifetime of a drop with a diameter of 50 µM. (5*10 -5 m) is equal to τ = 5*10 -4 hours or 0.072 seconds. Accordingly, knowing the lifetime of a drop, its flight speed in space, the speed of gas flow and the geometric dimensions of the gas duct, one can easily calculate the irrigation system for the gas duct.

Below we will consider the implementation of the system design taking into account the relations obtained above. It is believed that the flue gas heat exchanger must operate depending on the outside temperature, otherwise the house pipe will be destroyed when condensation forms in it. However, it is possible to manufacture a heat exchanger that operates regardless of the street temperature and has a better heat removal from exhaust gases, even to subzero temperatures, despite the fact that the temperature of the exhaust gases will be, for example, +10 0 C (the dew point of these gases will be 0 0 C). This is ensured by the fact that during heat exchange the controller calculates the dew point, heat exchange energy and other parameters. Let's consider the technological diagram of the proposed system (Fig. 9).

According to the technological diagram, the following are installed in the heat exchanger: adjustable dampers a-b-c-d; heat exchangers d-e-zh; temperature sensors 1-2-3-4-5-6; o Sprinkler (pump H, and a group of nozzles); control controller.

Let us consider the functioning of the proposed system. Let the exhaust gases escape from the boiler. for example, a temperature of 120 0 C and a dew point of 60 0 C (indicated in the diagram as 120/60). The temperature sensor (1) measures the temperature of the boiler exhaust gases. The dew point is calculated by the controller relative to the stoichiometry of gas combustion. A gate (a) appears in the path of the gas. This is an emergency shutter. which closes in the event of equipment repair, malfunction, overhaul, maintenance, etc. Thus, the damper (a) is fully open and directly passes the boiler exhaust gases into the smoke exhauster. With this scheme, heat recovery is zero; in fact, the flue gas removal scheme is restored as it was before the installation of the heat exchanger. In operating condition, the gate (a) is completely closed and 100% of the gases enter the heat exchanger.

In the heat exchanger, the gases enter the recuperator (e) where they are cooled, but in any case not below the dew point (60 0 C). For example, they cooled down to 90 0 C. No moisture was released in them. The gas temperature is measured by temperature sensor 2. The temperature of the gases after the recuperator can be adjusted with a gate (b). Regulation is necessary to increase the efficiency of the heat exchanger. Since during condensation of moisture, the mass present in gases decreases depending on how much the gases have been cooled, it is possible to remove up to 2/11 of the total mass of gases from them in the form of water. Where did this figure come from? Let's consider the chemical formula of the methane oxidation reaction (3).

To oxidize 1m 3 of methane, 2m 3 of oxygen is needed. But since the air contains only 20% oxygen, 10 m 3 of air will be required to oxidize 1 m 3 of methane. After burning this mixture we get: 1m 3 carbon dioxide, 2 m 3 of water vapor and 8 m 3 of nitrogen and other gases. We can remove from the exhaust gases by condensation just under 2/11 of all exhaust gases in the form of water. To do this, the exhaust gas must be cooled to outside temperature. With the release of the appropriate proportion of water. The air taken from the street for combustion also contains minor moisture.

The released water is removed at the bottom of the heat exchanger. Accordingly, if the entire composition of gases (11/11 parts) passes along the path of the boiler-recuperator (e)-heat recovery unit (e), then only 9/11 parts of the exhaust gas can pass along the other side of the recuperator (e). The rest - up to 2/11 parts of the gas in the form of moisture - can fall out in the heat exchanger. And to minimize the aerodynamic resistance of the heat exchanger, the gate (b) can be opened slightly. In this case, the exhaust gases will be separated. Part will pass through the recuperator (e), and part through the gate (b). When the gate (b) is fully opened, the gases will pass through without cooling and the readings of temperature sensors 1 and 2 will coincide.

An irrigation system with a pump H and a group of nozzles is installed along the path of the gases. Gases are irrigated with water released during condensation. Injectors that spray moisture into the gas sharply increase its dew point, cool it and compress it adiabatically. In the example under consideration, the gas temperature drops sharply to 62/62, and since the water sprayed in the gas completely evaporates in the gas, the dew point and the gas temperature coincide. Reaching the heat exchanger (e) hidden thermal energy stands out on it. In addition, the density of the gas flow increases abruptly and its speed decreases abruptly. All these changes significantly change the heat transfer efficiency for the better. The amount of water sprayed is determined by the controller and is related to the temperature and gas flow. The gas temperature in front of the heat exchanger is monitored by temperature sensor 6.

Next, the gases enter the heat exchanger (e). In the heat exchanger, the gases cool down, for example, to a temperature of 35 0 C. Accordingly, the dew point for these gases will also be 35 0 C. The next heat exchanger on the path of the exhaust gases is the heat exchanger (g). It serves to heat combustion air. The air supply temperature to such a heat exchanger can reach -35 0 C. This temperature depends on the minimum outside air temperature in a given region. Since some of the water vapor is removed from the exhaust gas, the mass flow of exhaust gases almost coincides with the mass flow of combustion air. Let the heat exchanger, for example, be filled with antifreeze. A gate (c) is installed between the heat exchangers. This gate also operates in discrete mode. When it warms up outside, there is no point in extracting heat from the heat exchanger (g). It stops its operation and the gate (c) opens completely, allowing exhaust gases to pass through, bypassing the heat exchanger (g).

The temperature of the cooled gases is determined by the temperature sensor (3). These gases are then sent to the recuperator (d). Having passed through it, they are heated to a certain temperature proportional to the cooling of the gases on the other side of the recuperator. The gate (g) is needed to regulate the heat exchange in the recuperator, and the degree of its opening depends on the outside temperature (from sensor 5). Accordingly, if it is very cold outside, then the gate (d) is completely closed and the gases are heated in the recuperator to avoid the dew point in the pipe. If it is hot outside, then gate (d) is open, as is gate (b).

CONCLUSIONS:

An increase in heat exchange in a liquid/gas heat exchanger occurs due to a sharp jump in gas enthalpy. But the proposed water spraying should occur in strictly measured doses. In addition, dosing of water into the exhaust gases takes into account the outside temperature.

The resulting calculation method allows one to avoid moisture condensation in chimney and significantly increase the efficiency of the boiler unit. A similar technique can be applied to gas turbines and other condenser devices.

With the proposed method, the design of the boiler does not change, but is only modified. The cost of modification is about 10% of the cost of the boiler. The payback period at current gas prices is about 4 months.

This approach can significantly reduce the metal consumption of the structure and, accordingly, its cost. In addition, the aerodynamic resistance of the heat exchanger drops significantly, and the load on the smoke exhauster is reduced.

LITERATURE:

1.Aronov I.Z. Use of heat from flue gases of gasified boiler houses. – M.: “Energy”, 1967. – 192 p.

2.Thaddeus Hobler. Heat transfer and heat exchangers. – Leningrad: State scientific publication of chemical literature, 1961. – 626 p.

Owners of patent RU 2436011:

The invention relates to thermal power engineering and can be used in any enterprise that operates boilers using hydrocarbon fuels. The objective of the invention is to increase the efficiency of using low-grade heat of condensation of water vapor contained in flue gases. The flue gas heat recovery device contains a gas-gas surface plate heat exchanger, in which the original flue gases are cooled, heating the dried flue gases in a countercurrent manner. Cooled wet flue gases are fed into a gas-air surface plate heat exchanger-condenser, where the water vapor contained in the flue gases is condensed, heating the air. The heated air is used to heat the premises and cover the needs of the gas combustion process in the boiler. The condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle. The dried flue gases are supplied by an additional smoke exhauster to the heater described above, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney. 2 n.p. f-ly, 1 ill.

The invention relates to thermal power engineering and can be used in any enterprise that operates boilers using hydrocarbon fuels.

A known boiler installation contains a contact water heater connected at the inlet to the exhaust gas duct of the boiler, and at the outlet through a gas outlet duct equipped with a smoke exhauster to the chimney, and an air heater with heating and air paths (USSR Copyright Certificate No. 1086296, F22B 1/18 dated 15.04. 1984).

Installation works in the following way. The main part of the gases from the boiler enters the exhaust flue, and the rest of the gases enter the heating duct. From the exhaust flue, the gases are directed to a contact water heater, where condensation of water vapor contained in the flue gases occurs. The gases then pass through the droplet eliminator and enter the gas outlet channel. Outside air enters the air heater, where it is heated by gases passing through the heating path, and is sent to the gas outlet channel, where it mixes with cooled gases and reduces the moisture content of the latter.

Flaws. Unacceptable quality of heated water for use in the heating system. The use of heated air only to supply it to the chimney in order to prevent condensation of water vapor. Low degree of heat recovery from flue gases, since the main task was to dry the flue gases and reduce the dew point temperature.

Known commercially produced by the Kostroma Heating Plant are heaters of the KSk type (Kudinov A.A. Energy saving in heat-generating installations. - Ulyanovsk: UlSTU, 2000. - 139, p. 33), consisting of a gas-water surface heat exchanger, the heat exchange surface of which is made of finned bimetallic tubes, strainer, distribution valve, drip eliminator and hydropneumatic blower.

Heaters of the KSk type work as follows. Flue gases enter the distribution valve, which divides them into two streams, the main gas flow is directed through strainer into the heat exchanger, the second - along the bypass line of the gas duct. In the heat exchanger, water vapor contained in the flue gases condenses on finned tubes, heating the water flowing in them. The resulting condensate is collected in a pan and pumped into the heating network feed circuit. The water heated in the heat exchanger is supplied to the consumer. At the outlet of the heat recovery unit, the dried flue gases are mixed with the original flue gases from the flue bypass line and sent through a smoke exhauster into the chimney.

Flaws. For the heat exchanger to operate in the condensation mode of its entire convective part, it is required that the heating temperature of the water in the convective package does not exceed 50°C. To use such water in heating systems, it must be additionally heated.

To prevent condensation of residual water vapor of flue gases in the flues and chimney, part of the source gases is mixed through a bypass channel with the dried flue gases, increasing their temperature. With such an admixture, the content of water vapor in the exhaust flue gases also increases, reducing the efficiency of heat recovery.

A known installation for recycling heat from flue gases (RF patent No. 2193727, F22B 1/18, F24H 1/10 dated April 20, 2001), containing a sprinkler installed in the gas duct with distribution nozzles, a recovery heat exchanger and an intermediate coolant heat exchanger, the heated path of which is connected at the inlet to the moisture collector. The sprinkler is located in front of the specified heat exchangers, installed one opposite the other at the same distance from the sprinkler, the nozzles of which are directed in the direction opposite to the heat exchangers. The installation is additionally equipped with a heat exchanger for reheating the irrigation water installed in the gas duct and located above the sprinkler, the heated path of which is connected at the inlet to the heat exchanger of the intermediate coolant, and at the outlet to the sprinkler. All heat exchangers are surface, tubular. The tubes can be finned to increase the heating surface.

There is a known method of operation of this installation (RF patent No. 2193728, F22B 1/18, F24H 1/10 dated April 20, 2001), according to which the flue gases passing through the flue duct are cooled below the dew point and removed from the installation. In the installation, water is heated in a recovery heat exchanger and distributed to the consumer. The outer surface of the recovery heat exchanger is irrigated with an intermediate coolant - water from a sprinkler with distributing nozzles directed towards the flow of gases. In this case, the intermediate coolant is preheated in a heat exchanger installed in the gas duct opposite the recovery heat exchanger and at the same distance from the sprinkler as the recovery heat exchanger. Then the intermediate coolant is supplied to a heat exchanger for reheating the irrigating water installed in the gas duct and located above the sprinkler, heated to the required temperature and sent to the sprinkler.

Two independent arcs of water flow in the installation: clean, heated through a heat transfer surface, and irrigating, heated as a result of direct contact with exhaust gases. A clean stream of water flows inside the tubes and is separated by walls from the contaminated stream of irrigation water. A bundle of tubes performs the function of a nozzle designed to create a developed contact surface between irrigation water and exhaust gases. The outer surface of the nozzle is washed by gases and irrigating water, which intensifies heat exchange in the apparatus. The heat of the exhaust gases is transferred to the water flowing inside the tubes of the active nozzle in two ways: 1) due to the direct transfer of heat from the gases and irrigating water; 2) due to condensation on the surface of the nozzle of part of the water vapor contained in the gases.

Flaws. The final temperature of the heated water at the outlet of the nozzle is limited by the wet thermometer temperature of the gases. When burning natural gas with an excess air ratio of 1.0-1.5, the wet bulb temperature of the flue gases is 55-65°C. This temperature is not sufficient to use this water in the heating system.

Flue gases leave the apparatus with a relative humidity of 95-100%, which does not exclude the possibility of condensation of water vapor from the gases in the exhaust tract after it.

The closest to the claimed invention in terms of use, technical essence and achieved technical result is a heat exchanger (RF patent No. 2323384, F22B 1/18 dated 08/30/2006), containing a contact heat exchanger, a droplet eliminator, a gas-gas heat exchanger connected according to a direct flow circuit, gas ducts, pipelines, pump, temperature sensors, control valves. Along the flow of circulating water of the contact heat exchanger, a water-to-water heat exchanger and a water-to-air heat exchanger with a bypass channel along the air flow are located in series.

Method of operation of the heat exchanger. The exhaust gases through the gas duct enter the inlet of the gas-gas heat exchanger, sequentially passing through its three sections, then to the inlet of the contact heat exchanger, where, passing through a nozzle washed by circulating water, they are cooled below the dew point, giving off sensible and latent heat to the circulating water. Next, the cooled and moist gases are freed from most of the liquid water carried away in a droplet eliminator, heated and dried in at least one section of the gas-gas heat exchanger, sent into a chimney by a smoke exhauster and released into the atmosphere. At the same time, heated circulating water from the sump of the contact heat exchanger is pumped into the water-water heat exchanger, where it heats cold water from the pipeline. The water heated in the heat exchanger is supplied to the needs of process and domestic hot water supply or to a low-temperature heating circuit.

Next, the recycled water enters the water-air heat exchanger, heats at least part of the blown air coming from outside the room through the air duct, cooling to the minimum possible temperature, and enters the contact heat exchanger through the water distributor, where it takes heat from the gases, simultaneously washing them from suspended particles, and absorbs some of the oxides of nitrogen and sulfur. The heated air from the heat exchanger is supplied by a blower fan to a standard air heater or directly to the firebox. Recycled water is filtered if necessary and processed by known methods.

The disadvantages of this prototype are:

The need for a control system due to the use of recovered heat for hot water supply purposes due to the variability of the daily schedule of hot water consumption.

Water heated in the heat exchanger, supplied for the needs of hot water supply or in a low-temperature heating circuit, requires it to be brought to the required temperature, since it cannot be heated in the heat exchanger above the temperature of the return circuit water, which is determined by the saturation temperature of water vapor in the flue gases. The low heating of the air in the water-to-air heat exchanger does not allow this air to be used for space heating.

The task set is to simplify the heat recovery technology and increase the efficiency of using low-grade heat of condensation of water vapor contained in flue gases.

This problem is solved in the following way.

A device for heat recovery from flue gases is proposed, containing a gas-gas heat exchanger, a condenser, an inertial drip eliminator, gas ducts, air ducts, fans and a pipeline, characterized in that the gas-gas surface plate heat exchanger is made according to a counterflow circuit, a surface gas-air plate heat exchanger is installed as a condenser, in An additional smoke exhauster is installed in the gas duct of cold dried flue gases; a gas duct for mixing part of the heated dried flue gases is installed in front of the additional smoke exhauster.

A method of operation of a flue gas heat recovery device is also proposed, according to which the flue gases are cooled in a gas-gas heat exchanger, heating the dried flue gases, water vapor contained in the flue gases is condensed in the condenser, a part of the blast air is heated, characterized in that in the gas-gas in a heat exchanger, the dried flue gases are heated by cooling the original flue gases using a counterflow scheme without regulating the gas flow rate, water vapor is condensed in a surface gas-air plate heat exchanger-condenser, heating the air and the heated air is used for heating and covering the needs of the combustion process, and the condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle, in the gas duct of cold dried flue gases, the aerodynamic resistance of the gas path is compensated by an additional smoke exhauster, in front of which a portion of the heated dried flue gases is mixed, excluding the condensation of residual water vapor carried away by the flow from the condenser; the temperature of the heated air is regulated using changes in the speed of the smoke exhauster depending on the outside air temperature.

The source flue gases are cooled in a gas-gas surface plate heat exchanger, heating the dried flue gases.

The difference is the use of a surface plate heat exchanger without any gas flow control devices, where the heating medium (the entire volume of wet flue gases) and the heated medium (the entire volume of dried flue gases) move in countercurrent. In this case, a deeper cooling of the moist flue gases occurs to a temperature close to the dew point of water vapor.

Next, the water vapor contained in the flue gases is condensed in a gas-air surface plate heat exchanger-condenser, heating the air. The heated air is used to heat rooms and cover the needs of the combustion process. The condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle.

The difference between the proposed method is that the heated medium is cold air supplied by fans from environment. The air is heated by 30-50°C, for example from -15 to 33°C. The use of air with a negative temperature as a cooling medium allows you to significantly increase the temperature pressure in the condenser when using counterflow. Air heated to 28-33°C is suitable for space heating and supply to the boiler to ensure the combustion of natural gas. Thermal calculation of the circuit shows that the flow rate of heated air is 6-7 times higher than the flow rate of the original flue gases, which makes it possible to completely cover the needs of the boiler, heat the workshop and other premises of the enterprise, and also supply part of the air to the chimney to reduce the dew point temperature or to a third-party consumer .

The aerodynamic resistance of the gas path in the flue of cold, dried flue gases is compensated by an additional smoke exhauster. To prevent condensation of residual water vapor carried away by the flow from the condenser, a portion of the heated, dried flue gases (up to 10%) is mixed in front of the additional smoke exhauster. The temperature of the heated air is regulated by changing the flow rate of the dried flue gases, by adjusting the speed of the smoke exhauster depending on the outside air temperature.

The dried flue gases are supplied by a smoke exhauster to the heater described above, where they are heated to prevent possible condensation of water vapor in the flues and chimney and are directed into the chimney.

The flue gas heat recovery device shown in the drawing contains a gas duct 1 connected to a heat exchanger 2, which is connected to a condenser 4 through a gas duct 3. The condenser 4 has an inertial drop catcher 5 and is connected to the condensate discharge pipeline 6. The fan 7 is connected to a cold air duct 8 with condenser 4. Condenser 4 is connected by air duct 9 to the heat consumer. The dry flue gas duct 10 is connected to the heat exchanger 2 through a smoke exhauster 11. The dry heated flue gas duct 12 is connected to the heat exchanger 2 and directed into the chimney. The gas duct 12 is connected to the gas duct 10 by an additional gas duct 13, which contains a damper 14.

Heat exchanger 2 and condenser 4 are surface plate heat exchangers made of unified modular packages, which are arranged in such a way that the coolant flows countercurrently. Depending on the volume of flue gases to be dried, the heater and condenser are formed from a calculated number of packages. Block 7 is formed from several fans to change the flow of heated air. Condenser 4 at the outlet of the dried flue gases has an inertial drop catcher 5, made in the form of vertical louvers, behind which a gas duct 10 is embedded. A damper 14 is installed on the gas duct 13 for the initial adjustment of the temperature reserve, which prevents condensation of residual water vapor in the smoke exhauster 11.

Method of operation of a flue gas heat recovery device.

Wet flue gases enter heat exchanger 2 through flue 1, where their temperature is reduced to a temperature close to the dew point. The cooled flue gases through the flue 3 enter the condenser 4, where the water vapor contained in them is condensed. The condensate is discharged through pipeline 6 and, after additional processing, is used to replenish losses in the heating network or steam turbine cycle. The heat of condensation is used to heat cold air, which is supplied by fans 7 from the environment. Heated air 9 is directed to the production room of the boiler room for its ventilation and heating. From this room, air is supplied to the boiler to ensure the combustion process. Dried flue gases 10 pass through an inertial drip eliminator 5, and are supplied by a smoke exhauster 11 to a heat exchanger 2, where they are heated and directed into the chimney 12. Heating of the dried flue gases is necessary to prevent condensation of residual water vapor in the flues and chimney. To prevent drops of moisture from falling out in the smoke exhauster 11, carried away by the dried flue gas flow from the condenser, part of the heated dry flue gases (up to one tenth) from the flue 12 through the flue 13 is supplied to the flue 10, where the entrained moisture evaporates.

The temperature of the heated air is regulated by changing the flow rate of the dried flue gases by changing the speed of the smoke exhauster 11 depending on the outside air temperature. With a decrease in the flow rate of wet flue gases, the aerodynamic resistance of the gas path of the device decreases, which is compensated by a decrease in the speed of the smoke exhauster 11. The smoke exhauster 11 provides a difference in the pressure of the flue gases and air in the condenser in order to prevent flue gases from entering the heated air.

A verification calculation shows that for a natural gas boiler with a power of 6 MW, with a flow rate of wet flue gases of 1 m 3 / s with a temperature of 130 ° C, the air is heated from -15 to 30 ° C, with a flow rate of 7 m 3 / s. The condensate flow rate is 0.13 kg/s, the temperature of the dried flue gases at the outlet of the heater is 86°C. The thermal power of such a device is 400 kW. total area heat exchange surface 310 m2. The dew point temperature of water vapor in flue gases decreases from 55 to 10°C. The boiler efficiency increases by 1% only due to the heating of cold air in the amount of 0.9 m 3 /s required for the combustion of natural gas. At the same time, heating this air accounts for 51 kW of the device’s power, and the rest of the heat is used for air heating premises. The results of calculations of the operation of such a device at various outdoor temperatures are given in Table 1.

Table 2 shows the results of calculating variants of the device for other flow rates of dried flue gases, at an outside air temperature of -15°C.

1. A device for recovering heat from flue gases, containing a gas-gas heat exchanger, a condenser, an inertial drop eliminator, gas ducts, air ducts, fans and a pipeline, characterized in that the gas-gas surface plate heat exchanger is made according to a counterflow circuit, and a surface gas-air heat exchanger is installed as a condenser plate heat exchanger, an additional smoke exhauster is installed in the gas duct of cold dried flue gases, a gas duct for mixing part of the heated dried flue gases is installed in front of the additional smoke exhauster.

2. The method of operation of the flue gas heat recovery device, according to which the flue gases are cooled in a gas-gas heat exchanger, heating the dried flue gases, condensing water vapor contained in the flue gases in the condenser, heating part of the blast air, characterized in that in the gas-gas in a heat exchanger, the dried flue gases are heated by cooling the original flue gases using a counterflow scheme without regulating the gas flow rate, water vapor is condensed in a surface gas-air plate heat exchanger-condenser, heating the air and the heated air is used for heating and covering the needs of the combustion process, and the condensate after additional processing is used to make up for losses in the heating network or steam turbine cycle, in the gas duct of cold dried flue gases, the aerodynamic resistance of the gas path is compensated by an additional smoke exhauster, in front of which a portion of the heated dried flue gases is mixed, excluding the condensation of residual water vapor carried away by the flow from the condenser; the temperature of the heated air is regulated using changes in the speed of the smoke exhauster depending on the outside air temperature.

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