RU2294487C2 - Method and device for heating air by flue gas - Google Patents

Abstract

FIELD: heat power engineering.

SUBSTANCE: method comprises heating air by means of its direct contact with the flue gas via convection and radiation. The device comprises gas box through which branch pipes for cold air pass and which is provided with slot nozzles for flowing cold air at its ends. The nozzles are parallel to the direction of the flue gas.

EFFECT: enhanced efficiency.

2 dwg

Description

The present invention relates to a power system, namely to the use of flue gas heat from boiler units and industrial furnaces when heating the air supplied to combustion.

A known method of heating air with flue gases by jet supplying it to a heat exchange surface in an air heater containing interconnected and adjacent to the gas duct air chambers, each of which is connected to its air collector through a perforated partition installed along the working wall (heat exchange surface) of the chamber at a given distance with staggered holes [1].

The disadvantages of the known method and device are the inability to use jet air distribution to increase the heat transfer rate by direct contact of air with flue gases and their radiation, as well as to reduce harmful impurities in flue gases, the creation by air chambers of an intermediate link between flue gases and the air supply system, the presence of a heat exchange surface, which increases the metal consumption of the device and, accordingly, complicates the design and increases the corrosion th metal wear increases the aerodynamic resistance of the device, which reduces its reliability and efficiency.

Closer in technical essence to the present invention is a method of heating air with combustion products (flue gases), comprising transferring heat to heated air, which is supplied tangentially to the air box with the formation of a cylindrical vortex, from flue gases by convective heat transfer and radiation through the heat exchange wall of the box, made from a metal mesh, by direct contact of a longitudinally moving stream of flue gases with an air cylindrical vortex, carried out in troystve (recuperator) comprising coaxially mounted inner and outer boxes forming the flue and the air channels separated by heat transfer wall made of metal mesh, and an air duct is provided with a tangentially mounted inlet and outlet [2].

The main disadvantages of this method are the tangential entry and exit of air into the air chamber, which determines the formation of a cylindrical air vortex in the zone of contact of the flue gas with the air flow, the formation of gas and air vortices at the edges of the mesh cells, which causes intensive mixing of the gas and air flows and, accordingly, averaging concentrations of the components contained in them, the absence of jet air distribution, reducing the contact surface of gas and air flows, h o, ultimately limits the heating air temperature.

The main disadvantages of the known device are the tangential placement of the inlet and outlet air pipes, the presence of a heat exchange wall made of a metal mesh separating the gas and air ducts and subject to intense corrosion wear, the absence of structures for air jet distribution, which reduces the efficiency and reliability of the device.

The technical problem to which the invention is directed is to reduce the mixing of gas and air flows while increasing the temperature of heating the air, as well as increasing the reliability and efficiency of the device.

The technical result is achieved by the fact that the proposed method includes the transfer of heat to heated air in direct contact with a longitudinally moving stream of flue gases by convective heat transfer and radiation, the air stream being distributed at the inlet of the gas box to flat jets moving parallel to each other and to the gas flow in one direction with it and dragging it along with it, which in the course of movement compress several times with the formation of a secondary jet and collect in one stream at the outlet of the gas oroba.

A device that implements this method includes a gas duct, air inlet and outlet, and at the entrance of the flue gases into the gas duct through its bottom from the cold air manifold, cold air nozzles are passed, ending in slotted cold air nozzles placed parallel to each other and directed to the side of the flue gas movement, coaxially opposite each cold air nozzle, intermediate confuser traps with inlet sections similar to the plane stream section are arranged, ending A series of slotted intermediate nozzles directed towards the movement of the flue gases, and at the exit of the flue gases from the gas duct opposite each intermediate nozzle, a series of hot air traps with inlet sections similar to the section of the confuser trap connected through hot air pipes to the hot air collector are placed.

The method is implemented in the device, which is shown in figures 1-4.

The device comprises a gas box 1, at the beginning of which cold air pipes 3 are passed through the bottom from the cold air collector 2, ending with slotted cold air nozzles 4 arranged parallel to each other and directed towards the flue gas movement, arranged coaxially opposite each cold air nozzle 4 intermediate traps-confusers 5, ending with intermediate slotted nozzles 6, and at the end of the gas box 1, a series of hot air traps 7 are placed, similar to the intermediate traps -confusers 5 and connected through hot air nozzles 8 to the hot air collector 9.

The proposed method is based on the properties of a flooded turbulent stream of air, in particular a flat stream, which, propagating in the direction of flow, mixes with the surrounding gas medium, and mixing is accompanied by the involvement of the masses of the gas medium, the message of the peripheral part of the gas medium moving in the same direction jets. At the same time, along with the mixing of the boundary layers of the air stream and the gas medium, intense heat exchange occurs between them, significantly exceeding the rate of heat transfer through the wall, since in this case there is no thermal resistance of the wall with contaminants and convective heat transfer is carried out directly between the air and gas particles, and also begins to play a significant role radiant heat transfer, which leads to a quick equalization of the temperature of the air stream and the gas environment [3, p. 326-339], [4, p. 50-60]. In addition, partial mixing of air with flue gases and the subsequent use of the resulting mixture for combustion can reduce the content of flue gases NO _x and SO _x [5, p. 457]. To reduce the mixing of an air stream with flue gases while ensuring a high temperature of its heating, re-compression of the same air stream is used, which allows to reduce the velocity gradient on the axis of the stream and, accordingly, the flue gas impurities in it [6, p. 378].

The proposed method for heating air with flue gases is carried out in the proposed device as follows. Flue gases at a vacuum corresponding to the operating mode of the boiler unit or industrial furnace, enter the gas box 1, where it is also a high-pressure fan through the cold air manifold 2 through the inlet pipes 3 from the slot nozzles 4, the amount of which is selected based on the conditions for creating stable flat jets with sufficient air speed for subsequent compressions and the formation of repeated jets, the air is supplied in the form of parallel flat jets heated from all sides by moving flue gases, carried away by mi jets, which then fall into the intermediate traps-confusers 5. In this case, in the boundary layers, a certain part of the flue gases is partially mixed and involved in the air jets, intense convective and radiant heat exchange between the flue gases and air and, accordingly, rapid heating of the air jets and flue gas cooling. Air jets heated to an intermediate temperature and partially mixed with flue gases in the intermediate confusion traps 5, the distance to which X is determined on the basis of the conditions for providing sufficient kinetic energy of the jet for re-compression and expiration of the repeated jets at a given intermediate mixing and the corresponding heating temperature, are compressed and flow from the intermediate nozzles 6 in the form of repeated flat jets. In this case, the inlet cross-sectional areas of the confuser traps 5 should correspond to a given air flow rate at a jet speed at a distance from the nozzle X, and the cross-sectional area of the intermediate nozzles 6 should create stable repeated flat jets with an air velocity less than the initial jet, but sufficient for subsequent compression and outflow which interact with flue gases in the same way as described above. Air jets heated to the required temperature with some admixture of flue gases, the amount of which is set from the required recirculation, fall into the rectangular inlet openings of the hot jets traps 7, located similarly to the confuser traps 5, with an area corresponding to the flow of hot air at the speed of entry of the jet into the traps 7 , where, through the outlet pipes 8 and the hot air manifold 9, air is supplied for the combustion process to the furnace of the boiler unit or industrial furnace, and the cooled flue gases leaving from the gas conduit 1 at a speed greater than that at the inlet of it, due to the transfer of part of the energy of the air jets.

The effectiveness of the proposed method and device can be illustrated by the example of the interaction of one plane stream of air with flue gases.

Initial data:

The velocity of the flue gases in the box 1, u _n = 10 m / s;

The velocity of the outflow of air from the nozzle 4, 6, u _about = 60 m / s;

The temperature of the hot flue gas, t _g = 170 ° C;

Cold air temperature, t _hv = 25 ° С:

The concentration of flue gases in hot air is 20 vol.%;

The width of the exit slit of the nozzle 4, 6, 2V = 0.02 m.

Decision

The calculation of the parameters of the air stream, the concentration of impurities (flue gases), the temperature of hot air was carried out according to the equations given in the literature [4, p.52-53], [6, p.374-380].

Lower jet length limits

где

Where

m = 2.62; n = 2.49 - complex coefficients.

The axial temperature of the jet at the entrance to the trap 5, 7

где

Where

θ ₀ is the axial temperature of the jet at the exit of the nozzle 4, 6, ° C.

Axial velocity at the entrance to the trap 5, 7

The change in the concentration of flue gas impurities in the jet at a distance X

где

Where

- безразмерная концентрация воздуха в струе на расстоянии X;

- dimensionless concentration of air in the stream at a distance X;

, где

where

- безразмерное измененние скорости струи относительно газовой среды на расстоянии X;

- dimensionless variation of the jet velocity relative to the gaseous medium at a distance X;

где

Where

где

Where

- безразмерное изменение скорости струи на расстоянии X;

- dimensionless variation of the jet velocity at a distance X;

The calculation results with a jet length of X = 0.15 m (X _{min (m)} = 0.137 m, X _{min (n)} = 0.124 m) are given in table.

Table No. nozzles in order Initial jet velocity Jet temperature, ° С The concentration of flue gases (air) in the stream, vol. share m / s At the exit of the nozzle At the entrance to the trap At the exit of the nozzle At the entrance to the trap one 60 25 38 0 (1.0) 0.047 (0.953) 2 57.5 38 fifty 0.047 (0.953) 0.088 (0.912) 3 55.2 fifty 61 0.088 (0.912) 0.13 (0.87) four 53.1 61 71 0.13 (0.87) 0.17 (0.83) 5 51.0 71 80 0.17 (0.83) 0.20 (0.80)

Thus, at a given concentration of flue gases in hot air (20 vol.%), Air can be heated from 25 ° C to 80 ° C (the flue gases are cooled from 170 ° C to 120 ° C) using 4-fold intermediate compression jets.

For similar conditions, we calculated the heating of the air stream from 25 ° C to 80 ° C without intermediate compression. The following results were obtained:

The length of the jet, X = 0.32 m;

The concentration of flue gases is 42 vol.%.

Comparison of the calculation results shows that re-compression of the air stream allows several times to reduce the concentration of impurities (flue gases) at the same temperature of heating the air.

Thus, the proposed method of heating air with flue gases and a device for its implementation can reduce the mixing of gas and air flows with a simultaneous increase in the temperature of heating the air and, therefore, improve the quality of hot air, environmental performance of cooled flue gases, reduce corrosion of metal, reduce aerodynamic device resistance and, ultimately, increase its reliability and efficiency.

Literature

1. A.S. USSR No. 964356, M class. F 23 L 15/04/1982.

2. A.S. USSR No. 1370372, M class. F 23 L 15/04, 1988.

3. A. D. Altshul et al. Hydrodynamics and aerodynamics. - M.: Stroyizdat, 1983, 415 p.

4. I.A. Shepelev. Aerodynamics of air flows indoors. - M.: 4 Stroyizdat, 1978, 145 p.

5. G. M. Delyagin and others. Heat-generating installations. - M.: Stroyizdat, 1986, 560 p.

6. G.N. Abramovich. Applied gas dynamics. - Science, 1976, 888 p.

Claims (2)

1. A method of heating air with flue gases, comprising transferring heat to heated air in direct contact with a longitudinally moving stream of flue gases by convective heat transfer and radiation, characterized in that the air stream at the inlet of the gas box is distributed on plane jets moving parallel to each other and the gas stream in one direction with it, carrying it along with them, which, in the course of their movement, are compressed several times and collected in one stream at the outlet of the gas box. 2. A device that implements the specified method, including a gas duct, inlet and outlet air nozzles, characterized in that at the inlet of the flue gases into the gas duct through its bottom from the cold air collector, cold air nozzles are passed, ending in slotted cold air nozzles arranged parallel to each other to the friend and directed towards the movement of flue gases, coaxially opposite each nozzle of cold air, intermediate confuser traps are arranged with input sections similar to the section of flat steel ruins ending with slotted intermediate nozzles directed towards the movement of the flue gases, and at the exit of the flue gases from the gas duct opposite each intermediate nozzle there is a series of hot air traps with inlet sections similar to the cross section of the confuser trap connected through hot air pipes to the hot air collector .

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