RU2253078C2 - Heat exchanger for cooling steam-gas mixture - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: heat exchanger comprises pipes with spiral-ring fins. The fins are provided with longitudinal slots. The pipes in the heat exchanger are arranged vertically.

EFFECT: enhanced efficiency.

3 dwg

Description

The invention is intended for energy and heating boilers burning gaseous fuels, and provides an increase in the efficiency of the boilers due to deeper cooling of the combustion products and condensation of the water vapor contained in them.

In a number of industries, heat exchangers are used, made of pipes with spiral-ring rolled or wound and welded ribs [Kuntysh VB, Kuznetsov NM Thermal and aerodynamic calculation of finned air-cooled heat exchangers. Energoatomizdat, St. Petersburg, 1988, 278 p.].

Recently, such heat exchangers have also been used for deeper cooling of flue gases behind steam and hot water boilers burning natural gas [Kudinov AA, Antonov VA, Alekseev Yu.A. Analysis of the effectiveness of the use of a condensation heat exchanger behind a steam boiler DE-10-14GM // Industrial Energy. - 1997, No. 8; Gomon V.I., Presich G.A., Navrodskaya R.A. Utilization of flue gas heat using surface (including condensation) and contact heat exchangers. On Sat “Materials of the workshop“ Modern boiler equipment - efficiency, safety and environmental friendliness ”” - Kiev, 1996, p.31-37].

When using water having a temperature below the dew point temperature as a cooling agent, water vapor contained in a gas-vapor mixture condenses on a fin surface. In this case, not only the “physical” heat of gases, but also the so-called “latent” heat of vaporization, whose value often exceeds the “physical” heat, is useful. So, when cooling natural gas combustion products leaving the boiler from 153 to 50 ° С with water at a temperature of 5-20 ° С, the amount of heat obtained by cooling dry gases is 2153 kilojoules per cubic meter of (normal) burnt gas, and the amount of heat , released due to condensation of water vapor, is equal to - 3320 kilojoules per cubic meter.

Due to the condensation of water vapor on cooling surfaces, the gas is dried during cooling. Figure 1 shows the design diagram of a heat exchanger for cooling a gas-vapor mixture (the heat exchanger is shown in the form of six transversely flowing ribbed tubes) and the formula we obtained gives the dependence of the concentration of water vapor C in a gas-vapor mixture on its temperature T at a known vapor concentration C _о and the temperature of the mixture T _about at the entrance to the heat exchanger and a given temperature T _{article the} surface of the fins of the heat exchanger. The concentration of steam C _article in the immediate vicinity of the surface is equal to the concentration of saturated steam at a temperature T _article .

The calculation results according to this formula are shown in figure 2 for the following values of the temperature of the mixture T _o at the inlet to the heat exchanger and the temperature of the cooling surface (wall of the ribs) T _article : 1 - T _about = 430 K, T _article = 290 K; 2 - T _about = 430 K, T _article = 280 K; 3 - T _about = 380 K, T _article = 290 K; 4 - T _o = 430 K, T _article = 280 K. Line 5 gives the dependence of the concentration of saturated steam (in g / m ³ ) on temperature (in ° C). The calculations were performed for natural gas combustion products containing 17% water vapor by volume at the inlet of the heat exchanger.

Figure 2 shows that when the natural gas combustion products are cooled to at least 40-50 ° C, the steam remaining in them is in an overheated state, that is, the saturation temperature of this steam (at its partial pressure) remains below the temperature (gas-vapor mixture) . This eliminates condensation on the uncooled walls of the gas ducts and the chimney, through which the combustion products cooled in the heat exchanger are released into the atmosphere.

The above considerations are valid in the absence of entrainment of water droplets and splashes from the heat exchange surface.

In the analogs described in the above references to the literature, the axes of the finned tubes are arranged horizontally in the heat exchanger, respectively, the ribs are in vertical planes, almost perpendicular to the axes of the tubes. In the tube bundle, the condensate formed on the edges of the overlying pipes flows in the form of droplets onto the underlying pipes, and from the pipes of the lowest row to the lower metal fence of the heat exchanger. At the same time, a part of small drops and splashes will inevitably be carried away by the gas flow.

Evaporation of entrained drops does not reduce the amount of heat received in the heat exchanger due to steam condensation if it (evaporation) occurs after the heat exchanger. However, it increases the degree of saturation of gases with water vapor, both due to an increase in the amount of steam in the gas, and due to a decrease in the temperature of the gas due to heat consumption for evaporation. All this can lead to condensation of water vapor on the inner walls of the flues and chimney.

The closest in technical essence and the achieved result to the proposed design should be considered the heat exchanger-heat exchanger described in [Kudinov AA, Antonov VA, Alekseev Yu.A. Analysis of the effectiveness of the use of a condensation heat exchanger behind a steam boiler DE-10-14GM // Industrial Energy. - 1997, No. 8], which is selected as a prototype.

This heat exchanger consists of bimetallic pipes with knurled fins located horizontally in the form of packets in the boiler duct. Cold water is pumped through the pipes at a temperature below the dew point temperature of the combustion products washing the pipes. Water vapor contained in the products of combustion condenses on the vertical ribs and flows in the form of drops from the ribs of the upper rows to the lower ones, while part of the droplets and splashes are carried away by a transverse horizontally directed stream of combustion products. This is a disadvantage of the prototype.

The proposed solution allows to eliminate the entrainment of drops and spray of condensate formed on the surface of the finned heat exchanger tubes. The essence of the proposed technical solution (Fig.3-5) lies in the fact that in the known device, which is a heat exchanger for cooling a vapor-gas mixture, consisting of vertical pipes with spiral-ring ribs, i.e. transverse with respect to the axis of the pipe, along the axis of the pipes 3, a longitudinal slot 4 is made in all the ribs 1. The slot 4 can be made, for example, by means of a disk cutter moved along the pipe. In this case, the metal of the supporting pipe 3 is not affected so that the structural strength does not decrease, only the ribs and the inserted pipe 2 are cut out, from which the ribs are rolled.

Since during screw rolling or winding, the ribs are slightly inclined to the axis of the pipe, in the vertical position of the pipe they have an inclination with respect to the horizontal, so that the condensate formed on them will drain under the action of capillary forces to the slot in the rib, and then through the gap to the lower pipe a heat exchanger board in which finned tubes are fixed. This eliminates the flow of condensate in the form of droplets from pipe to pipe, and therefore, the entrainment of drops.

Disruption of a film of water from a horizontal surface by a parallel flow of combustion products washing it begins according to [J.J. van Rossum Experimental investigation of horizontal liquid films // Chemical Engineering Science, 1959. V. 11. P. 35 - 52] at a flow velocity exceeding 18 m / s, even with a condensate film thickness of 2.5 mm. With a decrease in the film thickness, the limiting speed of its breakdown increases. Such high speeds of the gas-vapor mixture in heat exchangers are not used due to a sharp increase in the aerodynamic resistance of the beam of ribbed tubes, which ensures that the film does not break off from horizontally located ribs.

In accordance with the proposed technical solution (figure 3), the pipes are fixed in the tube plate so that the slots 4 in the ribs are placed along the rear with respect to the incoming gas stream 5 of the forming pipe. This reduces the likelihood of a dynamic breakdown of the condensate draining along the slot.

The lower tube sheet delimits from above the chamber into which cooling water enters, flowing from this chamber through the finned tubes.

If the water has a temperature below the dew point of the water vapor contained in the vapor-gas mixture (and only in this case condensation of steam in the heat exchanger is possible), the condensate flowing from the pipe to the grate will not evaporate. Conversely, steam from a gas-vapor mixture will condense on a grate in the same way as on ribbed pipes.

To ensure free flow of condensate from the pipe grate to the outlet pipe, the grate should be tilted a few degrees to the horizon and have small flanges that prevent it from flowing around the perimeter of the grate.

The technical result of the invention is:

- obtaining additional heat by deep cooling of the combustion products of natural gas due to the condensation of the water vapor contained in them;

- obtaining demineralized water from the combustion products (about 1 liter of 1 m ^{3 of} burnt gas), which, after deaeration, can be used to power steam boilers or other purposes;

- water vapor remaining in the combustion products is in an overheated (not saturated) state, which eliminates its condensation on the inner surfaces of insulated flues and chimneys.

This design was tested in the boiler room of the Experimental Production Plant of the Ural State Technical University.

Claims (1)

A heat exchanger for cooling a gas-vapor mixture, consisting of pipes with spiral-annular ribs, characterized in that longitudinal slots are made along the axis of the pipes in all the ribs, and the pipes in the heat exchanger are arranged vertically.

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