DE102016122016A1 - heat exchangers - Google Patents

Abstract

Heat exchanger for use in the flue gas path of a furnace 1 comprising a plurality of metallic heat exchanger tubes 12, which are designed to be flowed around inside by a cooling medium and externally by flue gas R, wherein the heat exchanger tubes 12 are provided on the outside with a sheath of functionalized perfluoroalkoxy polymer (PFA) or functionalized and chemically modified polytetrafluoroethylene (PTFE) are surrounded, wherein the sheath contains 1.0 wt .-% to 5.0 wt .-% of a homogeneously distributed in the sheathing ceramic material.

Description

The invention relates to a heat exchanger for use in the flue gas path of a furnace according to the feature of patent claim 1.

The removal of pollutants from flue gas is done to reduce the environmental impact. In a multistage dedusting process, most of the dust in the first stage is removed from the flue gas. This is usually done with a fabric filter or with an electrostatic precipitator. In the electrical dust separation, dust particles are negatively charged by means of spray electrons in gas stream and deposited on opposite precipitation anodes.

For better separation rates, either the filter can be increased or the operating volume flow can be reduced. The latter can be achieved by lowering the temperature of the flue gas before the filter. However, burning combustible fuels increases the risk of water vapor in the flue gas reacting with sulfur dioxide and sulfur trioxide to form sulfurous acid and sulfuric acid. During operation, corrosive attack on components of the flue gas path can occur due to condensed acids. Also by the formation of condensate dust encrustations are favored.

In addition, environmental regulations are becoming ever stricter, so existing facilities often need to be retrofitted. If there was no danger in the original mode of operation of a firing plant to reach the area of the sulfuric acid dew point, no corrosion-resistant, more expensive materials were used in the flue gas path. Arrangements in the flue gas path, however, must be retrofitted if the temperature of the flue gas is to be lowered into the area of the sulfuric acid dew point. However, heat exchanger tubes made of corrosion-resistant steel are very cost-intensive and in the long run not resistant. Heat exchanger tubes made of acid-proof plastic, however, are not as resistant to pressure as heat exchanger tubes made of steel. Also, the relatively soft plastics PTFE or PFA are not as abrasion resistant as heat exchanger tubes made of steel. The thermal conductivity of PTFE and PFA is also much lower than that of steel, so that thicker-walled plastic heat exchanger tubes would not be able to provide the desired heat exchanger performance in a very limited space.

Proceeding from this, the present invention seeks to provide a heat exchanger for use in the flue gas path of a furnace, which has a high mechanical resistance to dusts and high chemical resistance, especially against acids and is compact at the same time. Furthermore, a method for producing such a heat exchanger will be shown.

This object is achieved with a heat exchanger having the features of patent claim 1.

Advantageous developments of the invention are the subject of the dependent claims.

The heat exchanger according to the invention is suitable for use in the flue gas path of a furnace, in particular in the area of a flue gas filter system, in particular in fossil-fired power plants. In particular, it is suitable for connection with an electrostatic precipitator with the possibility of lowering the temperature of the flue gas before it enters the filter into the area of the sulfuric acid dew point. The heat exchanger is designed to withstand high dust loads and at the same time to be sulphurous acid resistant.

The invention solves this problem, inter alia, by the use of jacketed, metallic heat exchanger tubes. They are designed to be flowed through by a cooling medium inside and outside to be flowed around by flue gas. The heat exchanger tubes are provided on the outside with a sheath. The sheath consists of functionalized perfluoroalkoxy polymer (PFA) or a functionalized and chemically modified polytetrafluoroethylene (PTFE). The functionalization consists in that the sheathing contains 1.0 wt .-% to 10.0 wt .-% of a homogeneously distributed in the shell granulated ceramic material.

Modified PTFE is an advanced PTFE that has improved properties compared to standard PTFE by chemical modification (incorporation of a modifier into the polymer chain). In particular, the weldability can be made possible and the permeation and thus the barrier behavior can be improved. Component life is increased, service intervals are extended, and system availability and reliability are improved.

The heat exchanger according to the invention makes it possible to be cooled with cooling medium, which is at a higher pressure than would be possible for pure plastic pipes. While hoses made of fluoropolymer plastic can be operated with a maximum of 6 to 7 bar internal pressure, it is possible for heat exchanger tubes made of metal, in particular steel, to drive pressures well above 40 bar.

The heat exchanger thus has the property of being compact, because the high internal pressure does not adversely affect the material that comes in contact with the flue gas. Rather, the metallic heat exchanger tubes are made of non-acid-resistant steels, since the outside of the tubes is protected by the sheath.

The sheathing is functionalized by incorporation of a homogeneously distributed ceramic material. This material significantly increases the abrasion resistance of the sheath. It should be noted that the heat exchanger according to the invention is preferably connected upstream of an electrostatic precipitator, i. in this type of use maximum exposure to dust is exposed. While the dust load downstream of the electrostatic precipitator should preferably be less than 100 mg per cubic meter, the dust load in the flue gas stream upstream of the electrostatic precipitator is in the range of several grams per cubic meter, i. it is many times higher. The abrasive load on the heat exchanger tubes is correspondingly high.

A further advantage of the sheath made of abrasion-improved PTFE is that the functionalization of the material causes only an insignificant influence on the diffusion through the sheathing. The basic material used is preferably chemically modified PTFE or PFA, since this material has been shown to fulfill the requirement of corrosion resistance. A sub-problem of the invention to be solved is the conflict of objectives that by the functionalization of the material to improve the abrasion stability, the diffusion through the Flourpolymerfolie, i. the barrier effect should not be adversely affected, because the thickness of the shell, which surrounds the heat exchanger tubes, should be as low as possible due to the low heat conductivity of PFA and PTFE. This conflict of objectives is achieved in the context of the invention by the admixture of a ceramic material in powder form. The ceramic material is admixed with the fluoropolymer granules prior to extrusion. The mixture is homogenized and then melted in the extruder. The result is a tube with embedded ceramic particles, which significantly increases the abrasion resistance. Particularly advantageous properties have resulted when the sheathing obtains 1.0 to 5.0 wt .-% of the ceramic material. Weight fractions of the ceramic material in a range from 1.20% to 2.50% are particularly advantageous. The maximum particle size of the powdered ceramic material should not exceed 100 micrometers. In addition, the mean grain size of the powdered ceramic material is a maximum of 60 microns. The ceramic material as an additive to the polymeric materials may itself be a heterogeneous mixture of different ceramic materials, so that the result is a compound with at least one ceramic material.

In the context of the invention, tests were carried out with different tube materials or sheaths, the properties of the sheath having been tested without internal metal heat exchanger tubes. The different diffusion properties were determined with a so-called helium leak test. Here, a hose made of the functionalized and optionally chemically modified plastic is sealed at the end and placed under helium under a defined pressure. The helium diffuses through the polymer. The collected amount of helium is measured.

The tests have shown that unaltered PFA tubing has the highest diffusion tightness. Post-annealing brings the permeabilities of modified PTFE to the order of unannealed, virgin, ie unaltered PFA. The diffusion-tightness is therefore not reduced by the incorporation of the ceramic material in the claimed size range in such a way that is to be expected when using polymeric sheaths with fundamentally poorer diffusion resistance.

The heat exchanger according to the invention is a shell-and-tube heat exchanger. Several U-shaped extending heat exchanger tubes are combined to form a tube package of the bundle. In this case, at least two adjacent heat exchanger tubes are arranged in a U-shaped deflection region so that their course intersects. The heat exchanger tubes are preferably arranged hanging.

The heat exchanger according to the invention is not exclusively, but especially for tight spaces of flue gas paths of combustion plants, especially designed by power plants for use in front of an electrostatic precipitator or fabric filter. In general, it is not planned in the design of a furnace system to use larger heat exchangers in this area of the flue gas path. Therefore, in the design of thermally adequately dimensioned heat exchangers in addition to the extreme dust loads still considerable space problems must be considered. It would be beneficial to arrange the heat exchanger tubes with high packing density. However, parallel tubes with diametrically disposed ends require separate water boxes at both ends of the heat exchanger. That's expensive. Plant engineering cheaper is a coolant supply and removal at only one end of the heat exchanger, the heat exchanger tubes are deflected for this purpose U-shaped.

However, the U-shaped deflection region, ie the bending radius of the sheathed heat exchanger tubes, can not be arbitrarily small, since otherwise it would come to the drapery in the region of the sheath. In the area of wrinkles, the structure of the plastic sheath and thus the Berrierewirkung disturbed or even destroyed, with the risk that flue gas and acidic condensate attack the inner metal heat exchanger tubes corrosive. The bending radii are therefore limited downwards. This in turn limits the packing density of the heat exchanger, so that the available space may possibly not be optimally utilized.

In order not to fall below the permissible bending radii, the invention provides that at least two adjacent heat exchanger tubes intersect in their U-shaped deflection region. Adjacent heat exchanger tubes therefore do not run parallel to one another in the deflection region. In the other areas in front of and behind the deflection area, however, the adjacent heat exchanger tubes should be completely parallel to one another. As a result, with sufficiently large bending radii high packing densities can be realized, so that the available space is optimally utilized.

The heat exchanger tubes are grouped into tube packages. Within the tube package, the heat exchanger tubes are arranged in rows for uniform distribution, which are flowed around in succession by the flue gas. Such a tube package may have an outer row of heat exchanger tubes and at least one inner row of heat exchanger tubes. Supply and return of the heat exchanger tubes of the inner row is between supply and return of the outer row of heat exchanger tubes. The bending radii should be as small as possible.

In an advantageous development, adjacent outer heat exchanger tubes of a tube bundle intersect in pairs. In addition, adjacent inner heat exchanger tubes of the tube package may intersect in pairs and preferably in groups. Such a tube package preferably has as many outer and inner heat exchanger tubes. For example, a tube package comprises 6 outer and 6 inner heat exchanger tubes, each with a U-shaped deflection, so that there are a total of 12 × 2 = 24 mutually parallel pipe sections whose ends are connected to a tube plate .. Adjacent outer heat exchanger tubes intersect in their U-shaped deflection in pairs, so that there are a total of three intersection points in the 6 outer heat exchanger tubes. The inner heat exchanger tubes are arranged between the outer heat exchanger tubes. If the bending radius of the outer heat exchanger tubes has already been selected as small as possible and can not be reduced, it is not possible for adjacent inner heat exchanger tubes to intersect the same way as the outer heat exchanger tubes. This is only possible with the at least next but one of arranged in a row inner heat exchanger tubes, so that intersect as a result groups of three adjacent heat exchanger tubes.

Several of these tube packages can be assembled into larger modules / bundles, which are connected to a common inlet and outlet of the coolant. The entire structure is therefore modular and easily scalable.

The invention further relates to a method for producing a heat exchanger for arrangement in the flue gas path of a furnace according to one of claims 1 to 8, wherein the at least one ceramic material admixed with the PFA or chemically modified PTFE prior to extrusion homogenized the mixture and then in an extruder is melted. Subsequently, a tube is extruded from the functionalized PFA on the metallic heat exchanger tube, wherein the sheathed tubes are then bent, cut to size and combined into tube pacts.

As an alternative to directly extruding the PFA tube onto the metallic heat exchanger tube, a tube is first extruded in the chemically modified PTFE. The extruded tube is expanded in diameter under the influence of heat by means of an oversized mandrel. The metallic heat exchanger tube is inserted into the expanded tube. Due to the restoring forces of the fluoropolymer, the tube shrinks on cooling to the underlying metallic tube.

A heat exchanger produced by these methods has a high diffusion resistance, can be operated at the same time with very high internal pressures, is due to the very low permeability of the sheath corrosion resistant and at the same time highly resistant to abrasion, so that in the smallest space, high dust load, high service life, a high heat exchange performance can be achieved during operation in the range of sulfuric acid dew point.

In particular, the invention relates to a combustion plant with a flue gas path, in which such a heat exchanger is arranged in the flow direction in front of a filter of the furnace. The filter is in particular an electrostatic filter or a fabric filter. It is preferably a furnace of a power plant. The use of the heat exchanger according to the invention is expressly not on limited to a special position in the flue gas path. A heat exchanger of this type is suitable for all areas of the flue gas path, in which the risk of corrosive attack is to be expected by acidic condensates. This can also be an area downstream of a filter stage.

The invention will be explained below with reference to the embodiment shown in the schematic drawings.

• 1 einen Teilbereich einer Rauchgasfilteranlage mit Wärmetauschern in einer Draufsicht;
• 2 einen Wärmetauscher der Rauchgasfilteranlage in einer Seitenansicht;
• 3 den Wärmetauscher der 2 in einer Ansicht von oben;
• 4 den Wärmetauscher der 3 entlang des Schnittes IV-IV;
• 5 den Wärmetauscher der 4 entlang der Schnittebene V-V in 4;
• 6 in vergrößerter Darstellung ein einzelnes Modul der 5 im Querschnitt und
• 7 nochmals in vergrößerter Darstellung den Teilbereich VII in 6.

It shows:

• 1 a portion of a flue gas filter system with heat exchangers in a plan view;
• 2 a heat exchanger of the flue gas filter system in a side view;
• 3 the heat exchanger of 2 in a view from above;
• 4 the heat exchanger of 3 along the section IV-IV;
• 5 the heat exchanger of 4 along the section plane VV in 4 ;
• 6 in an enlarged view a single module of 5 in cross-section and
• 7 again in an enlarged view the sub-area VII in 6 ,

1 shows two heat exchangers 1 a flue gas filter system 2 , The heat exchangers 1 are electrostatic precipitators 3 upstream in the flow direction of the clean flue gas. The installation space in this area of the flue gas filter system 2 is very limited, so the heat exchanger 1 have to build very compact.

2 shows such a heat exchanger 1 , In the upper part of the heat exchanger 1 are connections 4 . 5 arranged for the supply and discharge of a coolant. The heat exchanger 1 comprises three stages connected in series in the flow direction 6 . 7 . 8th , as seen from the top view of 3 is to recognize. Every level 6 . 7 . 8th each includes two identical modules 9 . 10 . 11 parallel to the flue gas R to be flowed around.

The 4 and 5 show the said modules 9 . 10 . 11 in side view and in section. The modules 9 . 10 . 11 are flown in the image plane from left to right, ie that with smoke particles laden flue gas R first hits the left in the image plane modules 9 , then flow through the middle modules 10 and finally the third modules 11 , It can be seen that the first flowed through module 9 has a different configuration. The first module 9 is operated above the sulfuric acid dew point. Its pipes are metallic on the outside and not sheathed. There are small bending radii in the deflection area 13 possible. Based on 5 it can be seen that the first module 9 of heat exchanger tubes 12 has a lower packing density. The heat exchanger tubes are grouped into groups of four 15 × 4 such tube packages 14 each form a module 11 , The downstream modules in the flow direction 10 . 11 come with that through the first module 9 cooled flue gas in contact. There is a risk of corrosive attack by acidic condensate of the flue gas. The following modules 10 . 11 are at risk of corrosion. The heat exchanger tubes 12 are therefore sheathed.

The modules 9 . 10 . 11 each have individual heat exchanger tubes 12 , which run in large numbers predominantly parallel to each other and in their lower in the image plane in each case a U-shaped deflection 13 have. There, the individual heat exchanger tubes are deflected by 180 ° and back to the connections 4 . 5 in the upper part of the heat exchanger 1 guided. The heat exchanger tubes 12 are arranged hanging, for example over a longitudinal extension of about 8 m. Each individual heat exchanger tube has in this example an outer diameter of 25 mm.

Based on the enlarged view of 6 and 7 it can be seen that the heat exchanger tubes 12 arranged in a specific 90 ° grid. The mutual center distance of adjacent heat exchanger tubes 12 is 50 mm in this embodiment. 7 shows that the heat exchanger tubes 12 in individual tube packages 14 are arranged. An arrangement of 4 × 7 tube packages 14 forms a single module 9 , Every tube package 14 consists of 12 heat exchanger tubes 12 together, the U-shaped inside the tube package 14 be redirected.

The special feature of the heat exchanger according to the invention 1 is that a certain bending radius in the deflection 13 not fallen below. Likewise, the packing density in the 90 ° grid is high. For this purpose, the individual heat exchange tubes 12 within a tube package 14 arranged in two mutually parallel rows. The special feature is that the a number of heat exchanger tubes 12 is deflected with a bending radius that a second row of the heat exchanger tubes 12 both with its inlet section and with its return section (in front of and behind the deflection area 13 ) between the outer heat exchanger tubes 12 lies. The internal heat exchanger tubes 12 are in a sense of the outer heat exchanger tubes 12 surrounded within the 90 ° grid. The flue gas stream therefore initially encounters a number of outer heat exchanger tubes 12 (Inlet section), in the further course on the row of inner heat exchanger tubes 12 (Inlet section), whereupon again a series of internal heat exchanger tubes 12 (Return section) follows and finally the return section of the outer heat exchanger tubes 12 , So that the bending radius of the inner heat exchanger tubes 12 not too small, intersect adjacent inner heat exchanger tubes 12 groups. In this embodiment, there are 2 × 3 inner heat exchanger tubes 12 , in the inlet section to the deflection 13 run in a parallel row. In the deflection area 13 run each 3 adjacent these inner heat exchanger tubes 12 parallel to each other and opposite to the other 3 inner heat exchanger tubes 12 so that these are in the deflection area 13 cross. In the picture plane the 7 run from the top left to the bottom right inner heat exchanger tubes 3 above. It then closes the down in the image plane, the second triad of inner heat exchanger tubes 12 at. Accordingly, this second group of internal heat exchanger tubes 12 slightly longer than the first, uppermost group of three heat exchanger tubes 12 ,

Below these inner heat exchanger tubes 12 each pairwise crossing the outer heat exchanger tubes run 12 , Two adjacent heat exchanger tubes 12 intersect in the deflection area 13 so that it is at the outer heat exchanger tubes 12 three pairs of intersecting heat exchanger tubes 12 gives. Even with the outer heat exchanger tubes 12 are located from the top right to bottom left in the image plane running heat exchanger tubes 12 above the heat exchanger tubes running from bottom left to top right 12 so it's in the deflection area 13 a total of four different height ranges exist in which the heat exchanger tubes 12 run.

In such an arrangement can metallic heat exchanger tubes 12 be used with a shell of functionalized PFA or functionalized and chemically modified PTFE without it in the deflection 13 wrinkling of the sheath comes. The invention therefore makes it possible to use compact and high-performance heat exchangers, in particular in the narrow space conditions in the flue gas path of a firing system, in particular before an electrostatic precipitator, which are suitable to pass through the sulfuric acid dew non-destructive and endure the extreme dust loads longer.

LIST OF REFERENCE NUMBERS

1 -

heat exchangers

2 -

Flue gas filter system

3 -

electrostatic precipitator

4 -

connection

5 -

connection

6 -

step

7 -

step

8th -

step

9 -

module

10 -

module

11 -

module

12 -

heat exchanger tube

13 -

deflection

14 -

tube package

R-

flue gas

Claims (11)

A heat exchanger for use in the flue gas path of a furnace (1) comprising a plurality of metallic heat exchanger tubes (12) adapted to be circulated internally by a cooling medium and externally by flue gas (R), the heat exchanger tubes (12) being externally covered by a jacket of functionalized perfluoroalkoxy polymer (PFA) or functionalized and chemically modified polytetrafluoroethylene (PTFE), the sheath containing from 1.0% to 10.0% by weight of a ceramic material homogeneously distributed in the sheath. Heat exchanger after Claim 1 , characterized in that the sheath contains 1.0 wt .-% to 5.0 wt .-% of the ceramic material. Heat exchanger after Claim 1 or 2 , characterized in that the maximum grain size of the powdery ceramic material is 100 microns. Heat exchanger according to one of Claims 1 to 3 , characterized in that the average grain size of the powdery ceramic material is at most 60 microns. Heat exchanger according to one of Claims 1 to 4 , characterized in that a plurality of U-shaped extending heat exchanger tubes (12) are combined to form a tube package (14), wherein at least 2 heat exchanger tubes (12) in a U-shaped deflection region (13) are arranged so that their course intersects. Heat exchanger after Claim 4 , characterized in that the tube package (14) comprises outer heat exchanger tubes (12) arranged in a row and arranged in a row inside Heat exchanger tubes (12), wherein the deflection region (13) of the outer heat exchanger tubes (12) limits the deflection region of the inner heat exchanger tubes. Heat exchanger after Claim 6 , characterized in that adjacent outer heat exchanger tubes (12) of the tube package (14) intersect in pairs. Heat exchanger after Claim 6 or 7 , characterized in that adjacent inner heat exchanger tubes (12) of the tube package (14) intersect in groups. Process for the preparation of a heat exchanger according to one of Claims 1 to 8th , characterized in that the at least one ceramic material is added to the PFA or chemically modified PTFE before extrusion, the mixture is homogenized and then melted in an extruder and then a tube of the functionalized PFA is extruded directly onto the metallic heat exchanger tube, or Hose made of functionalized and chemically modified PTFE under the influence of a heat treatment with an oversized mandrel radially expanded and then shrunk after insertion of the metallic heat exchanger tube by restoring forces existing in the PTFE on the metallic heat exchanger tube, wherein the heat exchanger tubes thus coated (12) subsequently bent, cut and be summarized to tube packages (14). Furnace with a flue gas path, in which a heat exchanger according to one of Claims 1 to 8th is arranged, characterized in that the heat exchanger (1) in the flow direction in front of a filter of the furnace (2) is arranged. Firing plant after Claim 10 , characterized in that the filter is an electrostatic filter or a fabric filter.

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