GB2183814A - Heat exchanger tube end plates - Google Patents

Abstract

In a heat exchanger for permitting the flow of heat between two corrosive gases e.g. flue gases a passage 10 along which one fluid flows is enclosed by tube supporting walls 14 through which pass tubes 12 carrying the other fluid. Each wall comprises a sandwich in which a sheet of deformable corrosion- resistant plastics material, e.g. PTFE, 22 is held between sheets 20 and 21 of temperature and corrosion- resistant rigid plastics material. A gas-tight seal around the tubes 12 is provided by the plastics sheet 22 which the apertures 26 whose diameter is less than that of the inserted tubes 12. The tube supporting walls 14 are composed of juxtaposed panels whose edges are shaped to receive one or more sealing strips 46. <IMAGE>

Description

SPECIFICATION Heat exchangers This invention relates to a gas-to-gas heat exchanger and in particular to a sealing support structure for tubes in such a heat exchanger.

One use of a heat exchanger is in waste heat recovery from fired process equipment, e.g. boilers, furnaces and incinerators, where the gases passing through the heat exchanger can be acidic and corrosive. These gases are commonly passed through a heat exchanger of the type comprising a multiplicity of tubes located transversely of a duct. One of the gases, normally air, is passed through the tube and the other gas, normally combustion products, passes through the duct around the tubes. Heat exchange takes place through the tube walls and recovered heat, in the form of preheated air, is received back to the combustion chamber.

At high temperatures, e.g. where the tube wall temperature is higher than abut 170 C, condensation from the gases onto the tube wall generally does not occur and steel or cast iron tubes can be used. However, at lower temperatures, giving greater heat recovery, the tube wall temperature may fall below the dewpoint temperature of the gases, giving rise to corrosive condensation on the tube surface. To resist corrosion in such applications, borosilicate glass tubes have been employed, those being seal-mounted through tubeplates forming the walls of the duct at the back end of the tubes.

One seal-mounting system which has proved to us very successful is that described in British Patent No. 1,197,409. In the arrangement described in that patent, the wall of the heat exchanger which carries the tube ends, e.g.

the tubeplate, is covered on its inside face with a PTFE sheet, the sheet being apertured to allow the introduction of heat exchange tubes. The diameter of the apertures is in the free state slightly less than that of the heat exchanger tubes so that when the tubes are inserted a fluid tight joint is formed with the peripheral region of the sheet forming a collar around each tube.

Such a heat exchanger is designed for use where the corrosive gas (e.g. combustion products) flow through the duct and a non-corrosive gas (e.g. atmospheric air) flows through the tubes. This has two consequences. First the inlet and outlet spaces for the air at the ends of the tubes do not contain corrosive gases, and secondly, it is important that the seals are good otherwise corrosive gas will tend to seep into these spaces. With these factors in mind it is in practice found desirable to add an extra seal at the end of each tube on the exterior of the tubeplate. This seal is constituted by a small annular disc of PTFE which is clamped between an annular cast iron ring and the outer face of the tubeplate by bolts.

Such an arrangement is not suitable, however, when the gases passing through the tubes are also corrosive. In this case the exterior face of the tube plate and the bolts securing the outer seals are exposed to the effect of the corrosive gases. On the other hand, I have appreciated that the second seal, which is invariably included in such construction, is redundant.

According to this invention I provide a heat exchanger for transferring heat between a first corrosive fluid and a second corrosive fluid, comprising a passage through which the first corrosive fluid passes, and heat exchange tubes extending across the passage between opposed tube-supporting walls of the passage and through which the second corrosive fluid passes, the tube supporting walls each comprising a sandwich formed by respective inner and outer plates of temperature and corrosionresistant rigid material, with a sheet of deformable corrosion-resistant plastics material, e.g.

PTFE, between them, the sheet having apertures into which the tubes extend to form a substantially gas-tight seal, and the plates having apertures at locations corresponding to the apertures in the sheet through which the tubes pass with clearance.

A preferred material for two plates is a synthetic plastics material sold under the trade mark "KEEBUSH" by Kestner Engineering Co.Ltd., Greenhithe, Kent, England, which is capable of operating at temperatures in excess of 150"C and is specially formulated to provide good acid resistance.

The invention will now be described in more detail, by way of example, with reference to the drawings in which: Figure 1 is a sectional view through the side wall of a heat exchanger embodying the invention illustrating how one heat exchanger tube is supported in the tubeplates; Figure 2 is a schematic perspective view of the tubeplates showing the location of a plurality of tube apertures; Figure 3 is an overall perspective view of the heat exchanger; Figure 4 is a sectional view similar to Fig. 1 of a known heat exchanger; and Figure 5 is a sectional view through the side wall of a modified heat exchanger embodying the invention illustrating how side wall panels may be joined together.

Referring to Fig. 1, the heat exchanger illustrated has a duct 10 through which one gas passes and a multiplicity of paraliel heat exchanger tubes 12 extending across the duct between side walls 14 of the duct. The other gas passes through the heat exchanger tubes 12 between inlet and outlet spaces 16 to either side of the duct.

The side walls of the duct comprise a sandwich formed by two outer and inner plates 20 of a temperature and corrosion-resistant plastics material, such as "KEEBUSH" mentioned above. Between those plates 20 is a sheet 22 of a deformable corrosion-resistant plastics material, such as PTFE. The sheet 22 has tube-receiving apertures and is probably preformed around the apertures to form a collar 24 which has an aperture 26. In the free state the diameter of the aperture 26 is slightly less than the diameter of the tube 12 so that a gas-tight seal is formed by the collar 24 around the tube. The pressure in the duct exceeds the pressure in the tube which assists in maintaining the seal. In cases where the pressure in the tube exceeds that in the duct, the seal may be reversed.The plates 20 have apertures 28 in registry with each other and with the apertures 26 in the sheet 22. The apertures 28 are large enough to accommodate the tubes 12 with a clearance and also accommodate the collar 24 in the sheet 22.

The tube mounting shown in Fig. 1 has substantial advantages over the conventional mounting shown in Fig. 4. In the Fig. 4 arrangement, the tube 112 is mounted in an aperture in a cast iron side wall 120 of the heat exchanger. A PTFE sheet 122 lies over the inner face of the wall 120 and carries a collar 124 which forms a seal on the tube. On the outer face of the wall 120 an extra seal 130 is mounted. This seal 130 is constituted by a small annular disc of PTFE which is preformed to seal against the tube and is clamped between an annular cast iron ring 132 and the outer face of the duct wall by bolts 134. Not only is this time-consuming to assemble but it also means that the outer face of the wall 120, the ring 132 and the bolts 134, are exposed to the gases passing through the tubes 112.

By way of example only, the tubes 12 in Fig. 1 may have an outer diameter of 50mm with a wall thickness of 2.5 mm. Their length can be anything from 0.8mm to 5.2mm. The sheet 22 may be about 1 mum thick with the collars 24 preformed to stand about 8mm proud of the face of the sheet. The plates 20 may range in thickness from about 35mm down to about 10mm depending on their length and height and the optimum thickness for any given application may need to be found empirically. The apertures 28 in the plate may be 55mm in diameter.

As shown in Fig. 2, panels may be formed of typically about 1.4m length by 0.5m height with many tubes 12 supported in each plate.

As shown, the tube ends are received in staggered horizontal rows spaced apart 73mm (between centres) in the rows and with the rows spaced by 75mm. A larger wall may be made by juxtaposing several panels. To this end the plates 20 may be grooved along their edges, as shown at 40 in Fig. 2, to receive sealing strips 42, or may be chamfered, as at 44 in Fig. 1, in which case a single sealing strip 46 seals against the four plates 20 of two adjoining panels. The panels are then supported as shown in Fig. 3 by channel section members 50 bolted together.

As shown in Fig. 3, the side wall 20 is formed by six panels mounted three high and two wide. Preferably the PTFE sheet is also at least two panels wide so that no additional seal is needed at the central vertical join. Sealing strips are required at the horizontal joint between panels. The peripheral edges of the panels are trapped by the bolted-together channel sections and a central support 54 provided additional rigidity to the structure.

The ends of the support 54 are secured to the adjacent channel sections and if desired bolts can also be included along the length of the support 54.

The inside faces of the channel sections 50 are covered by PTFE and form a corrosionresistant flange around the periphery of the heat exchanger to which the incoming and outgoing ducts, which would carry corrosive fluids to and from the exchanger, would be attached. The central support 54, including bolts, is also covered by PTFE. Other fixings are exterior to the gas flow paths and need not be corrosion-resistant.

Fig. 5 shows modification of the heat exchanger incorporating a method of accommodating thermal expansion caused in the sheets of PTFE membrane the width of which spans more than two panels. A loop 23 is formed in the membrane 22 between adjacent panels 60 and 61 which allows for thermal expansion or contraction without stress within the membrane fabric. The inner plate 20b is thicker than the outer plate 20a, the inner plate providing the major part of the structural support.

It will be noted that in both the illustrated embodiments effective sealing is achieved without the need for a second seal, as in the prior art arrangement of Fig. 1, and the exchanger is capable of accepting corrosive gases through both of its flow paths. The exchanger may for example be used in the reheating of flue gases from a combustion process, after wet scrubbing to remove or reduce pollutants, in order to avoid "plume" formation at the outlet of the chimney.

Claims (12)

1. A heat exchanger for transferring heat between a first corrosive fluid and a second corrosive fluid, comprising a passage through which the first corrosive fluid passes, and heat exchange tubes extending across the passage between opposed tube-supporting walls of the passage and through which the second corrosive fluid passes, the tube supporting walls each comprising a sandwich formed by respective inner and outer plates of temperature and corrosion-resistant rigid material, with a sheet of deformable corrosion-resistant plastics material between them, the sheet having apertures into which the tubes extend to form a substantially gas-tight seal, and the plates having apertures at locations corresponding to the apertures in the sheet through which the tubes pass with clearance.

2. A heat exchanger according to Claim 1 in which the deformable, corrosion-resistant plastics material in PTFE.

3. A heat exchanger according to Claims 1 or 2 in which the inner and outer sheets comprising the tube supporting walls are composed of juxtaposed panels.

4. A heat exchanger according to Claim 3 in which the inner and outer plates of temperature and corrosion-resistant rigid material are grooved along the panel edges to receive sealing strips placed between facing edges of adjacent inner panels and between facing edges of adjacent outer plate panels.

5. A heat exchanger according to Claim 3 in which the two plates of temperature and corrosion-resistant rigid material are chamfered along the corners of their respective panei edges adjacent to the central sheet of plastics material to form a single groove into which a sealing strip is inserted.

6. A heat exchanger according to any of Claims 3 to 5 in which the panels are supported by bolted pairs of channel section members placed over the joints, one of each pair being on the inside of the wall and the other being on the outside of the wall.

7. A heat exchanger according to any of Claims 3 to 6 in which each sheet of plastics material extends over two or more panels.

8. A heat exchanger according to Claim 7 in which each sheet of plastics material is formed into a loop between adjacent panels which it spans in order to accommodate distortion therein due to thermal expansion.

9. A heat exchanger according to any of Claims 1 to 8 in which the apertures in the sheet of plastics material in the free state have a slightly smaller diameter than that of the tubes passing therethrough in order to create a gas tight seal when the tubes pass through them.

10. A heat exchanger according to Claim 9 in which the sheet of plastics material is so shaped that differential gas pressure acting across the seal further secures the sheet around the tubes.

11. A heat excanger according to any of Claims 1 to 10 in which the tubes are received by the tube supporting walls in staggered horizontal rows.

12. A heat exchanger substantially as described with reference to the accompanying drawings.