GB2133524A

GB2133524A - The heat exchanger

Info

Publication number: GB2133524A
Application number: GB08333118A
Authority: GB
Inventors: Serge Charlier; Edmond Demillecamps
Original assignee: SAG ECHANGEURS CHALEUR; Setif SA D Etude De Te Ste
Current assignee: SAG ECHANGEURS CHALEUR; Setif SA D Etude De Te Ste
Priority date: 1982-12-24
Filing date: 1983-12-12
Publication date: 1984-07-25
Also published as: LU84557A1; FR2540615B3; GB8333118D0; FR2540615A3; DE8337506U1; BE898449A

Abstract

The heat exchanger consists of one or more pairs of outer and inner tubes 1 and 3 respectively connected to superposed first and second tube plates 5 and 7 respectively. The outer tube 1 is closed at that end 2 remote from the first tube plate 5 and the inner tube 3 is open at that end 4 remote from the second tube plate 7 to provide a fluid connection between the tubes. The outer tube 1 is made from a fluorinated thermoplastics polymer and, in use, provides the heat exchange surface through which heat is transferred between a fluid surrounding the outer tube 1 and a fluid circulating through the tubes 1, 3. The heat exchanger has particular application to corrosive environments, especially where one of the fluids, that surrounding the outer tube, is corrosive. Several installations are described, notably for heat recovery from hot concentrated sulphuric acid to generate steam and for heat recovery and depollution of fluegas produced by sulphur containing fuels. <IMAGE>

Description

SPECIFICATION Heat exchanger The present invention relates to heat exchangers, and in particular to heat exchangers for effecting heat transfer between two fluids separated by a heat exchange surface.

In heat exchangers of the type referred to, it is well known to provide a heat exchange surface which consists of a metal such as copper, aluminium, steel of their special alloys. Whilst such constructions are acceptable for many applications they are not suitable if one or both of the fluids is highly corrosive.

In these circumstances it is necessary to use materials for the heat exchange surface which are chemically inert to the effects of the corrosive fluid(s). For example, metals such as titanium, zirconium and tantalum have been used but suffer from disadvantage of being more expensive.

Graphite has also been used but suffers from the disadvantage of being more fragile and often porous.

When highly chemically resistant synthetic resins such as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropene copolymers, became available it was thought that these could also be used. However, their poor mechanical properties and the fact that they can be welded only with great difficulty or not at all have imposed limits on their practical application.

In fact, the feeble resistance to pressure of tubes made from such polymers and their tendency to "flow" have restricted their use to very small diameter tubes of great wall thickness which is of course eminently prejudicial to the heat transfer process. Furthermore, such apparatus can only be subjected to very limited pressure values.

Moreover, the well proven constructions in which the tubes forming the heat-exchange surface are welded to connector plates could not be used because of the problems in welding such polymers. Instead more expensive constructions had to be used, for example a threaded connection for each tube or compressing the tubes together to seal them tightly relative to each other.

It is an object of the present invention to provide a heat exchanger which mitigates at least some of the problems and disadvantages aforementioned.

According to the present invention a heat exchanger for effecting heat transfer between two fluids separated by a heat exchange surface comprises first and second superposed connector plates, an outer tube of fluorinated thermoplastics polymer connected to the first plate and closed at the end remote from the plate, an inner tube connected to the second plate and opening into the outer tube, the arrangement being such that, in use, the outer tube is surrounded by one of two fluids and the other fluid is circulated through the tubes.

The heat exchanger of the present invention is particularly suited to applications in which at least one of the fluids is corrosive. Thus the fluorinated thermoplastics polymer of the outer tube is chemically inert to the effects of such fluids enabling the outer tube to be surrounded by the corrosive fluid without risk of attack. A further advantage of the fluorinated thermoplastics polymer is that it can be easily welded thereby permitting connection of the outer tube to the first plate by welding.

Further details and characteristics of the invention will be apparent from the following detailed description of preferred embodiments given, by way of example only, with reference to the accompanying drawings wherein: Figure 1 illustrates the basic construction of the heat exchanger according to the invention; and Figure 2 illustrates a two-pass construction of heat exchanger according to the present invention.

Referring to Fig. 1, a heat exchanger is illustrated comprising a pair of concentric outer and inner tubes 1 and 3 respectively corresponding one ends of which are welded to first and second relatively superposed connector plates 5 and 7 respectively. The outer tube 1 is closed at the end 2 remote from the connector plate 5 and the inner tube 3 is open at the end 4 remote fom the connector plate 7, i.e. within the outer tube, to provide a fluid connection between the tubes.

The free annular space between the tubes 1 and 3 opens into a first fluid chamber or conduit 6 defined between the connector plates 5 and 7 and the inner tube 3 opens into a second fluid chamber of conduit 8 defined between the connector plate 7 and a cover plate 9.

The outer tube 1 is made from a fluorinated thermoplastics polymer such as ethylenemonochlorotrifluoroethylene copolymer (ECTFE) or vinylidene polyfluoride (PVDF).

In use the outer tube 1 is plunged into one of two fluids between which heat transfer is to be effected and the other fluid is circulated through the tubes 1, 3 in the direction indicated by the arrows. Fluid circulation could be in the opposite direction to that shown.

The tubes 1, 3 may be of any desired length, the most appropriate length being defined by thermal and mechanical calculations, and may have any cross-section.

Likewise based on such calculations are determined the relative dimensions of the tubes 1, 3, the relative separation of which may vary between a few millimetres to several centimetres depending on acceptable charge loss figures.

By way of example the following diameters of outer and inner tubes of circular cross-section may be noted without prejudice: -outer tube: dext: 15 mm dints :13.4 mm -inner tube: deX,: 10 mm dint 8 mm -outer tube: dext 12 mm dints 10.4 mm -inner tube: dent: 6 mm dint#: 4 mm -outer tube: dent: 8 mm dint~: 6.4 mm -inner tube: dent: 4 mm d,##: 3 mm A heat exchanger may comprise a single pair of outer and inner tubes as above described but usually there will be a plurality of pairs of outer and inner tubes with each pair being of similar construction. A plurality of pairs of tubes may be arranged in line or in alternately staggered quincunx formation.

By judicial partitioning of the fluid chambers or conduits it is possible to construct heat exchangers with a single-pass or several passes.

Fig. 2 shows a double-pass heat exchanger positioned in an opening in a duct. For the sake of clearer representation, only one pair of concentric tubes is shown for each pass.

Like reference numbers are used in this figure to those used in Fig. 1 for identical or similar construction elements and parts and the direction of circulation of the fluid through the tubes is as indicated by the arrows.

The heat exchanger according to this invention has a great number of advantages, of which the following may be mentioned.

In contrast with other types of heat exchangers heretofore constructed from plastics materials, the exchanger according to this invention lends itself perfectly to the realisation of large heat exchange surface areas by association of unit or modular elements arranged side-by-side.

The length of the tubes may be chosen in accordance with the thermal problems to be solved and with available space.

In the case of heat recovery from a hot aggressive liquid (see Example 1 below), such as sulphuric acid, the tubes may be arranged to hang vertically in the enclosed space containing the hot liquid. The space may be a parallelipiped or round tank, a duct, sump or any other type of container. In such an arrangement the tubes may be several metres long.

In the case of heat recovery from a corrosive hot gas (see Example 2 below), it is also preferred to arrange the tubes to hang freely in a flue or pipe through which the gas circulates or flows. Alternately, the tubes may be arranged to extend obliquely in an exhaust flue or chimney. In that case the tubes will be shorter and their length may not exceed 20 or 30 cm.

In the case of condensation of corrosive vapours (see Example 3 below), the tubes may be arranged to extend horizontally, for example at the top of a separation column.

Another advantage of the heat exchanger according to this invention resides in that it allows each tube to dilate and contract freely in response to the temperature conditions to which it is subjected without there being any need for the provision of compensatory devices. Such compensatory devices which are needed with other types of apparatus, such as heat exchangers with straight tube nests, are complicated to build and add to the overall costs.

In the treatment of polluted or dirty fluids which carry solid matter in suspension or of slurries, it is difficult to prevent such solids from being deposited on the tubes. The heat exchanger according to this invention can be very rapidly cleaned because the outside surface of the outer tube is immediately accessible simply by lifting the connector plates. Moreover, the fluorinated thermoplastics polymers which are used for the outer tubes are not very favourable to such incrustation and the tubes can be efficiently cleaned simply by washing them.

Lastly, the construction of a heat exchanger according to this invention is cheap by comparison with the cost of conventional apparatus of the type in question. In fact, in the very frequently encountered application of heat exchange between a corrosive fluid and a noncorrosive fluid, only one of the two connector plates, namely the one to which the outer tubes are secured must be made wholly or partly of a fluorinated thermoplastics polymer, whilst the inner tube which comes into contact only with the other, non-corrosive fluid, and its connector plate may be made from a less valuable and therefore cheaper material than fluorinated thermoplastics polymer. Such a material is, for example, depending on the thermal and mechanical problems involved, polypropylene, polyvinylchloride (PVC), copper, aluminium, brass, carbon steel or stainless steel.

For applications where both fluids are corrosive however, the inner tube would also be made from a fluorinated thermoplastics polymer as would the exposed surfaces of the second connector plate and cover plate.

The tubes may be secured to their connector plates by any known means or methods such as welding, adhesive fixation, threaded connection etc., but in principle welding is preferred by the Applicants.

Since fluorinated thermoplastics polymers are expensive materials it is desirable, from the point of view of cost efficiency, to limit their actual application to those parts which technologically demand such materials.

For example, for constructing the connector plate to which the outer tubes of fluorinated thermoplastics polymer material are secured it is possible to use a plate consisting entirely of fluorinated thermoplastics polymer of the required thickness for mechanical strength and the execution of the welded joints. However, it is preferred, where possible, to use-a composite connector plate comprising a minimal thickness of fluorinated thermoplastics polymer, just enough for the welded joints to be made and for the required chemical resistance, reinforced on that side thereof which is in contact with the non-corrosive fluid by a metal such as steel or a heat-setting resin, for example a resin of the polyester or epoxy-type.

The following examples serve to illustrate the application of the heat exchanger according to the invention but do not limit its scope.

Example 1: Heat recovery from 98.5% sulphuric acid at 140 C.

A tank receives an inflow of sulphuric acid at 98.5% concentration at the rate of 300 m3/hour. This acid enters the tank at a temperature of 140 C. The cover of this tank consists of an assembly of 25 heat exchanger modules each comprising 1000 tubes 3.5 m long, that is to say, 25,000 tubes in all, constructed as follows: -outer tubes (dent 8 mm, dint 6.4 mm) and associated connector plate made from ECTFE.

-inner tubes (dent 4 mm, dint 3 mm) and associated connector plate and cover plate made from carbon steel.

Such a heat exchanger has an effective heat exchange surface area of 2,200 m2.

In contraflow with the acid, water under pressure and at 100 C flows into the exchanger modules. It leaves the exchanger at 120 C and at a pressure of 4 bars absolute at the rate of 420 m3/h. It can then be expanded into steam at 120 C. The exchanged heat is 4,880 KW.

Example 2: Recovery of heat from a fluegas of a boiler fired with a sulphur-containing fuel.

The sulphur which is contained in the fuel oxidises to form sulphur trioxide, and sulphuric acid is formed which corrodes conventional installations as soon as dew-point is reached.

In order to solve this problem a heat exchanger according to this invention will be used under the following conditions.

The fluegas flows through the smoke channel at the rate of 1.2 kg/sec. and at a temperature of 250 C. The top of the smoke channel or stack is made up from an assembly of 42 juxtaposed heat exchanger modules each comprising 100 tubes 50 cm long, constructed as follows: -outer tubes (dent 15 mm, dint 13.4 mm) and associated connector plate made from PVDF.

-inner tubes (dent 10 mm, dint 8 mm) and associated connector plate made from carbon steel.

Such a heat exchanger has an effective heat exchange surface area of 100 m2.

Through these modules flows water at the rate of 10 kg/sec., entering at 70 C and being heated up to 74.5 C. The fluegas is cooled to 95 C and more than 90% of the sulphuric acid contained therein is condensed, thus achieving a marked depollution. The heat exchange in the installation amounts to 186 KW.

The whole installation further comprises a safety device which interrupts or diverts the gas flow in the event of a failure in water circulation.

Example 3: Partial condensation of vapours from a mixture of chlorinated and fluorinated solvents.

It is desired to partially condense, at 45 C 600 kg/h of chlorinated and fluorinated vapours rising in a separating column or tower.

This is done by putting at the top of the tower a heat exchanger module which is arranged for a double pass as schematically illustrated in Fig. 2. Each pass comprises 220 tubes 50 cm long, constructed as follows: -outer tubes (dent 12 mm, dint 10.4 mm) made from PVDF.

-inner tubes (d,, 6 mm, dint 4 mm) made from stainless steel AISI 316.

Such a heat exchanger has an effective heat exchange surface area of 5 m2.

A methanolic brine at -18 C flows through the horizontally arranged tubes and 400 kg/h of vapour of one composition are condensed whilst 200 kg/h of vapour of another composition flows into another column.

Example 4: Heating and thermostatic control at 35 C of a reservoir of diethanolamine.

A reservoir, or tank, contains 7.5 m3 of diethanolamine the temperature of which must be maintained at 35 C. The available heating fluid is water at 75 C.

The objective is achieved with the aid of a heat exchanger module which is arranged for a single pass and consists of 360 tubes 1 m long, constructed as follows: -outer tubes (dent 12 mm, dint 10.4 mm) made from ECTFE.

-inner tubes (de,t 6 mm, dint 4 mm) made from copper.

Such a heat exchanger has an effective heat exchange surface area of 13.5 m2.

The exchanged heat is 19.6 KW.

The module may be arranged on the tank cover with the tubes extending vertically, or on the tank side with the tubes arranged horizontally.

Claims

1. A heat exchanger for effecting heat transfer between two fluids separated by a heat exchange surface comprises first and second superposed connector plates, an outer tube of fluorinated thermoplastics polymer connected to the first connector plate and closed at the end remote from the plate, an inner tube connected to the second plate and opening into the outer tube, the arrangement being such that, in use, the outer tube is surrounded by one of two fluids and the other fluid is circulated through the tubes.

2. A heat exchanger according to claim 1 wherein the outer and inner tubes are arranged concentrically.

3. A heat exchanger according to claim 1 or claim 2 wherein the fluorinted thermoplastics polymer is vinylidene polyfluoride.

4. A heat exchanger according to claim 1 or claim 2 wherein the fluorinated thermoplastics polymer is ethylene-monochlorotrifluoroethylene copolymer.

5. A heat exchanger according to any one of the preceding claims wherein the inner tube is made from a material other than fluorinated thermoplastics polymer, for example polypropylene, polyvinylchloride, copper, aluminium, brass or steel.

6. A heat exchanger according to any one of the preceding claims wherein the first connector plate is of composite construction in which the side exposed in use to the fluid into which the outer tube is plunged is made of a fluorinated thermoplastics polymer, preferably the same as that of the outer tube, and the other side is made of a material other than fluorinated thermoplastics polymer, for example steel or a heat-setting resin.

7. A heat exchanger according to any one of claims 1 to 4 wherein the inner tube is made from a fluorinated thermoplastics polymer.

8. A heat exchanger according to claim 7 wherein the exposed surfaces of the connector plates are made from a fluorinated thermoplastics polymer.

9. A heat exchanger according to any one of the preceding claims wherein the tubes are welded to the associated connector plate.

10. A heat exchanger according to any one of the preceding claims comprising a plurality of outer tubes connected to the first connector plate and a corresponding plurality of inner tubes connected to the second connector plate, each inner tube opening into an associated outer tube.

11. A heat exchanger according to claim 10 wherein the outer and inner tubes are arranged in line.

12. A heat exchanger according to claim 10 wherein the outer and inner tubes are arranged alternately in quincunx formation.

13. A heat exchanger according to any one of claims 10 to 12 wherein the outer and inner tubes are arranged for a single pass.

14. A heat exchanger according to any one of claims 10 to 12 wherein the outer and inner tubes are arranged for several passes.

15. A heat exchanger according to any one of the preceding claims including a cover plate superposed the second connector plate.

16. A heat exchanger according to claim 15 wherein first and second fluid conduits or chamber are defined between the first and second connector plates and between the second connector plate and the cover plate respectively.

17. A heat exchanger for effecting heat transfer between two fluids separated by a heat transfer surface substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

18. A heat exchanger for effecting heat transfer between two fluids separated by a heat transfer surface substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

19. An installation incorporating a heat exchanger according to any one of the preceding claims.

20. An installajion substantially as hereinbefore described with reference to Example 1.

21. An installation substantially as hereinbefore described with reference to Example 2.

22. An installation substantially as hereinbefore described with reference to Example 3.

23. An installation substantially as hereinbefore described with reference to Example 4.