EP0396277A1

EP0396277A1 - Gaskets

Info

Publication number: EP0396277A1
Application number: EP90304177A
Authority: EP
Inventors: Peter John Children; Patrick Richard Thody; John Richard Watts
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-04-29
Filing date: 1990-04-19
Publication date: 1990-11-07
Also published as: GB9008798D0; GB2231396A; GB8909930D0

Abstract

A heat exchanger Gasket 11 comprises a core 13 of elastomeric material contained within an outer sleeve 14 of chemically and/or heat resistant plastics material, preferably a film forming fluoroplastics material heat shrunk onto the elastomeric core. A plate heat exchanger including such gaskets and a method of making the gasket are also disclosed.

Description

This invention relates to gaskets, and more particularly to plate heat exchanger gaskets, and to a method of making same.
Plate heat exchangers commonly employ elastomeric sealing gaskets of complex shape between adjacent plates. These plate heat exchangers are however limited in their maximum temperature of continuous operation by the heat resistance of the gaskets. For temperatures in excess of the maximum heat resistance of elastomeric gaskets, a gasket of compressed asbestos fibre is often used. Asbestos gaskets are also often used when there is no elastomer with sufficient chemical resistance to the product being passed through the heat exchanger.
Asbestos gaskets suffer from a number of drawbacks. Firstly, they are punched from sheet material and this results in considerable wastage of material. Punching from sheet material also imposes restrictions on the shape of the gasket section. Secondly, because of the relative incompressibility of asbestos as compared with elastomeric material, it is more difficult to create a good seal and it is often necessary for the heat exchanger to be made stronger so that increased sealing forces can be applied. Thirdly, there is an increasing reluctance to use any product which contains asbestos because of the potential health hazard.
The invention seeks to provide a heat exchanger gasket which overcomes these drawbacks.
Accordingly, in a first aspect, the invention provides a heat exchanger gasket comprising a core of elastomeric material contained within an outer sleeve of chemically and/or heat resistant plastics material.
Preferably, the elastomeric core is of synthetic rubber and, advantageously, the outer sleeve is of a film forming fluoroplastics material, such as polytetrafluoroethylene (PTFE), fluorinated ethylene polypropylene (FEP) or per fluoro-alkoy (PFA).
Preferably, each gasket comprises a plurality of unconnected parts one of which is of non-circular shape when viewed in plan.
Preferably, the gasket is of non-circular cross-section and in this case the gasket typically comprises a substantially flat base portion and a convex upper portion.
Preferably, the outer sleeve has been heat shrunk onto the elastomeric core.
Conveniently, the elastomeric core has been extruded and joined at its ends.
Conveniently, the outer sleeve is a seamless tube.
In a second aspect, the invention provides a plate heat exchanger comprising a plurality of plates and a gasket according to the first aspect of the invention between adjacent plates.
In a third aspect the invention provides a method of making a heat exchanger gasket comprising the steps of:

(a) forming a strip-like core of elastomeric material,
(b) locating the core within an outer sleeve of flexible, chemically and/or heat resistant plastics material, and
(c) subsequently joining the ends of the strip-like core together.

Preferably, the method also comprises the step of heat shrinking the outer sleeve onto the inner core.
Preferably, the core is extruded although it could be moulded. If the core is extruded it is preferably formed into the required shape by placing it in a former and then curing (vulcanizing) it.
Preferably, the sleeve is slit and peeled away from the core at its ends prior to joining the two ends of the core together.
Preferably, the ends of the sleeve are joined together at least on the inner circumference of the core.
The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a plan view of one embodiment of a heat exchanger gasket according to the first aspect of the invention,
Figure 2 is a section taken along the line II-II of Figure 1, on a much enlarged scale,
Figure 3 is a perspective exploded view of part of a plate heat exchanger incorporating gaskets as shown in Figures 1 and 2,
Figure 4 is a scrap view showing part of the gasket of Figure 1, in the course of being made, and
Figure 5 is a scrap view showing part of the gasket of Figure 1, in the course of being made by an alternative method to that shown in Figure 4.

Plate heat exchanger gaskets are of complex shape and an example of a fairly simple plate heat exchanger gasket is shown in Figure 1. The gasket shown in Figure 1 comprises three parts 10, 11 and 12 which are conventionally joined together and formed as an integral moulding. However, when making this gasket by a method according to the present invention the gasket is formed in three separate parts, parts 10 and 12 being circular as viewed in plan and part 11 being of non circular shape, and indeed being non-symmetrical about any two mutually transverse planes, as viewed in plan.
As best shown in Figure 2, each part 10, 11 and 12 comprises an inner core 13 of elastomeric material, e.g. synthetic rubber, and an outer sleeve 14 of chemically and/or heat resistant material, preferably in the form of a film forming, heat shrinkable fluoropolymer such as, for example, polytetrafluoroethylene (PTFE), fluorinated ethylene polypropylene (FEP), per fluoro-alkoy (PTA), PVDF, PVF or CTFE.
The core 13 may be moulded in conventional manner and then cut transversely prior to drawing it through the sleeve 14, but it is more economical to extrude the core 13. As shown in Figure 2, the core 13 is of non-circular cross-section and typically, as shown, comprises a flat base portion 15 having outwardly flared lower side walls 16 joined to a convex or peaked upper portion 17. The core 13 is extruded to this cross-section and an appropriate length of core material is then placed in an aluminium former having an endless channel corresponding to the required shape of the gasket part. The core 13 is then vulcanized in high temperature steam in an autoclave or by any other appropriate method.
The core 13 will thus be formed as an endless band and it is cut transversely, stretched into a strip-like length, and whilst in a stretched condition it is drawn through a tubular length of flexible, heat shrinkable fluoropolymer which will form the outer sleeve 14. The core is then allowed to return to its required shape. The ends of the sleeve 14 are then slit over a short distance along the outer circumference of the core in the direction of the longitudinal extent of the sleeve 14 and the ends of the sleeve 14 are peeled back, as shown in Figure 4, to expose the two ends of the core 13. The ends of the core 13 are then joined, such as by injecting a shot of rubber between the two ends of the core while pressing the ends together, and by then vulcanizing the joint. The peeled back ends of the sleeve 14 are joined by welding at least along the inner circumference of the core 13 so that there is an uninterrupted layer of fluoropolymer where each gasket part is to be exposed to heat exchanger fluid.
The sleeve 14 is then heat shrunk down on to the elastomeric core 13. Typically this is accomplished by using air heated to a temperature of about 120°C. Finally, stresses which have formed during the moulding and shrinking stages are relieved by placing the gasket parts in a jig which supports the gasket parts in their required shapes, raising the temperature of the gasket parts to about 150°C, and allowing the gasket parts to cool while supported in their required shapes.
As an alternative to peeling back ends of the sleeve 14, as described with reference to Figure 4, the sleeve 14 is heat shrunk down onto the elastomeric core 13 before joining the ends of the core 13. The two ends of the sleeve 14 are then cut back as shown in Figure 5. A short sleeve 24 of similar material to sleeve 14 is then placed over one end of sleeve 14. The two ends of the core 13 are then joined as before, and the sleeve 24 is then drawn over the exposed part of the core 13 and secured, such as by welding, at opposite ends to opposite end portions of the sleeve 14. This sleeve 14 may then be heat shrunk down onto the core 13.
Plate heat exchanger gaskets are commonly bonded with adhesive to one plate of the heat exchanger so as to facilitate assembly and, at some stage during the manufacturing process, the sleeve 14 is etched to ensure that adhesive will bond to it.
Figure 3 shows part of a plate heat exchanger. The heat exchanger comprises end plates 17 (only one of which is shown), and a plurality of heat transfer plates 18 (only two of which are shown). The heat transfer plates 18 are arranged in a stack and clamped between two end plates 17. A gasket, made up of three parts 10, 11 and 12, is bonded to one face of each heat transfer plate 18 and makes sealing contact with the opposite face of an adjacent plate. These gaskets serve to separate two fluids in conventional manner.
A heat exchanger gasket made as described above will retain elastomeric properties at much higher temperatures than has been possible hitherto and in aggressive liquids and gases which can not be sealed with "rubber like" gaskets.
The fluoropolymer film protects the elastomeric core from high temperature oxidation which would otherwise destroy the molecular structure of the elastomer and render it unserviceable as a seal at high temperature. Thus it is possible for the temperature resistance of the gasket to be increased without having to use a compressed asbestos gasket or some other non- elastomeric seal.
Fluoropolymers have very high resistance to a wide range of aggressive chemicals and so the gasket described above can be used to seal plate heat exchanges which are designed for use with such chemicals.
At present, when chemically resistant elastomeric gaskets are required, a fluoro-elastomer, such as VITON (RTM), is normally used, but this material is extremely expensive. By contrast, the gasket described above can use a relatively cheap core which is chemically protected by the fluoroplastics sleeve.
However, it is not the intention to exclude the use of a fluoro-elastomer as the core of the gasket as this in conjunction with the chemically protective sleeve would give maximum possible chemical resistance.
A plate heat exchanger gasket comprising an inner core of elastomeric material and an outer sleeve of chemically and/or heat resistant plastics material can be made in other ways. For example, a liquid elastomer could be injected into an endless fluoroplastics tube which is formed to a required shape, and then vulcanized in steam or in a mould.

Claims

1. A heat exchanger gasket comprising a core (13) of elastomeric material contained within an outer sleeve (14) of chemically and/or heat resistant plastics material.

2. A heat exchanger gasket as claimed in Claim 1, wherein the elastomeric core (13) is of synthetic rubber.

3. A heat exchanger gasket as claimed in Claim 1 or Claim 2, wherein the outer sleeve (14) is of a film forming fluoroplastics material.

4. A heat exchanger gasket as claimed in any one of the preceding claims, wherein the outer sleeve (14) has been heat shrunk onto the elastomeric core (13).

5. A heat exchanger gasket as claimed in any one of the preceding claims, wherein the elastomeric core (13) has been extruded and joined at its ends.

6. A heat exchanger gasket as claimed in any one of the preceding claims, wherein the outer sleeve (14) is a seamless tube.

7. A plate heat exchanger comprising a plurality of plates (17, 18) and a gasket according to any one of the preceding claims between adjacent plates.

8. A method of making a heat exchanger gasket, comprising the steps of:

(a) forming a strip-like core (13) of elastomeric material,

(b) locating the core within an outer sleeve (14) of flexible, chemically and/or heat resistant plastics material, and

(c) subsequently joining the ends of the strip-like core together.

9. A method as claimed in Claim 8, further comprising the step of heat shrinking the outer sleeve onto the inner core.

10. A method as claimed in Claim 8 or Claim 9, wherein the joint between the two ends of the core is covered by a short sleeve (24) of flexible, chemically and/or heat resistant plastics material, which is secured to opposite end portions of the first mentioned sleeve (14) after the ends of the core (13) have been joined together.