The present invention relates to a method to increase the life of gaskets intended to be used in plate heat exchangers. The gaskets are placed in gasket grooves in the plates which constitute the plate heat exchanger and the gaskets are on one side in contact with a medium giving off or taking up heat and on its other side with surrounding. atmospheric air.

In some applications the temperature of the medium giving off heat is very high. In such applications where the gaskets are subjected to a high temperature on one side and to atmospheric air of lower temperature on its other side an oxidative degradation of the surface of the gasket takes place, which gives a surface layer, which forms a protection for the gasket.

The gaskets in a plate heat exchanger must have such an elasticity that they are compressed when the plate heat exchanger is tightened by means of clamping bolts, are pressed out and give a tightening force such that the medium giving off or taking up heat can not leak out from the plate heat exchanger. A usual gasket material for high temperature applications is fluorocarbon rubber (FKM), which is a material which may withstand high temperatures (180 °C), but which is very expensive.

Another high temperature material is HNBR (hydrogenated nitrile) which may be used at temperatures up to 160 °C. This maximum temperature is valid when the fluid giving off heat consists of water or steam.

Another material which has found use in plate heat exchangers working at a high temperature is EPDM a polymer which is a terpolymer of ethylene, propylene and an ethylidenenorborene. This material may be

used at temperatures up to 160 °C and has at this temperature a life of about one year. At temperatures below 130-140 °C the surface of the gasket is not attacked. In continuous operation at 130-140 °C the gasket has a life of about 2-3 years.

When the temperature of the medium giving off heat exceeds 130-140 °C the problems with the life will become considerable also for these heat resistant EPDM-materials. As has been mentioned above the surface layer which faces the surrounding air becomes oxidised relatively rapidly and constitutes principally a reinforcement of the gasket. The oxygen in the atmospheric air does not only influence the outer part of the gasket but influences also the parts of the gasket which are turned against the plates bearing against each other. As a consequence of the very small movements between the plates and the gaskets, which never may be eliminated oxygen will penetrate into the slit between the ageing gaskets and the plates. The gasket material in the outer part of the slit is oxidised and this oxidation front moves inwards along the surface of the gasket. When the oxidation front has reached the centre line of the gasket, the gasket will no longer be able to tighten. At an operation temperature of 160 °C it will take around one year until the damages on the gasket will be so great that they cause a leakage.

It has now surprisingly been found that the life of gaskets used for applications with such high temperature of the medium giving off heat may be prolonged in a considerable degree.

According to the invention a thin layer of a fluoropolymer is applied at least on the parts of the gaskets which bear against adjacent plates and are situated at the part of the gasket which is turned against surrounding

air alternatively that the bottoms of the gaskets grooves in the plates on both sides of the plates at least in its outer part turned against the surrounding air are coated with a thin layer of fluoropolymer. Each gasket bears with one of its sides against the bottom of the gasket groove and with its other side against the lower side of the gasket groove of an adjacent plate. Each plate shall consequently be coated both in the gasket groove on one side of the plate and on the back (lower side) of the gasket groove on the other side of the plate.

If so is desired the thin layer of fluoropolymer may be applied on the whole part of the gasket which abuts the bottom of the gasket groove and the lower side of the gasket groove of an adjacent plate. If the gasket grooves in the heat exchanger plate instead are coated with fluoropolymer the bottom of the groove may at its whole width be coated with fluoropolymer.

Of course the gasket may if so is suitable be coated with fluoropolymer over its whole circumference, the important thing is however that there is a layer of fluoropolymer on the side turned against the surrounding air. In the same way the heat exchanger plate may, if so is suitable, be completely coated with fluoropolymer.

The layer of fluoropolymer applied to the surface of the gasket has a thickness which is below 0,25 mm, preferably 0,1 mm.

The fluoropolymer used for the coating may consist of polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF) or fluorocarbon rubber (FKM).

It is known per se to produce gaskets of fluoropolymers and to provide gasket material with a protecting layer of fluoropolymer. The purpose of the earlier described use of fluoropolymer is however to make use of the high chemical resistance of fluoropolymers in surroundings where aggressive fluids are treated.

Such gaskets are however not suitable when one desires a gasket with a high elasticity, since a gasket of fluoropolymer is rigid and at that cumbersome and lacks the flexibility which is needed in order to obtain a tightening between the plates in a plate heat exchanger.

It is not known before that such a thin layer of fluoropolymer, that is now suggested and moreover does not need to cover the whole gasket or be applied on a part of a heat exchanger plate situated adjacent the gasket, could be able to protect against oxidation.

The fluoropolymer is suitably applied on the surface of the gasket by painting or spraying or by dipping the gasket in a mixture of fluoropolymer and a binder followed by curing or vulcanization. The fluoropolymer may be dispersed in a binder which after curing binds the fluoropolymer to the surface of the gasket. If the coating material consists of fluorocarbon rubber this may be dispersed in a solvent with an addition of vulcanizing agent to bind the polymer to the gasket surface.

According to the invention one may use a gasket material which retains its flexibility also under difficult operation conditions as has been described above and which life through the coating is at least doubled.

This effect may probably be attributed to the low permeability for oxygen in the fluoropolymer.

The invention also comprises use of plates or gaskets coated according to the method of the main claim in plate heat exchangers intended for heat exchange at a high temperature.

The invention will be described further with reference to the attached drawing, figure 1 and 2 of which show a cross section of a gasket coated according to the invention, while figure 3 and 4 show how the plates in a plate heat exchanger are coated according to the invention.

Figure 1 shows a gasket 1 in cross-section. The gasket is on its upper and lower side coated with a thin layer 2 of fluoropolymer. The gasket has been coated with fluoropolymer only on a part of its upper and lower side.

When the gasket is arranged in the gasket groove on the heat exchanger plate the right side of the gasket should be turned against the surrounding atmosphere. The thickness of the coating shown on the drawing has been exaggerated for drawing technical reasons, since the thickness of the coating according to the invention is below 0,25 mm.

Figure 2 shows a gasket 3 which on its whole upper and lower side has been coated with a thin layer 4 of fluoropolymer.

Figure 3 shows two neighbouring plates 5 in a plate heat exchanger. The plates 5 each have a groove intended for the gasket which shall seal the space 7 between the plates, where a medium giving off or taking up heat is brought to flow. The space 8 on the other side of the gasket groove is open against the atmospheric air.

According to the invention the part of the gasket groove that is turned against the atmospheric air has been coated with a thin layer 9 of fluoropolymer. Also a part of the lower side of the gasket groove has

been coated with a layer 10 of fluoropolymer. Also in this figure the proportions between the thickness of the plates and the layer of fluoropolymer are incorrect, since it should be impossible to be able to see the layer of fluoropolymer in a correctly executed drawing.

Figure 4 shows two neighbouring plates 11 in a plate heat exchanger, where however the whole bottom 12 of the gasket groove on both sides has been coated with a layer 13 of fluoropolymer.