METHOD OF PROVIDING HEAT TRANSFER PLATES WITH A LAYER OF A SURFACE PROTECTING MATERIAL

Abstract: For the application of a surface protecting layer of plastics onto heat transfer plates the plates are first assembled to a plate heat exchanger (1). After this a gaseous medium containing the plastic material, preferably in an evaporated state, is introduced into the plate interspaces. In a manner known per se the plastic material is caused to deposit onto the heat transfer plates in the plate interspaces to form a surface protecting plastic layer.

Description: Method of providing heat transfer plates with a layer of a surface protecting material. Heat transfer plates are made of many different materials which are more or less resistant to corrosion or other influence by different heat exchange fluids. As to heat transfer plates of metal, which for a certain purpose are not sufficiently resistant to corrosion, it is technically possible, but in practice difficult, to increase the resistance to corrosion of the plates to an acceptable cost by providing the plates with an outer layer of another metal. The possibility of providing metallic heat transfer plates with a protecting layer of plastics, which would be cheaper than a layer of metal, does not seem to have been used to any large degree. This may be for several reasons. Thus, heat transfer plates of thin sheet metal are often provided with corrugations or other protuberances in their heat transferring portions, which in a plate heat exchanger abut against each other by a large force at a great number of contact places distributed across the heat transferring portions. If the plates were covered by thin plastic layers, such layers would easily be broken at the said contact places. The possibility of providing heat transfer plates intended for brazed plate heat exchangers with thin plastic layers has been excluded because such plastic layers would obviously be destroyed in connection with the brazing together of the heat transfer plates. The present invention relates to a novel and inexpensive way of providing heat transfer plates with a layer of a surface protecting material, which method is characte- rized in that the heat transfer plates are first assembled to a plate heat exchanger, in which plate interspaces for the through flow of two heat exchange fluids are formed, and that a gaseous medium containing the surface protecting material is then introduced into the plate heat exchanger at least in those plate inter¬ spaces being intended for through flow of one of said heat exchange fluids, the surface protecting material in a way known per se being caused to form a layer on the surfaces of the heat transfer plates. The method according to the invention may be applied on any kind of plate heat exchanger but is particularly intended for plate heat exchangers having permanently assembled heat transfer plates, e.g. welded or brazed plate heat exchangers. In brazed plate heat exchangers the invention provides in addition to a surface pro¬ tection for the heat transfer plates also a surface protection for the brazing material used between the heat transfer plates. This means that the area in which brazed plate heat exchangers may be used can be extended, since the brazing material sometimes consti¬ tutes an obstacle for such heat exchangers to be used in connection with certain liquids. It is particularly suitable in connection with use of the invention that plastics is used as a surface pro¬ tecting material. The plastic material then may be introduced into the plate heat exchanger either in the form of a mist, i.e. in the form of small liquid drops suspended in a gas, or in an evaporated form. In the latter case the gaseous medium being introduced into the plate heat exchanger may completely consist of evapora¬ ted plastics. A technique that may be used is described in the patent specification SU-A 1151546. According to this technique a substance, di-p-xylylene, may be transformed from a solid state to a gas by sublimation in a first chamber at a pressure of 1 mm Hg and a temperature of 200°C, after which the substance is subjected to pyrolysis in a second chamber at 600°C. A substance, e.g. di-p-xylylene, treated in this manner would then according to the invention be sucked into an assembled plate heat exchanger kept at a relatively low temperature, e.g. room temperature, so that the gaseous substance is condensated onto the surfaces of the heat transfer plates in the plate heat exchanger and, simul¬ taneously, a polymerization takes place. The sucking of the gaseous plastic material into the plate heat exchanger may be performed in several steps for obtain- ment of several layers of plastics on the surfaces in question. If necessary for a sufficient covering of all the surfaces by layers of plastics, the suction of the gaseous plastic material into the plate heat exchanger may be performed alternately from the inlet and the outlet, respectively, of the plate heat exchanger for the heat exchange fluids. The invention is described in the following with refe¬ rence to the accompanying drawing, which schematically shows a plant for covering of heat transfer plates with a layer of plastic material by the method according to the invention. In the drawing there is shown a permanently assembled brazed plate heat exchanger 1 of a conventional kind, which has an inlet 2 and an outlet 3 for a first heat exchange fluid and an inlet 4 and an outlet 5 for a second heat exchange fluid. The plate heat exchanger may be for instance of the kind described more closely in WO 88/09473 or GB-A 2.005.398. The drawing shows schematically an apparatus 6 for evaporation of a plastic material. This apparatus communicates through a conduit 7 with an apparatus 8 for a pyrolysis of evaporated plastic material. The apparatus 8 in turn communicates through conduits 9 and 10 with the inlet 2 of the plate heat exchanger for said first heat exchange fluid. The plate heat exchanger outlet 3 for the same heat exchange fluid communicates through conduits 11 and 12 with a liquid trap 13 which in turn through a conduit 14 communicates with a vacuum pump 15. Through conduits 16 and 17 - shown by dotted lines in the drawing - also the plate heat exchanger inlet 4 and outlet 5 for said second heat exchange fluid may be connected to the conduits 12 and 9, respectively. The shown plant is intended to operate in the following manner. The evaporating apparatus 6 is charged batchwise or continuously with a plastic material in a solid or liquid state. By means of the vacuum pump 15 a sub- pressure is generated in the apparatus 6 corresponding to an absolute pressure of about 1 mm Hg. When a desired subpressure prevails in the apparatus 6, this is heated to a temperature of between 150°C and 200°C, the supplied plastic material being evaporated. By means of the vacuum pump 15 the formed vapour is sucked through the conduit 7, which may comprise a suitable throttle, into the apparatus 8 in which there is maintained by means of the vacuum pump a subpressure corresponding to an absolute pressure of about 0,5 mm Hg. A temperature of between 600 and 700"C prevails in the apparatus 8, so that a pyrolysis is obtained of the incoming vapour of plastic material. By means of the vacuum pump 15 the vapour of plastic material is sucked further on through the conduit 9, which may contain a suitable throttle, and through the conduit 10 into every second plate interspace in the plate heat exchanger 1. Within these plate interspaces a subpressure corresponding to an absolute pressure of 0,1 mm Hg is maintained. The whole plate heat exchanger is kept at a relatively low temperature, e.g. room temperature, vapour condensating onto one side of each of the plates in the plate heat exchanger. Vapour also condensates onto such inner parts of the plate heat exchanger which delimit inlet and outlet channels to and from, respectively, the plate interspaces. A certain excess amount of vapour may be sucked further on through the conduits 11 and 12 to the liquid trap 13, in which it is condensed. In connection with the condensation of the evaporated plastic material onto the inner surfaces of the plate heat exchanger a polymerization of the plastic material occurs, so that a continuous solid layer of plastics is formed on the surfaces. If all of the surfaces in the plate heat exchanger are to be covered by plastics, also the conduits 16 and 17 are used. Either, all of the conduits 10, 11, 16 and 17 may be open for simultaneous through flow, or the through flow may be controlled by means of not shown valves such that evaporated plastic material is first introduced only into every second plate interspace of the plate heat exchanger and, thereafter, evaporated plastic material is introduced only into the rest of the plate interspaces. In a similar manner it is possible, in a plant of the kind shown in the drawing, to connect in parallel with the conduits 9 and 12 several plate heat exchangers by means of branch conduits corresponding to the conduits 10, 11, 16 and 17. If so, valves are preferably arranged at least in the branch conduits corresponding to the conduits 10 and 17, so that the different plate heat exchangers, or parts thereof, may be successively connected to the apparatus 8. In this manner a rela- tively small vacuum pump may be used even if a large number of heat exchangers are connected to the plant. If valves are arranged in all of the branch conduits, i.e. even those corresponding to the conduits 11 and 16, it is possible during operation of the plant to remove treated heat exchangers successively from the plant and replace them by new heat exchangers to be treated. The plant thus may be kept in operation continuously as long as desired. The above briefly described technique concerning evaporation, pyrolysis and condensation (polymerization) of plastic material is previously known, and no further description thereof would be necessary. Thus, technique of this kind is marketed for instance by an Italian company, Himont Italia, under the registered trade mark GALAXYL, and by two American enterprises Para Tech Coating Company and Paratronix, Inc. According to the technique thus previously known objects to be covered by a plastic layer are placed within an evacuated chamber. The said technique is also described in the patent specification SU-A 1.151.546.

Claims: Claims 1. Method of providing heat transfer plates with a layer of a surface protecting, e.g. corrosion resistant, material, c h a r a c t e r i z e d i n that the heat transfer plates are first assembled to a plate heat exchanger (1), in which plate interspaces are formed for through flow of two heat exchange fluids, and that after this a gaseous medium containing the surface protecting material is introduced into the plate heat exchanger (1) at least into those plate interspaces being intended for through flow of one of said heat exchange fluids, the surface protecting material in a manner known per se being caused to form a layer on the surfaces of the heat transfer plates. 2. Method according to claim 1, c h a r a c t e ¬ r i z e i that the assembled heat exchanger (1) is connected both to a subpressure source (15) and to an apparatus (8) containing said gaseous medium with the surface protecting material, the gaseous medium with the surface protecting material then being sucked into the plate interspaces of the plate heat exchanger (1). 3. Method according to claim 1 or 2, c h a r a c ¬ t e r i z e d i n that the surface protecting material is made gaseous (6) before it is introduced into the plate heat exchanger (1), after which it is caused to condensate onto the surfaces of the heat transfer plates in the plate interspaces. 4. Method according to any one of the preceding claims, c h a r a c t e r i z e d i n that plastics is used as a surface protecting material. 5. Method according to any one of the preceding claims, c h a r a c t e r i z e d i n that the heat trans¬ fer plates are assembled permanently, e.g. by brazing or welding, before the gaseous medium with the surface protecting material is introduced into the plate inter¬ spaces.

Inventors: NOVAK LADISLAV, NOVAK, LADISLAV
Applicants: ALFA LAVAL THERMAL, ALFA-LAVAL THERMAL AB
Application year: 1992
Main classification: F28F19
IPC codes: B05D7, F28F19