NO164130B - PROCEDURE FOR PREPARING A PLATE HEAT GROWTH - Google Patents

Description

The invention relates to a method for producing a plate heat exchanger which contains at least three parallel, spaced apart plate elements consisting of graphite and a fluoropolymer and comprising a plate and a frame which surrounds the plate, and means for supplying and removing the heat exchanging fluids which flows through the space between two plates.

Plate heat exchangers constructed according to the principle of filter presses consist of at least three and usually of a large number of plates which are designed with channels and which are connected to each other in a stack with the individual plates lying at a distance from each other. The stack and each plate are provided with means for supplying and removing the heat-exchange fluids that flow through the spaces between the plates. The distance between neighboring plates is determined by spacers which

is inserted between two and two plates. Preferably, the spacers are formed at the edges of the plates, e.g. by deep drawing (see DE application pattern 82 08 878), and it is also known in the plan for the plates to form secure, waffle patterns and other surface structures that increase stability and improve the distribution of the fluids. The gussets can be so deeply designed in the material that they touch the neighboring plate and thus contribute to bearing. This precaution allows the use of very thin sheets, above all of thin sheet metal and foils and also of thermoplastics. Sheets of thermoplastic can be easily produced and are usually more resistant than metals to corrosive media. This is especially true for fluoropolymers. There are e.g. known a plate heat exchanger of polytetrafluoroethylene (PTFE) which contains a number of plates with channels and special heat exchanger walls (FR-A-2 373 025). As the thermal conductivity of PTFE is relatively small, it has been proposed to incorporate graphite powder into the fluoropolymers (FR-A-2 080 563). Such plates contain approx. 80% PTFE and only 20% graphite (FR-A-1 055 271), so that the improvement in thermal conductivity is relatively limited. According to DE-OS 21 41 019, work is carried out with a content of 5 - 45 percent by weight of graphite particles in polyfluorinated plastics when manufacturing pipes for tube bundle heat exchangers

by extrusion. Although the graphite particles are introduced in a special granulometry and a defined distribution of the graphite particles in the plastic base mass is established: by means of a special mixing process, the increase in thermal conductivity achieved in this way is not satisfactory. Heat exchanger walls with a graphite content of up to 90% are known from DE-A-2 705 807, but the binder or base mass of the plates consists of thermoset resins such as phenolic resins, epoxide resins and furan resins, whose corrosion resistance is less than that of fluoropolymers.

Fluoropolymers, on the other hand, have less stiffness and a higher creep rate. It has been proposed to improve these properties by mixing carbon fibers into polyvinylidene fluoride (PVDF), but fiber proportions of over 30% reduce the fluidity of the mixture to such an extent that processing with normal molding methods is no longer possible (see GB-PS 1 324 424). Finally, one method is known for the production of bodies, where mixtures containing expanded graphite and corrosion-resistant resins such as PVDF are pressed (US-PS 4 199 628). In this connection, the graphite particles are arranged mainly in a plane perpendicular to the pressing direction, which in plate heat exchangers, due to the manufacturing process, is the direction of heat flow. As a result of the anisotropy of the graphite grains, this necessarily results in plates with a high heat resistance in the direction of the heat flow and thus heat exchangers with a poorer heat transfer capacity. Such shaped bodies are therefore only used as corrosion-resistant liners etc.

Finally, a heat exchanger plate for fuel cells is known where the plate consists of 85 - 95% graphite powder' and 5 - 15% PVDF (US-A-3 801 374)•. To produce the plates, PVDF is dissolved in a solvent such as methyl isobutyl ketone and the solution is mixed with graphite powder. 'Before pressing, the de-icing agent must be removed, which requires a great deal of effort to avoid environmentally harmful emissions. It is also not known what the thermal conductivity of the process is' manufactured plates have.->

The invention is based on the task of providing a method for producing a plate heat exchanger of the type mentioned at the outset, whose plate elements are easy to produce without environmental impact and have a low thermal resistance.

This task is solved by a method of the kind indicated at the outset in the way that appears from the characteristic in claim 1.

In this way, the plate can be produced with a very small thickness. They have a significantly lower brittleness than plates that consist exclusively of graphite, and the reversible and irreversible changes in shape are significantly smaller than the length and volume changes of plates that are made of plastic or of plastic with fillers. The favorable material properties for plate heat exchangers can obviously be traced back to the fact that the fluoropolymers in the form of thin films surround the graphite grains and connect them to each other, and that the connection has a high shear strength. Thermoplastically processable fluoropolymers are particularly suitable as a binder, e.g. copolymers of tetrafluoroethylene or trifluorochloroethylene, polyvinyl fluoride and polyvinylidene fluoride, which, due to their easy workability when mixed with graphite powder, are preferred as binders.

The mechanical and thermal properties of the plate elements, which are made from graphite powder and a binder from the group of fluoropolymers, can be widely adapted to the current operating conditions, e.g. by changing the proportion of binder or another manufacturing parameter. Sheets with a bulk density of 2.0 - 2.1 g/cm^ and a tensile strength of 20 - 40 MPa are particularly advantageous. Plates of this type have a wide application area and, as elements with large surfaces, also have sufficient stiffness. Under special operating conditions, e.g. high solids content in the heat exchanging fluids, the wear rate of the plates may possibly increase to a higher value as a result of the lower hardness of the graphite. To reduce wear and tear, it is advantageous under these conditions to use plates in which wear-resistant particles, which are known per se, are dispersed. The proportion of the particles depends on the wear load on the plate, and the grain size of the particles is suitably less than half the thickness of the heat-exchange plates . Particularly suitable are particles of silicon carbide, which have high hardness and favorable thermal conductivity and practically do not affect the corrosion resistance of the plate elements and can therefore be present in the elements in a proportion by weight of up to 50%. In the case of particularly strong corrosion attacks, the addition of coke particles is preferred. For heat exchangers that work at higher pressures or are subjected to stronger mechanical loads, it is advantageous to have plates containing carbon fibers as reinforcement, preferably as short-cut fibers in a concentration of 2 - 20%. A reinforcement of frames consisting mainly of a fluoropolymer and graphite particles, in bipolar electrode devices in electrolytic cells with randomly distributed carbon fibers is known from US-A-4 339 322. As the fiber reinforcement is particularly effective in the flat areas of the plates, the content should of fibers is enriched in the frames and in the supply and removal areas of the plates.

An important parameter for the performance of heat exchangers is the velocity distribution of the fluids between the heat exchanger floats. To regulate the fluid flow, the plate elements are preferably designed with knobs which are arranged in a predetermined pattern and lie closer in the supply areas for the fluids. They extend, e.g. in rows across the flow direction of the fluids. Appropriately, the lugs, which can be in the form of truncated cones, touch the neighboring element, whereby a further stiffening of the plate stack is obtained. According to another embodiment, the plate elements are stiffened by plate-like carbon fiber structures such as webs, tapes or prepreg layers with carbon threads that extend in one or two directions. The flat fiber structures are glued to the surface of the plates, e.g. with molten polyvinylidene fluoride, or pressed into the elements during their manufacture. It is preferably the flat areas of a plate element, from which the distributor lugs also proceed, that are braced.

Gaskets of a flexible and corrosion-resistant material, e.g. gaskets of a fluoropolymer. Gaskets of graphite foil, whose cold flow is very small, are preferred. The use of such gaskets is known from DE-A-24 06 522. The gaskets made of expanded graphite, whose bulk density is approx. 0.8 - 1.2

q/ cm?, is suitably pressed together to a thickness of 0.1 - 0.3 mm when the plate stack is clamped together. The removability of the gaskets from the plate frame is facilitated by a coating of fluoropolymer, e.g. dispersions containing tetrafluoroethylene copolymers, which are applied to the sealing rafts.

The plate heat exchangers produced according to the invention exhibit the outstanding resistance to corrosive substances which is special for graphite, but do not exhibit the brittleness of graphite which makes the manufacture and use of thin-walled elements on a technical scale practically impossible. The plate elements' strength is not limited in time, as is the case with heat exchangers that only consist of polymer. Other advantages are the larger heat conduction area and the simple adaptation of shape and properties to the respective area of application by adding granular or fiber-shaped substances.

To produce the plate elements, graphite powder with a grain size of less than 0.5 mm is mixed with a fluoropolymer with a grain size of less than 0.001 mm in a ratio of between 95:5 and 70:30 at a temperature of 200 - 350°C. The graphite component in the mixture consists of natural graphite such as flake graphite or earth graphite or electrographite which is produced by graphitizing petroleum coke, needle coke or coal tar pitch coke. The grain size of the graphite is less than 0.5 mm. In this way, for the setting of a specific packing density, several grain fractions are preferably also mixed together, e.g. fractions of 0 - 0.063, of 0.063 - 0.1 and of 0.1 - 0.3 mm. After cooling to room temperature, the aggregates formed by the mixture are ground to a grain size below 0.3 mm. The ground material is then, possibly after the addition of a pressing aid such as stearic acid or a metal soap, filled into a pressing matrix containing a core. Both the core and the press piston are profiled, and the distance between the die face and the core is at any point proportional to the thickness of the pressed plate element. If e.g. the ratio between the frame height and the thickness of the heat exchanger surface is 4:1, the matrix and the core in the corresponding places are likewise at a distance from each other in the ratio 4:1. The layer of powder filled between the matrix and the core is thus four times as thick in the edge zones as in the middle, so that when the powder is pressed to the correct dimension, it is compressed equally in every place. The press pressure is at least 200 bar and the press temperature at least 200°C. During the pressing, the frame, the heat exchanging surfaces and any knobs or the like for controlling the fluid flow are connected to each other by connection between the materials (stoffschlussig).

The invention will be described in more detail below with reference to the drawing and the examples. - Fig. 1 is a schematic axonometric view of a plate heat exchanger.

- Fig. 2 shows a plate element in elevation.

- Fig. 3 is a section through a stack containing several plate elements.

In fig. 1, plate elements 1 are displaceably arranged in a framework 2. Each plate element has holes 3 through which heat exchanging fluids are supplied to the corrugated part 4 of the plate. Several elements are combined to form a plate heat exchanger 5 which is closed by a front plate 6 and a back plate 7. In the front plate 6, there are nozzles 8 inserted flush with the holes 3 through which heat exchanging fluids F^ and F2 are supplied and carried away. The plate elements, the front plate and the back plate are tensioned together by means of tension rods which are not shown in the drawing. Each plate element consists as shown in fig. 1 of a frame 9 and a corrugated part 4 which is surrounded by the frame. In the frame, a groove 10 is formed for guidance in the framework 2 (fig. 1), and between the holes 3 and the wavy part 4 there is arranged a knob 11 for controlling the fluid flow. Bead 12 also serves to distribute the fluid. As shown in fig. 3, a flat gasket 13 consisting of a flexible, corrosion-resistant material, especially graphite foil, has been inserted between oppositely facing frames 9.

Example 1

In a trough mixer with a sigroa mixing tool, 80 parts by weight of electrographite powder (size distribution: 60% between 60 and 200 micrometers and 40% less than 60 micrometers) were mixed with 20

parts polyvinylidene fluoride powder, whose grain size was less than 1 micrometer. The mixing temperature was 220°C and the mixing time one hour. After cooling to room temperature, the mixture was ground on a pin mill to a grain size of less than 0.3 mm. Then, as described above, it is filled in a press mold and pressed into a continuous plate element. The pressing pressure was 400 bar, the temperature 270°C and the pressing time 10 minutes. The following material values were measured:

Rot density - 2.02 g/cm<3>

Bending stiffness - 5 0MPa

Tensile strength - 30MPa

Thermal conductivity - 25W/m.K

Permeability coefficient 3 x 10~^ cm^/s

Seventeen plate elements were clamped together with a front plate and a back plate as shown in fig. 1, as a 0.3 mm thick flat gasket was inserted between each frame. The plate heat exchanger was used for cooling diluted hydrochloric acid (approx. 20% HC1) which was contaminated by organic solvents. The fluid temperatures were as follows: acid inlet 116°C, acid outlet 52°C, cooling water inlet 7°C, cooling water outlet 40°C. The heat transfer figure was 2000 - 3000 W/m.K. Corrosion damage and leaks did not occur at an operating pressure of 3.5 bar.

Claims (5)

1. Method for manufacturing a plate heat exchanger containing at least three parallel, spaced apart plate elements consisting of graphite and a fluoropolymer and comprising a plate and a frame surrounding the plate, and means for supplying and removing the heat exchanging fluids that flow through the space between two plates, characterized in that (a) graphite powder with a grain size of less than 0.5 mm is mixed with a fluoropolymer with a grain size of less than 0.001 mm in a ratio of between 95:5 and 70:30 at a temperature of between 200 and 350°C, (b) the mixture after cooling to room temperature is ground to a grain size of less than 0.3 mm, (c) the ground material is pressed into plate elements at a temperature of at least 200°C and a pressure of at least 200 bar and (d) the plate elements are assembled into a heat exchanger and flexible flat gaskets are arranged between the flat surfaces of neighboring frames. 2. Method as stated in claim 1, characterized in that the plate elements of the heat exchanger consisting of plate and frame are pressed from a mixture containing a graphite powder and powdered polyvinylidene fluoride. 3. Method as stated in claim 1 or 2, characterized in that the plates are at least partially reinforced with surface-like carbon fiber structures. 4. Method as specified in one of the preceding claims, characterized in that gaskets of graphite foil with a thickness of 0.1 - 0.3 mm are inserted between the frames of neighboring plates. 5. Method as stated in claim 4, characterized in that the seals are coated with a fluoropolymer.