Air coolers are described in Perry's Chemical Engineers'handbook, 7th edition, 1997, pages 11-47 to 11-52. Air coolers are for example used in refinery, petrochemical and chemical processes to cool or condense process fluids flowing at the tube side with air. Air coolers typically include a tube bundle, wherein the tube is provided with spiral-wound aluminium fins and a fan, which fan moves air across the tubes.

A disadvantage of these air coolers is that, when corrosive liquids, vapours or gasses are to be cooled or condensed, severe corrosion of the metal tubes will occur. For example in a process to remove hydrogen sulphide from waste water by means of steam stripping air coolers having metal tubes are used to condense the vapours leaving the top of such a steam stripper column.

It has been found that due to corrosion the tubes of these air coolers have to be replaced every 0. 5 to 2 years.

It is known from for example US-A-4474233, US-A-4941692 and US-A-5036903 to use graphite tubes to overcome the corrosion problem in so-called shell-tube heat exchangers wherein the shell side medium is a liquid. These tubes cannot be easily applied in an air cooler application because the heat exchanging capacity of the tubes is too low.

It is known from US-A-5195578 to use vertically oriented graphite tubes having a helical of fins on the exterior of the tubes in a so-called trickling film

evaporator. In this apparatus liquid descends by gravity along the tubes in the helical channel formed by the helical of fins. The examples illustrate a graphite tube having graphite fins. The tube was made by machining a cylindrical block of graphite. It will be recognised that such a fabrication method is time consuming, laborious and therefore induces high costs to the resulting tube.

In the description other methods of making such tubes are mentioned, like gluing of the fins to the tube. No examples or further information is however supplied on how this could be performed.

US-A-4514900 describes an apparatus to manufacture a finned tube. This so-called finning machine helically winds an aluminium fin stock having a L-shaped cross-section around a metal tube. In the introductory part of this publication the use of an adhesive is mentioned as a possible method of securing the fin stock to the tube. The use of an adhesive is however not preferred, according to this publication, because of the insulative effect of the adhesive present between the fin stock and the tube. A further disadvantage of the method of US-A-4514900 is that only relatively strong tubes can be provided with fins. Less strong materials, for example tubes having very thin walls or certain brittle non-metal materials, cannot be used in the finning machine because they will break or bend during operation.

Japanese patent application 08010902 describes a method to make a finned tube by first preparing a coil-shaped fin member of phosphor bronze or stainless steel. This fin-member is subsequently extended or compressed to arrive at the desired pitch and a tube is inserted into the cavity of the fin-member. The fins and tube are subsequently fixed by means of solder.

The present invention is aimed at providing an improved method to make finned tubes. Especially directed

to tubes which cannot directly be used in a finning machine as for example disclosed in US-A-4514900.

This object is achieved by the following method.

Method to construct a tube having metal fins by inserting the tube in a tube like interior of a pre-fabricated helical of metal fins and assembling the tube with the helical of fins by means of an adhesive.

It has been found that with the method according the invention finned tubes can be prepared starting with tubes, which normally cannot be finned using a finning machine as described above. In this manner a simple method is obtained to provide, for example, finned graphite tubes which can advantageously be used in air coolers for cooling corrosive liquids, vapours or gasses. These air coolers can be used during a prolonged period of time without having to replace the tubes due to corrosion. It has further been found that these finned tubes have a comparable heat exchanging capacity as finned tubes obtained by prior art processes, such as described by US-A-4514900. This is surprising because one skilled in the art would expect a considerable less efficient heat exchanging capacity due to the use of an adhesive. Other advantages will become apparent when reading the below detailed description of the invention.

The tube is preferably a tube which cannot be directly used in a finning machine because of its reduced mechanical strength. Two groups of tubes are typical examples of such a tube. The first group of tubes are thin walled metal tubes, having a wall thickness of less than 1 mm, and more specially between 0. 3 and 0. 9 mm. The metals are for example titanium, tantalum, zirconium or molybdenum. The second group of tubes are of a non-metal material. These materials are more brittle than metals and are therefore less suitable to be directly used in a finning machine. The non-metal material of the tube is

preferably a material having a sufficient thermal conductivity in tile radial direction of the tube.

Preferably the radial conductivity is higher than 30 W/m/°K. An example of a non-metal material is tungsten carbide, silicon carbide or graphite. Preferably the non- metal is graphite. These graphite tubes can be made by extrusion of fine grain primary carbons, followed by curing and graphitizing at a temperature of around 3000 °C. The tubes are suitably impregnated with a suitable resin, for example a resin of the phenolic, epoxy or furan type, to fill the pores of the non-metal material and increase its strength. Graphite tubes for use in conventional shell and tube liquid/liquid heat exchangers are known and for example described in US-A-4474233. Examples of graphite tubes are GRAPHILOR 3 (GRAPHILOR is a trade mark) of Carbone Lorraine (having its registered offices in Pagney sur Moselle (FR)), Diabon type of Sigri GmbH (having its registered offices in Meitingen (DE)) and KARBATE (KARBATE is a trade name of the Union Carbide Corporation). One of the advantages of the present invention is that such commercially available graphite tubes can be easily used in an air cooler.

With the present invention it is possible to fix fins on tubes which have a lesser mechanical strength by making use of a pre-fabricated helical of fins. The tube can be easily inserted into the tube like interior of the pre-fabricated helical of fins. This method of construction can be performed without putting any substantial strain onto the tube, thereby reducing the risk of breakage.

The pre-fabricated helical of fins is preferably fabricated by first tension wrapping a strip around a mandrel tube. The metal fins preferably have an L-shaped cross-section, in a plane along the tube axis, as

exemplified in the above cited Perry's and in US-A-4514900. The L-shaped base will be positioned on the exterior of the mandrel tube and the remainder of the strip will extend outward from the exterior of the tube in a radial direction. This is the actual fin. The mandrel tube is of course stronger than the tube on which the fins are eventually assembled. Preferably the mandrel tube is a metal tube, more preferably the metal is of the same sort usually used to make prior art finned metal tubes on said finning machines. By subsequently relieving the tension of the thus formed helical of fins and removing the mandrel tube from the interior of the helical of fins the pre-fabricated helical of fins is obtained. This method is very advantageous because existing finning apparatuses and methods can be used to obtain the pre-fabricated helical of fins. Suitably the pre-fabricated helical of fins, after being freed from the mandrel tube, is put on a carrier tube having a slightly smaller diameter than the tube like interior of the helical of fins. This enables handling of the helical of fins without risking that the helical falls apart.

Before assembling the pre-fabricated helical of fins onto the tube, the interior of the helical of fins and the exterior of the tube is preferably prepared in the following manner. First the surface is cleaned, followed by abrasion of the surface and finally cleaned again.

Cleaning of the interior surface of the helical of fins can be performed by spraying a cleaning fluid, for example methyl ethylketone, acetone or iso-propanol, through a tube, which tubes is moved axially within the helical of fins. The cleaning fluid is subsequently removed from the interior of the helical of fins before abrading the interior surface. Abrading can be performed by methods well known to one skilled in the art. For example by applying an abrading fluid, for example

SCOTCHBRITE (SCOTCHBRITE is a trade mark of 3M corporation) in the same fashion.

The fins can be of any metal, for example copper.

Preferably aluminium fins are used. The metal fins may be coated with epoxy to avoid atmospheric corrosion attack in a manner known in the art.

In order to prevent galvanic corrosion of the metal fins it has been found important that the adhesive forms a physical barrier between the tube and the metal fins.

The adhesive layer is suitably present between the L-formed base and the exterior of the tube. More preferably the adhesive also forms a physical barrier between the exterior of the tube and the exterior of the finned tube. Galvanic corrosion may occur when a water bridge forms between the metal fin and the surface of the tube. By forming the above mentioned barriers the formation of such water bridges and the resulting galvanic corrosion is avoided. The adhesive layer is preferably between 0. 1 and 1. 5 mm thick. Thinner layers could result in that the galvanic corrosion occurs and thicker layers would impart an undesirable decrease in thermal conductivity between the tube and the fins.

The adhesive may be any adhesive which effectively bonds the metal fin to the surface. Suitable adhesives have a good adhesion to both the tube material and the metal fins, have a service temperature above the operation temperature wherein the tube will be used and are not very sensitive to moisture. Normally however the operational temperature of an air cooler comprising the tubes obtainable by the method of the present invention is high enough to ensure that any moisture present in the air will evaporate before it can significantly effect the strength of the adhesive layer. It has furthermore been found advantageous when the adhesive is capable of filling gaps between the strip and the non-metal tube.

Finally the adhesive layer should preferably have some flexibility in order to overcome the strains which may occur, due to the difference in thermal expansion of the metal fins and the tube material, when heating and cooling the tube.

Examples of adhesives are described in Adams, R. D., J. Comyn, W. C. Wake, Structural Adhesive Joints in Engineering, 2nd Edition, Chapman & Hall 1997, ISBN 0-412-70920-1, Messler, Robert W. Jr., Joining of advanced materials, Butterworth-Heinemann 1993, ISBN 0-7506-9008-9 and Habebicht, Gerd, Kleben : Grundlagen, Technologie, Anwendungen. 3. Vollig neubeart.

Und erw. Aufl. Springer 1997, ISBN 3-540-62445-7.

Examples are amide-cured epoxy, vinyl phenolic, nitrile phenolic, acrylics, aliphatic amine-cured epoxy, phenolic, aromatic amine-cured epoxy, acid anhydride-cured epoxy, bismaleimide, polyimide, silicone and polybenzimidazoles type adhesives.

Especially when a non-metal tube is used, especially a graphite tube, and optionally the metal fins are aluminium the adhesive is preferably an epoxy based adhesive. Suitable epoxy based adhesives may be one or two component epoxy adhesives like for example ARALDITE 2014 (AW 139/XB 5323), ARALDITE 2015 (AV 5308/HV 5309), ARALDITE AV 118 (ARALDITE is a trade mark of Ciba Geigy), SCOTCHWELD DP490, SCOTCHWELD 2214 (SCOTCHWELD is a trademark of 3M Corporation), A140 (H9940) of Axson and ESP108, ESP110 (ESP108 and ESP110 are trade marks of Permabond).

The dimensions of the tube and the fins may be those known for finned metal tubes which are commonly used in air coolers. Examples of typical tube outside diameters are between 15 and 50 mm. Typical fin dimensions such as thickness, height and pitch are preferably those used for the state of the art metal finned metal tubes. For

examples, typica ; fin thickness is between 0. 2 and 0. 3 mm, typical-in height is between 5 and 20 mm and typical fin pitch is between 5 and 15 fins per inch.

Typical wall thickness of non-metal tubes is between 3 and 10 mm.

The invention is also directed to a graphite tube provided with metal fins, wherein the fins are fixed to the graphite tube by means of an adhesive. Choice of graphite tube, metal fins and adhesive are as exemplified above. The tube may be obtainable by the method described above. Alternatively the fins can be assembled on the tube by fixing a plurality of individual, for example L-based, rings onto the graphite tube.

An example of the above described tube is illustrated in Figures 1-4. Figure 1 shows a cross sectional view along the axis of a tube having individual rings or a helical of fins with a continuous L-shaped base. Figure 2 is a cross sectional view AA'of Figure 1 illustrating a tube having a plurality of rings and Figure 2b is a cross sectional view of Figure 1 of a tube having a helical of fins. Figure 3 shows a cross sectional view along the axis of a tube having individual rings with a non-continuous or intermitted L-shaped base along the inner circumferential.

Figure 1 shows a cross sectional view along the axis (5) of a graphite tube (1) having individual rings (2) or a helical of fins (2) with a continuous L-shaped base (3). Between the L-shaped base (3) and the exterior of the tube (1) a layer of adhesive (4) is present, which layer (4) substantially covers the exterior of the graphite tube (1).

Figure 2a shows the cross-sectional view AA'of Figure 1 in case the fins (2) are assembled on the graphite tube (1) as individual rings. The ring (2) shown in Figure 2a has a L-shaped base along the whole of the

inner circumference of the ring (2). The reference numbers have the same meaning as in Figure 1.

Figure 2b shows the cross-sectional view AA'of Figure 1 in case the fins (2) are assembled as a helical of fins. The reference numbers have the same meaning as in Figure 1.

Figure 3 shows a cross sectional view along the axis (5) of a non-metal tube (1) having individual rings (2) with a non-continuous or intermitted L-shaped base (3) along the inner circumference of the ring (2).

As can be seen in Figure 3 and 4 not all of the L-shaped base (3) covers the exterior of the tube (1). Figure 4 illustrates that the adhesive layer (4) forms a continuous physical barrier between the non-metal tube (1) and the exterior (6) of the tube.

The invention is also directed to an air cooler consisting of tubes as described above. The air coolers according the invention may be of the same design of existing air coolers having metal tubes. Examples of such designs are mentioned in the above cited Perry's Chemical Engineers'handbook, 7th edition, 1997, pages 11-47 to 11-52. Typically the air coolers will consist of a bank of tubes and a fan, which fan will, when in use, move cooling air across the bank.

The air cooler according to the invention can be obtained by retrofitting existing air coolers having metal tubes by replacing the bank of metal tubes with the bank of finned tube as described above or as obtainable by the method described above.

The air coolers according to the invention can suitably be used to cool or condense fluids, vapours or gasses. The air coolers are advantageously used when such fluids, vapours or gasses are corrosive for the commonly used tube materials, for example stainless steel and corrosion resistant alloys (so called CRA's). Examples in

which the air cooler according to the invention can be used are units for processing organic and inorganic acids, for example sulphuric acid and nitric acid, or processes in which such acids are used. An example of a suitable application is an air cooled overhead condenser of a distillation process. More preferably the distillation process is the atmospheric distillation of a petroleum crude feed stock in a refinery operation. The overhead vapour obtained in such a distillation process is very corrosive due to the presence of hydrochloric acid and/or hydrogen sulphide. Another suitable use of the air cooler is as condenser in a process to remove hydrogen sulphide from waste water by means of steam stripping. In the condenser the corrosive vapours leaving the top of such a steam stripper column are cooled and condensed.

The invention will be illustrated by the following non-limiting examples.

Example 1 A standard finning machine was used to produce a helical of aluminium fins with fin height of 16 mm and an external tube diameter of 19 inch (31. 7 mm), respectively, thickness of 0. 2 mm and pitch of 11 fins/inch (2. 5 mm). During manufacturing the helical of fins were wrapped'around a standard steel mandrel with an external diameter of 1 inch. After manufacturing, the helical of aluminium fins were stripped from the steel mandrel and put on a carrier tube with a slightly smaller diameter than the original steel mandrel.

The helical of aluminium fins were prepared for bonding in three steps, (1) cleaning with isopropanol, (2) abrading with SCOTCHBRITE and (3) final cleaning again with iso-propanol. In order to be able to abrade and clean the internal surfaces of the fins and to keep

the spiral-wound fins into a straight position, an external mould of a soft elastomeric material was used. A 2-component epoxy, ARALDITE 2014 from Ciba Geigy, was applied to the inside of the fins, using a dosing nozzle and rubber spreader. To control both distribution and thickness of the adhesive, an orifice plate was used.

After cleaning and abrading the external surface of a graphite tube of type GRAPHILOR from Carbone Lorraine having an external diameter of 1 inch (nominal diameter, tolerance +/-3%) the tube and the helical of fins as obtained above were assembled by inserting the graphite tube into the interior of the helical of fins. Cleaning and abrading of the graphite tube was performed in the same manner as for the helical of fins.

Examples 2-4 Example 1 was repeated except that a different 2-component epoxy was used, respectively ARALDITE 2015, ESP108 and ESP110.

Example 5 To determine experimentally the thermal performance of the finned graphite tube, as obtained in Examples 1-4, temperature measurements and thermal cycling tests have been performed, according to standard testing methods ASTM D 1183 and ASTM D 1151. The results of these test give a measure of the cooling performance of the finned tube. If the results are in the same order as for the state of the art aluminium finned metal tubes, then application in commercial air coolers of such finned non-metal tubes is possible. The cycling test was repeated 50 times. The results were compared to the results obtained according to the same tests on a aluminium finned carbon steel tube having the same dimensions as the aluminium finned graphite tubes obtained in Examples 1-4. From the results, summarised in Table 1, it was concluded that thermal cycling resulted in only margina :'changes in thermal performance, comparable to the changes observed for a state of the art aluminium finned carbon steel tube.

Table 1 Tube according to adhesive Oil Temp. Fin Temp. (°C) Fin Temp. Relative Example (°C) before cycling (°C) change after cycling 1 ARALDITE 2014 150 52.1 52.6 1% 2 ARALDITE 2015 150 51.5 53.5 4% 3 Permabond 150 51.4 54.7 6% ESP108 4 Permabond 150 51.9 53.9 4% ESP110 Aluminium fins on no adhesive 150 54.0 56.1 4% Carbon steel tube

Example 6 To predict the thermal performance of the aluminium finned graphite tube as obtained in Example 1 finite element (FE) calculations have been performed. The calculations were compared to the same calculations performed for an aluminium finned carbon steel tube having the same dimensions. The results are summarised in Table 2. The outcome of the FE analysis suggests that the cooling capacity of the finned graphite tube is approximately 15% lower than for a finned carbon steel tube. However, compared to corrosion resistant alloys (CRA's) which are most often used in air cooler applications to cool or condense corrosive liquids, gasses or vapours, it is expected that the performance of the finned graphite tube will be equal or better.

Table 2 Description Finned-tube Finned-tube graphite carbon steel Liquid 130 130 temperature (°C) Air temperature 20 20 (°C) External fin 55. 2 62 temperature (°C) Outward Heat Flow 8. 8 10. 5 (Watt) Thus based on the above described results it can be concluded that an air-cooler having a bank of aluminium finned graphite tubes will have a comparable heat exchanging capacity as the state of the art air coolers having a bank of aluminium finned metal tubes. Because a graphite tube material is used the expected life time of the air cooler will be significantly extended when used to cool or condense corrosive liquids, gasses or vapours.