COATING METHOD FOR METAL TANKS WITH COIL-WOUND HEAT EXCHANGER

The object of the present invention is an electrostatic powder coating method suitable for metal tanks intended to contain water or similar fluids and internally provided with one, or more, coil-wound heat exchangers also metallic and constrained by welding to the walls of the same tank.

In particular, the method refers to the electrostatic deposition of enamel powder of the vitreous type, said coating consisting more correctly in enamelling.

The adoption of the electrostatic powder enamelling in place of the previous wet enamelling, applied by spray or by immersion, has been one of the keys to obtain an ecological process while maintaining the competitiveness of enamelling.

Essential features of such well known technology are:

• imparting an electrostatic charge to the dry powder enamel;
• dispensing such powder on the piece to be enamelled through a suitable dispensing gun (also called "lance");
• drawing and total recycling of the powder that does not adhere to the piece.

The composition of the powder is a very important factor in order to ensure both its adherence with the metal substrate and the duration of the coating over time.

The presence of too many fine particles, for example, does not allow a correct charge and leads to excessive accumulation on the corners of the piece to be enamelled. On the other hand, the presence of too many particles of large granulometry leads to a disproportionate charge attraction with relative nonuniformity of thicknesses.

As known to the men skilled in the art, therefore, the enamels, for a correct application, must have preferred distributions of various granulometries and a suitable electrostatic charge otherwise known undesirable situations are produced such as the "Faraday cage" effect or the phenomenon of "back ionization"; phenomena that negatively affects the result of the enamelling. In fact, this requires a distribution of enamel free of gaps or local accumulations of powder, with thickness comprised between a lower limit, below which the layer applied cannot be considered as protective, and an upper limit, above which enamel gaps are very likely.

By way of an example, for pressurized liquid storage tanks a thickness of enamel comprised between 180 and 500 µm is considered as adequate for the purposes.

The understanding of the phenomena related to the application of this technology has made significant advancements since its beginnings in the mid-seventies of the last century, however, it failed to resolve difficulties related to the application of enamel on complicated surfaces and provided with many shaded zones compared to the dispensing lance.

The case to which the present invention aims to provide a solution or at least clear improvements, that is the enamelling of steel tanks provided with one or more coil-wound exchangers is particularly difficult due to the need of coating with enamel the walls supporting the coil, which the latter keeps for a good part shaded with respect to the lance and due to the fact that the turns of the coil, side by side, make a uniform application difficult, presumably due to undesired "Faraday cage" effects.

The known method of application of the electrostatic powder enamelling for the case in question, which will be later illustrated, is therefore very painstaking and time consuming and however does not produce very satisfactory and constant results from piece to piece, often leaving dangerously incomplete zones in the end turns of the coil, such that it is substantially unused yet preferring to wet enamel such tank, by immersion in enamel muds, giving up all the advantages of dry enamelling; not least the complete recovery of the powder that has not adhered to the piece.

An object of the present invention is to provide a method for the application of electrostatic powder enamelling in steel tanks provided with an inner heating coil also of steel that ensures a coat of enamel of a thickness comprised between the lower and upper limits required on the entire inner surface.

A further object of the present invention is that of providing a method that reaches the previous object through steps of a reduced number and simpler than the method currently known.

Further features and advantages of the present invention shall be better highlighted by the following description of a basic version of the method according to the invention in accordance with the main claims and to some preferred variants in accordance with the dependent claims, the whole illustrated, by way of a non-limiting example, in the annexed drawing tables, wherein:

• Figure 1 shows a cutaway view of a storage tank capable of being enamelled according to the method of the invention;
• Figures 2 (a) to 2 (f) show, for the same storage tank of Figure 1, the subsequent steps of application of the powder enamel according to the prior art;
• Figures 3 (a) to 3 (c) show, for the same storage tank of Figure 1, the subsequent steps of application of the powder enamel according to the invention.

Figure 1 shows, of tank 1, the covers 2 joined by means of welding seams 3 to the cylindrical wall 4 (called in jargon "shell"); a single coil 5, welded to said shell 4; the water inlet and outlet tubes 6 and 7 respectively, the opening 8 intended for maintenance inspections, the optional insertion of temperature control and heating means and, as regards the present invention, the insertion of the lance for dispensing enamel powders to be applied electrostatically (not shown).

The drawing of tank 1 is not necessarily to scale; in particular the turns of coil 5 may actually be thicker than as shown.

The tank 1 is shown arranged with vertical axis, with the opening 8 below, which is the position that it is made to take during the enamel application process; such a position is not necessarily that taken in operating conditions.

Spatial orientation terms optionally used below such as "lower"/"upper", "above"/"below", refer to the position as illustrated, that is, the set-up required for the application of the electrostatic powders.

Both the known methods and the method according to the invention, which shall be described later, provide multiple steps each of which consisting of one or more upwards and downwards passes of the powder dispensing lance in predetermined stretches, the number of passes depending on the quantity of powder that can be deposited in each of them and, ultimately, on the thickness of enamel that is desired to be achieved.

Of tank 1 there are indicated some zones identified by corresponding vertical stretches and horizontal planes of separation.

The total height Hscb consists of, from top to bottom, a first stretch Hc that goes from the top of the upper cover 2 to the welding seam 3, lying on an upper welding plane α; an intermediate stretch D follows that goes from said welding plane α to an upper plane β where coil 5 begins; the central stretch Hsp follows that goes from said upper plane β to a subsequent lower plane P where coil 5 ends; then there is another intermediate stretch D from said lower plane β to a lower plane α where the welding seam 3 of the shell 4 to the lower cover 2 lies.

That is, there have been identified, vertically, end stretches Hc corresponding to the vertical development of the covers 2; adjacent innermost stretches D relative to the two stretches of shell 4 not occupied by the coil 5 and finally the central stretch Hsp occupied by the same coil 5.

It is known from the evidence of the facts that the zones on which it is more difficult to correctly apply the enamel (hereinafter "critical zones") are the zones of the welding seams 3 (zones from just above to just below the planes α), due to the rather rough surface thereof, and the end turns of the coil 5 (the higher and lower turns).

On the other hand, there are no particular difficulties to make the enamel dust reach also the stretch Hsp of the shell 4 comprised between said coil 5 start and end planes β: although this stretch Hsp of the shell 4 is partially hidden to the lance, the powders reach it easily guided by the lines of force of the electric fields due to the electrostatic charges.

Accordingly, the known methods of application of electrostatic powders concern, in particular, spreading the same powders over such critical zones.

The method substantially used up to now is shown in Figs. 2 (a) to 2 (f) that illustrate each one of the consecutive four to six steps provided.

With reference to Fig. 2 (a), the first step consists in applying powder exclusively in the stretch Hsp occupied by the coil 5 (that is, in the space comprised between the two coil 5 start/end planes β).

With reference to Fig. 2 (b), the second step consists in applying powder in a stretch H.sup that goes from a centre plane µ to about half the height of the coil 5 up to an end plane κ inside the upper cover 2 and sufficiently close to the top of the same cover 2 to allow powders to cover it entirely. Therefore, this step has the purpose of depositing additional powder on the upper turns and covering entirely the interior of the cover 2.

With reference to Fig. 2 (c), the third step, geometrically symmetrical to the previous one, consists in applying powder in a stretch H.inf that goes from the same centre plane µ up to an end plane κ sufficiently inside the lower cover 2 to allow the entire coating with the powders.

With reference to Figs. 2 (d) and (e), there may be provided a fourth and fifth step consisting, respectively, in the further application of powder in the stretches D1 and D2 corresponding to the zones of the shell 4 free from the coil 5 (zones comprised, at the top and bottom, between the planes α and β). These are intended to thicken the enamel in the critical zones (welding seams 3 and end turns of the coil 5).

With reference to Fig. 2 (f), the last step consists in passing again the entire tank 1 for a stretch that goes from the interior of the upper cover 2 to the interior of the lower cover 2 (that is, from the one to the other end plane κ).

Despite this care in the application of the powder, by repeating the operation in the most critical zones, this does not deposit easily on the end turns, especially the upper ones, or, deposited during a step, it falls during the subsequent ones.

The powder application method according to the invention has solved such problems by providing the following simple one or more steps.

With reference to Figs 3 (a), (b) and (c), there are identified two planes ϕ at the top and the bottom in the zones of the shell 4 free from the coil 5 (that is, planes comprised, at the top and bottom, between the corresponding coil 5 start/end welding planes α and β).

The method according to the invention provides, in the preferred variant, the following three steps and preferably in the order in which they will now be listed.

With reference to Fig. 3 (a), a main step consists in applying powder in the stretch H1 comprised between two limit planes ϕ which are planes spaced from coil 5 start/end planes β only by the amount necessary to ensure that the lance is able to apply powder on the entire surface of the coil 5, including the upper and lower surfaces, respectively, of the outermost turns.

For instance, according to a preferred embodiment, said limit planes ϕ are spaced from coil 5 start/end planes β by a distance substantially equal to around 30 mm; said measure allows an uniform and complete deposition of the powder on the surface of the coil.

With reference to Fig. 3 (b), a complementary step consists in applying powder in the stretch H2 comprised between the upper limit plane ϕ and the interior of the upper cover 2 up to the upper end plane κ as defined above.

For instance, according to a preferred embodiment, said upper end plane κ is spaced from the top of the upper cover 2 by around 20 mm.

With reference to Fig. 3 (c), a further complementary step consists in applying powder in the stretch H3 comprised between the lower limit plane ϕ and the interior of the lower cover 2 up to the lower end plane κ as defined above.

For instance, according to a preferred embodiment, said lower limit plane ϕ is spaced from the bottom of the lower cover 2 by around 40 mm. In this case, the lower limit plane ϕ is more spaced from the lower cup 2 in respect of the similar distance between the upper end plane κ and the top of the upper cover 2, since the powder falls by gravity and is deposited uniformly on the surface of said lower cup 2.

As it can be noted, the method according to the invention provides a main step (that illustrated in Fig. 3 (a) ) in which the powder dispensing lance runs entirely through the stretch H1 concerning the coil 5, with a margin above and below the same coil 5 just sufficient to ensure the coating with powder of the entire coil 5.

As stated above, according to a preferred embodiment by way of a non-limiting example, said margin is substantially equal to around 30 mm above and below the coil 5.

In addition to this main step, there may be other possible complementary steps in which it is avoided depositing powder on the coil 5; that is, steps, in which the lance does not dispense powder on that stretch H1 in which it has already previously operated.

It is noted that the complementary steps in which the lance does not dispense powder in the stretch H1 are only optional and not mandatory because the main step, although only the stretch H1 has been processed, might also be sufficient to coat satisfactorily with powder also the entire inner surface of one or both covers 2, if these are sufficiently compressed or the coil 5 comes sufficiently close to them.

From what described it is deduced that a feature of the method according to the invention is that no step substantially provides, except unavoidable slight overlaps to the ends of each stretch H1, H2 and H3 of application, to deposit powder where it has already been deposited in an earlier step.

The invention, which has been illustrated for simplicity with reference to a tank 1 provided with a single coil 5, can now be clearly generalized to a tank 1 provided with one or more coils, specifying that the method according to the invention provides

• as many main steps as said coils 5 and in each of which the powder is dispensed exclusively for a vertical stretch that concerns only a corresponding coil 5, with a margin above and below the same coil 5 just sufficient to ensure the entire coating thereof with powder,
• and as many optional complementary steps each aimed at applying powder in corresponding stretches not sufficiently covered in the main steps. Preferably, each of the stretches not reached by sufficient powder in two main steps that concern two consecutive coils 5 is entirely coated in a single complementary step.

Not necessarily, according to the invention, the one or more main steps and any complementary steps follow one another in a particular chronological order, the second ones being able, if necessary, to be put before the first ones.

It has been found that, by carrying out steps of application of the enamel powder exclusively according to the invention, a thickness of enamel within the desired limits is obtained and there are not powder drops in any zone during steps subsequent to those of deposit.

It can be noted how the steps provided by the method according to the invention are of a reduced number compared to the known technology.

This translates into a considerable reduction in the treatment times and in an easier tuning of each step of the process as regards as, for example, programming the movements of the dispensing lance (which is obviously robotised), amount of powder to be dispensed at each pass, electric charge of the powders, electric fields applied, etc.

The invention, in addition to cylindrical tanks 1 as illustrated, is also suitable for tanks of spherical or ellipsoidal shape provided with coil 5 with axis substantially parallel to the axis according to which it can move, inside, the dispensing lance penetrating through the opening 8. In fact, even tanks of such geometrical shapes, make it possible steps in which the powder dispensing lance runs entirely through a stretch H1 as defined above and any further steps in which such stretch H1 is ignored by the lance.

It is clear that the method according to the invention is particularly useful for the application of enamels of vitreous type, where thickness control is a fundamental requirement or there would be enamel gaps in operation.

The method easily allows a thickness of vitreous enamel comprised between 180 and 500 µm as generally desired.

However, the method is perfectly usable for the electrostatic application of any powder, also of a polymeric nature where an excessive thickness cannot affect the reliability of the corrosion protection but nevertheless constitute a waste of material.