GB2124659A

GB2124659A - Hot dip galvanising bath

Info

Publication number: GB2124659A
Application number: GB08316818A
Authority: GB
Inventors: Frederick Carr
Original assignee: GKN Birwelco Ltd
Current assignee: GKN Birwelco Ltd
Priority date: 1982-06-26
Filing date: 1983-06-21
Publication date: 1984-02-22
Also published as: GB8316818D0

Abstract

Apparatus for galvanising elongate articles such as tinned tube for heat exchanges, comprising a bath through which the articles pass in a straight line beneath the level of zinc established therein by an overflow. Zinc is continuously pumped from a store to the bath and the overflow zinc is collected and returned to the store so that coating of the articles is assisted by the flow of zinc in the bath. The store and bath are laterally adjacent one another at substantially the same level, to minimise splashing of the zinc.

Description

SPECIFICATION Galvanising elongate articles This invention relates to galvanising elongate articles.

The invention has been developed in relation to the galvanising of finned tube, in which as described hereafter, particular problems arise, but is applicable generally to elongate articles requiring to have a coating of zinc applied thereto.

Tube for use, for example, in heat exchangers, having fins on its exterior surface to increase the surface area available for heat transfer, typically requires to be galvanised, if made of a ferrous metal, to prevent corrosion. The galvanising may also assist in securing the fins to the tube.

Hitherto, galvanising of such finned tube has generally been carried out by immersing discrete lengths of the finned tube in a bath of molten zinc.

The ends of the tube must be plugged to prevent the zinc reaching the interior thereof. This method is not convenient for a tube which is produced continuously, since it must be cut into suitable lengths for the gavanising process, and in addition requires a bath containing a large quantity of zinc.

When we refer to continuous tube it will be appreciated that the length of tube will have ends but we mean that its length is much greater, e.g. a number of times greater, than can be accommodated in the galvanising bath.

Further problems which are encountered in galvanising finned tube are the formation of dross crystals, and the satisfactory drainage of molten zinc from the finned tube after it has left the bath.

These effects can partially block the spaces between the fins and impair flow therethrough, as well as detracting from the appearance of the tube. The dross which forms during galvanising is an alloy of zinc and iron, and has a higher melting point than zinc alone.

It is the object of the present invention to overcome or reduce one or more of these difficulties.

According to the invention, we provide an apparatus for galvanising elongate articles, comprising: a bath containing molten zinc; a store for molten zinc disposed adjacent said bath at substantially the same level as said bath; means for feeding molten zinc to said bath from said store; an overflow for excess zinc from said bath, to establish an operating level of zinc therein; means for returning said excess zinc to said store; and an entrance and an exit for an article to enter and leave said bath, beneath said operating level of zinc therein.

By the use of a bath through which molten zinc is flowing rather than remaining static therein, the time for which the article is required to be in the bath to ensure satisfactory coating is reduced.

This is important in the case of finned tubes, where penetration of the zinc to the roots of the fins is necessary. A bath of great length is not required in apparatus according to the invention, giving economy in the use of zinc and requiring minimum heat input to keep the zinc in the molten state.

Preferably the zinc is fed into the bath adjacent said exit therefrom, and said overflow is adjacent said entrance to the bath. Preferably said zinc is fed into the base of said bath in a generally upward direction, which further assists coating of finned tubes.

The apparatus may further comprise a settling chamber for impurities in the molten zinc, receiving zinc before return thereof to said store.

This ensures that the zinc flowing to said bath is as far as possible free of dross crystals.

Inevitably molten zinc will escape through the entrance and exit of the zinc bath, and the apparatus preferably comprises a channel arranged to collect such escaping zinc and to return same to said store with said overflow zinc from the bath.

The manner in which molten zinc flows in the apparatus, particularly in terms of the store for molten zinc being disposed at substantially the same level as the bath thereof, reduces to a minimum any tendency for splashing of the zinc and hence formation of oxides (ash) therein.

Nevertheless, the apparatus may further comprise a cover by which an inert atmosphere is maintained in contact with the molten zinc.

It has been found that the provision of galvanized finned tubes of a good surface finish and free from harmful accumulations of dross crystals requires operating conditions in and associated with the molten zinc bath as follows: 1. Zinc temperature must be accurately maintained constant 2. Provision must be made for dross crystals to be continuously separated from the flowing molten zinc 3. The period of total immersion of the tube in molten zinc must preferably be in a section of the bath immediately adjacent to the exit aperture 4. The molten zinc flow entering the above total immersion section should be as nearly as possible free of dross crystals.

5. The molten zinc flow in the above total immersion section should have in addition to the horizontal velocity component directing it along the tube, an upward velocity component tending to prevent dross crystals from settling on the uppermost edges of the fins 6. On passing through the exit aperture, the finned tube should enter a clearing section in which the excess molten zinc is drained or cleared from the fins 7. The above clearing section should be maintained at the same temperature as the molten zinc 8. There should be an efflux of molten zinc into the clearing section from the exit aperture. The clearing section should be so arranged that the lowermost edges of the fins are clear of the molten zinc surface of the reflux stream 9.Provision may be made for shaking, vibrating, rotating and brushing of the fins in the clearing section, to assist in clearing the surplus zinc.

The apparatus according to the present invention conforms as far as possible to these requirements, and is thus capable of achieving satisfactory galvanising of finned tube as well as other elongate articles.

The invention will now be described by way of example with reference to the accompanying drawing, which is a diagrammatic representation of apparatus according to the invention.

The illustrated apparatus comprises an assembly of refractory or ceramic materials enclosed in a steel casing. The following elements are defined within the refractory or ceramic material A galvanising bath 1 0.

An entrance passageway 11 for an elongate article to enter the galvanising bath 10.

An exit passageway 12 for the article from the galvanising bath. Both this and the entrance passageway 11 have a cross-sectional shape through which the article is a close fit.

Zinc collecting troughs 13, 14 adjacent the entrance and exit passages 11, 1 2 respectively.

A zinc collecting gulley 1 5 extending the length of the apparatus, and connecting troughs 1 3 and 14.

An over-flow weir 1 6 leading from galvanising bath 10 into gulley 15.

A settling sump 1 7 which zinc from gulley 1 5 can enter via a weir 18.

A pump sump 1 9. This communicates with the settling sump 17 by a weir 20, and accommodates a submersible pump for pumping molten zinc.

A tundish 21. This receives molten zinc delivered by the pump in sump 19, and communicates with the galvanising bath 10 adjacent the exit passage 1 2 therefrom, by an inverted syphon passage, not shown.

The illustrated apparatus could be enclosed within a chamber in which is maintained an inert atmosphere to prevent oxidation of the zinc.

Means would be provided for maintaining a suitable temperature within the chamber, so that the zinc remains molten. The construction from refractory material enables the apparatus to be used with zinc at a relatively high temperature, so that relatively thick coatings of zinc can be built up.

The means for maintaining the temperature of molten zinc may, for example, comprise an immersion heater or heaters which may conveniently be disposed in sump 19 which provides the main storage of molten zinc in the apparatus. Alternatively, inductive heating of the molten zinc in the sump 19 could be utilised. As yet a further possibility, radiant heaters in the upper part of the chamber containing the apparatus may be employed.

In use of the apparatus, the article, e.g. finned tube, which will have been subjected to conventional pickling or other pre-treatment processes, enters the apparatus at 22 and leaves at 23. By means of the pump, not shown, a flow of molten zinc is established from sump 17, to sump 19 and tundish 21 to enter the galvanising bath 10 adjacent the exit 1 2 thereof. A certain amount of molten zinc escapes via the entrance passage 11 to the collecting trough 13 and gulley 15, and rather more molten zinc is carried out of exit passageway 12 by the finned tube to drain from the tube into collecting trough 14.The passage of finned tube through the entrance and exit passageways acts somewhat in the manner of a labyrinth seal, to restrict escape of the zinc at these points.The excess of zinc delivered to the bath 10 over that which leaves via passageways 11 and 12 reaches gulley 1 5 by way of weir 1 6. From gulley 1 5, the zinc returns to sump 17 by way of weir 18, and dross tends to settle in sump 1 7. The levels of weirs 16, 18, 20 are such as to provide for progressively lower levels of zinc from the bath 10 to sumps 17, 1 9.

The sump 17 has a sloping floor 24 to enable settled dross to be mechanically removed by a raking process.

After emerging from the exit passageway 12, the finned tube is subjected to a shaking or vibration to assist excess molten zinc to drain from it. Alternatively or in addition, a mechanical wiping process may be employed to assist such removal of excess zinc. This may be done by, for example, one or more brushes which engage the finned tube. As a further possibility, a jet of gas or vapour may impinge upon the finned tube to blow excess zinc therefrom, such jet being at an elevated temperature to prevent the tube from being chilled and freezing the excess zinc before it can be removed.

It will be noted that splashing of molten zinc does not occur in the bath 10, or in the flow of zinc to sump 1 9. This is achieved by virtue of the side-by-side disposition of these elements.

Provided that a splash-free transfer of zinc from sump 19 to tundish 21 is achieved, the apparatus enables the molten zinc to be maintained in optimum condition for satisfactory coating of articles.

The tube may be pushed through the apparatus by suitable means which grips the tube to advance it before reaching the apparatus.

If the fins on the tube are circular, the tube may be rotated while it is passing through the apparatus. The apparatus is particularly suited to the galvanising of continuously produced tube, but if a short length of tube is passed through the apparatus the ends of such length may be plugged to prevent zinc reaching the inside of the tube. If the tube is manufactured in individual lengths and required to be galvanised as a continuous tube, individual lengths may be welded together before they pass through the apparatus.

Claims

1. Apparatus for galvanising elongate articles, comprising: a bath containing molten zinc; a store for molten zinc disposed adjacent said bath at substantially the same level as said bath; means for feeding molten zinc to said bath from said store; an overflow for excess zinc from said bath, to establish an operating level of zinc therein; means for returning said excess zinc to said store; and an entrance and an exit for an article to enter and leave said bath, beneath said operating level of zinc therein.

2. Apparatus according to Claim 1 wherein said zinc is fed into said bath adjacent said exit therefrom, and said overflow is adjacent said entrance thereto.

3. Apparatus according to Claim 2 wherein said zinc is fed into the base of said bath, in a generally upward direction.

4. Apparatus according to any one of the preceding Claims further comprising a channel arranged to collect zinc escaping from said entrance and exit and to return such zinc to said store thereof with said zinc from said overflow.

5. Apparatus according to any one of the preceding Claims further comprising a settling chamber for impurities in said zinc, receiving zinc from said overflow before return thereof to said store.

6. Apparatus according to any one of the preceding Claims further comprising pump means for raising molten zinc from said store thereof to a tundish from which it can flow by gravity through a passage to said bath.

7. Apparatus according to any one of the preceding Claims wherein said bath, overflow, channel, tundish, store and settling chamber are defined by a structure of a refractory material.

8. Apparatus according to any one of the preceding Claims further comprising a cover adapted to maintain an inert atmosphere in contact with molten zinc in the apparatus.

9. Apparatus according to Claim 8 further comprising radiant heating means in said cover, to maintain said zinc in a molten condition.

10. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.