GB2069674A - Process and apparatus for drying bulk items within a perforate treatment barrel - Google Patents

Abstract

In-barrel drying of a load of bulk items which have undergone electrolytic and/or chemical surface treatments within a perforate barrel is achieved by a process in which drying air is drawn downwards through the load and out through the perforate barrel casing underlying the load by suction applied externally of that section of the casing. Drying station apparatus for carrying out this drying process comprises support means 26 for supporting a perforate treatment barrel 23 at a drying station 1, and a suction box 11, sealing flaps 12, air-extractor fan 13, droplet-separator 14 and air-reheating heating elements 15. <IMAGE>

Description

SPECIFICATION Process and apparatus for drying bulk items within a perforate treatment barrel The present invention concerns a process and apparatus for drying bulk items, for instance small articles or components formed of metal or metallized-plastic materials, within a perforate barrel, such as for instance the barrels used in barrel-electroplating or some kinds of chemical surface-treatment.

Above all the invention relates to the drying of bulk items within rotatable barrels of the kind currently employed in the electroplating and associated chemical surface pretreatment of small metal items such as metal screws, pins and so on.

The electroplating barrels currently used for electroplating small metal and metallized-plastic items in bulk are of various kinds; but broadly-speaking they all tend to consist of a perforate casing including an opening, closable by means of a removable lid, through which the barrel can be loaded and unloaded with the bulk items. Indeed, nearly all such barrels consist essentially of two end sections, arranged perpendicular to the rotational axis of the barrel, and a perforate casing of polygonal cross-section (including a removable lid in one or more of its faces) extending parallel to that rotational axis between the two endsections.

Typically such a barrel might be loaded with metal screws, which are say to be electro-galvanized, and then made to pass through the appropriate series of treatment stages-which in a typical case might consist of a series of treatment solutions designed to achieve successively the chemical degreasing, the pickling, the electro-galvanization and, if appropriate, the passivation of the screws.

Such a treatment cycle is moreover also likely to include intervening water-rinsing stages, between the individual chemical treatment solutions just mentioned, and a final waterrinsing stage at the end of the cycle. In such a treatment cycle the individual chemical solutions and probably the water-rinses will be contained in tanks in which the barrel will be successively immersed; and each such tank represents a treatment-station in the treatment cycle. Although the barrel and its load can be moved from one treatment-station to another while still wet, when at last they emerge from the last treatment-station the load is still wet with water, and must be dried.For that purpose, despite the obvious convenience of drying the load within the electroplating barrel, and indeed despite the proposals whiCh have been put forward for achieving such iri-barrel drying and which are discussed below, it is so far as I know still today the universallyadopted practice in industry to transfer the load of wet items from the electroplating barrel into a centrifuge, and there to spin off the water from the bulk load and thus dry it.

The fact that such a transfer and separate drying operation has substantial disadvantages has till now been outweighed by the practical inefficiency of all previously-suggested in-barrel drying techniques and/or their other concommitant disadvantages.

One known proposal for drying the bulk load of items within an electroplating barrel requires a pipe, provided with nozzles, to be disposed along the axis of the barrel, through which both liquids (predominantly for the purpose of rinsing) and also air can be supplied to the interior of the barrel. Good barrelplating practice however requires that the bulk load should occupy only about one-third of the volume of the barrel, and thus the bulk load positions itself by gravity below this pipe.

The airflow from the pipe nozzles thus impinges on the upper surface of the bulk load, and is only forced through the bulk load to a very small extent. Anyway the amount of air which can readily be blown via the pipe into the barrel is quite restricted, and the spatial distribution of the air-flow within the barrel is not favourable. For all these reasons this arrangement failed to achieve the intended degree of drying within a reasonable time. It is moreover quite costly to provide the barrel with an additional central pipe, whose presence along the axis anyway interferes greatly with the introduction by means of a cable and/or other structural elements of the electrolytic current which must be carried within the barrel.This defect is not one to be underrated, because of course the conduction of the electrolytic current to within the barrel is of absolutely fundamental importance to the successful performance of any barrel-electroplating process.

Another known proposal for drying the bulk load of items therewithin requires a barrel having a perforate casing in which the crosssection of the perforations is conical, with the basal area of the "cone" opening towards the outside of the casing. It was envisaged that this arrangement would present the least possible resistance to the passage through the perforations of an air blast directed against the barrel from outside and hence impinging upon its casing, and thus promote airflow through the perforations and into the load.

Alas this arrangement however achieves little or nothing; because, as is known from fluiddynamics, the circulation of gaseous fluids in passages of narrow cross-sections leads to local turbulences, which restrict the area through which gas can flow freely to something considerably less than might have been supposed from the geometrical shape and area of the cross-section. In fact the suggested conical shape of perforation tends to intensify turbulent flow patterns, superimposing localized subordinate eddy currents upon the main flow, thus resulting in a substantial increase in flow-resistance, and therefore a corresponding reduction in the flow-volume. Again therefore this proposal turns out to be far less effective than might have been hoped.

Nor is this really surprising. The load within the barrel is a conglomerate mass of small items in bulk; and the problem to be solved resides in the fact that what must be dried is the entire surface area of all the items making up the load-most of which are not, at any given instant, exposed upon the outside of the load. So unless the airflow can penetrate and flow through the load, it cannot exert its drying effect except upon those relatively few items which at any given instant happen to be exposed. The exposed items represent only an extremely small fraction of the total surface area of the items which are to be dried.Since neither of the above-discussed prior proposals can, at best, achieve more than merely the drying of the exposed surfaces in the load, it is hardly surprising that both of them turn out to have only a very small drying efficiency, quite inadequate for drying a load of items in bulk within the barrel itself for industrial purposes.

Yet another known proposal attempts to find a way round these problems, by forcing air through the perforations in the casing of the plating barrel when mounted in a prismatic chamber, denoted as "a drying station". A so-called air-input box. whose shape is adapted to the polygonal barrel casing, and through which air can be forced into the barrel via the perforations in its casing, is fixedly connected to the chamber at one of its lower, side corners. When the barrel arrives at this so-called drying station it is positioned with only about one quarter of its entire external surface area contacting the air-input box. Two airflow restrictors, formed bf flexible material, are mounted upon respectively the upper and the lower edge of the air-input box, and these make sliding engagement with the barrel-casing as it is rotated at the drying station.Accordingly air blown into the airinput box at the drying station is prevented by the air-flow restrictors from escaping, and is thus forced into the inside of the barrel through the perforations in the quarter-section of the barrel casing which contacts the airinput box.

One of the consequences of this arrangement is that the airflow through the perforations undergoes some flow-mechanically conditioned bafflings, which cause a corresponding reduction in the volume of air entering the barrel, and thus inevitably a reduction in the drying effect.

Nor is that the only consequence. In accordance with conventional good barrel-plating practice the load will occupy about one third of the internal capacity of the barrel, which can be rotated in either of two senses.

When the barrel is rotated in such a sense that.the tumbling load locates itself away from the air-input box, then the air which is forced into the barrel will only graze along over the top surface of the load, that is to say the surface down which the constantly-mixing bulk items will tumble as the barrel rotates. In that event, as already discussed above in connection with other previous proposals, the drying effect will be applied to only an extremely small part of the total surface area of the load to be dried, and thus the drying efficiency of the process is inadequate.

When alternatively the barrel is rotated in the opposite sense, then the tumbling load will locate itself in that quarter-area of the barrel casing which directly faces the air-input box; and in that event the air forced into the barrel is made to flow not only through the barrel casing but also through the load-and one would therefore expect that the drying efficiency should be improved. However, the air enters the barrel through the perforations so forcibly that it can sometimes cause mechanical damage to small and fragile items which happen to be situated in the region of the barrel casing wall; and anyway the improvement in drying efficiency is much less than might have been expected.

At the time when the barrel and the load within it reach the drying station both are still very wet. Experience shows that, as a generalization, when using a medium-sized barrel loaded in the conventional manner to onethird of its capacity the so-called "drag-out" loses (i.e. the amount of water or other liquid still adhering to the barrel and to the load after their removal from the previous tank) will usually be of the order of from 1.5 to 2.5 litres. All this liquid has somehow to be removed; and some of it is still dripping out of the barrel at the time when the barrel reaches the drying station.If however upon rotation of the barrel at the drying station the tumbling load locates itself immediately facing the airinput box, then any water droplets which are, trying under gravity to drip out of the load will tend to be thrown back up into the load by the air flow through the peforations, an effect which is diametrically the opposite of what is desired, and which largely negates the drying effect of the air flow. Thus even this lastdescribed proposal has failed to achieve the drying efficiency needed for industrial purposes.

It has however now been found that one can successfully dry bulk items within an electroplating or similar barrel by means of air which is drawn through the load, while avoiding the disadvantages attendant upon the previously-proposed techniques which have been discussed above.

According to one aspect of the invention there is therefore provided a process for drying a bulk load of items within a treatment barrel having a perforate outer casing, in which drying air within the barrel is drawn in a generally-downwards direction through the bulk load and the resultant moisture-laden air is then withdrawn from the barrel through the perforations in its casing underlying the bulk load by suction applied externally of the perforate barrel casing.

The barrel, in accordance with conventional practice, should be filled to about one-third of its internal volume with the conglomerate mass of items making up the bulk load.

Desirably the barrel will be rotated during at least part of the drying process, and air is drawn through the bulk load and withdrawn from the barrel in a generally-downwards but non-vertical direction substantially normal to the tumbling plane of the surface of the bulk load when in the position which it adopts during its rotational movement at the drying station.

To achieve this, a sub-atmospheric pressure is developed in a vacuum chamber or suction box mounted adjacent and effectively in sealing engagement with the portion of the barrel casing at which the tumbling load positions itself. In this way air is drawn from within the barrel (and thus from the space therein which is not filled with the load) downwardly through the load and out through the underlying perforated barrel casing, so that a stream of drying air is drawn into and through the conglomerate mass of items and thereafter drawn out through the perforate barrel casing.

Since the drying air flows completely through the load it thus contacts virtually the entire surface area of all the items to be dried, the air-stream thus branching and meandering around virtually each such item, and hence the maximum drying effect is thereby achieved. Moreover, because the air-stream is diffused throughout the load it arrives at the perforations in the barrel casing from many different directions and can be drawn off therethrough in something more nearly akin to laminar flow, thus with a minimum of flow resistance and hence with the maximum possible flow volume.

There may of course be some local turbulences within the load itself, but if so that is not detrimental, since it will assist the drying air to distribute itself throughout the load and to become laden with the moisture from the wet items making up the bulk load. Any local turbulences, moreover, will not persist for any length of time, since rotation of the barrel will grind and mix the load within that barrel as it rotates, and hence the position of the items relative to each other within the load will constantly change.

Because the load will of course rest under gravity upon the barrel casing, it therefore necessarily follows that the air-stream drawn through the load will be directed generally downwards, i.e. in the same direction as that in which the liquid naturally tends to drip away. Thus the effect of the air-stream upon liquid droplets and the effect of gravity upon the dripping of the liquid which wets the load will both tend to work largely in the same direction, and thus act in conjunction rather than against each other. The indrawn airstream consequently helps to accelerate the removal of liquid droplets from the load; and for this further reason brings about a substantial increase in the drying efficiency.

The fact that the air is drawn downwardly through the load has the further advantage that, because the pressure differential tends to press the items in the same direction as gravity onto the barrel casing, there is no longer any significant danger of the items in the load being thrown away from the perforated casing and thus suffering damage (as is the case with prior art barrels) and there is thus no longer any constraint of this kind upon the flow volume which can be used-which therefore can be greatly increased.

The passage of very high volumes of air leads of course to a correspondingly very high capacity for moisture-uptake and thereby to a very rapid removal of moisture from the surface area of the wet load of items-or in other words, leads to intensive and very rapid drying.

It should be self-evident that the process of this invention may be practised quite successfully no matter whether the barrel and load are rotating continuously, rotating only intermittently, perhaps (in some circumstances) merely rocking back and forth, or even not rotating at all but simply remaining stationary.

It also deserves mention that insofar as the barrel is stationary or only rocking back and forth, the process can still be operated even if the barrel lid has been removed, and the drying air then flows in freely through the inlet opening into the barrel.

It is in fact a preferred feature of the process of this invention that the barrel should be rotated during only part of the drying process; and specifically that the barrel should remain stationary during the initial period of drying and only subsequently be set in rotational motion during the final period of drying.

Thus, after the barrel has reached the drying station one passes drying air through it without simultaneous rotation during a first timephase, and commences rotation only during a second time-phase of the overall drying operation. There is of course nothing to prevent one from using several alternating stationary and rotational drying periods-but the most crucial consideration is that the initial drying period shall preferably be a stationary one. It can be shown that the off-flow of liquid from the surfaces of wet solid items is promoted when their spatial positions remain unchanged.Of course, during this initial station ary drying period the indrawn drying air should flow through the load of items in bulk with its full intensity, since the natural flow direction of the droplets under gravity and the directional effect of the indrawn air-stream will then combine to accelerate the outflow of the liquid to be eliminated. When the rate of liquid outflow significantly diminishes, then is the moment for this initial stationary drying period to be brought to an end. and the barrel to be set once more in rotation so as to continue the drying process, and usually to complete it, during this second, rotational drying period.Although theoretical considerations can be invoked to explain this observation. it is nevertheless still surprising that the combination of such successive stationary and rotation drying periods should contribute so substantially to the reduction of the overall times required for the drying process.

According to another aspect of the invention there is also provided drying station apparatus for drying a bulk load of items within a perforate treatment barrel by means of the drying process herein described, which apparatus comprises support means capable of supporting the perforate. load-containing barrel at the drying station, and suction means so arranged that upon operation they apply suction to the exterior of part only of the perforate casing of the load-containing barrel when thus supported, said casing part to which suction may be applied being located at least mainly below the casing where it can be overlaid at least partially by the bulk load, in order that upon operation of the suction means drying air within the barrel will be drawn by suction through the bulk load in a generally-downwards direction and the resultant moisture-laden air will then be withdrawn through the perforations in that casing part where it is overlaid by the bulk load.

The support means may be associated uniquely with the drying station apparatus; but probably will most advantageously form part of a programmed transportation system.

The most up-to-date installations for the "inbarrel" chemical and/or electrolytic surface treatment of items in bulk generally are provided with a programmed transportation system, which automatically moves the individual barrels through the entire installation in accordance with predetermined time-path sequences. The still-wet items in bulk emerging from the last treatment stage must however currently be transferred manually from the barrel into a drying centrifuge. because neither the transfer nor even the centrifuging are operations which in practice are suitable for mechanisation.By making possible the adequate drying of the load of items in bulk within the barrel itself, within a time-span which meets with the time-phase requirements of an automated transportation system, the process and apparatus of this invention represent a great practical advance in the automation of in-barrel chemical and/or electrolytic treatment installations.

The support means will of course normally and desirably include means capable, upon energisation, of rotating the supported barrel in at least one sense, and above all in the sense that results in the bulk load positioning itself adjacent the suction means.

Advantageously the suction means will include a vacuum chamber, having an opening therein adjacent the casing part of a supported perforate load-containing barrel to which suction is to be applied. sealing means mounted upon the vacuum chamber adjacent the opening therein and biassed for sliding engagement with the casing exterior of the perforate barrel, so as to establish an effective' seal between the vacuum chamber and the casing exterior, and air-extractor means for withdrawing moisture-laden air from said vacuum chamber.

The apparatus will also normally and desirably be so arranged that moisture-laden air withdrawn from the perforate barrel is subsequently passed through de-humidifying means, which should consist of or include a moisture droplet separator and/or air-reheating means. Those air reheating means may conveniently be a heat-exchanger and/or electric heating elements.

From what has been said above it can be appreciated that because the drying air is drawn from the barrel, rather than forced into the barrel, it is possible for large volumes of air to flow through the perforate casings of such barrels, even those of them whose perforations are of very small cross-section. Such very small perforations are needed when plating small items such as pins, small screws or nails, transistors etc.; but if one attempts to force drying air into such barrels from outside, the development of turbulence at the perforations can effectively block them almost completely against any significant entry of air into the barrel interior.

In order that the invention may be well understood one preferred embodiment thereof will now be described in more detail, though only by way of illustration, with reference to - the accompanying drawings, wherein: Figure 1 is an end-elevational view of a barrel, transverse to its rotational axis, mounted at a schematically-illustrated drying station; and Figure 2 is a side-elevational view of the barrel and schematically-illustrated drying station arrangement shown in Fig. 1.

Referring both to Figs. 1 and 2, the arrangement comprises a drying station vacuum chamber etcetera, generally indicated 1, and a perforate treatment barrel, generally indicated 2. The vacuum chamber 1 constitutes the drying station of an electrolytic treatment installation (not shown) which will desirably be one operating with an automated transportation system.

The barrel 2 is between one-third and onehalf full of a load, generally indicated 21, of small items in bulk; and it is rotated (by conventional means not shown) in the sense of the arrow at a rate of n rotations per minute.

The barrel 2 consists essentially of a perforate casing 22, a removable perforate lid 23 and two end-sections 24. The periphery of one of the two end-sections 24 is toothed to constitute a gear wheel, and is driven via a pinion 25 by a drive motor (not shown in the drawings).

The barrel 2 is rotatably mounted upon two support arms 26. The perforate barrel-casing 22 and lid 23 are preferably formed of some highly thermally-resistant material, for example a polypropylene resin reinforced with glass fibres. The perforations should be arranged grouped closely together, and so far as possible (dependent upon the nature of the items to be treated therein) should each be of large cross-section, so as to produce as little resistance as possible to the passage of air therethrough.

The barrel 2 is positioned at the drying station 1 only after both the load 21 and the barrel 2 have first been water-rinsed, preferably with hot water-say at a temperature of about 90"C. Consequently both the load 21 and the barrel 2 are already themselves at a high temperature on arrival at the drying station 1, and this high temperature naturally has a favourable effect upon the speed with which liquid will evaporate therefrom.

A vacuum chamber or suction box 11 is so arranged that the load 21 upon rotation of barrel 2 in the sense of the arrow will position itself directly opposite the suction opening of this suction box 11. Two flexible flaps 1 2 rest in sliding contact with the casing 22 of the rotating barrel 2, and form a reasonably effective seal therewith. When the pressure in the thus-sealed suction box 11 is reduced by means of the radial air-extractor fan 13, air is then positively drawn in through the load in the direction of the suction air stream 31 shown by the series of black arrows. The indrawn air stream 31 is positively made to flow straight through the section of the perforate barrel casing 22 overload by the load 21.

The arrows in the suction stream 31 are blackened to symbolize the strong uptake of moisture by the indrawn air stream 31, which thereafter while still moisture-laden continues downwards in a vertical direction, until it reaches and passes into and through a first dehumidifier, namely droplet separator 14, in which much of the moisture content of the moisture-laden airstream 31 is removed, as symbolized by hatched arrows 32.

The efficiency of the droplet-separator 1 4 is dependent upon the rate of throughput and the size of the smallest removable drops, i.e.

the so-called border-line drops. When the moisture-laden airstream 31 is made to flow through the tortuous flow-grid of the droplet separator 14, then the droplets entrained therein are caused by inertia to impinge upon its walls, and to a large extent separate out thereon, leading to a partial extraction of the moisture content of the airstream. The degree of extraction achieved depends upon the airinflow speed of the moisture-laden air entering the droplet separator, upon the moisture content of that moisture-laden air, and upon the geometrical arrangement of the moisture-extraction grid.However, since the arrangement of this invention permits the air-inflow speed to be high, and ensures a high-moisture content in the moisture-laden air entering the droplet separator, the latter can operate rather efficiently, and thus the drying of the items in load 21 is especially intensive and rapid.

After leaving the droplet separator 14, the relatively dry dehumidified air 33, indicated and symbolized by means of the white arrows, then passes onwards to a further dehumidifier, namely the heat exchanger (or heating elements) 15, where it is reheated to become drying air, which then is sucked into the radial air-extractor fan 13, and driven thereby towards the upper part of the drying station 1.

Broadly-speaking, as indeed appears from the drawings especially Fig. 1, the air flows round and round in a continuous cycle; but with the construction illustrated a certain proportion of the hot dehumidified airstream 33 escapes and is lost to the surrounding atmosphere. That loss however is replaced by fresh air drawn in from the atmosphere, which joins the remainder of the hot, dehumidified air, and these together are drawn back into the barrel 2 through its perforate outer casing 23, as shown symbolically in Fig. 1.

Because of the partial loss of the air-output from extractor fan 1 3 to the atmosphere, and its replacement by atmospheric air. it is possible to operate the drying process without the droplet separator 1 4 and/or without the heating elements 1 5-but the arrangement as previously described, wherein both these forms of dehumidifier are present, is of course generally to be preferred.

Claims (11)

1. A process for drying a bulk load of items within a treatment barrel having a perforate outer casing, in which drying air within the barrel is drawn in a generally-downwards direction through the bulk load and the resultant moisture-laden air is then withdrawn from the barrel through the perforations in its casing underlying the bulk load by suction applied externally of the perforate barrel casing.

2. A process as claimed in claim 1, in which the bulk load occupies substantially one-third of the internal volume of the barrel.

3. A process as claimed in claim 1 or claim 2, in which the barrel and its bulk load are rotated during at least part of the drying process and air is drawn through the bulk load and in a generally-downwards but nonvertical direction substantially normal to the tumbling plane of the surface of the bulk load when in the position which it adopts upon rotation of the barrel

4. A process as claimed in claim 3, in which the barrel is rotated during only part of the drying process, the barrel remaining stationary during at least one initial stationary period of the overall drying process and being rotated during at least one subsequent rotational period of the overall drying process.

5. A process as claimed in claim 4, in which the initial stationary period of the dry ing process is terminated when the rate of egress of liquid water significantly diminishes, and is then succeeded by a rotational period of the drying process.

6. A process as claimed in any of the preceding claims in which the moisture-laden air withdrawn by suction from the barrel is treated to reduce its moisture-content and is thus reconstituted as drying air, at least part of the drying air thus reconsistituted being permitted to re-enter the barrel through its perforations elsewhere, and being thus recycled through the bulk load and once more moisture-laden then again withdrawn through the perforate casing by suction.

7. A process as claimed in any of the preceding claims and substantially as herein described.

8. A process as claimed in any of the preceding claims and substantially as herein described with reference to the accompanying drawings.

9. Drying station apparatus for drying a bulk load of items within a perforate treatment barrel by means of a process as claimed in any of the preceding claims, which apparatus comprises support means capable of supporting the perforate, load-containing barrel at the drying station, and suction means so arranged that upon operation they apply suction to the exterior of part only of the casing of the perforate, load-containing barrel when thus supported, said casing part to which suction may be applied being located at least predominantly below the casing where it can be at least partially overlaid by the bulk load, in order that upon operation of said suction means drying air-within the barrel will be drawn by suction through the bulk load in a generally-downwards direction and the resultant moisture-laden air will then be withdrawn through the perforations in said casing part where it is overlaid by the bulk load.

1 0. Apparatus as claimed in claim 9, in which the support means include means capable on energisation of rotating the supported barrel in at least one sense.

11. Apparatus as claimed in claim 9 or claim 10, in which the suction means includes a vacuum chamber, having an opening therein adjacent the casing part of a supported perforate load-containing barrel to which suction is to be applied, seal means mounted upon the vacuum chamber adjacent the opening therein and biassed for sliding engagement with the casing exterior of said perforate barrel so as to establish an effective seal between the vacuum chamber and the casing exterior, and air-extractor means for withdrawing moisture-laden air from said vacuum chamber.

1 2. Apparatus as claimed in any of claims 9 to 11, arranged so that moisture-laden air withdrawn from the perforate barrel is subsequently passed through de-humidifying means.

1 3. Apparatus as claimed in claim 12, in which the de-humidifying means is or includes a moisture droplet separator.

1 4. Apparatus as claimed in claim 1 2 or claim 13, in which the de-humidifying means is or includes air-reheating means.

1 5. Apparatus as claimed in claim 14, in which the air reheating means is a heatexchanger and/or electric heating elements.

1 6. Apparatus as claimed in any of claims 9 to 1 5 and substantially as herein described.

1 7. Apparatus as claimed in any of claims 9 to 1 6 and substantially as herein described with reference to the accompanying drawings.