GB2178158A

GB2178158A - Arrangement for the cooling of surfaces

Info

Publication number: GB2178158A
Application number: GB08614951A
Authority: GB
Inventors: Rainer Buhl
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1985-06-20
Filing date: 1986-06-19
Publication date: 1987-02-04
Also published as: DE3613801A1; GB8614951D0; NL8601236A; FR2583860A1; JPS61295474A

Abstract

The cooling medium is introduced through nozzles tangentially into the cooling medium space deliminated by an essentially cylindrical shell, so that the cooling medium is set in rotation and is led away again via a duct arranged near the axis of rotation. The formation of vapour bubbles is thereby largely suppressed. <IMAGE>

Description

SPECIFICATION Arrangement for the cooling of surfaces The present invention relates to an arrangement for the intensive cooling of surfaces.

The necessity of a particularly intensive cooling is present, for example, in the case of those surfaces which are encountering highpower electrical discharges or are being bombarded by intensive particle radiation. The idea is known, of making the surface to be cooled as the wall of a cavity through which flows a liquid cooling medium; in doing so the maximum power which can be dissipated from this wall by the cooling liquid is limited by the formation of bubbles of vapour. Surfaces to be cooled, for which the invention is particularly suitable are, e.g., carriers for the evaporation of materials in vacuum by electron bombardment and the cathode sputtering of targets by ion bombardment.

Since a laminar flow of the cooling medium is not suitable for the dissipation of large amounts of heat at high power densities because the volumes of liquid heated by the hot wall persist in skirting this hot wall and soin particular if the wall forms a horizontal upper boundary of the cooling cavity-scarcely any exchange of heat takes place with the colder parts of the laminar flow of liquid, it has already been proposed for the purpose of better cooling action to bring about a turbulent flow, which is done, e.g., by increasing the pressure, whereby a higher speed of flow and thereby the formation of eddies results. The boundary region between laminar and turbulent flow according to opinion hitherto should be particularly effective for cooling heat exchangers of large area (and consequently is striven for, e.g., in the building of power stations).

For the present purpose of the present invention this known measure is quite inadequate.

For the generation of eddies the flow must be adapted to the geometry of the surface which is to be cooled and possibly in addition to a different loading locally, which is realized through guide members arranged longitudinally and transversely. Yet these guide members too impede the flow particularly at edges and corners, which favours the formation of extensive vapour bubbles. If these vapour bubbles grow into areas of stronger flow they may partially be torn off but often the liquid cooling gets completely paralysed through vapour bubbles. Again, the deposit of lime reduces the heat transfer in such critical regions and demands frequent cleaning.As already said, it is known that vapour bubbles may be torn off from the hot surface through high speed of flow of the cooling medium as long as they are still small in which case for the short time during which they adhere to the wall they impair only a iittle the contact with the cooling medium. But this can be achieved only by a high rate of flow of cooling medium, i.e., a correspondingly high pressure difference between the inlet and output of the cooling medium. In the case of the employment of cooling water containing lime it is to be noted in addition that the lower limit of the necessary amount of water per unit of time is given by the maximum temperature arising at places of diminished flow and above which the precipitation of calcuim carbonate increases strongly.

As is well known, this temperature is a function of the hardness of the water.

Because of the conditions mentioned the loading of the costs of operation through a high consumption of cooling medium is often considerable and it is one problem of the present invention to reduce these. Also from the ecological aspect it is desirable to solve the problem of the cooling with a lower consumption of cooling medium.

The problem of the invention is solved by the means specified in Claim 1.

Underlying the invention is the recognition that the cooling cavity at least in the region of the surface which is to be cooled, where this is being acted upon at high power density, must be free of guide members. For every member of that kind reduces the speed of flow of the cooling medium, that is, particularly severely in the edges and corners of the guide members. Their influence might indeed be reduced through good heat-conductive connections to the cooling surface and through careful design of the heat transfer (the reduction in the speed of flow is partially compensated by the increase in the cooling contact surface). But the cost of production for such a solution would be considerable.

The surprisingly high cooling action in the case of the arrangement in accordance with the invention may probably be explained by the fact that with the inlet of the cooling medium by means of a nozzle a sufficiently rapid flow is generated, necessary to the tearing off of the vapour bubbles, by the pressure difference in the cooling medium between the inlet and outlet being converted almost without loss into kinetic energy. A larger cross-section of the admission and discharge ducts is to be recommended in order that even in these no too great a part of the pressure drop available gets consumed through the formation of turbulent flow.The solution in accordance with the invention may be realized particularly well in the case of surfaces which are to be cooled which are in the form of circular discs, in which case the admission of cooling medium into the cavity provided for the cooling may be effected tangentially to the inner wall of the latter.

It further contributes to the high efficiency of a cooling arrangement in accordance with the invention, that in the case of the vapour bubble once torn away from the wall the condensation into the liquid phase is mainly into the surroundings since no further supply of heat exists and consequently a reformation of vapour is not possible so that the vapour bubble rapidly collapses on itself. The small amount of permanent gases which was dissolved in the water and does not again go into solution out of the collapsing bubble, gets forced inwards through the centrifugal force exerted upon the liquid cooling medium. Also for the same reason such gases get forced along the bottom of the crucible towards the centre whence they may be sucked away through the cooling flowing out.

Thus whilst in the case of known cooling arrangements it was required that the walls of the cavity through which the cooling medium flowed should be made as structured as possible in order to achieve the formation of eddies, in the sense of the invention it is more favourable if the walls are as smooth as possible. For in this way the flow losses are kept small and velocity of the cooling medium is maintained nearly up to the centre. Since the square of the velocity determines the kinetic energy whereas the momentum is decisive for the releasing of vapour bubbles, the outstanding action may be observed up to the centre of the cooling surface. The conservation of the angular momentum in addition opposes the flow losses of the cooling medium during the transition to still smaller radii so that an increase in the rotational frequency results.

The centrifugal force K=mw2r thereby remains high down to very small radii and thereby also the force which the cooling medium exerts against the wall which is to be cooled so that the abstraction of heat becomes independent of the position in the construction.

The invention is explained in greater detail below with the aid of a simple embodiment.

There is shown in: Figure 1 a vertical section through a crucible cooled in accordance with the invention, for the evaporation of a material by means of an electron beam; Figure 2 a section along the line A-A in Fig.

1.

As may be seen from the drawing, the crucible body 1 comprises a baseplate 2 and an upper part 3 connected to the baseplate 2, the two together defining a cavity 4 through which may flow a liquid cooling medium usually water. The cooling medium is admitted via a pipe 5 and is led away again via a central pipe 6 surrounded by the former. The material 8 which is to be melted and evaporated lies in the crucible and during operation is bombarded and thereby heated by a highenergy electron beam generated by an electron gun which is not shown. In doing so a very high electric power is transmitted to the comparatively small area 9 of the bottom of the crucible, which for the greater part must be led away through the walls of the crucible by cooling in order to avoid overheating and thereby melting of the crucible.In order to guide the cooling medium in the sense of the present invention against the underside 9 of the bottom of the crucible which is to be cooled in particular, an insert 10 is provided, which lies on an annular elevation 11 on the baseplate and is connected to it, whereby a distribution space 13 for the cooling medium is formed. The ring 11 comprises a number of channels 14 (two are shown) through which the cooling medium flows into the cavity 4. It was found to be advantegous if these channels 14 are shaped in the form of Laval nozzles. The sum of the cross-sectional areas of all of the outlet openings from the aforesaid nozzles is to be small in relation to the cross-sectional area of the cooling medium in the inlet and outlet pipe and in relation to the diameter of the cylindrical cavity or the area of the bottom of the crucible.The direction of the jets of cooling medium leaving the nozzles 14 should run as far as possible tangentially to the inner wall of the cavity and in this embodiment as far as possible in parallel with the underside of the bottom 9 of the crucible.

In that case the cooling medium rotating about the axis 15 of the arrangement is constantly sweeping radially from the outside inwards across the underside of the bottom of the crucible which is to be cooled and gets carried away again via the central pipe 6. The necessary rate or velocity of flow of the cooling medium follows from these design outline conditions and also from conditions influencing the process (temperature of the crucible, temperature of the cooling medium at the inlet etc.) in a specific case.

The progress achieved by the invention may be seen from measurements which were performed upon electron beam evaporators designed in accordance with the invention. For this purpose the copper crucible of the evaporator was acted upon by an electron beam of a power of 10kw. In that case with a temperature of 11"C of the admitted cooling water (4 litre/min) a temperature of 46"C resulted for the discharged cooling water, that is, a total rise in temperature of 35"C. On the other hand in the case of a conventionally cooled crucible of the same size and at the same power of the electron beam at least 12 litre of cooling water per minute were necessary. Thus the saving in cooling water in the case of the arrangeinent in accordance with the invention amounted to two thirds.

In another example the cooling water which a target in a cathode sputtering installation was cooled in a conventional way, was heated by 7"C if the power transmitted to the target through the cathode sputtering amounted to 40 kW. But in spite of this low heating, in the case of an attempt to increase the power above the aforesaid power limit a continuous pounding against the water hoses occurred and from time to time even the elastomer seals were burnt, which were provided for sealing of the target with respect to the cooling water channels. After the incorporarton of a cooling system in accordance with the invention the amount of cooling water could be reduced from 67 litre per minute to 19 litre per minute and at the same power of 48 kW the heating amounted to 35.5"C. In that case at an inlet temperature of the cooling water of 13"C the input power could be raised even considerably above the aforesaid limit without any operational trouble occurring at all. Thus in this example the progress from the invention was shown in that a considerably higher temperature rise of the cooling water and thereby a better utilization of it could be permitted without the functioning of the device having become impaired.

Claims

1. An arrangement for the cooling of surfaces, where the surface to be cooled is made as part of the wall of a cavity which has an essentially cylindrical shell and through which flows a liquid cooling medium and a duct for admission of the cooling medium emerges in the neighbourhood of the shell and further duct is provided for leading the cooling medium away from the cavity, wherein the mouth of the admission duct is made as a nozzle and is so arranged that the cooling medium enters the cavity tangentially to its shell and is set in rotation about an axis and the duct for removing the cooling medium is arranged near the aforesaid axis.

2. An arrangement according to Claim 1 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawing.