Heating – Work chamber having heating means – Having means by which work is progressed or moved mechanically
Patent
1996-08-26
1999-04-13
Walberg, Teresa
Heating
Work chamber having heating means
Having means by which work is progressed or moved mechanically
34 78, F26B 2106, F27B 902
Patent
active
058937099
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a heat-transfer device for use in a convective-heat installation, in particular in a convective soldering installation.
BACKGROUND OF THE INVENTION
Convective soldering installations are used for reflow soldering. The problem occurring in this connection is that it is difficult to stabilise the system "soldering item/soldering installation" on account of the relatively poor heat transfer. In particular when working soldering items having a relatively large mass, the control of the system is so slow that only low throughput velocities can be achieved.
The heat-transfer devices presently in use in convective soldering installations (forced-air or convective furnaces) transmit heat by means of forced gases, as a rule with air. They have in general several temperature zones. Since this heat transfer from the furnace to the soldering item is achieved relatively slowly (small .alpha.), the installations are mostly quite long and the electrical power of the systems is high. Electrical power values of 90 kW and more occur quite often.
In order to accelerate the heat transfer, combined systems are available in which gas convection is used in addition to a direct infrared-radiation heating. The gas convection is to level out the drawbacks of the pure infrared radiation, such as shading effects etc., so that the workpiece is prevented from partially overheating.
The problem with most of the workpieces, which constitute the soldering items, is that they have to be heated rapidly as well as reliably while an overheating of individual parts must be excluded. The demand for rapid heating leads to a high temperature difference (.DELTA.T) between the heat source and the soldering items. In order to exclude an overheating of the peripheral area or, e.g., of protruding components of a componentry, it would be desirable to keep .DELTA.T between the heat-supplying gas and the desired maximum temperature at the workpiece as low as possible.
The only possibility to solve this contradiction is to select .DELTA.T such that it is as low as possible and to let the heat-transfer gas pass the workpiece as quickly as possible.
The main problem of the convective systems available today is that it is difficult to control temperature fluctuations in the system. In particular if large masses on a low temperature level are transported into the furnace, heat is withdrawn from the interior space. This heat loss has to be compensated for as quickly as possible. At the same time, the control must not overshoot too much so that the maximum system temperature is not exceeded.
In contrast, it is the object underlying the present invention to provide a heat-transfer device, in particular in a convective soldering installation or in a drying installation, which has a uniform thermal behaviour and controls the thermal equilibrium as quickly as possible.
This object is achieved by the features of claims.
SUMMARY OF THE INVENTION
For solving this problem, the invention is based on the idea of arranging at least one hollow body containing a heated fluid in the vicinity of the workpiece, e.g. the soldering item; the fluid transmits heat via the wall of the hollow body to the workpiece. The fluid can be vapour or a liquid such that a very precise temperature adjustment is guaranteed and the heat capacity is high. The heated fluid is occluded and separated from the workpiece to be heated.
The advantage of the present invention resides in the fact that it is possible to restrict the maximum furnace temperature exactly and in a simple way and that at the same time the dissipated heat is compensated for extremely quickly and the system cannot overshoot. Moreover, the occluded fluid does not get lost so that environmental damage can be prevented.
BRIEF SUMMARY OF THE DRAWINGS
In the following, the invention will be illustrated by means of the drawings, in which:
FIG. 1a shows a schematic view of an embodiment according to the invention; and
FIG. 1b shows a cross-section through the embodiment of
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Rauchfuss, Jr. George W.
Walberg Teresa
Wilson Gregory A.
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