Muffle convection brazing/annealing system

Abstract

A convection muffle furnace for brazing and / or annealing a workpiece includes a muffle having towers formed integrally therewith and being mounted movable relative to a top wall of a furnace. Circulation fans are mounted in the towers so as to move along with the muffle during its expansion and contraction for producing forced convection heat transfer.

Description

BACKGROUND OF THE INVENTIONThis invention relates generally to heat-treating furnaces used for brazing and / or annealing of workpieces. More particularly, it relates to an improved method and apparatus for brazing and annealing of a workpiece which includes a furnace having a muffle formed therein and circulation fans disposed in towers which are an integral part of the muffle so as to produce forced convection heat transfer.As is generally well-known in the art of heat-treating, brazing and annealing require a close tolerance on the temperature control in order to achieve a desired temperature profile or to maintain temperature uniformity within a workpiece. Convection heat transfer has been found to provide faster heating or cooling of the workpiece on a more uniform basis. In such convection heat transfer, the hot gases are typically directed at the workpiece by means of jets or fans, with the fans being the more common means to impart velocity to the gases.Further, the brazing an...

AI Technical Summary

Benefits of technology

Accordingly, it is a general object of the present invention to provide an improved muffle convection brazing / annealing furnace which is relative simple and economical to manufacture and assemble, but yet overcomes the disadvantages of the prior art furnaces.

A plurality of towers are provided, each being formed integrally with the roof of the muffle in each brazing zone and are mounted movably relative to the top wall of the heating chamber A plurality of fans are provided, each being mounted and sealed in a corresponding one of the plurality of towers for creating an atmosphere convection current in each of the brazing zones. Direct-fired burners are provided for heating the exterior of the muffle so as to increase the temperature of the atmosphere convection current. As a result, the plurality of fans in the towers are movable along with the muffle during its expansion and contraction so as to provide increased heating rates and high thermal efficiencies.

Problems solved by technology

The use of the inner liner suffers from the disadvantage in that it is required to be rigidly positioned with respect to fans, doors, and heating / cooling devices since these items are mounted and sealed to the furnace outer shell.

Another drawback lies in the fact that the need for rigid mounting of the inner liner imposes stresses thereon as a result of differential thermal expansion and contraction between the inner liner and the furnace casing.

While various mechanisms have been used heretofore so as to absorb or counteract the stresses due to the differential expansion and contraction, these stresses eventually lead to destruction of the integrity of the liner due to the temperatures normally encountered in the brazing and annealing processes.

This problem becomes even more complex because replacement of the liner then becomes necessary which is a very expensive task since the furnace must essentially be completely disassembled for removal of the existing liner and building of a new one in its place.

Moreover, another difficulty results from the necessity of supplying heating or cooling through the radiant tubes which isolates a gaseous media from the furnace atmosphere.

This results in low heat transfer rates from these items to the recirculating atmosphere.

Also, radiant tubes are temperature limited by the materials of construction that can be used, thereby further reducing heating capacity.

Leakage of gases through the radiant tubes and leakage around penetrations are also additional sources of atmosphere contamination.

While there has also been implemented in the prior art muffle-type furnaces in order to overcome the disadvantages of the traditional lined controlled-atmosphere furnaces, these muffle-type furnaces are still not free from all problems.

In particular, since the muffle has the capability of being freely movable with the thermal forces there has not been heretofore developed a suitable means for equipping a muffle-type furnace with recirculating fans for imparting convection heat transfer to the workpiece within the muffle so as to produce increased heating rates as well as high thermal efficiencies.

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