Friday, July 10, 2009

Building vacuum furnace

Imported vacuum furnaces are very expensive. Hence we decided to build a vacuum furnace in-house.
A vacuum furnace can be divided in to three subsystems:
1. A double walled chamber to create vacuum and maintain vacuum.
2. Heating system consisting of electrical heating elements and shields to reflect the heat inwards.
3. Power supply and control gear to maintain required temperature in the furnace.

We looked at our strengths and weaknesses to build the vacuum furnace.
We had people who are good at building vacuum tight vessels.
We had people who can design the vacuum system as well as size up the required
vacuum pumps
One of our engineers came forward to design the heating elements.

What we lacked was people who can design the control system.

We decided to off-load control system to a professional company.
Taking in to account the brazing requirement we firmed up the size of the vacuum furnace as follows;

External size : 1200mmID x 800mm long.
Hot zone: 450mm x 450mm x 450mm
Vacuum: 5 x 10-4 mbar
Maximum power required: 60kw.
We arrived at this figure to take in to account the rapid rate of of heating needed for aluminium brazing.
Largest size that can be loaded: 250mm x 250mm x 250mm.

The furnace was double walled furnace with the inner shell of stainless steel and the outer shell of carbon steel.Water circulated in the annular space. Several feed throughs were provided for electrical connections,thermocouples and vacuum gauge.
Vacuum chamber with all the feed throughs were subjected helium leak test.
Vacuum system consisted of a fore vacuum pump, roots blower and a diffusion pump.
The control system was designed to take care of starting the pumps, raising the temperature in the furnace at a controlled rate,holding the temperature constant during brazing and shutting down the heating at a controlled rate. In addition temperature was constantly monitored from the thermocouples fixed in the vacuum chamber. We calibrated the furnace and started our brazing activities.
We built one more high temperature vacuum brazing furnace later to vacuum braze stainless steel These furnaces are in no way inferior to imported furnaces.
We will give details of heat exchangers brazed in the next blog.


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