Vacuum Furnace  Maintenance Tips

Diagnosing and Repairing Water Leaks
in Vacuum Furnaces

The chart below is from a vacuum furnace showing a classic example of the pressure curve in a furnace with a water leak.  When the pressure in the furnace is reduced to a point below the vapor pressure of the water, the water begins to boil, causing the pressure to rise.  The boiling action removes the latent heat of vaporization from the water, causing its temperature to drop.  The vapor pressure of water, as with most substances, is lower at lower temperatures.  When the temperature of the water goes low enough that its vapor pressure is higher than the furnace pressure, the boiling stops and the vacuum level starts down again.  The vacuum level will continue to drop until the vapor pressure of the water at the new temperature is reached when it will start to boil again.

This cycle is repeated several times with the vacuum level required to boil water becoming lower with each cycle due to the heat which was taken out of the water.  This process is represented on the graph as peaks and valleys going from left to right, with each peak being slightly lower than the previous one.  With this particular furnace, the pressure rose to a point where the control system shut down the heating system, causing the saw tooth shape of the heating curve.  When the pressure dropped to an acceptable level, the control system allowed the heat cycle to resume, resulting in the ‘saw tooth’ pattern being reflected in the heating curve.  Eventually sufficient heat is removed from the water that it freezes, resulting in the leveling out of the pressure curve. 

Small water leaks may not be large enough to cause the heating system to cycle on and off.  The pumpdown will slow until the water freezes, and the pumpdown will continue.  It may not seem that there is a problem, because the required vacuum level can be reached, but this is misleading.  The leak is still there and it needs to be fixed.    

The possible sources for leaks include:

1. Welds
2. Older vessel inner wall corrosion
3. Heat exchanger (internal or external)