Purpose of Evacuation:
Before steam is often entered into the autoclave, the autoclave must be evacuated by reducing the pressure within the autoclave with the utilization of vacuum from the air pump or steam ejector.
Hereby the existent atmosphere within the autoclave is removed.
By keeping the evacuation pressure below the saturation pressure of water, which is out there within the green cakes, steam is released from the green cakes as a desired side effect.
Air and hydrogen which are still present within the green cakes are scavenged from the autoclave by this self-formed saturated steam.
Why the vacuum should be done for AAC Blocks?
The vacuum is mandatory.
This will remove the entrapped air from autoclaves.
Otherwise, liquid steam marks are going to be visible after autoclaving.
Those blocks are sensitive and have a tendency to break easily. If it's not broken, please keep the separate stacking/drying for a couple of days.
If there's no vacuum, do the purging of steam.
Purging is nothing but the entry of steam from one side and the exit from other side.
By evacuation, the air is removed from the autoclave and the cakes.
Evacuation (vacuum) should be done up to -400 mbar (for 45 minutes).
Normally it (from 0 to -400 mbar) takes 10 to 15 minutes to achieve the same.
It should be maintained between -300 to -400 mbar for the next 30 minutes.
This is done to achieve the uniform temperature distribution of the cake.
Common Mistake operators do between Evacuation and Rising of Autoclaves
Generally, operators switch off the Vacuum pump immediately after Evacuation.
Afterwards, they proceed for closing the Valve.
This is the common mistake noticed in most of the factories.
Make sure that the Valve (Vacuum) should be in closed condition before switching off the pump.
Otherwise it will suck the atmosphere air and our purpose of evacuation will not be met.