if i want to replace an 800A watercooled disc type thyristor (for spotwelding m/c),with an air cooled thyristor with heat sink,what could be d rating of thyristor and the heat sink.can anyone suggest???

 

Air cooling is much less effective than water cooling. With that level of power (I squared times any R is huge), you would have to use multiple thyristors with large fan-forced heat sinks, plus the problem of synchronizing multiple triggering circuits. What could only be called a can of worms.

I wouldn't want to try it unless there was absolutely no choice.