Please clarify what type of device you are talking about.
Usually a rectifier is an arrangement of semiconductors specified at least by output voltage, output current and application. Rectifiers aren't rated in kVA.
In general water cooled rectifiers are very reliable when designed properly. Check if the company offering the system has experience in this area because designing such a unit needs some knowledge not common to most electrical engineers.
"some knowledge not common to most electrical engineers"
I'll second that.
In fact, I'd guess there's a lot of EE's out there, like me, who would just throw up their hands and say that water and electricity don't mix too well, and if you wanna use water cooled, it's your baby.
But maybe you can find someone like electricuwe (who generally seems to know what he's talking about) to get something like this to work for you better than air-cooled would.
Hey peebee, have you ever seen a REALLY big transmitter. I mean something in the megawatt range. The wires are copper pipes with water flowing in them to cool them.
Above a certain size, water-cooling becomes necessary to eliminate hot spot temperatures. This is a time-proven technology, but obviously adds another level of complexity to the system. De-ionized water is often used, requiring some knowledge of water chemistry, treatment, etc.
At 800 kVA, I would think air-cooling is still an option and might be preferable if you have the physical space and can stand the noise of the fans.
I have to diagree to your statement since Saut didn,t supply enough information to give such a specific advice. For a rectifier rated 800V/1000A, maybe for traction application, I would never use water cooling but for supplying e.g. 16V/50000A e.g. for electroplating it's the only available option.
Although my employer competes with them on other levels, we stay clear of water cooled designs since it is such a specialized application. We refer all of our customers who need water cooling to them. Several of their (water cooled) competitors have gone under recently so be carefull if you are considering used equipment. The original manufacturers may not be around for support.
By the way for those who were debating the merits based strictly on size, the other reason to consider it is environment. If the rectifier needs to operate in a high ambient and / or the air is corrosive or contaminated, water cooling is often the only answer, regardless of size.
