Is it safe to use a screw clamp on my immersion chiller if it comes into contact with boiling wort for 15 minutes?
The short answer is - if the clamp is stainless steel then no problem. If it is not stainless, then you will obviously start developing rust which I try to avoid.
The bigger issue however, is why is the clamp in contact with the wort? A properly designed IC always has the ends of the copper tubing well higher than the depth of the kettle. If you are a handyman, you should try to bend the copper to get the ends up. Look at any picture of a retail IC to see what it should look like. The other option, if your IC is shorter than the kettle, is to simply stretch it out like a slinky until it is long enough to clear the top of the kettle.
Just my thoughts anyway.
I have a screw clamp holding my wort return copper tube to my IC so that the wort flows past the coil. Think of the recirculating wort chiller, I think in the real version it is welded to the IC. Any bad stuff that could leach into my beer?
Like Uncle Tom said, if it is all SS or SS and aluminum, your OK. The clamp will say all SS. I use one in my MT to hold my false bottom to my dip tube.