I’ve been homebrewing for over 30 years, but I’m still learning. All my education and information says you don’t cover your brew kettle, in order that the bad stuff can evaporate. But it just occurred to me that every brewery I’ve every visited has a top on its brew kettle. Most of them are like an inverted funnel, and I assume they’re feeding the evaporate up a smokestack to the outside. So what is the purpose of this lid/funnel? Does it contribute to some condensate falling back into the brew kettle? Does bad stuff fall back into the kettle also? It is to keep the brewery from becoming too hot and humid, or does it have some other purpose? Should I partially cover my brew kettle?
It is to keep the humidity out of the brewery. There’s generally a condensate trap to drain the condensation off the side of the stack.
at home a lid can speed up getting your strike water to temp and getting the wort to a boil. although I’d be nervous to take my eyes of the wort while it was coming up to temp for fear of boilover.
More surface area give a higher boil off rate so that has to be controlled., the commercial breweries stacks get rid of the volatiles, some systems have a trough that collects condensate high in DMS and such off of the bottom of the ‘hat’ and that has a drain as Sean says.
Thanks, guys. You have answered my question. 
My kettle (12 barrel bk) has a stack, no trough for run off. I leave the manway open during the last 30 minutes of boil. I also always boil for 90 minutes. No DMS in my beers.
Love a picture of this and any suggestions on how to make one.
I’ll post a picture asap. Had my HVAC guy install it though, Be aware, it is for a 12 barrel commercial brewhouse.