Hi Jack, it's been ages since you made this post, but perhaps someone else might find this reply useful? There are a couple of other threads that pretty much cover this issue, but briefly:

Linear Heat Detection and IR seem to be good candidates from what I've found out. See my responses to a couple of other threads for links to some interesting we sites. That said, we're considering pilot sprinkler heads because we're looking at a fast, reliable way to operate sprinklers in a high airflow environment (the belts are in the intake).

I hope that jelps you or someone in some way. ■