So, doing an extract demo tomorrow. Planning on 2.5-gallon boil, and adding 2.5 gallons after whirlpool to drop the temp. (I'll be brewing outside without my usual access to sink full of icewater-my normal stovetop/chill method). What temp should the liquor addition be to drop my temp down to 140 or so (the upper limiy on my fermenter)?

To a reasonable approximation:

2.5*T

_{w} + 2.5*T = 5*140 -> T

_{w} + T = 280 -> T = 280 - T

_{w}where T

_{w} is the temperature of the wort after whirlpooling. This is an approximation because the specific heats of water and wort aren't quite the same, but you'll be within a few degrees.

Or 68°F if the wort is at 212°F, but it won't be after whirlpooling.

Edit: I'm not sure why you'd chill the wort to just 140°F though. Instead I freeze the (boiled, sanitary) water and come out with wort at ~60°F.