Are you really sure that you need a minimum volume of boil off? DMS boils at 99°F (37°C) so it's going to boil off pretty much as fast as it is created during the boil. So, how fast DMS is evaporated is going to be controlled primarily by how fast the SMM precursor is converted to DMS. SMM doesn't convert to DMS significantly at temps below 70°C (158°F) [ref: http://www.milkthefunk.com/wiki/Dimethyl_Sulfide.] The interesting thing is that DMS creation continues as you cool from boiling down to about 70°C (158°F), and that DMS will continue evaporating from the wort almost as fast as it is created. However, the spray condenser is not going to be evacuating much from the BK headspace once steam generation slows way down. So, to minimize the potential for DMS retention in the wort, you probably want to remove the lid from the BK so that the DMS created during initial cool down can escape before the BK cools to the temperature where the evaporated DMS can start condensing on the BK walls and running back into the wort. The linked reference makes the point that open cooling is critical to residual DMS minimization, and my opinion (which could change after some additional study) based on the reference is that open cooling is more important than boil time or boil off rate.
That's my understanding. I want to dig deeper into the references of that article to see if they have anything about whether or not boil off rate affects the evaporation rate of DMS. Doesn't seem like it should have a large effect, but I could be wrong as this is new territory for me.
The reference I linked earlier (http://www.milkthefunk.com/wiki/Dimethyl_Sulfide} does note a pH dependency for conversion of SMM to DMS, but not as strong a dependency as you seem to notice. They list a SMM half life of 38 minutes @ pH 5.2 vs. 32.5 min @ 5.5 pH both at 100°C (212°F.) SMM half life increases pretty rapidly as temperature drops.
Getting rid of DMS is pretty easy at boiling temps, once it is formed from the SMM. The thing that takes longer is converting most of the SMM to DMS so that it can boil off. Residual SMM in the wort/beer can turn to DMS later if the level isn't reduced enough during the boil. So, time at boiling temp is more important the vigor of the boil. If you understand the charts in the reference, they show that DMS evaporates pretty rapidly even below boiling temp (zero boiling vigor.) Boiling vigor probably has some effect on the evaporation rate of DMS, but even at zero boiling vigor, DMS still evaporates faster than it is formed from SMM. According to the reference, the most important things to worry about are boiling long enough (regardless of vigor) and, allowing free evaporation of DMS during the early part of the cooling process (including any whirlpool/hop stand), by having the boil vessel uncovered.
You might want to check probrewer for info since stout sells a similiar condenser unit for their 3bbl and up systems and has been for quite some time. I almost bought one from sungood machinery and I remember it had multiple spray nozzles along the side of it.
Just brewed my 10th or so batch with mine, working great. 9th-ish batch was a parti gyle, boiled 2 in one day. Love my condenser!
Agree! This system works great. We took third place for an IPA we brewed in the first round of the National Homebrew Competition using the condenser. Don't hesitate to use BrunDog's condenser.
I worked up a spread sheet using a picture you posted as a reference.
