[airspirit]

The reason the water at the bottom is coldest is for one reason: energy is drawn from the coolant as it evaporates. As a portion of the droplet goes up in smoke, it takes energy with it as the water becomes a more excited state: steam. This leaves the water droplet colder than it was before, and the steam is warmer than the droplet was. No energy is lost: some is just transferred during the phase change.

As those cooled droplets accumulate in the bottom of the reservoir, the coolant mass is colder than the surrounding air. By re-warming it via a heat exchanger (removing energy from the secondary PC loop), it will now be easier to evaporate when re-sprayed through the evaporation chamber.

There is more than just temperature involved, though. At any given room temperature, there will be a desired humidity factor that the water will try to balance at. The less humid the room, the more water will evaporate, and the cooler the water that is collected will be. Similarly, the warmer the room, the cooler the water will be in comparison. Ideally, if you put a vacuum over a water mass, the water will evaporate like a mofo, leaving nothing but the minerals that was dissolved in it. You can get your best results if you run a dehumidifier in your room as well, or if you live in a dry environment, by pumping the steam outside.

By submerging the heat exchanger, you are simply putting it in the place where the primary bong coolant is at its coldest. By putting it at the coldest point, you will get the most effective heat transfer possible. The fact that it would also reduce scale buildup is just an added benefit.

BTW, one of those heat exchangers used before the pump and after the bong res would give you spectactular results. HEs are something that can be found by many manufacturers all over the place, and would not only increase overall efficiency of the combination system, but would give you one less point of failure. When it comes time to remove scale from the HE (every six months or less), just flush it with CLR and you're all set.