[krazy]

Gooserider laid out all the major issues in plain english. I agree with what he said, but I'll add some info that nobody brought up anyways.

The most significant source of heat transferred into the water by a pump is from the warmth of the motor. There is theoretically a certain amount of heat generated just by the motion and friction of the water being turned and thrust around by the pump, but this is negligible for our purposes. Different types and even models of pumps transfer different amounts of heat from the motor to the water. Some transfer almost none, and some can really heat up the water passing by. Cooling the pump's motor to channel away some of this heat is the obvious solution, and there are several ways to do it.

An easy way is to make sure the pump's motor housing is in the flow path of a fan. A lot of setups I have seen have the pump mounted in front of the heat exchanger's fan to carry away any excess heat the pump produces. This will probably be sufficient to prolong the life of the pump by making the motor a little cooler, but it will in no way take away a lot of the heat.

Another method uses a submergable pump (many pumps, like the commonly-used Eheims and Danners are capable of being used both in-line or submerged). Build a reservoir that has the whole pump right inside of it. The pump's inlet can be simply exposed, but the power cord and outlet plumbing will need to be run out through some type of watertight seal. This way, any heat the pump makes has to go into the water. Your water temperature will be increased, but running the outlet directly to the heat exchanger will make this a moot point. Unlike the fan option, this does not aid in keeping your water cooler at all. It increases it. The main advantage of this setup is to avoid potential pump leaks and cool the pump with hopes of a longer life.

The only other thing I can think of doing with the extra passages in the radiator you describe is to run some kind of cold liquid through it. Don't bother making a water chiller go through, because you would get much better efficiency by simply running this chilled water directly through the blocks. The main forseeable advantages the second loop might facilitate is running either liquids you want to keep out of your waterblocks for corrosion, contamination, sediment, etc. reasons, or to get by with a slower flow in the secondary loop and a fast flow on the block loop.

You could try building some kind of in-ground loop system to exchange heat with cold sand, or maybe run a long loop of tubing through your attic (if it's wintertime) to chill the water below ambient and hopefully cool the clean, fast-flowing block loop's water further than a normal radiator could with room temperature air.

The next issue that comes up is that the double-loop radiator is not really designed to transfer heat from one loop to the other. It is designed to transfer heat out of both loops and into the air. If you really wanted to try to chill one circuit of clean/fast water with another separate circuit of less-ideal water (slower, dirtier, not-water, etc.), then a parallel flow exchanger would be the best choice. (someone please correct me if I'm using the wrong term for this)

A parallel flow exchanger is designed to transfer heat from one liquid pass directly into another liquid pass without having to go through air. The easiest style to make is just a copper tube inside of a larger tube. The small tube has the warm water in it, and the bigger outer tube has the cooled water in it. The longer the nested tube setup is, the more room you have to play with for heat exchangure.

Parallel exchangers are most efficient when the two loops are run in opposite flow directions. Think about it: when the warmest water comes into the end, it first encounters the chilled water that is about to leave the exchanger, and is already as warm as it can get. It will still be cooler than the warm water, so some heat is transferred out. As the warm loop's water moves down the exchanger, it is jacketed by progressibely cooler and cooler water, until the warm circuit's flow reaches the far end of the exchanger, where the chilled flow's water is at it's coldest, allowing the temperature differential to stay up.

If you ran the flows in the same direction, the warmest water in the hot loop would immediately see the coldest water in the cold loop, which would have a great differential, but by the time you got to the far end of the exchanger, the warm would have been cooled down, and the cool would have been warmed up. The differential will be worse.