Wort Chiller Flow Rate

Made my 5th batch last night and while it was cooling with the wort cooler I had a thought.

If I reduce the amount of water going through the wort cooler, it comes out much warmer since it spends more time in the cooler collecting heat. More heat per volume of water, but less water.

When I increase the amount of water it comes out much cooler since it spends less time collecting heat. Less heat per volume of water, but more water.

Has anyone done the experiment varying the flow rate through the wort cooler to determine if less or more flow rate will cool faster? If the different is negligible, I'd prefer to use less water simply to cut down on the water bill. But my gut instinct is that cooler water absorbs heat at a faster rate than hot water, so a higher flow rate will probably cool better.

Just curious

I haven't done the research or the calculations, but I do have a fundamental understanding of thermodynamics and heat/mass transfer. For my setup using a plate chiller I run the wort at a constant speed, typically full throttle (I recirculate it back into the BK to help with a whirlpool). On the cooling side, I start the cooling water at a fairly high volume. The delta between the wort and the cooling water is fairly high, on the order of 150-170* depending on ground water temps, so the water will quickly absorb heat energy from the wort. As the wort temperature drops, I slow the water rate down. As you pointed out, this leaves the water in contact with the wort for a longer period of time, giving it more of a chance to absorb the heat. This is important as the delta is now much smaller, so the energy transfer will not be as efficient. Yes, you could leave the hose on full throttle and get a slightly better heat transfer, but the amount of water you're wasting is not worth the slight increase in cooling. Slowing the flow down doesn't do much to the heat transfer, but does save you quite a bit of water.

If you had a closed system loop on the cooling side, such as a pump in a bucket with some ice water, I'd leave it on full throttle the whole time to get the most heat transfer via the highest possible mass-flow rate through the cooling side. I assume you're using an open system, like me - and all the left over cooling water becomes 'waste' (or cleaning water).

Someone with more time / energy can pull up the equations and prove me right or wrong... but this technique has worked for me and gets me down in temps quite quickly without having to dump a ton of water down the drain.

-Kevin

Previous poster is correct, the delta between the wort and the cooling coil (chiller) is what matters here. Increasing water flow rate will decrease the chiller temp, as you pointed out, thus increasing the delta and increasing heat transfer. Faster water flow rate will without a doubt chill the wort faster, all else constant.