I was referring specifically to an immersion chiller, tho all the equations are related and mostly start with Newtons law of cooling; there is a reasonably good overview in an old

BYO Mr Wizard, unfortunately all in imperial numbers.

When we look at an immersion chiller there are three main components to the equation that we can control, the flow rate of the coolant, the difference between the coolant and the wort lastly the material between the two i.e. what the chiller is made of and how much surface area there is (really going to be a choice between copper and stainless not going to make as much difference as people thing, we keep hearing how crap a conductor stainless is and it is for a metal but tends to be thinner and easier to keep clean and doesnt add Cu to your wort swings and roundabouts).

So once we have an immersion chiller we can control the flow rate of the coolant and by adding a pre-chiller the difference between the wort and the coolant.

Increasing the flow rate of the coolant increases the differential uses more coolant.

Increasing the differential increases the rate of heat transfer uses more energy.

Thats where it starts to get interesting!

If the flow rate is such that the water leaving the cooler is at (well nearly) the temperature of the wort it has taken up as much heat as it can thats the effective minimum flow rate.

The last variable being the differential, at the start if your cooling water was at 20oC and the wort was at 100oC D=80oC, when the wort cool to say 30oC D falls to 10oC, by driving that back up (cooling the coolant) we can get faster heat transfer.

I last played with the equations 15 odd years ago while doing a dip Chem, just had a quick look at whats on

Wikipedia and I would have to spend a lot of hours refreshing before I could make use of the equations.

If someone wants to spend the time and construct a spreadsheet it would be quite easy to see the colorations, I do remember drawing up the graphs for how fast a

cup of coffee cools (in theory).

Mark