PA 120.3 and MCR320 in parallel

[Cathar]

Won't slower flow make the temp delta (between incoming and outgoing water) greater ?

(sigh)

I'll pull out ye olde racetrack analogy.

If I'm a molecule of water, circulating around a water-cooling loop, I'm just going around and around and around. It's just like being a race-car on a race-track at a fixed speed. The time I take to travel through the radiator is like the time spent travelling down the main straight of the race-track.

If the race-track is 5 miles long, of which the main-straight (radiator) is 1 mile long, in the space of 1 hour, how much time per hour do I spend on the main-straight if I'm travelling at a fixed speed of 60mph, 120mph, or 180mph?

The answer is that the speed doesn't matter of course. I spent 1/5th of my time on the main straight. 'cos the straight is 1 mile long out of 5. Even if I travel at twice the speed around the circuit, it'll take me half the time to get from the start to the end of the main straight, but because I'm going twice as fast everywhere else, I'll be on the main-straight twice as often. i.e. No matter how fast I travel, I'll be on the main-straight for 12 minutes out of every hour.

Now common physics tells us that the rate of heat-exchange is proportional to the temperature difference between an object, and something else that's cooler than the object. If I'm a molecule of water, and air is cooling me (by way of the air cooling the metal tubes inside the radiator that I'm flowing through), then the longer I spend in the radiator, the cooler I will get (that's good), BUT, the cooler and closer I get to the air-temperature, the less quickly I'll lose heat (that's bad). The two cancel each other out. So why does higher flow results in better performance?

The more quickly I rush around, the more likely I'm going to be tumbled about (think white-water rapids as opposed to a smooth slowly flowing river). This means that I'm going to get tossed against the cool metal walls of the radiator more often, rather than just cruising along down the middle of the tube, only passing heat slowly to water molecules beside me that are only a little cooler than I, because they are beside another molecule, and then beside another molecule, before we get to the cold metal wall. i.e. water sucks for transferring heat if it's not getting mixed about and thrown against the cold walls.

Hope that explains it in a "simple" manner that is intuitive, obvious, and directly counters the "slower flow is better" argument.