Not to put too fine a point on it but. . .
There's a lot more going on here than simply looking at the water-air delta-T. If you wish to make things simple, all you need to do is consider the equation for convection. It goes something like this:
q = h * A * delta-T
People are quick to point out that delta-T is "high" entering the first radiator and "low" entering the second radiator. This is overly simplistic. "High" and "low" have no meaning outside of one having a greater value than the other.
Perhaps is would be better to ask a different question.
Say you have one radiator (call is "A") that has twice the internal surface area of another (call is "B"). Now suppose that you have two of radiator "B". Is this enough information to predict the relative performance of "A" against 2 of "B" (in either series or parallel)? Heck no.
Even conducting a test whereby you measure the outlet water temperature versus the inlet water temperature is incomplete.
You could have water 1°C over ambient, but if it's only flowing at 10 gph it will not yield an acceptable chip temperature. You could have water 10°C over ambient and your chip would do fine if the flow rate was 100 gph.
Man I'm getting off track here. Anyway, if you look at the convection equation you can state that the area is the same whether radiators are in series or parallel. So it comes down to a question of "h" and "delta-T". Each of these are variables, ie they change as you travel through the radiator.
"h" is largely a function of velocity. Velocity will always be higher when you plumb in series. Delta-T is simply the water temperature minus the air temperature. Such a statement belies the complexity of calculating what delta-T will be versus total flow rate, total heat load, total air flow, etc., etc.
The gist of what I was getting at before is that you need to consider the effect of higher surface velocity in the radiator (kudos to series plumbing) vs the higher overall system flow rate (kudos to parallel plumbing). With low restriction radiators, overall flow rate won't take much of a hit from plumbing them in series. In this instance (and this is the minority of cases) plumbing in series will actually result in improved chips temperatures. With relatively high restriction in the radiator (most fall into this category) plumbing in parallel will yield enough of an overall system flow rate boost that chip temperatures will be better this way.
The conventional wisdom that parallel radiators is preferred will usually be correct, but not always. Blindly stating parallel to be better is not right. Honestly a little background testing on head loss would let you make the right call probably 95+% of the time.
Really this is no different than the argument about series versus parallel for dual systems. The "best" option depends largely on the restrictiveness of the blocks used. Highly restrictive blocks will do better in parallel while low restriction blocks may do better in series.
I suppose what boggles my mind is that people so readily acknowledge that flow velocity in blocks is paramount yet fail to recognize that the same rules apply to radiators. I guess a partial explanation is that radiator area is practically unrestricted in comparison to block area, meaning that you can compensate for lousy water or air flow with a really big radiator. This isn't the case with blocks where all power will come from a tiny area whether we like it or not.
I've said it before and I'll say it again. They only way to be 100% certain is to test the options.
Originally Posted by nn_step
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Anyway, if you know the basic principals of what is happening with rads (or anything really) in series vs parallel you should be on the way. Ladderman has his rads in series, as do I. The only way to know for sure what will perform better is to test it both ways.
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