Running radiators in parallel vs. series...

[Kevin]

Everyone says that running two radiators in parallel is best. The logic with this argument is that by splitting the flow in half, the water spends a longer time in the actual heatercore before it moves on and meets up with the other stream. Running them in series, however, seems just as effective to me. Water goes into the first one and cools to some point in between ambient and the water's previous temp. After entering the second heatercore, the temp then drops further below that point. "Logically," I could see how parallel seems better, but has anyone ever done any tests to prove it?
-Kevin

[jtroutma]

The one thing that you are missing is that radiators are more efficent when the deltra-T (difference in temp of the water to surrounding air) is greater. SO if you cool the water with the first radiator and then dump it into the second radiator, that second radiator wont be running at its highest possible ability in your system.

I say forget all about the "slows the water down" (thats just a perk) and think of it as you want the warmest water in your radiators as possible. That way the radiators are actually allowed to disperce the most heat from the water as possible before it returns to the blocks.

The heat disapated from the radiators is a exponental curve till you hit the radiators max output where it goes logrithmic. SO when the water temp is that much higher than the ambient air, you get the most "performance" out of your radiator.

In series, your first radiator does most of the work and your second radiator can only do half the work that the first radiator was doing (1 + 1/2 = ~1.5). In parallel, both radiators are working the same and thus twice the potential performace. (1 + 1 = ~2)

I am sure that BigBen will be in here shortly to confirm or debate my posting very soon

[Skulemate]

It also has to do with the amount of flow resistance that the pair create... the pair in parallel will have a lesser impact on the flow than they will in series (though the two splitters could certainly negate this bonus depending on what type are used... I'm not really sure). Though I believe that this is probablly just a secondary effect, and that the main performance boost comes from what jtroutma described in his post above.

[bowman1964]

well i will be the first to say .you hit the nail on the head .right on.

the delta-t is what counts and running 2 in parallel is the way to go .this can also be the quietest way too.2 rad in parallel with slow fans will remove quite a bit of heat and quite effecient too.

[Kevin]

Originally posted by jtroutma
The one thing that you are missing is that radiators are more efficent when the deltra-T (difference in temp of the water to surrounding air) is greater. SO if you cool the water with the first radiator and then dump it into the second radiator, that second radiator wont be running at its highest possible ability in your system.

I realize that. And it sounds reasonable to me... But at this point it's just theory to my knowledge. I haven't seen any tests or anything. My dual heatercore rig should be finished tonight. I don't have a mobo w/ a thermal diode in it, but i'd love to eventually re-route my hose and do tests both ways. As of now, it is running in series as things are already getting messy... more splitters and stuff would be too big of a mess.
-Kevin

[Gerwin]

I tried something similar when I build my own rad: First I had 4 copper waterpipes running in serie. My Eheim 1046 could barely make the water flow fast enough. When I changed the construction so that the 4 tubes were parallel, the flow through the system was much better. Even though I can't measure it, I have to conclude that the flow through each copper pipe was slower as well, which should increase cooling.
One thing I would never do, although some people do it: running your coolers in parallel: How on earth do you know if all coolers get enough water?

[bigben2k]

Originally posted by jtroutma
I am sure that BigBen will be in here shortly to confirm or debate my posting very soon

Roughly correct

The question was wether anyone tried it.

We went through this recently where a guy pushed the flow rate way up there, and got worse temps.

Check it out

[Kevin]

I may be missing something, but it looks like he doesn't have two raditaors. If you are speaking just as far as is more flow better, then you would be correct to say it is not necessarily (as seen in that guy's experiment). So nobody has tried parallel vs. series next to eachother. I'd like to do that some time then.
-Kevin

[bigben2k]

Yes, it was only one rad. Did you run a search? I'm pretty sure someone tried it.

[g.l.amour]

one has to also consider ergonomics. in my case where one rad is mounted on top of case, and the second at the bottom; it is nearly impossible to join them in parallel. i'm speaking of hose routing. the hose length in my system (which already is alot) would easily double. that way my eheim 1048 would definetely not be upto the task anymore. with 1 rad i was not able to cool the lot passively. now with the two rads when i'm idling (kazaa, soulseek, surfing, no folding) my temps stabilise at 47C for mobo and CPU, cpu is at 1.8V and 1660MHz. the only fan turning at that moment is PSU fan. i bet if i had a container sized case i would be able to join them parallel. but i have to work with the case i have so....

[bigben2k]

Actually, the extra hoses are not very restrictive at all. I agree though, it's a lot more tubing.

Since you would be reducing the total restriction by setting up the rads in parallel, the pump would actually work better.

If you added hoses going up and down, put a tee or Y somewhere in there to complete your loop, you'd be all set.

[bigben2k]

Here's what I mean:

[g.l.amour]

and yet another nice idea. i'll have alot of looking into it to do. waiting for my mail order water wetter (1.5 month already); to start modding for the better again.

[myv65]

Theory will only take you so far. As to the comment of the water coming out of the first radiator and into the second being cooler than in parallel, so what? An equivalent statement could be the water coming out of the second in series would be cooler than exiting the two in parallel. Hey, it must be since it's entering cooler, right?

Truth be told there is no single answer. It will depend on the rest of the setup.

In general, if the radiators are comparatively insignificant in developing head loss then running them in series should give better results. If the radiators are responsible for a lot of the total system head loss, running them in parallel will yield better results.

Again, generally speaking, radiators are fairly restrictive so you should see better results running in parallel. This will be as a result of higher overall system flow, however, as opposed to more efficient radiator performance.

Other than some odd exceptions that BillA's testing turned up, radiator performance will increase with increased flow rate. I'll guarantee you that flow rate per radiator will be higher in series than in parallel. Overall chip temperatures, however, may be worse due to lower flow (and lower convection) in the blocks.

The only way to know for a given setup is to try both.

[Kevin]

Originally posted by myv65
The only way to know for a given setup is to try both.

Absolutely. That's why I started this post, to see if anyone had actually tried it. I knew there was plenty of theory about them both already. Thanks to all for the mass amounts of educated guesses and speculation . I'm not sure, though... I'd really like to give this a go some time.

-Kevin

[jtroutma]

Well then Kevin, we all eagerly await your discoveries and details.

[airspirit]

I'm going to jump off a cliff to see if gravity affects my current bodily setup (I did gain a few pounds recently).

Radiators are more efficient with a larger dT. If you put hot water through one radiator in a 2way series, it'll cool it decently through the first and the second won't do crap because of the low dT. In parallel, you'll have more efficient use of both because the dT is at the maximum for both, and the flow rate is lower in both (more time for thermal transfer in the rad). It doesn't take a rocket scientist to see that if the theory overwhelmingly supports one choice over another that odds are the one supported by theory is most likely going to do better in practice.

Oh, and if you get them working even EQUALLY well in series, you have a seriously screwed up system, i.e. you're not blowing air over your rads when they're in parallel or something. There shouldn't be any series configuration that does even equally well if all things such as air flow across the rads and mean system water flow/pressure are the same.

Personally, I like pushing the bounds of logic and trying things that are counterintuitive, but only to a point. It doesn't take jumping off a cliff to prove to yourself that gravity works. Neither does it take lighting your car on fire to prove that it is generally combustible or eating nightshade to prove that it is poisonous. If you want to waste the time, that is your perogative and I'll look forward to seeing how your results back up real science and putting this debate to rest, but there is no sense further beating on a dead issue. The only reason someone should even consider doing it this way is for aesthetic reasons, and even that is normally a stretch since, as Ben pointed out, even the oddest configurations can be set up either way.

My $0.02.

I'd like to open up debate on why we can't use a couple spare 80mm fans and an AT power supply to make a personal helicopter. I'm sure that in the right configuration those fans will give me enough lift to fly ....

(sorry, I'm just in a b!tchy mood today)

[bigben2k]

Originally posted by airspirit
I'm going to jump off a cliff to see if gravity affects my current bodily setup (I did gain a few pounds recently).

Bad news, no matter how much you weigh, you'll fall at the same rate, LOL!

You're right on though.

When I get around to it, I'm going to try it all with a blower, instead of an axial fan. As BillA indicated, .25 in. water of pressure is optimal, but it's also the kind of pressure where axial fans deliver zero CFM.

[jtroutma]

Airspirit:

You wont fall any faster..........but you will reach terminal velocity a bit faster

[Brians256]

myv65, I'm curious about why you believe that radiators running in series could ever perform better. You really seem to know something about this subject so I feel like I'm missing something.

Bear with me as I try to describe what I'm thinking about the subject.

The rate of cooling is an inverse function: dT/dt = K/x where K is just some system-specific constant, right? This should be right because the amount of instantaneous heat removed is proportional to the delta-T. As the temperature goes down, the cooling rate decreases in proportion.

Assume that there is zero headloss, which is an impossible situation, but it can help prove the point. Then, the amount of energy removed by a radiator would be the integral of a 1/x function.

So, with two radiators in parallel, it would be like this:
Integral(t1 .. t2) of 2K/x
which is equivalent to
2 * Integral (t1 .. t2) of K/x

With two radiators in series, it would be like this:
Integral (t1 .. t2) of K/x plus Integral(t2 .. t3) of K/x

I believe that the first integral would be greater because Integral (t1 .. t2) of K/x is greater than Integral (t2 .. t3) of K/x.

Basically, the series solution would be adding an earlier portion of the heat removal curve to a later portion. The later portion is less effective, because the lower delta-T moves less heat per unit time.

Someone check my math on this, please? I'm not trying to be ultra-precise, but I believe the gist is correct.

[Brians256]

I can think of a situation where running radiators in parallel could be worse, however. Depending upon the mass flow, if one radiator is significantly worse than another, the system could run worse than if it was running them in parallel.

Taking the concept to the extreme, imagine half of the water bypassing the radiator entirely. You would end up with half of the cooling. At some point, a mismatch becomes bad enough that it overcomes the efficiency advantage of running the radiators in parallel. That would be TOTALLY system specific, however.

[myv65]

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.

[Brians256]

Whew! Thanks myv65! I knew I was missing something: the velocity effect on the transfer rate. Good job for having the patience to stick with the thread and help us out.

[airspirit]

(My gravity quip was facetious. I was trying to grasp for another absolutely ridiculous test of physics.)

This is precisely, though, the argument against running radiators in series. Flow rate is also a factor. Not only will a higher dT give you faster thermal transfer through the rad, but the time that the water mass remains in the radiator, the more heat will be removed from it. Those are really are the two pivotal factors, assuming everything else is the same, correct? I am purposefully leaving out the surface area of the rad, boundary layer issues, radiator material, and other strange issues because if they are identical in both setups, they do not affect the outcome in any appreciable way. Follow me so far?

Lets say we are running a 1/2" system. Our two radiators are both identical in interior size. It doesn't matter whether they are 1/2" or 3/8" or what for the moment. Just try to visualize these two setups for now and imagine that they have equal flow velocity and pressure. If the water is running in either configuration, it will either as an entire water mass pass through both radiators in two units of time, or it will be split in half and each half of the water mass will spend one unit of time in ONE radiator. If this was the case, then the series configuration could be consideres superior because the water is, as a mass, spending more time in the radiators, and the dT argument is quashed.

As you can see from the condition I made at the beginning making flow and pressure identical between the two, this is PURE FANTASY. This is junk science/logic/reasoning. Returning to reality, you will note that if you are running 1/2" in your system and you SPLIT the system into two parallel paths, the flow will be split. Now, if the system is ENTIRELY 1/2", the water velocity will be halved in the parallel configuration as opposed to the series configuration. With this in mind, you will see that whether the system is in parallel or series configuration, ALL of the water, regardless of path, will spend an identical amount of time in one or more radiators. Following this logic, the dT factor now will select the winner of this debate. Since the dT is maximized in both radiators in the parallel configuration, the parallel configuration is guaranteed to be most efficient REGARDLESS OF THE SIZE OF TUBING, SIZE OF RADIATORS, AND PRESSURE OR FLOW RATE OF THE WATER!!!!!!! There is NO ARGUMENT otherwise.

The only way that this can be proven otherwise is if you have two wildly dissimilar radiators. Think one Black Ice Micro and one Heater Core from a F1 racer. If you have two similar/identical radiators, you would be extremely foolish to put them in series expecting better results. It is quite easy to visualize how this process works, and it is a wonder that we are still arguing it.

BTW, has anybody thought about my helicopter idea? I have a case from an old PC I can bolt the fans to and use as a chair ... just so I don't have to use an extension cord, I'm thinking of painting my body with photosensitive paint and plugging the PSU into my ... ear. Yeah, my ear.

[Brians256]

Localized water velocity will affect the transmission of heat since the skin effect will be increased or decreased. Myv65 is just saying that the series radiator MIGHT work better if the water's skin effect is a significant component of the thermal resistance in the radiator.