The biggest factor is your delta-T calculation...it just depends on what components you select to reach those numbers. Budget is usually the single most restrictive part of designing a good watercooling loop.
What two components would make the biggest difference?
Loop size/tube thickness
And as far as budget goes, that where deciding what two components are most important NOT to skimp on quality for price.
Rads and blocks are pretty basic and you usually will not go wrong. The main reason I put these here is 1) you want to make sure you have ENOUGH radiator for the watt-load of your loop and 2) you want to stay with most major block manufacturers and ensure you are getting quality components.
Calculating surface heat loss, wall area would be the surface area of the processor?
There is a detailed write-up in the watercooling sticky that I added on how to calculate what size/how many radiators your loop will need based on TDP of the components being cooled by the loop.
From the sticky:
How many radiators (and what kind?) do I need for my loop?
When it comes to figuring out how much radiator you need for your specific loop, you have to start doing some math. I know that we all have been building a loop and thought, ‘how many and what kind of radiators do I need for this loop to stay cool like I want?’
First tip: Google is your best friend.
Finding out what the TDP or your CPU or GPU is can be as simple as doing some searches by searching for ‘i7 2600k TDP, ‘GTX 580 TDP’, or ‘AMD 6970 TDP’. Remember to account for all components…if you run a multi-card graphics setup, you need to include the TDP values for all cards in the total. For example, our i7 2600k has a stock TDP of about 95 watts at 100% load (estimated). If we have a 2x SLI setup of GTX 580’s, we are looking at about 244 watts at 100% load, per card. Total? About 583 watts in heat that these three components can potentially produce when at 100% load, simultaneously. (This also translates very closely to wattage when you need to consider a power supply for your system, but you need to account for the remaining components: motherboard, fans, hard drives, DVD drives, etc. To help calculate a full system TDP, you can use a tool like the Extreme PSU Calculator (link))
Once you have calculated your total loop TDP potential, you need to consider radiators that dissipate heat in watts depending on flow rate of your loop and fans being used and their speeds/power. For this task, we almost always refer to Skinneelabs.com/radiators (link) for all of this crucial information, graphs and comparisons.
For example, I am going to reference the XSPC RX360 radiator for this loop. Given the total TDP of 583 watts, I want to know if this single radiator is enough for my loop, or if I should consider another radiator.
Looking at this chart, we can see that the maximum amount of heat this radiator can dissipate is around 555 watts using 2800 rpm fans (very fast, very loud). You could get better results in a push/pull scenario, but that’s even louder; you may be able to live with a 15-20° delta and loud fans if you went this route. Remember the link above on Delta-T? Basically, it’s a mathematical derivative of your ambient room temperature, flow rate, heat to be dissipated (in watts) and the ability of your radiator to dissipate heat (in watts) depending on fans used. You’ll notice this chart has a listing of different fans in the upper-left corner: this determines the angle of the graph and the temperature delta on the left side of the graph. Lower fan speeds correlate to a higher delta-T as you add more heat in watts to the loop. The more heat you produce, the more important it is to remove it from the loop; and fans help accomplish this goal. If you notice the actual temperatures on the lines of the graph at the determined points (around 300 watts of load and around 555 watts), you’ll see that the fan speed allows the heat dissipation to be rather normalized. However, the further to the right (and up the graph you go), you’ll also notice that your delta-T rises. Below a 5° is incredibly good, 10° is still very good and even 15° deltas are very much the norm. If we wanted to run this loop at a 10° delta, we would need to run two of these RX360 radiators to keep the heat load in watts below 300 watts dissipated per radiator with fans of 600-2800 rpm (in a single-fan setup; push/pull would allow some leniency here…perhaps a RX360 and an RX240, instead).
Granted, TDP and determining our delta-T isn’t an exact science, but it gets us pretty close. It’s somewhat more difficult when overclocking, but again…Google should be your first place to search. However, I’ve found a couple of great calculations to help CPU overclocking and estimated TDP:
OC Wattage = TDP * ( OC MHz / Stock MHz) * ( OC Vcore / Stock Vcore )^2 (Thanks to GL on the guru3d.com forums for this info) (link) It’s a few years old, but the concepts still apply.
How do I know if I need watercooling or air cooler upgrade?
My temps seem to be hot on my CPU and/or GPU!!...I need a new cooler!
Not so fast. I’m not trying to discourage anyone from going out and contributing to the economy by spending their money on a new cooler or full-fledged watercooling loop. However, here is a simple way to test whether or not your cooler needs updating or if you simply have poor case airflow, in which a cooler upgrade would still have little effect (watercooling loop would still provide improvement, but still dependent upon ambient temps and radiator placement/airflow).
Remove the side panel of your case. Take a house or desk fan- turn on HIGH and blow air into your case. Run your benchmarks, game, Fold, encode video, etc…see what your temps are. Compare with the temps you get with your case side on, and no fan. If the temps drop 5C or more, you have a case airflow issue inside your case that should be addressed before ever replacing an air cooler with another air cooler. Watercooling might help, but depending on your desired temp ranges and budget, updating/adding fans might be the only thing you need to improve to get temps where you want them. If your temps remain the same or within 1-3C, you might have a minor airflow issue (or ambient temps are different than when you ran your baseline test without the fan). If you still are unhappy with your temps, this is when considering a cooling upgrade is a good decision.
Flow Rates- Pump/Radiator/Block flow and performance estimation tool from Martin of MartinsLiquidLab.org (link)
You'll need to download one of the linked Excel sheets to use and input your various watercooling loop components to help determine flow and performance expectations for your loop. While this is not 100% accurate or fool-proof, it does provide an excellent performance baseline of your loop in terms of flow rates and ultimately, calculating your delta-T based on radiators used, flow rates and ambient temps as well as other variables for your specific setup.
Note- links were not preserved in this copy, see WC sticky for links.