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myv65 · 07-13-2002, 01:55 PM

[sarcasm mode on]
You guys really need to brush up on your thermodynamics.
[sarcasm mode off]

Some have mentioned that some of the energy gets put into the fluid as mechanical energy. So what? What do you suppose happens to that energy eventually?

There's a very basic law of thermodynamics that goes essentially like thus:

Input + produced = output + destroyed + stored

The "produced" and "destroyed" parts of the equation don't generally apply when discussing heat transfer. These apply more to availability and entropy, a couple of other thermo concepts. "Stored" doesn't apply to steady-state operation.

For a pump, it gets energy input in the form of electricity. This input gets split into a few different forms. By far the two dominant ones are mechanical energy into the water and heat. Heat gets produced directly by motor inefficiency and friction. Most electric motors that I'm familiar with run around 75-80% efficiency. Most pumps that I'm familiar with (industrial stuff in the 100-5000 gpm range) peak between 60 and 75% efficiency. As choked as we tend to run water cooling pumps, their efficiency is even lower.

So we've got some heat produced directly. As someone already noted, submerged pumps put all of this into the water. Inline pumps get rid of much of this motor inefficiency heat through convection to air. The remainder of it is due to impeller/volute inefficiency which generates a lot of water currents that aren't going where we wish them to go. These secondary flows are basically nothing more than non-productive churning of the water. A little of this can get out as convection off the pump casing, but with the water temperature so close to ambient, it isn't much.

OK, so we're left with the "useful" work produced by the pump, namely volumetric flow rate multiplied by pressure rise over the pump. This, along with resistance, is what determines our flow. Now comes the real crux of the matter. Where does the energy go that produced this flow? Hello, it can't just build up indefinitely. It has got to get out just the same as the thermal energy from our chips. Assuming a conventional system of pump, radiator, tubing, and block, then virtually all gets out through the radiator as thermal energy exchange.

You need to remember that energy isn't so different from water or any other physical entity. It can't "just disappear". If you could draw an imaginary box that totally enclosed your cooling system, then the energy entering the box must ultimately be balanced by the energy leaving the box. Energy can come and go in various forms, but what comes in must get back out eventually. Knowing this, figure out how it can get out. For all practical purposes, all of it goes in through electricity to the pump. For all practical purposes, all of it gets out as heat transfer. Some of it directly off the pump, for inline pumps, but the majority of it through the radiator.

07-13-2002, 01:55 PM	#26
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	[sarcasm mode on] You guys really need to brush up on your thermodynamics. [sarcasm mode off] Some have mentioned that some of the energy gets put into the fluid as mechanical energy. So what? What do you suppose happens to that energy eventually? There's a very basic law of thermodynamics that goes essentially like thus: Input + produced = output + destroyed + stored The "produced" and "destroyed" parts of the equation don't generally apply when discussing heat transfer. These apply more to availability and entropy, a couple of other thermo concepts. "Stored" doesn't apply to steady-state operation. For a pump, it gets energy input in the form of electricity. This input gets split into a few different forms. By far the two dominant ones are mechanical energy into the water and heat. Heat gets produced directly by motor inefficiency and friction. Most electric motors that I'm familiar with run around 75-80% efficiency. Most pumps that I'm familiar with (industrial stuff in the 100-5000 gpm range) peak between 60 and 75% efficiency. As choked as we tend to run water cooling pumps, their efficiency is even lower. So we've got some heat produced directly. As someone already noted, submerged pumps put all of this into the water. Inline pumps get rid of much of this motor inefficiency heat through convection to air. The remainder of it is due to impeller/volute inefficiency which generates a lot of water currents that aren't going where we wish them to go. These secondary flows are basically nothing more than non-productive churning of the water. A little of this can get out as convection off the pump casing, but with the water temperature so close to ambient, it isn't much. OK, so we're left with the "useful" work produced by the pump, namely volumetric flow rate multiplied by pressure rise over the pump. This, along with resistance, is what determines our flow. Now comes the real crux of the matter. Where does the energy go that produced this flow? Hello, it can't just build up indefinitely. It has got to get out just the same as the thermal energy from our chips. Assuming a conventional system of pump, radiator, tubing, and block, then virtually all gets out through the radiator as thermal energy exchange. You need to remember that energy isn't so different from water or any other physical entity. It can't "just disappear". If you could draw an imaginary box that totally enclosed your cooling system, then the energy entering the box must ultimately be balanced by the energy leaving the box. Energy can come and go in various forms, but what comes in must get back out eventually. Knowing this, figure out how it can get out. For all practical purposes, all of it goes in through electricity to the pump. For all practical purposes, all of it gets out as heat transfer. Some of it directly off the pump, for inline pumps, but the majority of it through the radiator.