Pumps and heat

[bigben2k]

Originally posted by Dix Dogfight
BB2k
I don't know what amount of flow a 1250 produces at a 1m lift. But using the maximum flow at the maximum lift results in the maximum kinetic energy added. A sort of worst case scenario if you will.

It was only a theoretical exaple to figure out a rough estimate of the energy added to the water due to pumping.

A: about 13.5 L/min, or 810 L/h.

Ok, so how do I run these numbers?

[Skulemate]

myv65... that's not a problem with the data, but rather how I used it

[murray13]

O.K...., just jumping in here if that's ok.


Dix or myv: For any given situation a pump WILL impart a given amount of heat to the fluid just in moving it around. @ >100% efficiency. If I'm starting to get this...

[bigben2k]

Originally posted by myv65
I think more to the point is the actual motor power. Motor nameplates merely state the maximum power the motor will consume. This does not mean that they consume that amount of power irrespective of flow and/or head.

Current draw can easily be measured, with the right meter.

So if I have an Eheim 1250, pushing 100 gph, which, according to the p/q curve results in a pressure drop of 1.7 meters (2.42 psi), and let's say that I (theoretically) measure the current draw at 0.20 amps, for an AC voltage of 117, which translates in a power consumption of 23.4 Watts.

Where can I go with these numbers?

[bigben2k]

Originally posted by murray13
O.K...., just jumping in here if that's ok.


Dix or myv: For any given situation a pump WILL impart a given amount of heat to the fluid just in moving it around. @ >100% efficiency. If I'm starting to get this...

Just jumping too!

The pump will induce heat regardless of its efficiency. The efficiency will however dictate how much of the total energy (supplied by the electric co) is going to actually move water. For a centrifugal pump, that should be around 50%, according to Dave (depending on the pressure drop, of course).

[murray13]

So, the REAL question is how to tell which pump to buy?!!!

What are the critical factors someone needs to consider when looking at manuf. specs. All that they have is head, flow, (max) input power?

If I specify that I will have 200g/h flow.

Then can I just look for head vs power @ 200g/h?

And find the highest head with lowest power?

It would help if I knew how much head I needed. I guess I'm going to have to break open the system and measure it.

I know that this is over-simplifying things a lot, but isn't that what others will really want to know. (other than those of us who enjoy the physics side of things)

[murray13]

Well actually calculate it.
I have the curve for my pump and I will just measure the flow and look at the chart.

[Alchemy]

Originally posted by myv65
OK, so we're left with the "useful" work produced by the pump, namely volumetric flow rate multiplied by pressure rise over the pump.

Remember that there will also be a change in kinetic energy of the fluid if the suction and discharge lines are different sizes.

I assume you're quite familiar with the Bernoulli equation?

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.

Where else can energy enter the system if there is no CPU heat load?

Alchemy

[Alchemy]

Originally posted by bigben2k
Yes, that's correct, so you were right.

As for the energy otherwise, it appears in the system in the form of pressure (and that's the answer to myv65). If you take a balloon for example, there is work being done, in every direction, so you don't see anything move, but the work is there.

Work is the integral of a force applied over a certain distance. If there is no movement, there is no work. If I try to pick up a heavy rock but can't budge it, I'm not doing any work, no matter how much force I exert.

In our case, our rigs have different pressure points, and that's the "mechanical" energy that the pump provides, with about 70% of the power it is provided with. Everything else (30%) is attributed to (not necessarily in order):
1-EMI
2-Heat
3-Noise
4-Friction (of water against water, water against inner surface, impeller against bearing, impeller against water, etc...)

It is friction that *causes* heat and noise. They are not independent forms of energy loss.

Note the friction in water due to its own turbulence is basically nil unless you have supersonic flow.

Alchemy

[bigben2k]

Thanks Alchemy.

I realized that I was wrong (your first point) , where if there's no movement, then there's no work.

For your second point, Yes, ultimately it comes down to friction, except for the heat (#2), which I didn't state clearly, comes from the motor coils. The same is true for #3, but would also include "water noise".

So now you've got me wondering: if "the friction in water due to its own turbulence is basically nil unless you have supersonic flow", then where does the heat come from? Friction of water to inner tube/channel surface?

[myv65]

Originally posted by Alchemy
Remember that there will also be a change in kinetic energy of the fluid if the suction and discharge lines are different sizes.

I assume you're quite familiar with the Bernoulli equation?

Many folks run closed systems with the same size tubing on both the suction and discharge. Depending on where you "cut the lines" to examine the system, the average fluid velocity may be the same. I'd agree that it is not uncommon to see port sizes vary with the discharge often a little smaller than the suction. And yeah, the Bernoulli equation is about as familiar as F=ma.

Where else can energy enter the system if there is no CPU heat load?

Alchemy

In the literal sense, anywhere. There's no such thing as a perfect energy barrier. In a more practical sense, the CPU and pump are responsible for virtually all energy that you have to get out of the fluid eventually. What's comical is seeing some folks use pumps that are rated as high or higher than the energy used by a CPU. At some point people need to realize that the closer you get a fluid to ambient, the more it takes to close the gap further still. It's also possible to go over the top where a pump adds so much energy that your final CPU temperatures begin to climb. Yeah, we're talking tenths of a degree here, but when the differences are that small, what is the point of using an ever larger pump?

I got a good chuckle over your "work" comments above. People outside engineering aren't held to the definitions that we construct. Work is, after all, simply another measure of energy and it is only because someone decided to call the integral of F*d "work" that we use that definition as engineers. If Ben was an engineer, I would have expected him to say "energy" rather than "work". Us engineering geeks are the only ones really concerned about the distinctions between potential energy, kinetic energy, work, etc., etc.

I am a little confused by your final comments about friction/turbulence in water. I guess I want to separate your comment from what happens within a pump. In a pump there is a lot of wasted energy because of secondary flows that don't contribute to system flow. The best of centrifugal pumps max out well less than 100% efficient with many "pond pumps" running under 50% efficient. The excess energy put into the impeller that doesn't generate flow generates heat. This heat isn't heat transfer but rather due to non-useful "churning of the water". So in this respect, it really is the internal friction of the water. It is not, however, quite the same as the turbulence in as established flow stream. I think this is what you were getting at in your post.

[Alchemy]

Originally posted by myv65
In the literal sense, anywhere. There's no such thing as a perfect energy barrier. In a more practical sense, the CPU and pump are responsible for virtually all energy that you have to get out of the fluid eventually. What's comical is seeing some folks use pumps that are rated as high or higher than the energy used by a CPU. At some point people need to realize that the closer you get a fluid to ambient, the more it takes to close the gap further still. It's also possible to go over the top where a pump adds so much energy that your final CPU temperatures begin to climb. Yeah, we're talking tenths of a degree here, but when the differences are that small, what is the point of using an ever larger pump?

Well, if you draw a boundry condition around the entire piping system, the only sources of energy entering the system would be the pump and the chips being cooled. Every point in the system will be at a higher temperature than ambient, so there would be no heat transfer into the system at all.

Actually, now that I think about it, any parts inside the case might possibly be cooler than the air inside the case. So I guess you're right after all.

Regarding pump sizing, I've been trying to understand how the so-called "sweet spot" is possible for cooling systems. Convective heat transfer resistance *must* reach the minimum limit as the flow rate through the system is brought to infinity.

My best guess of why maximum heat tranfer does not coincide with minimum CPU temperatures is because at very high flow rates (and correspondingly high TDH) the pump duty becomes large enough to overtake the benefits of improved heat transfer.

If you pick the most important part of the system and consider the change in fluid temperature across the radiator as Q=UA(deltaT), UA is increasing very slightly as flow rate increases, but if you have to increase Q more than that to match it, you're going to end up with an even larger temperature gradient. Not good.

What do you think?

Us engineering geeks are the only ones really concerned about the distinctions between potential energy, kinetic energy, work, etc., etc.

I'm still a bit wary of calling pressure change in the system "potential energy" because it really isn't, but I'm going with it here b/c I think it's too pedantic and too complicated to call it anything else, or explain why it can be expressed in terms of height.

I assume from your posts you're a mechanical engineer?

I am a little confused by your final comments about friction/turbulence in water. I guess I want to separate your comment from what happens within a pump. In a pump there is a lot of wasted energy because of secondary flows that don't contribute to system flow. The best of centrifugal pumps max out well less than 100% efficient with many "pond pumps" running under 50% efficient. The excess energy put into the impeller that doesn't generate flow generates heat. This heat isn't heat transfer but rather due to non-useful "churning of the water". So in this respect, it really is the internal friction of the water. It is not, however, quite the same as the turbulence in as established flow stream. I think this is what you were getting at in your post.

Well, perhaps I should have said "due *entirely* to its own turbulence," that is, not turbulence created in the pump.

If you think about it, when you dead-head a centrifugal pump so that the impeller does no shaft work, you're still doing work on the fluid. It's essentially the same setup Joule used to prove the first law of thermodynamics.

On that note:

Originally posted by bigben2k
So now you've got me wondering: if "the friction in water due to its own turbulence is basically nil unless you have supersonic flow", then where does the heat come from? Friction of water to inner tube/channel surface?

Yep. Exactly. And since most plastic tubing is very smooth, the vast majority of the heat is being created at the fittings.

Alchemy

[bigben2k]

Interesting.

Since you brought up fittings, maybe you can answer something that's been bugging me for the longest time:

We all know that a 90 deg elbow is very restrictive. What we end up doing, as much as possible, is taking the 90 degree bend over a larger radius, by bending a tube.

My question: how much of a benefit is this, really?

I mean, the mass of water will still be re-directed 90 degrees, so the same amount of work is applied. I do understand that because of the sharp bend, the flow will tend to "keep going straight", which is restrictive, but isn't the same being done over a much larger radius?

[utabintarbo]

Originally posted by bigben2k
Interesting.

Since you brought up fittings, maybe you can answer something that's been bugging me for the longest time:

We all know that a 90 deg elbow is very restrictive. What we end up doing, as much as possible, is taking the 90 degree bend over a larger radius, by bending a tube.

My question: how much of a benefit is this, really?

I mean, the mass of water will still be re-directed 90 degrees, so the same amount of work is applied. I do understand that because of the sharp bend, the flow will tend to "keep going straight", which is restrictive, but isn't the same being done over a much larger radius?

Centripedal acceleration?

[myv65]

Originally posted by Alchemy
Regarding pump sizing, I've been trying to understand how the so-called "sweet spot" is possible for cooling systems. Convective heat transfer resistance *must* reach the minimum limit as the flow rate through the system is brought to infinity.

My best guess of why maximum heat tranfer does not coincide with minimum CPU temperatures is because at very high flow rates (and correspondingly high TDH) the pump duty becomes large enough to overtake the benefits of improved heat transfer.

If you pick the most important part of the system and consider the change in fluid temperature across the radiator as Q=UA(deltaT), UA is increasing very slightly as flow rate increases, but if you have to increase Q more than that to match it, you're going to end up with an even larger temperature gradient. Not good.

What do you think?

The "sweet spot" is, IMHO, largely imaginary unless one uses a ridiculously powerful pump. Within reason, more flow will yield temperatures that continue to approach ambient. As you surmise, once the power of the pump becomes a dominant source of the total energy, it'll cause the chip temperatures to begin climbing (unless you also have a ridiculously large radiator). With a very large pump, you may be better off throttling flow as pump power will drop with flow in spite of the higher TDH. I can see it now, someone is going to begin selling the "Spaceballs" system with ludicrous pump, radiator, and water block. LOL.

I assume from your posts you're a mechanical engineer?

Yes. And unlike most folks that get the degree, I have chosen not to use it as a stepping stone to other areas. The common joke at most companies that I know (at least the ones that employ engineers) is that "you must check your calculator at the door" when you transfer to any other department.

This alone does not make me any brighter than any other engineer. In fact some would argue I'm thick in the head for staying on the engineering side indefinitely. It has allowed me to continue in my field, learning new things and keeping my hands directly involved with engineering issues on a daily basis. I now manage the mechanical engineering department for a small OEM company and have ~15 people reporting to me. Fortunately, they fly pretty well on their own and I get to spend a decent part of my time thinking about designing and technical issues and not too much on "managing".

I gather you've already got an engineering degree of some fashion or another, too. I recall you said something about a "licensing exam" when you first began posting here.

[Alchemy]

Originally posted by bigben2k
Interesting.

Since you brought up fittings, maybe you can answer something that's been bugging me for the longest time:

We all know that a 90 deg elbow is very restrictive. What we end up doing, as much as possible, is taking the 90 degree bend over a larger radius, by bending a tube.

My question: how much of a benefit is this, really?

I can't find any numbers on that, despite checking three of my texts. Sorry. The best I can tell you is that Perry's Chemical Engineers Handbook says it is "significant."

I mean, the mass of water will still be re-directed 90 degrees, so the same amount of work is applied. I do understand that because of the sharp bend, the flow will tend to "keep going straight", which is restrictive, but isn't the same being done over a much larger radius?

I *think* the issue is that the fitting serves to convert some of the fluid's energy into creating a radial angular acceleration. The force required to change the fluid's direction would be equal to the velocity of the fluid squared over the radius of the bend. I *believe* that that would be integrated over the angle of 90 degrees, so the work would be less when the radius is larger.

Just a guess, though.

Alchemy

[Alchemy]

Originally posted by myv65
I gather you've already got an engineering degree of some fashion or another, too. I recall you said something about a "licensing exam" when you first began posting here.

No, just a senior in college. The licensing exam was the "Fundamentals of Engineering," the first half of qualification as a registered professional engineer. The second part I can't do until I get out in the real world and gain this strange item called "experience."

I'm looking at grad schools right now. It would probably be easy for me to get a management position once I get out, but since I'm not terribly interested in that, I'd rather get a doctorate and do consulting work.

If I could make a living hanging around boards like these and doing serious engineering consulting, I would.

Alchemy

[myv65]

Originally posted by Alchemy
If I could make a living hanging around boards like these and doing serious engineering consulting, I would.

Alchemy

I hear ya. Consulting is nice work when you can find it. You can also work for a consulting engineering firm, but then the "middle men" take a good portion of what your work is worth. I've done some independent consulting based upon my prior career. The pay was phenomenal, but required travel. That was OK prior to kids, but not my bag any longer.

I forgot about Ben's elbow question above. The gist of it is a question of momentum change vs time. A shorter radius equals a quicker change. Rapidly changing momentum in a fluid tears up energy. Same thing you said above in a slightly modified wording.