good idea? - Page 2

bigben2k · 07-23-2002, 03:35 PM

Quote:

Originally posted by Cova

I also read most of that thread as it was being made, and though I disagree with a few small points in it, I didn't feel like posting about them at the time. Anyways - which part of my idea did you "used to think that way" about, that you now think differently? The part about bigger pumps adding more heat for increased flow, or the part about friction of the water not making a difference worth worrying about?

The part about water friction not adding a significant amount of heat.

In short, since mag drive centrigugal pumps are about 70% efficient (i.e. 70% of the power supplied to them is turned into water motion), that energy is countered by the flow restrictions, and induced in the water in the form of heat. So in the case of the Eheim 1048, a 7 Watts heat source is added to the loop.

In the case of an Eheim 1250, rated for 28 (?) Watts, assuming same efficiency (probably lower though), it would add 19.6 Watts of heat to the water.

(of course there's also heat added from part of the remaining 30% of the pump energy, as motor coil heat).

Cova · 07-23-2002, 03:50 PM

You see, thats the part of that thread I don't agree 100% with.

I can't get into specific numbers so much, but I think that virtually all of the heat that the pump puts into the liquid is transfered from the motor/pump housing getting hot. As good of a insulator as our tubing is, I'd bet that the tubing and block can radiate off as much heat as the friction of the water puts in, leaving the rad to deal with heat from the motor and CPU.

They give examples in that thread of pumps running overnight with all the fans off, and the water being hot in the morning. My system does this too - water-temp in the morning usually being in the high 30's, low 40's. You ever put a temp-probe on the pump itself? My pump in the morning after running like that is usually around 55 - with my coolant running through something that hot it doesn't suprize me at all that it warms up over-night.

myv65 · 07-23-2002, 04:24 PM

I'm certain not everyone will agree with my statements. So be it. Here's an engineer's take on the situation and why BigBen2k has the right idea.

You can look at a radiator as basically a series of thermal resistances. You have to get thermal energy from the fluid to the tubing. You have to convey that thermal energy from the tubing to the fins. You have to dissipate that thermal energy from the fins to air. Along the way you run into interface resistances between the tubing and fins as well as the potential for fouling of the tubing via scale and crud in your fluid.

The only part of the picture that fluid flowrate can affect is energy exchange from the fluid to the tubing walls. Convective energy exchange is defined by the equation q = h * A * delta-T where "q" is energy, "h" is convective coefficient, "A" is surface area, and delta-T is the local temperature differential between the fluid and tubing wall. For you calculus knobs out there, we're actually talking heat flux integrated across the entire surface area where delta-T is a function of position.

"A" is fixed by our radiator. If we assume for a moment that "q" is fixed, ie no change in energy dissipation versus flow, then we recognize that h * delta-T must be a constant. "h" is a function of many things, but the dominant factor for a given fluid that determines this is the fluid's velocity. If velocity goes up, "h" goes up with it. This tells us that delta-T should drop as velocity increases.

If this is true (and it is), then higher flowrate would be better provided you could get it without adding more energy to the system. Therein lies the rub. You can't get more velocity without putting more energy into the system. So now the question becomes, "How do I find the radiator's sweet spot?"

The only practical answer is through experimentation. Qualitatively, you can describe the process, but quantitatively is another story.

"h" does not increase linearly with velocity. "h" tends more to a fractional power on the order of velocity ^ (1/2). This means that increasing velocity by a factor of ~4 is required to double the convection coefficient. Power required to generate flow doesn't go up linearly either. Unfortunately, it requires ~ 4 times the pressure to double the flow. Translation: At some point, power that you put in goes up more quickly than the benefit you gain from higher convection.

Let's also remember that flowrate * delta-T (fluid in vs fluid out of radiator) is constant for a constant energy input to the system.

In the real world, this means that at very low flow rates, the fluid leaving the radiator will be practically at ambient temperature, but must be pretty darn warm entering. This is obviously not ideal. At extremely high flow rates, it takes so much energy to push the flow around our loop that the fluid will be almost a constant temperature going through the radiator, but that constant will be well above ambient. In the "sweet spot", flow is high enough for good velocity, yet not so high that copious excess energy gets dumped into the fluid to pump it around.

The differences are pretty minor and it isn't like there's a sharp point defining the optimum flow rate. You will find that radiator dissipation is more like a gentle hill with a gradual drop in performance from its peak.

Where you'll get bit is if the radiator is simply too small or the airflow rate too low to dissipate the necessary power.

Cova,

I'll also be brutally blunt. Virtually zero of the pump's ineffiency gets released from the housing as heat. What you feel when the casing gets hot is the motor's ineffiency. I am sorry that you will no doubt find this offensive, but I'll take the concerted agreement of all those that work in the pump field over the unstudied opinion of an anonymous pump user any day.

At some point I gotta say, sorry, but that's simply the way things are. If after all the explanation in the other thread you still disagree, that is certainly your call. But if you wish to continue believing this try finding some hard data to back up the position. Other than heresay from others without solid background, you will find nothing.

bigben2k · 07-23-2002, 04:44 PM

An experiment should be able to prove this.

Swap the pump for a shaft driven one (noisy as heck, but it will prove the point). Make sure that no heat is transmitted through the motor shaft.

Alternatively, try a belt driven gettho assembly. Bearing friction aside, it should all be there.

Cova · 07-23-2002, 05:07 PM

Actually - I don't find it offensive at all, and I think that you might have interpreted what I typed earlier a bit differently than I meant it. I don't (and never have) disagreed that if my pump housing is hot that that energy is coming from the electrical motor inside it. But I don't think that the impeller/water-filled area of the pump is really that well insulated from the motor area that a significant amount of that heat is transfered to the water in the cooling system, whether the pump is inline or submerged.

It's quite possible that I'm wrong, that that friction through the cooling system is creating a noticeable amount of heat - but that just sounds wrong to me.

An easier experiment to test this (as opposed to bigben's ideas) would be to take an inline pump that is submergeable, hook it into a loop with a rad and some tubing (and hell - wrap a towel around the rad so it can't dissipate much heat at all), and run it like that for a while. Do this with the pump submerged in water that is being cooled separatly to remove any heat put out by the pump motor - keep the pump itself cool and see if the rest of that first loop heats up much/at all.

myv65 · 07-23-2002, 05:10 PM

Sounds fair enough to me. Anyone feel like doing the test?

Cova · 07-23-2002, 05:16 PM

Afraid I only own 1 pump, 1 rad, and 1 block - and all 3 of them are in use in my computer ATM.

07-23-2002, 04:24 PM	#28
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	I'm certain not everyone will agree with my statements. So be it. Here's an engineer's take on the situation and why BigBen2k has the right idea. You can look at a radiator as basically a series of thermal resistances. You have to get thermal energy from the fluid to the tubing. You have to convey that thermal energy from the tubing to the fins. You have to dissipate that thermal energy from the fins to air. Along the way you run into interface resistances between the tubing and fins as well as the potential for fouling of the tubing via scale and crud in your fluid. The only part of the picture that fluid flowrate can affect is energy exchange from the fluid to the tubing walls. Convective energy exchange is defined by the equation q = h * A * delta-T where "q" is energy, "h" is convective coefficient, "A" is surface area, and delta-T is the local temperature differential between the fluid and tubing wall. For you calculus knobs out there, we're actually talking heat flux integrated across the entire surface area where delta-T is a function of position. "A" is fixed by our radiator. If we assume for a moment that "q" is fixed, ie no change in energy dissipation versus flow, then we recognize that h * delta-T must be a constant. "h" is a function of many things, but the dominant factor for a given fluid that determines this is the fluid's velocity. If velocity goes up, "h" goes up with it. This tells us that delta-T should drop as velocity increases. If this is true (and it is), then higher flowrate would be better provided you could get it without adding more energy to the system. Therein lies the rub. You can't get more velocity without putting more energy into the system. So now the question becomes, "How do I find the radiator's sweet spot?" The only practical answer is through experimentation. Qualitatively, you can describe the process, but quantitatively is another story. "h" does not increase linearly with velocity. "h" tends more to a fractional power on the order of velocity ^ (1/2). This means that increasing velocity by a factor of ~4 is required to double the convection coefficient. Power required to generate flow doesn't go up linearly either. Unfortunately, it requires ~ 4 times the pressure to double the flow. Translation: At some point, power that you put in goes up more quickly than the benefit you gain from higher convection. Let's also remember that flowrate * delta-T (fluid in vs fluid out of radiator) is constant for a constant energy input to the system. In the real world, this means that at very low flow rates, the fluid leaving the radiator will be practically at ambient temperature, but must be pretty darn warm entering. This is obviously not ideal. At extremely high flow rates, it takes so much energy to push the flow around our loop that the fluid will be almost a constant temperature going through the radiator, but that constant will be well above ambient. In the "sweet spot", flow is high enough for good velocity, yet not so high that copious excess energy gets dumped into the fluid to pump it around. The differences are pretty minor and it isn't like there's a sharp point defining the optimum flow rate. You will find that radiator dissipation is more like a gentle hill with a gradual drop in performance from its peak. Where you'll get bit is if the radiator is simply too small or the airflow rate too low to dissipate the necessary power. Cova, I'll also be brutally blunt. Virtually zero of the pump's ineffiency gets released from the housing as heat. What you feel when the casing gets hot is the motor's ineffiency. I am sorry that you will no doubt find this offensive, but I'll take the concerted agreement of all those that work in the pump field over the unstudied opinion of an anonymous pump user any day. At some point I gotta say, sorry, but that's simply the way things are. If after all the explanation in the other thread you still disagree, that is certainly your call. But if you wish to continue believing this try finding some hard data to back up the position. Other than heresay from others without solid background, you will find nothing. Last edited by myv65; 07-23-2002 at 04:29 PM.

07-23-2002, 04:44 PM	#29
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	An experiment should be able to prove this. Swap the pump for a shaft driven one (noisy as heck, but it will prove the point). Make sure that no heat is transmitted through the motor shaft. Alternatively, try a belt driven gettho assembly. Bearing friction aside, it should all be there. __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101

07-23-2002, 03:50 PM	#27
Cova Cooling Savant Join Date: May 2002 Location: Canada Posts: 247	You see, thats the part of that thread I don't agree 100% with. I can't get into specific numbers so much, but I think that virtually all of the heat that the pump puts into the liquid is transfered from the motor/pump housing getting hot. As good of a insulator as our tubing is, I'd bet that the tubing and block can radiate off as much heat as the friction of the water puts in, leaving the rad to deal with heat from the motor and CPU. They give examples in that thread of pumps running overnight with all the fans off, and the water being hot in the morning. My system does this too - water-temp in the morning usually being in the high 30's, low 40's. You ever put a temp-probe on the pump itself? My pump in the morning after running like that is usually around 55 - with my coolant running through something that hot it doesn't suprize me at all that it warms up over-night.

07-23-2002, 05:07 PM	#30
Cova Cooling Savant Join Date: May 2002 Location: Canada Posts: 247	Actually - I don't find it offensive at all, and I think that you might have interpreted what I typed earlier a bit differently than I meant it. I don't (and never have) disagreed that if my pump housing is hot that that energy is coming from the electrical motor inside it. But I don't think that the impeller/water-filled area of the pump is really that well insulated from the motor area that a significant amount of that heat is transfered to the water in the cooling system, whether the pump is inline or submerged. It's quite possible that I'm wrong, that that friction through the cooling system is creating a noticeable amount of heat - but that just sounds wrong to me. An easier experiment to test this (as opposed to bigben's ideas) would be to take an inline pump that is submergeable, hook it into a loop with a rad and some tubing (and hell - wrap a towel around the rad so it can't dissipate much heat at all), and run it like that for a while. Do this with the pump submerged in water that is being cooled separatly to remove any heat put out by the pump motor - keep the pump itself cool and see if the rest of that first loop heats up much/at all.

07-23-2002, 05:10 PM	#31
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	Sounds fair enough to me. Anyone feel like doing the test?

07-23-2002, 05:16 PM	#32
Cova Cooling Savant Join Date: May 2002 Location: Canada Posts: 247	Afraid I only own 1 pump, 1 rad, and 1 block - and all 3 of them are in use in my computer ATM.

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