I'm confused, Radiator Flow Rate, More not always better?

[Erasmus354]

I think they are using flow rate and delta across the radiator to come up with the heat dissipated. Then they probably use delta from the air in sensors to the average of the two water sensors.

I still struggle with the whole Q = M X CP X dT equation myself with units but I think I got it.

To simplify it further and convert it to flow rate:

Q in watts = 263.43 x (Flow rate in GPM) x (dT in C)

So you need 263 watts to raise the delta 1 degree at 1 GPM

Or if you want to solve for dT

dT in Celcius = (Q in watts)/(263.42 x (Flow rate in GPM)

So for example, I've been testing with a 590 watt heat load at 1.5 GPM, so my dT would be 1.49 degrees across the radiator.

I think I need a beer, this stuff hurts..

Oh I understand the math and physics just fine, always have. I don't find it that complicated either. Protip: Stop using imperial measurements. The calculations are much simpler if you use liters per minute. What I don't understand is whether that is actually what they are doing. A google translation of their methodology page:

This temperature difference between water input and output is only a few degrees at most at very low speed and a big power to dissipate. It is less than 1 ° C typically (flow> 2 L / min). With this method, it requires appropriate instruments to measure just 0.01 ° C or better. Indeed, every hundredth of a degree equivalent to several watts after calculation, especially where high-speed input-output gap is reduced, which reduces the sensitivity of the measure:
English version

The advantage of this method is that we know exactly the power dissipated by each radiator. There is no loss side or powers unknown variables introduced in the measuring circuit (eg a pump.). Here, everything happens only between the entry and exit of the radiator.

The method is to measure the temperature rise of water compared to air in a single loop, where we introduce a constant heating power, is another method. It is valid if we take some precautions because it introduces factors more or less unknown, but it does not require tools for high resolution. Each method has its advantages and disadvantages vis-à-vis what is sought and what was available. Either one works at constant power deltaT to find the water-air, or are working on water-air deltaT to find the constant power.

Notice how they are pointing out the need for highly accurate sensors to measure the way you describe, and they go on to mention the other way of calculating it (which is using the known heating element). They never actually state, or at least I can't tell from the translation, which of the two methods they are actually using. If they are using the second method I can see how they might have made the same mistake you did.

[Martinm210]

Thanks for this, Martin.
I'm going to have to change the calculations in my recent tests again, then. I'm quite curious if it will make a significant difference in my results.

It depends on the heat load and flow rate. A smaller heat load and small difference in flow rate may not amount to much, but it made a huge difference in my test where I ran a 600 watt heat load over a range of flow rates from .5 to 3.4 GPM. My original test was showing a 22% improvement at .5GPM vs 3.4GPM, but once corrected the higher flow rate shows a 2% improvement over the low flow rate. My results were really messed up, but all better now. I think I'll switch out one of my water sensors over the inlet side though regardless to start recording the average water instead of water out.

I'm thinking three simple points will do if the results are flat. Maybe one at .5GPM, 1.5GPM, and Max flow. This will help average out the three tests and give a good indication if there is any trends regarding flow rates.

Oh I understand the math and physics just fine, always have. I don't find it that complicated either. Protip: Stop using imperial measurements. The calculations are much simpler if you use liters per minute. What I don't understand is whether that is actually what they are doing. A google translation of their methodology page:



Notice how they are pointing out the need for highly accurate sensors to measure the way you describe, and they go on to mention the other way of calculating it (which is using the known heating element). They never actually state, or at least I can't tell from the translation, which of the two methods they are actually using. If they are using the second method I can see how they might have made the same mistake you did.

I see, yeah I'm not sure you can tell by the data presented, but I think they did it right.

My error was causing a very pronounced parabolic like curve starting high at low flows similar to what you see on a block C/W curve. Their data seems to be very different in that regard (more linear where it progresses downward) but there's not alot of number checking since they don't provide any of the data collection details...only the watts and deltas.