Flow rates and heat transfer hr

hr

Got to thinking about the posts regarding heat transfer and flow rates. Recently a poster here asked about the output, or lack of , from his hydronic baseboard. It was suggested that a smaller pump be added to slow the flow thus allowing the hot water to stay in the pipes longer and release more heat!

Actually the opposite is true. Looking at baseboard output tables, you will notice as the flow rate increases, so does the heat output. Same for hydronic coils in air handlers.

I dug out an article that explains this better, see the link below.

This would be the same for flow thru a boiler. There is a trade off, however. As the flow rate increases so does the head loss. Also there is a velocity issue as flow rate increases. The CDA gives guidelines for velocity limits in copper tube. Also most copper boilers have a 7-8 FPS limit to prevent erosion of the tubes.

Chapter 6 in “Modern Hydronic Heating” goes into this in more detail. It gets into turbulent and laminar flows and Renyolds numbers that indicate this. Interesting reading if you have the time.

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Duncan_2

I think of it this way.

When I think about stuff like this, I consider the two extremes.

One extreme would be pumping a zillion gallons a minute of 180° water through the base board.

The baseboard would be essentially the same (hot) temperature from beginning to end.

The other extreme would have the water crawling through the pipes at such a low flow rate that the water would be room temperature by the time it got to the end of the baseboard.

So assuming twenty feet of baseboard at different flow rates, which will give the most heat?

Twenty feet of baseboard, every inch of whose length is at 180°?

Or twenty feet of base board whose temperature dwindles to 70° by the time it gets to the end?

In the first scenario, you have a zillion gallons of water per minute flowing, each gallon dropping temperature a zillionth of a degree. Quantity.

In the second scenario, you have one gallon of water a minute dropping 110°. Quality.

Actually, I like the latter. Smaller pump, and you're wringing out every last drop of heat produced by the boiler. Within reason, of course... the heat needs to be even throughout the living space.

I've seen some variations in flow/output of baseboard, but the above is the general idea.

Balancing temperatures by flow usually means a lot of tweaking. Maybe it's easier if you use one of those balancing valves with a tapered port opening on the ball of the ball valve. I'd be interested in hearing experiences (or experiments!) with those.

hr

One problem

There is a point, below renyolds 2300 wher the flow turns to laminar. Engineers use this when sending heated or chilled fluids long distances, between buildings for example. The laminar flow with it's boundary layer would not give off much heat. Your latter example may be pushing this number. Siggy's software has a program that allows you to play with this equasion.

Hydronics need to be designed around turbulant flow.

On the other end of your example the velocity of that type of flow would not be workable.

At least that is how I understand it. Any engineers care to weigh in on this?

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PJO

I agree...

Although it is pretty rare to have true laminar flow in an enclosed pipe with decent pressure, it is possible. The other end is more likely...but not at a zillion fps :-)

Basically, I guess the argument is better for slightly higher flow when it comes to an increase in heat output. I would think that the only problem is in velocity noises and faster wear on certain components...99% of the problems are probably here, due to the American method of "bigger is better" when it comes to pumping (as well as other things!).

Not to go on a side stream, but doesn't the temperature feeding the radiation move slightly up and down anyway? Let's say you have a typical low-tech set-up...the boiler cycles on and off and while doing so, the temperature delivered also raises and lowers some...right? Therefore, the output of the radiation differs slightly though the cycle...and also, if you have baseboard, doesn't the resulting convection slow down as the air temperature differential is lowered (the cool air and heated air become closer in temperature)? This further adds to the confusion/wondering in my little (but interested) brain. I would think the difference between a little more or less flow velocity would be noticed about as much as these items...any comments?

I think I have raised more questions/comments...sorry.

Take Care, PJO

hr

Not only does

the temperature fluculate with the boiler, but the baseboards at the end of a multiple baseboard loop see lower water temperatures. So actually you would need to have more footage to provide the same output at the end of the loop boards.

Once again the Hydronics Toolkit Software has a program to analyze series string baseboard loops, and demonstrate how the varing temperature effects the entire loop.

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Mark J Strawcutter

so how do you reconcile this with

steamhead's tech topics article about the overheating boiler due to excessively high flow?

Mark

Gary Fereday

and this?



Your search - "+www.pmmag.+com/CDA/ArticleInformation/features/BNP" - did not match any documents.
thnaks to google and explorer!

hr

Mark, show me the numbers

on how you change the laws of physics, or principles of thermaldynamics. Saying it, and proving it are different things. I'm certainly not a mechanical engineer. And I don't profess to be a wizard or guru, more of a radiant jester actually

But I know folks who are When I have a principle I can't quite grasp I call them or go to the textbooks.

Installer techniques or info gathered from years in the field I'm better at, and will offer that info as "my opinion" only. The engineering side, I call in the smart guys with the degrees for backup

Show me on paper how one could remove the 007 from his hydronic baseboard system, replace it with a smaller 003, changing nothing else, and increase the bb output and I'll contest.

What say you Slant Fin factory folks here??

Just trying to understand and keep the info accurate. When I see info given out here that is completly false or impossible to work, I have to speak out. Once again "just my opinion"

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hr

bigugh

if you copy and paste that link to your browser window and click, it should take you to the article. It's working for me.

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Duncan

How do I reconcile it?

I don't. Something else was going on there. What it was, I have no idea... maybe something to do with the way the water went throught the boiler, or a messed up pump. Steamhead and I tried hashing this on out once, and I'll be damned if I could figure an answer.

Did my (first post) explanation make sense?

Duncan

Mark J Strawcutter

didn't mean to say you were wrong

I know less than anyone here - based on either experience or books. So when I read two expressions of opinion, by people whose opinions I value highly, and they seem to contradict each other, curiosity gets the better of me.

I purposely used the word "reconcile" since it implies that either side or both sides may need adjustment to bring them into agreement.

So, one way to reconcile your statement with Steamhead's article is to say he's got something wrong :-)

Mark

hr

The most important formula

Q= f times 490 Times deltaT

Q being the rate of heat transfer into or out of a stream of water

f= flow in gpm

490= constant for water

delta t = temperature change as the water passes through the HX device

Suppose a radiator with 1/2 gpm flow. 180 supply- 165 return

Q= .5 X 490 X (180-165)


Q= 3675 BTU/hr

Plug in 1 gpm( makes the math easy :)t maybe lower the delta t to 180-170 due to increased flow


Q= 1 X 490 X (180-170)



Q= 4900 BTU/hr at 1 gpm

Yes I agree adjusting flow is trickey at best. Ball valve radiant manifolds are virtually impossible to accuratly balance, in my opinion.

Robert Bean can explain equal flow and valve authority much better than anyone, I'll leave that to him.

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Mark J Strawcutter

yes, it did

and it's consistent with the gpm=btu/(500*deltaT) whereas Steamhead's article isn't. I thought perhaps there was some other missing piece of information that explained it. Perhaps something that only had an effect with grossly oversized pumps - like turbulent and laminar flows and Renyolds numbers (which I know nothing about).

Mark

Mark Eatherton1

Weighing in...

Good eye HR. I saw the same quote by the person of power that you did, and I too questioned it in my minds eye. I'd think I'd examine exactly what it was I was saying or implying before I said it or implied it:-)

I too have no hard and fast answers, and depend on proven scientific data and examples like Reynolds numbers etc. But it does make you think... The Germans have used a larger delta T (lower flow rates) successfully for years, and they've even taken it as far as reversing the direction of flow in order to equal out the hot and cold spots in the house. It makes you wonder why it is we Americans are so enamored with a 20 degree delta T...

As for Steamheads boiler, is suspect that the pressure differential across the boiler had more to do with its' inefficiency. Water is basically lazy and dumb. It knows how to be wet and how to hold heat. Given an opportunity to be lazy, it will always follow the path of least resistance. That said, when Steamies boiler was seing a big differential in pressure, the water was rushing quickly between the inlet and the outlet of a boiler that wasn't made to see any pressure differential at all, and it was bypassing the majority of the boilers wetted heat input surfaces. Probably safe to say that it had a high stack temperature with the high flow. Once the flow was slowed down, the wetted heating surfaces were allowed to contribute more to the thermal stream output of the boiler, and I'd bet a dozen Krispy Kream glazed donuts that his stack temps dropped drasticly.

I have heard of jobs that had a high velocity pump on BBR with a cast iron boiler that wouldn't put out the right amount of heat until a smaller pump was installed, or the output of the pump severely restricted.

But, then again, what do I know. I'm just another hot water plumber with lots of wild ideas...

Keep thinking around the box.

ME

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hr

The X files

that's where the answers is.

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Duncan

Hunh!

I wonder how much, what order of magnitude, the Reynolds Number affects heat transfer, in btu/hr ? Enough to make a difference, if its used in piping design, I guess.

My "analysis" was exaggerated to extremes just to help understand what's going on with delta t. When I try to understand whats happening with a varying quantity, looking at each extreme helps me understand what's going on. Exaggerated reductionism works better than calculus for me. ;-)

Just when you think you understand it, you get some other factor like Reynolds' factor (which I thought had something to do with making hats).

hr

Sounds like Steamheads

boiler was similar to a hot water heater with the dip tube removed. We all know how that effects heat transfer and output.

I don't have enough info to answer the question on Steamies job.

If the changes he made fixed the problem and didn't cause others, I'd chalk that up as a win for all involved and a kudo to Steamie for the clever sleuthing.

My post wasn't to challange Steamheads fix. It was to challange some questionable claims made by a rep in regards to baseboard output, and the changes gpm make

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