Thank you for the detailed analysis.
I do have some doubts about your suggestion that the stratification temperatures in the typical DHW tank will be no more than on the order of 5 degrees.
At least during the periods when hot water is actually being consumed the difference can be much greater. How long that high differential will be maintained is another question though.
I am working on the assumption that there is a practical reason for having a top and bottom heater in an electrical DHW tank with interlocked thermostats that do not energize the bottom element unless the top of the tank (from which hot water is drawn for use) has already reached the cutoff temperature.
The design of the cold water inlet dip tube is intended to minimize active mixing of water in the tank as hot water is being drawn off.
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Having done the heat transfer and fluid/hydraulic as well as the mechanical pressure vessel design for at least/probably several hundred of them, that's pretty much what I did.
Actually, the biggest issue with performance for this situation is not whether the entry is top or bottom, but one of heat exchange (coil) area. See the eq. below.
Using 2 coils instead of one will (approximately) double the heat transfer (Q). That's a pretty much must do. Not doing so is a waste.
As for top vs. bottom entry:
Q = U*A* LMTD
where Q = heat transferred
U = heat transfer coeff.
A = heat transfer area.
LMTD = Log Mean Temperature Difference. (see any decent heat transfer text or Google).
1.) For a well mixed tank (no temperature stratification) and the same set up, say, using 2 coils of equal size in series, top or bottom entry for the collector fluid will result in the identically same performance because the LMTD ( Log Mean Temperature Difference) will be mathematically the same for either setup.
2.) For large temp. differences say, 20 dg. C. or more between the tank and the coll. loop fluids, the LMTD will be nearly identical for top or bottom entry, and so, as a practical matter, make little to no measureable difference in the total heat transfer.
3.) As the temp. diff. between the hot and cold fluids becomes small, say, 2-3 deg. C., the LMTD's for the top entry arrangement get slightly larger than the LMTD's for the bottom entry arrangement, giving the top entry a slight thermal advantage, but only for rather large tank fluid temp. stratifications of say ~ 5 deg. or so. That's a pretty large stratification for a practical DHW tank. This is an advantage in theory for the top entry, but it's highly unlikely to be more than something like a few % for that high level of stratification and less likely for situations commonly encountered, with the limit being the well mixed tank having no advantage for either arrangement. The top entry arrangement may, in theory anyway, have the slight advantage of increasing stratification, but my guess is that's more theory than practical, and anyway will come at the expense of less heat transfer due to lower top tank LMTD. Probably/mostly secondary effects and not worth worrying about.
4.) The (practical) advantage of a bottom entry is that, aside from the sort of backhanded way of looking at it as no real/practical disadvantage, if tank stratification does exist, such as in the A.M. assuming little/no overnight disruption of any tank stratification, the collector loop will be able to fire up sooner and operate more effectively through a larger temp. diff. between the fluid at the bottom of the tank and the higher collector loop fluid temp., which temp. can now be lower. The same situation may well be true in late afternoon, but probably less likely due to hot H2O draw during the day that will tend to upset the tank fluid stratification (increase mixing) This added gain is probably/usually enough to offset any small gain from top entry LMTD advantage for mid day operation, especially if a tendency toward a mixed tank is common.
So, the big advantage, if it can be called that, to bottom entry is not a better heat transfer rate due to effective temp. diff. advantage, but perhaps, at least under some common scenarios, a better daylong performance. In most practical situations either top or bottom may not really matter much. Surface area and fouling are more important in my book, but given the choice, I'd go bottom entry for a better chance of a kick on the tails of a day.
5.) As for a series or parallel arrangement for the coils, series hookup will result in slightly better heat transfer rates due to an approx. doubling of fluid velocity and an increase of probably about 20% or so in the heat transfer film coeff. inside the coil. Not a big deal because the outside coil surface heat transfer coeff. will be about an order of mag. smaller, and so will be the controlling coeff. anyway, but, every little bit helps. But the biggest advantage to series flow will be more likely that of a cleaner inside tube surface from increased fluid velocity and less resulting fouling, much like the diff. in fast and smooth running streams, fast ones running with hard(er) bottom, that is cleaner. The inside of the coil will be cleaner and transfer more heat ( actually offer less resistance to heat transfer due to a smaller/thinner fouling layer) resulting in more heat transfer due to less fouling resistance to heat transfer for more years, and so, a chance for a longer service life. The penalty for the series arrangement will be slightly higher pressure drop, but (as a guess) that being small enough through either coil so as to be a small portion of the pressure drop and probably about unmeasureable, that's not something I'd worry about. I would shoot for an increased probability of a cleaner coil interior by keeping the inside fluid velocity as high as possible.Leave a comment:
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I'm currently feeding just the bottom coil; I figured that made sense as the warm water would rise. After setting everything up, and seeing relatively warm water coming out of the tank going back to the roof I thought of using both coils.
Whay are you suggesting bottom first, or top first? The coldest water is at the bottom of the tank, is it not?
Thanks again.Leave a comment:
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I disagree. Feeding the top coil first will give you the greatest temperature differential through both coils thus allowing you to extract more BTUs in the process.
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Thanks Inetdog. I was thinking series, but I like your idea of parallel with valves even better. I will try it and see what happens.Leave a comment:
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The only reason that I see is that by running heated water through the top coil you will not be as effectively transferring heat from panels to tank in the case where you have consumed hot water and the bottom section of the tank contains the cooler makeup water.
If you feed only the bottom coil, you will start by heating the coolest water first, extracting more heat from your panels at a lower working temperature.
Connecting to top and bottom coils might allow you to heat water in the top part of the tank to a usable temperature faster than if you tried to uniformly heat all of the water in the tank.
If you put both coils in series, the hot water from the panels should enter the top of the top coil and exit from the bottom of the bottom coil.
If you put both coils in parallel (via tees or manifolds) I would include valves to allow you to direct the flow only to the top or to the bottom coil and compare your results.
Do not try to measure the heating effect by measuring the temperature only at the top of the tank (or indeed at any one height in the tank.)
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Run through both coils on tank?
Just recently installed an Oventrop evacuated panel system, 8 tubes, with an 80 gallon two coil tank. I am using the tank to feed my old electric hot water tank, so only one coil is connected to the panels.
My question: should I run the water through both coils? I'm thinking this would get more heat into the tank, and I'm not using the top coil anyway. Is there a reason not to do this?
Great forum, thanks.
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