Run through both coils on tank?

[Graycenphil]

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.

[inetdog]

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.

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.)

[Graycenphil]

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.

[J.P.M.]

Feed through both coils in series, bottom coil first.

[Wy_White_Wolf]

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.

WWW

[Graycenphil]

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.

[J.P.M.]

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.

WWW

Under review.

J.P.M.

[inetdog]

Under review.

J.P.M.

Consider the counterflow heat exchanger as you review.

[J.P.M.]

Consider the counterflow heat exchanger as you review.

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.

[inetdog]

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.

[J.P.M.]

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.

Understood. You're welcome.

As you probably know, the stratification of water as f(temp.) is (or can easily be made to be) a complicated subject, quite application specific as to effects and degree, and probably not manageable in a forum type setting. Small example: it is affected by, among many other things, tank wall thermal conductivity.

On an experiential/experimental basis at my home - not as part of my prior engineering practice, I've got an 80 gal. ( ~ 5 ft. of liquid col. height) single electric (4.5 kW) element. I replaced a prior thermal collector about 8.5 yrs. ago, kept the relatively new tank (less than 1yr.), repiped and instrumented the entire (new) thermal collector loop w/ several thermometers, two rotometer type flow meters, and a bunch of pressure gauges and other thermometers. Long, boring story. The collector uses potable water directly from the tank without a HX. The system runs well with no problems.

FWIW, I believe I know more about heat exchangers than I know about solar energy. Knowing that, I do not believe the currently available technology, or quality for residential heat exchangers used in solar thermal energy applications is either cost effective, practical or serviceable. Most such products are junk and cause more problems due to fouling and performance penalties than they are worth. Most homeowners are clueless about them and service personnel are only marginally better, if at all.

Anyway, as you may guess, I have not stopped my old ways of piddling with stuff to see what practical knowledge I can squeeze from such efforts. I usually wind up confirming what the textbooks say, but there are always surprises 1X/awhile.

Two things I have among several toys on the system are thermowells w/thermocouples at the top/bottom of the tank, the bottom one in the outlet to the collector about 2 pipe dia. from the tank, the top one in one of 2 anode locations, the other anode being there of course, and checked annually.

Commonly, in the morning and with usually no hot H2O draw since maybe 10 P.M. the prior evening, the undisturbed tank temp. differential - top to bottom runs about 3 - 4 deg. F.

If hot water draw does occur, the cold water from mains replacement (usually ~ 60 - 70 F. depending mostly on season), does indeed tend to stay in the bottom of the tank as put there by the dip tube as you describe. There is usually not a lot of hot H2O draw around here as you might guess, so the level of cold water makeup in the tank tends to be below the bottom foot or so of tank level. Technically speaking, that results in a lot of stratification, but it is not as linear as naturally occurring stratification with a sharp "knee" at the cold/warm interface - and again, quite low in the tank and probably below any HX coils that might be in a similar tank. I sort of confirm that "sharpness" in the interface by being able to use the collector pump to recirculate tank H2O, bypassing the collector loop altogether. Then, at the tank return (at the top), another arrangement using a 3 way valve allows me to put the return back to the tank via direct injection at a variable rate of up to about 7.4 GPM through a 1/2" orifice pointing downward, or sideways horizontally and approx. at/along the tank wall to create a circular fluid motion in the tank, or some combination of the two. In so doing, and at a throttled rate and a horizontal return, the transition from the colder added water to the much warmer "upper" water is always quite sudden, as indicated by the 10 sec. or so it takes to go from say 80 deg. F. up to, say, 110+ deg. F., indicating to me that the cold water does indeed stay low and the demarcation cold/hot does tend to stay somewhat sharp for longer than one might think if stratification is left undisturbed.

Once collector circulation begins in the A.M., the tank stratification is quickly destroyed. That's due not only to the fragility of the stratification, but for other reasons as well. For my situation, that reason is mostly due to flow rate. I suppose this may spark some discussion, but I circulate through 2 ea. 4 X 8 collectors plumbed in series. I can circulate at up to about 4.3 GPM through the collectors, but usually run at about 1.7 GPM and keep the collector fluid Reynold's # in the transition zone for what I believe are several sound/practical engineering reasons, but that's sort of off topic. Details on request.

[Graycenphil]

Thanks again for all the thought. Perhaps a dumb question: At night (when no hot water is being used or made) does the tank become more stratified, due to lack of movement? Or does it become less stratified, due to the hot water on top warming the cold water below?

Less dumb question: My tank has two temp sensor locations, upper and lower. The controller compares this to the temp of the solar array, and decides when to turn on the pump. Once I plumb in both coils, should I still use the lower sensor, as I do now with just the lower coil in operation.

[LucMan]

I suggest that you connect only to the top coil, an 8 tube array is under sized for an 80 gallon tank. A 30-40 gallon tank would be the correct match for the array you have installed. Using the top coil only will give you usable hot water at your faucets.

[Graycenphil]

Thanks Lucman. The way I have it set up, the 80 gallon tank feeds my 35 gallon electric tank. We don't use a great deal of hot water. My hope is that I can get the 80 gallon tank above the temp of the 35 gallon thermostat, and keep it there so the electric never comes on. Whether I ever reach that goal, I'm using less electricity by starting with warmer water.

[J.P.M.]

Thanks again for all the thought. Perhaps a dumb question: At night (when no hot water is being used or made) does the tank become more stratified, due to lack of movement? Or does it become less stratified, due to the hot water on top warming the cold water below?

Less dumb question: My tank has two temp sensor locations, upper and lower. The controller compares this to the temp of the solar array, and decides when to turn on the pump. Once I plumb in both coils, should I still use the lower sensor, as I do now with just the lower coil in operation.

First, and respecting LucMan's opinion while still having a largely opposite view on this particular issue, I'd keep both coil's connected. More surface area inside the tank will result in more heat transfer which is the goal.

Any additional heat in the working fluid will then transfer that additional heat to the tank via the lower coil. Without using the lower coil, that heat will be retained in the working fluid and only serve to lower the collector efficiency by raising the average temp. of the collectors. By leaving both coils connected, the worst that will happen (assuming the rest of the control system functions correctly) is more heat will wind up in the 80 gal. tank.

To your questions, which BTW are both far from dumb:

1.) When no heat is being added to or withdrawn from the tank, the water will cool by heat loss to the surroundings. The stratification will decrease a bit, somewhat in proportion to the overall temp. decrease of the tank To a very loose 1st approx. if the tank temp. drops, say by 10% of the difference between the tank water and the tank's surroundings, the difference in tank stratification top to bottom will probably decrease by about 10% or so. The stratification is mostly caused by specific gravity differences of the water at different temperatures. In a strict sense, the temp. gradient in a still tank is not linear, but follows about the same path as how the specific gravity of the fluid - in this case H2O - varies as f(temp.), and that is not quite linear in a strict sense. Also, as I mentioned in another post, the thermal conductivity of the tank wall, which is usually about an order of magnitude greater than that of water will contribute to reducing or eliminating stratification. Het will flow from the top fluid to the tank wall making that portion of the tank warmer than the lower portion of the tank. Since heat flows from warmer to colder regions, the lower portion of the tank will thus warm up and transfer its heat to the lower portion of the tank fluid, which will then rise and reduce the stratification.

In the Thermodynamic limit, if left long enough, the tank will be at a uniform temperature equal to the surroundings with no stratification.

2.) The second of your non dumb questions is one I might defer to LucMan as he likely has more practical and hands' on experience than I, which is close to invaluable. I'm quite sure he's seen more practical/actual residential situations than I have.

However, I'd think if the lower coil was the first one the fluid returning from the collector saw, which is how I'd plumb it for reasons given yesterday, and, provided there was not much actual stratification as my experience, measurement and education leads me to believe, I'd still use the lower sensor. It would seem to me that, again, a worst case would be that the tank sensor seeing a temp. that has the highest probability of being the lowest temp. would keep the pump running longer and removing more heat. As a practical matter, I'd SWAG it that the tank stratification under operating conditions will be small enough to make sensor temps. close to the same, maybe even within the controller deadband, but I'd also guess that a cold startup in the A.M. with a slightly stratified tank would benefit from using the lower sensor.

[inetdog]

... Also, as I mentioned in another post, the thermal conductivity of the tank wall, which is usually about an order of magnitude greater than that of water will contribute to reducing or eliminating stratification. ....

I would guess that varies between a straight metal tank and a glass lined tank, which is a common option in DHW units. Even if the glass lining cracks and loses its effectiveness in protecting the tank wall from corrosion it will still be able to decrease the thermal shunt across the height of the tank.