Tom in Maine said:
If I recall correctly, that was a study done on a solar DHW system.
You might have feed water in such a case that is 40F in the winter into such a system.
This is not as significant an issue with a large wood fired system.
The paper I was referring to is quite a monster, it was 44 pages, and seems to be more of a "survey of the field" - I actually misremembered - it is chapter 6 of a book entitled "Thermal Energy Storage Systems" - seems to be one of those tomes intended to substitute for sleeping pills - lots of equations and "this author said this, and the other guy found that..." It seemed to be an effort to create a theoretical model of how the tanks worked, compared to real world results... However they appeared to be using a wide range of studies, using systems applications ranging from chilled water coolers to solar, to boilers. About the only thing shared in common was that they were all using water, or some sort of water based solution, with no phase changes involved. They said that the different systems behaved similarly, with the big difference being that the greater the ΔT, the easier it was to maintain good stratification. The other common factor appeared to be that the tanks were circulating the fluid as in a pressurized system as opposed to the "coil in still tank" method that the SolarTechnics tanks use.
They typically found 3 "zones" in a tank - the hot zone on top, cold zone on the bottom, and a "mixed" zone at the boundary - and that the narrower that mixed zone could be kept, the better the tank would perform. This was both because of the obvious idea that the narrower the mix zone, the wider the other two, and because it seemed to give a more constant temperature in the entire depth of the hot and cold zones, thus the desired zone would stay closer to the target temperature.
I would be skeptical believing that there are major savings with diffusers, etc on a wood fired system.
Btus is btus. The are more factors at play here than one might think of.
The article seemed to say the biggest design controllable factor in the way a tank was constructed that influenced the thickness of the mixed zone was the way the fluids were introduced into the tank - i.e. diffusers. It seemed more important that there was a gentle and distributed flow that discouraged mixing, than the specific design of the diffuser... The other big factor was the heat transfer in either direction through the tank walls, and thus a tank with walls of low thermal conductivity (insulation and plastic) would maintain better stratification than one with highly conductive (metal) walls - obviously this would be a negative for the popular LP tanks...
How is the DHW interfaced with two pressure tanks in series? Are you using a radiant floor or cheap hot water BB?
How frequently are you firing the tank?
They weren't going into this - just what happened inside the tank, and they generally only used one tank... However the suggestion was that it was best to get the biggest ΔT possible - thus you were better off to circulate the water around as few times as possible, so as to get more ΔT out of each pass... They were also just using a single heat change - water was pulled out of the tank, temperature changed, and then returned in most cases, which would be different than the multiple I/O coils that we like to use if we can...
I can tell you that I have significant stratification on a 345 gallon tank. But half of my heating load is DHW. My installation is only 3.75' tall.
Their data seemed to confirm what we have been saying for a while - that tall and skinny is slightly preferable to short and fat, but that there were definite limits before the increased losses due to larger surface areas exceeded the benefits from increasing the stratification. I believe they said ideal seemed to be around 2-1 height vs. width...
An educated guess is that you might pull another 10-15% out of a properly designed stratified system. But I'll bet that
cold in the bottom and hot out the top is all the stratification design you need.
Baffles and diffusers are not going to give the kind of systems we discuss here even a 10% performance bump.
10-15% more out of a system would be a BIG improvement - on the order of one or two additional hours of heating between fires at peak load - significantly worth the effort. How close to that ideal we can get with our setups can be debated, but presumably it's worth pursuing as much as we can get....
Gooserider
If someone is going to prove or disprove this theory for wood boilers, they need to do both parallel and series pressure tanks, with degree day data
and some serious thermocouple installations to sort it out.