stratification

[Jimboy]

Does stratification in a closed storage container really have much benefit?
In an open system, using coils within a stationary body of storage water, I can see where the benefit would be great. When charging the storage, the cooler water at the bottom will remove more heat from the boiler water in the coil. When removing heat from storage, the hotter water at the top will increase the water temperature going into the house. In this design, using a coil in the storage allows the storage water to be undisturbed and convection will ensure that the hotter water ends up at the top.
Using two propane tanks in a closed system, the benefits are not so clear. A 125k boiler needs about 14 gpm to remove the heat from the boiler. This continuous flow through the tanks will prevent stratification. Any zone calling for heat will probably cause a 5gpm flow.
The tank that the boiler water enters first will end up being the hotter of the two. This will be ok as long as the flow is reversed to heat the house using this hotter water first.
If frequent water movement through the tanks prevents convection, and therefore stratification, what difference does it make if the tanks are mounted vertical, one above the other, or side by side?
What is the opinion of the people who know more about this stuff than I?
Jim

 

[Duetech]

Some of the plumbing diagrams I have seen hook the tanks in parallel which reduces the flow effect. Also some use large inlet pipes to decrease velocity of the water to reduce mixing. I don't have storage yet so I am just an outsider looking in but those are a coupl of things I have seen and there's a lot more out there.

 

[2.beans]

i hooked my tanks in parallel and the water enters each tank at the same temp and supplies my primary loop leaving the tanks at the same temp. the cold water leaving my tanks, return to the boiler , are also the same temps but much cooler thanks to the stratifacation which makes my boiler happy.

 

[jebatty]

Does stratification in a closed storage container really have much benefit?

One point of view. Depends on how much your hydronic system needs the hotter water that stratification can provide. If you're feeding mainly radiant floor with 120F or less water, you're mixing down anyway, and stratification is not very impt. If you need 160F+ for baseboard, then stratification may be much more impt. Similar with dhw needs, if any.

If your pressure vessel does not have much vertical height, I don't think you're going to get much stratification no matter what you do, especially at a high supply/return flow. Regardless, if stratification is the goal, then hydraulic separation is impt. Moving 14 gpm or more is going to cause a lot of turbulence in the vessel unless a real effort is made to achieve hydraulic separation. That volume of water will change a 1000 gal tank in a little over an hour. Similarly, if three zones and each is moving 5 gpm.

Cave2k's response holds the key, I think. To achieve stratifcation what is needed is large and distributed inlets/outlets, IMO. I have a 1000 gal LP tank horizontal. The tank supply/return is fed from a plate hx, about 10 gpm. My return from the tank is from a dip tube reaching near the bottom, and the supply to the tank is directly and vertically into the top, about 1' from the dip tube. It is this way because I used existing tank fittings. For me stratification is not impt and this supply/return works well. I still do achieve about 20-30F stratification.

For a horizontal tank, my thought would be a 2-4" horizontal pipe running the full length of the tank at the top and bottom, with fittings on each end (tank would be bored and pipe welded in), and each pipe having a series of holes drilled into the pipe facing up on the top pipe and down on the bottom pipe, the holes being of smaller to larger sizes to achieve fairly even distribution of hot water along the full length of the tank and ditto for the bottom pipe. The boiler would feed hot water into the tank at one end of the upper pipe and draw from the tank at one end of the bottom pipe. The system zones similarly would draw hot water from the opposite end of the upper pipe and return to the tank at the opposite end of the bottom pipe.

If the boiler was supplying hot water, and zones calling for hot water, most boiler hot water would simply flow through the pipe directly to the zones. If boiler not supplying hot water, then the zones would be drawing from the top of the tank. I think this arrangement would result in maximum stratifcation and maximum output to system.

If a vertical tank, then similar supply and return manifolds (circular might be best) in the tank, constructed similarly, but I think the supply manifold would be installed about 1/3-1/4 down from the top of the tank.

 

[Fred61]

Continuing to work on my tank.
What should a stratification baffle look like in a non pressurized cylinder? Disc a little smaller than diameter or same diameter with holes or what? Also location in tank?

 

[Nofossil]

This is experimental territory. My guess is that you want a plate near the top and bottom to deflect flow into a horizontal direction and diffuse it as much as possible. There are lots of ideas and precious little hard data that I've been able to find.

 

[Fred61]

Thanks NOFO.
I'll sleep on possible designs and pick one. I'll post my results which won't be until next heating season when it goes into service. My water to the flat plate HX will be returning through a bulkhead fitting about 4 inches off the bottom of the tank and returning to the top (of course!) of a 6 foot tall tank.. Perhaps I can find a was to minimize mixing and possibly eliminate the baffle which would be expensive because it will probably need to be teflon sheet.

 

[timerak]

Here's some interesting info. (not for a propane tank with nozzels)

(broken link removed to http://www.iea-shc.org/publications/downloads/task32-d2.pdf)

 

[Nofossil]

Here's some interesting info. (not for a propane tank with nozzels)

(broken link removed to http://www.iea-shc.org/publications/downloads/task32-d2.pdf)

VERY cool stuff - great find!