stratification in horizontal tanks

[kuribo]

No doubt many here are using surplus propane tanks for their storage vessels. I am curious as to how well stratification develops in horizontal tanks. Anyone have a well instrumented set up who can relate their experience? I am looking to use 2 500 gal tanks side by side, plumbed in parallel, with a two pipe arrangement to feed floors and the tanks from the boiler at the same time if demanded. I plan to use large headers per Siegenthaler, diffusers on inlets/outlets, etc. to encourage stratification. Any experienced input appreciated...

 

[maple1]

My stacked 330's stratify great. No diffusers. The piping comes in pretty horizontal though.

One key is to not over-flow through storage. Which kind of depends on other system factors. Charging is done with a 15-58 on low speed, load circ is an Alpha on low DP setting. 3 of my 4 zones are throttled a bit.

 

[kuribo]

Thanks for the info. Unfortunately, I don't think I can stack this tanks so I will have to use them side by side.

Based on research I have read, it seems the following play an important role in achieving stratification:

- inflow/outflow velocites (which you mention). This is why I want to use a 2 pipe, rather than 4 pipe tank plumbing- a portion of the boiler supply can go straight to the loads rather than into the tank, reducing velocites into the tank. Also, by paralleling the tanks, each tank only sees half the flow, reducing velocities when there is no load draw.

- temp difference between the water temp into and out of the tank. The larger the temp difference, the better the stratification.
This is one reason why I want to use a "one and done" tank charging scheme- my supply at 180F, my return at 80F.

- horizontal flows into the tank.

Would appreciate hearing others experience with horizontal tanks....

 

[maple1]

I only see a 20dT on charging with mine. Well, through the boiler. Depending how depleted they are, it could be 40-60 through the tanks on lap 1. 20 on lap 2. I usually call it charged enough with not quite 2 laps through. It seems to stratify well the first lap though. Well, second too.

The biggest impact would usually come on depletion. I actually don't know what my dT is there, have never measure temp of my load return water. But I can see a dT of 40 on the tank when depleting - my low flows also making for that. I think 20 is usually considered a standard on both charging & depletion through a load, but this works for me. I am lucky though that heat is most needed on the first part of all of my zones (luck, not planning on my part). If I needed more heat at the ends of the zones, I would need to bump flows up.

When I first started this stuff, I just had a sidearm for DHW. It needed to pump for quite a while to heat the DHW tank up. Which did mess up my stratification. So - I would avoid a sidearm HX. Flat plate works very good.

 

[Woodman1]

I have a single horizontal 1000 gallon propane tank that stratifies extremely well. Also set up in 2 pipe configuration I can have 160 degree water going to my house within half an hour of starting a fire. I had it fitted with a 2" extended elbow on the top of one end that is directed upward in the tank. Then on the opposite end on the side another 2" extended elbow that is directed downwards within the tank. Plumbing to and from the boiler is all 1.5". Everything was kept close and simple to the eko 60. I use a taco 0010 circulator on speed 1 which ends up being 18 gpm. With this setup I can maintain a 20-25 degree delta through the boiler and never idle. Return protection is through a 140 degree danfoss.

 

[warno]

I have 3 250 gallon propane tanks stacked horizontal. Works great. I used headers on mine that all my supplys and returns go to. I have 9 temp sensors, 3 per tank, and one on each header.

Here's a pic.

This is during a charging cycle but it does separate out during usage.

Left column is top tank - top, middle, bottom
Middle column is middle tank - top, middle, bottom
Right column is bottom tank - top, middle, bottom
Far right 2 are supply header and return header

 

[jebatty]

Here's a link to how my 1000 gal horizontal LP tank charges during a burn with no draws on storage. Extreme stratification. Draws from the tank are from supply at the top middle of the tank and return to the bottom middle of the tank.

 

[kuribo]

I have 3 250 gallon propane tanks stacked horizontal. Works great. I used headers on mine that all my supplys and returns go to. I have 9 temp sensors, 3 per tank, and one on each header.

Here's a pic.

View attachment 237998

This is during a charging cycle but it does separate out during usage.

Left column is top tank - top, middle, bottom
Middle column is middle tank - top, middle, bottom
Right column is bottom tank - top, middle, bottom
Far right 2 are supply header and return header

View attachment 238000

Thanks! So it appears you are able to get the entire set of tanks to within 7F from top to bottom. Once you start supply zones, without input from the boiler, what sort of top, middle, bottom temps do you see?

 

[warno]

Thanks! So it appears you are able to get the entire set of tanks to within 7F from top to bottom. Once you start supply zones, without input from the boiler, what sort of top, middle, bottom temps do you see?

It ends up with all the hot at the top. I've seen 20 degrees or more difference from top top bottom. I'll try to get a pic today after work of what it looks like after running for a day. I didn't light a fire this morning so it should be separated out nicely.

 

[warno]

@kuribo here's a picture of my readouts this afternoon. No load was running at the time.

The bottom right sensor is on the bottom tank bottom but it isn't covered by insulation so the monitor reads much cooler than the water actually is. But you can see that each tank stratisfies and the whole stack does as well.

 

[kuribo]

What I am trying to find out is if you have your tanks fully charged top to bottom at say 180F and you draw from them to supply your loads, how well does the return water stay at the bottom of the tanks without mixing with the 180F water in the tank? After a few hours of drawing 180F water off the top, and returning say 80-90F water to the bottom, is the top still close to 180F and the bottom at 80-90F???

 

[Woodman1]

Yes. With boiler off and house circulator set up on demand the tank stays stratified until it is depleted

 

[warno]

Yes the top always stays the hottest. You will not likely be sending 90 degree water back to storage if you are supplying 180. That is a huge delta T. Not saying it's not possible but not likely. But yes when loads are running you are pulling from the top, hottest, and returning to the bottom, coolest. Heat will always rise.

 

[maple1]

He could return 90 if he mixes for low temp emitters or in floor.

 

[kuribo]

Yes the top always stays the hottest. You will not likely be sending 90 degree water back to storage if you are supplying 180. That is a huge delta T. Not saying it's not possible but not likely. But yes when loads are running you are pulling from the top, hottest, and returning to the bottom, coolest. Heat will always rise.

I would be using injection mixing to take that 180F water from the top of the tanks and mix it down with 80-90F water returning from my floors....

My design is based on very well stratified tanks so I am always supplying 80-90F return water from the bottom of the tanks to the boiler. Yes, I know about boiler protection- this cold water would be sent back to the tanks using a variable speed temperature set point circulator to only return enough water at 80-90F to mix the boiler loop water down to 140F....When the boiler is in start up or at the end of the burn, the circulator would slow down to not over cool the boiler return, and speed up when the boiler is putting out rated btu's....

Thanks for your reply.

 

[warno]

Ok. That makes sense. I didn't think about the lower temps of in floor heat or other emitters. But yes horizontal tanks stratisfy just fine.