Storage Stratification - Would this work?

[WoodNotOil]

I plan to use a flat plate hx for charging and drawing from my storage tank. To maintain stratification, my current plan is to use on the tank side of the hx two pumps and two zone valves to charge and draw in either direction through the same piping. That way when charging the water will come from the bottom and return to the top and when drawing from the top and back to the bottom. (See the system piping tab on my webpage for more details.)

However, I have also been racking my brain wondering if there is perhaps another less expensive way to accomplish stratification. Here is a picture of what I came up with. There are still two pumps, one that draws from the bottom and one from the top, but they both return to the middle of the tank eliminating the need for two zone valves. Any chance this would maintain stratification? If not, how much less efficient would it be? Thanks for the help.

 

[Nofossil]

(Holding my hands over my ears to drown out the cries of outrage from the real plumbing experts....)

I wonder if you could have a single path with two pumps facing in opposite direction, without check valves? In that case, one pump would be pumping water through the other in the reverse direction. Simple - two pumps, no valves, perfect stratification.

 

[Eric Johnson]

I've thought of that, too. I'm not sure how well water would pump through the idle (and opposing) pump, but it might work just fine. A somewhat more complex alternative would be to plumb in a bypass around each pump, perhaps using a couple of zone valves wired into each pump's power supply.

 

[free75degrees]

3 way zone valves might be another option, although they are a bit pricey. They would be simpler than using two on/off zone valves.

http://www.pexsupply.com/Categories.asp?cID=700&brandid;=

 

[leaddog]

I've thought of that, too. I'm not sure how well water would pump through the idle (and opposing) pump, but it might work just fine. A somewhat more complex alternative would be to plumb in a bypass around each pump, perhaps using a couple of zone valves wired into each pump's power supply.

Instead of using zone valves use check valves on the by pass. than the water would flow thru the bypass and not the other pumps. Swing checks are quite cheap compared to zone valves plus very little resistance.
leaddog

 

[WoodNotOil]

Leaddog - Can you be more specific on how a swing check can be used in place of a zone valve. I like the idea if it would work. Here is an image of my original plan with zone valves.

 

[leaddog]

Leaddog - Can you be more specific on how a swing check can be used in place of a zone valve. I like the idea if it would work. Here is an image of my original plan with zone valves.

The pumps would be in series pumping in oposite directions with a bypass going around each pump with a check valve in it. checks should be horizonal so they would work well. I don't have a prog. to draw a pic. sorry Hope that makes sence.
leaddog

 

[free75degrees]

The pumps would be in series pumping in oposite directions with a bypass going around each pump with a check valve in it. checks should be horizonal so they would work well. I don't have a prog. to draw a pic. sorry Hope that makes sence.
leaddog[/quote]

would that cause each pump/bypass loop to create a little mini circulation where the water just gets pumped through the pump, then back through it's bypass then back through the pump again without going through the rest of the system?

 

[WoodNotOil]

I was having the same thought. I don't think it would work. Here is an image that shows the piping.

 

[Nofossil]

I think you'd get a local recirculation loop through the circ and the check valve. I think the lowest cost guaranteed solution is the 2 pump / 2 zone valve approach. However, most zone valves are designed to provide a good deal of head loss - a 1/4" orifice is not uncommon. You'd want to make sure you had a zone valve with a bit more flow.

It may be that the head loss pumping backwards through the inactive circ would actually be less. The HX has very low head loss, and there shouldn't be any restrictive plumbing anywhere else in the loop. Maybe worth a call to Taco.

 

[WoodNotOil]

I am a little confused by something, perhaps you all can help. When I talked to a tech from a heating supply website he talked me through sizing the plate and pumps:

"The Tarm Solo Plus 40 produces 140,000 BTUs and with a sending temperature of 180° and a returning tempurature of 160° water will need to travel at 15gpm through the 30 plate heat exchanger to charge the tank. . . In order to accomplish the 15gpm I will be using Grundfos 15-58 variable speed pumps probably set on the highest setting. "

Is the pump sizing the same on both sides of the plate or does this refer to just the boiler side? can smaller pumps be used on the tank side?

Nofossil - is 10-12 feet of head loss small? How much head loss does a zone valve add to all this?

 

[Nofossil]

I don't know about the amount that the zone valve would add - maybe the manufacturer has a spec for it, but I've never looked.

The flow rate calculations are somewhat straightforward, other than the unit conversion. A gallon of water weighs 8.3 lbs, and a BTU raises one gallon of water by one degree. If you have 140,000 BTU/hr and a 20 degree temperature difference, you can calculate the required flow rate:

140,000 BTU/hr = 2333 BTU/minute.

At at 20 degree temperature difference, you would need to move 2333/20 = 117 pounds per minute. At 8.3 pounds per gallon, that's 14 gallons/minute.

If you only had a 10 degree temperature difference, then you would need 28gpm.

On the tank side, you'll need to think about the temperature difference between the bottom of the tank and the boiler output. Note: This is one of the reasons why storage stratification is SO important. You'll have to assume some temperature difference between the boiler outlet and the hot side of the tank -> HX loop - you'll never get the heated output of the HX to the same as the hot inlet. If we assume a 10 degree difference, then we can look at several scenarios:

Scenario 1 - storage is at a chilly 100 degrees, Heated water out of the HX is at 170. Delta T is 70 degrees. To transfer 140,000 BTU/hr under these conditions, you'd need about 4gpm. (substitute 70 for 20 in the equations above).

As the temperature at storage bottom increases, you need higher flow rates to transfer the same amount of energy.

Scenario 2: storage bottom is now at 150 degrees. This gives us a delta T of 20 degrees, same as the boiler side. We'll need 14gpm in that case.

Now it gets ugly. You'd like to get the storage near 180 degrees. However, it's clear that you won't be able to return 160 degree water to the boiler as the storage starts to get hot. Now you'll need to have a bigger circ on the boiler side. Fortunately, and the tank bottom temperature increases, the HX is better able to provide a heated outlet temp closer to the heat input temp from the boiler. We'll start using 5 degrees for the difference. Our heated water outlet from the HX will now be considered to be 175 degrees.

Scenario 3: storage bottom at 165 degrees. Delta T is 10 degrees, requiring a 28gpm circulator.

Circs in the range of 30gpm use a lot of power and require larger plumbing. A more realistic approach might be to consider that as your storage gets hotter, your boiler will have to idle a bit. With practice, you'll have a sense of how much wood to add so that it's starting to die down as the tank gets hot. Keep in mind that 140,000 BTU/hr is the peak output. Your long-term average will be a good deal less.

With perfect stratification - no mixing, no turbulence - you could maintain a 50 or 60 degree delta T until the entire tank is filled with 170 degree water.

 

[WoodNotOil]

Thanks Nofossil - I take it that a pump capable 15gpm is needed on the tank side of the hx as well. Since it will need to range between 4-28 gpm, a 15gpm seems a good compromise.

Now you have me curious about this head pressure thing. It certainly would be cheaper to reverse the flow through the pumps, especially if the head loss is similar to running through a zone valve. I will have to research that one some more. Anyone have any ideas on that?

Also, people have suggested a lot of things, but no one has commented on my original question about returning to the middle of the tank. Can I assume no one thinks that would work? I thought it was a long shot as it would probably cause mixing.

 

[Nofossil]

Thanks Nofossil - I take it that a pump capable 15gpm is needed on the tank side of the hx as well. Since it will need to range between 4-28 gpm, a 15gpm seems a good compromise.

Now you have me curious about this head pressure thing. It certainly would be cheaper to reverse the flow through the pumps, especially if the head loss is similar to running through a zone valve. I will have to research that one some more. Anyone have any ideas on that?

Also, people have suggested a lot of things, but no one has commented on my original question about returning to the middle of the tank. Can I assume no one thinks that would work? I thought it was a long shot as it would probably cause mixing.

I'd call Taco and talk to an applications engineer about the reverse flow. I think the middle of the tank idea would cause more mixing than you'd want. Solar people have put a large vertical pipe in a stoarge tank with multiple holes along its length. The idea is to have VERY slow flow within the pipe. Water then finds its height, and will tend to come out at the right level in the tank. That's the theory, anyway. I doubt you could get that effect with the flow rates we're talking about.

 

[Dave T]

Could you quickly increase the size of the dip tubes base like from 3/4 just inside the tank to 2" to limit the volocity and possibly stir up less of the stratification? DAVE

 

[leaddog]

Also, people have suggested a lot of things, but no one has commented on my original question about returning to the middle of the tank. Can I assume no one thinks that would work? I thought it was a long shot as it would probably cause mixing.

I'd call Taco and talk to an applications engineer about the reverse flow. I think the middle of the tank idea would cause more mixing than you'd want. Solar people have put a large vertical pipe in a stoarge tank with multiple holes along its length. The idea is to have VERY slow flow within the pipe. Water then finds its height, and will tend to come out at the right level in the tank. That's the theory, anyway. I doubt you could get that effect with the flow rates we're talking about.

I don't under stand why returning the water to the middle would mix more than returning it to the bottem. If your tank was 170* at the top and 140* at the bottem returning 160* water to the middle would seem to mix less than returning 160* to the bottem as it would have to rise thru all the cooler water. Maybe I'm missing something as I was going to return my house zone to the middle also thinking that would be better. My boiler would be supplyed from the bottem and returning to the topwith 160* or higher.
leaddog

 

[deerefanatic]

Here is what I'm doing for this problem..... 1 pump and 4 fullport ballvalves with motor-turners on them.......

Pump -> tee with two valves..... One goes to top of tank, one to bottom.

return line -> tee with two valves, same as above......

So, to charge the tank, the pump comes on and the valve attached to the pump tee puts the hot water into the top of the tank, and the return line tee opens the valve attached to the bottom to draw off the cold water..........

Then when I need to draw from it, the pump tee closes the valve going to the top and opens the valve going to the bottom to dump the cold water there..... The return line tee does the opposite to return hot water to the heat loads........

Forget about zone valves.... I got 1.5" Apollo brand ball valves with 24vdc Valve turner motors for ~$23 each on eBay..... brand new....... T316 Stainless I might add.....

They don't show up every day, but if you're persistent, industrial ball valves (usually stainless) with turner motors show up on eBay every few weeks or so... Do a search for "valve actuator" for starters.........

 

[WoodNotOil]

That is another way that would work. Are those valves like the taco:
http://www.pexsupply.com/Categories.asp?cID=349&brandid;=

From what I have read these EBV's have a low head loss so they would be a good choice. I am trying to envision what you explained. Is the image below an accurate representation? I think I could use EBV in my original plan with two pumps and two valves as well. Not sure which would save the most money. Probably depends on what is available for cheap on Ebay.

 

[free75degrees]

That is another way that would work. Are those valves like the taco:
http://www.pexsupply.com/Categories.asp?cID=349&brandid;=

From what I have read these EBV's have a low head loss so they would be a good choice. I am trying to envision what you explained. Is the image below an accurate representation? I think I could use EBV in my original plan with two pumps and two valves as well. Not sure which would save the most money. Probably depends on what is available for cheap on Ebay.

Not sure what type of tubing you are going to use to go down into the tank, but if it is expensive you can get away with only 2 instead of 4 using a layout like the one I posted in this thread: https://www.hearth.com/talk/threads/18879/

 

[deerefanatic]

Ok, WoodNotOil: Yes, that is exactly what I'm doing.. Only I only have two tank fittings.... the bottom pipes share a tank tapping and the top pipes share a tapping...... Otherwise, you've got it right on.....

Yes, the valves are electrically actuated ball valves.... But not necessarily "hydronic ball valves" just stainless steel run of the mill full-port valves with motors on them.

 

[WoodNotOil]

deerefanatic - Is this image closer?

 

[deerefanatic]

Yep, that's exactly it.......

 

[Bob Rohr]

another way. use a 4 way valve with an operator. I did it with a Paxton valve and an off the shelf actuator. In one position flow goes as show, and reverses when the operator moves to the other position.

1 pump 1 motor, and a thermostat or whatever to switch the actuator.

www.paxtoncorp.com V4R1 valve of your choice, VM062 operator

I've used this assembly to reverse flow through real long radiant loops to even heat output.

hr

 

[WoodNotOil]

That is an intersting way to go. I don't see any prices. Do you know about how much the valve and actuator would cost?

 

[Bob Rohr]

lots of companies make those actuators, Johnstone Supply has them for 120 bucks or so. The 4 way valve, depending on size, go with iron body not brass, I guess 150 or so??

I'd bet the whole package would be less $$ then 4 zone valves, plus it has a much higher Cv (flow rate) through the 4 way. Even a 1 or 1-1/4 zone valve would typically have a 7.5 gpm, or 75,000 BTU/ hr flow rate.

hr