storage tank pressurised or not

[john54]

I just bought a 700 gallon stainless steal milk cooler tank for hot water storage . Im not sure if i should use it like it is non pressurized set up or convert it to pressurized set up by welding the holes shut and installing the appropriate size fittings.I don't have a furnace yet i wanted to get a Garn but they are to much so Orlan, Eco burn Tarm. This is my first post. iIam going to get this project started. I am very green but would really appreciate advice on how to do this

 

[pybyr]

It's highly unlikely to be able to handle pressure safely if it is sealed up; tanks designed for pressure are thicker and have shapes designed to deal with the pressure. Someone here in the boiler room did some calculations some time in the last year and explained the forces that even modest pressure levels create within a vessel, and it is nothing you want to mess with, especially when you're dealing with hundreds of gallons of scalding hot water.

 

[North of 60]

Without pressurizing your storage you will have to have it located above the highest point ABOVE your heating system or only above the boiler if you are using a heat exchanger. Also think of freezing for storage location.
N of 60

 

[john54]

so unpressurized with this type of tank is the safest.What size fittings do you suggest for top and bottom of Tank there is a Man hole in the top and several other holes that can be welded shut 1inch with a shut off is this the right size top and bottom?

 

[pybyr]

The size of the holes really depends on a lot of other things that you need to nail down first.

1) how big is your heat load, and how large a wood boiler are you likely to put in? Reason to look at this is to see how many BTUs you are going to need to move into and out of the tank, which will then affect how much water you'll have to move in a given interval, which (if you look at some hydronic heating rules of thumb) will then point to the pipe size you'll need. Too small a pipe for a given flow rate creates too much head for pumps, and also the resulting high velocity in the pipes isn't a great thing (noise, potential erosion of the pipes) plus unless you run a coil type heat exchanger in the tank, high velocities (from small diameters) will tend to mix your tank, and destroy the thermal layering/ stratification that you want to keep.

If you simply need to go ahead and do something, it seems likely that 1.5 or 2 inch ports are likely to work, and you can always use reducing fittings (better that than have undersized ports that you can't expand)

2) are you going to do (a) a coil heat exchanger immersed in the tank or (b) an outboard heat exchanger such as a flat plate? This will/ may affect how many ports and what size

3) you may want to keep the hatch on the top of the tank in case you need to access it to check or repair something-- clamping and perhaps caulking will be easier to reverse than welding.

hope that's a helpful start

 

[flyingcow]

Without pressurizing your storage you will have to have it located above the highest point ABOVE your heating system or only above the boiler if you are using a heat exchanger. Also think of freezing for storage location.
N of 60

I probably misunderstood. But my tank is about 5 to 6ft below my boiler. Distance between boiler and tank is about 100ft. Works fine.

 

[flyingcow]

I just bought a 700 gallon stainless steal milk cooler tank for hot water storage . Im not sure if i should use it like it is non pressurized set up or convert it to pressurized set up by welding the holes shut and installing the appropriate size fittings.I don't have a furnace yet i wanted to get a Garn but they are to much so Orlan, Eco burn Tarm. This is my first post. iIam going to get this project started. I am very green but would really appreciate advice on how to do this

Not a vessel for pressurized system. Should make a nice non-pressurized tank. Be able to install coils easily.

 

[pybyr]

Without pressurizing your storage you will have to have it located above the highest point ABOVE your heating system or only above the boiler if you are using a heat exchanger. Also think of freezing for storage location.
N of 60

I probably misunderstood. But my tank is about 5 to 6ft below my boiler. Distance between boiler and tank is about 100ft. Works fine.

Height of unpressurized storage in relation to boiler would only matter if you were running the same water in both boiler and storage tank, without a heat exchanger in between.

With most boilers, you wouldn't want to do that anyway, as, under such conditions, you're likely to have (and keep) a lot of oxygen in the boiler water, leading to premature corrosion of the boiler.

Once you have some type of heat exchanger separating the boiler water from the storage tank water, vertical difference in having the tank below the boiler becomes mostly irrelevant, unless you were talking huge vertical differences.

 

[john54]

Thanks for your reply s .Heat load is a 2storie older farm house that is easy to heat 1400 sq feet and a 30x 40 12foot high slab heated floor work shop Garage that isn't built yet. Hope to start the garage this summer .I would like to use a plate heat exchanger to transfer heat to the tank.The stove that i am thinking of is the Eko 25 or 40 probably the 25 is big enough i don't know.I WOULD REALLY LIKE TO PUT THE FURNACE IN THE HOUSE SO I DON'T HAVE TO GO OUT SIDE TO PUT WOOD IN THE FURNACE. Currently i have a forced air wood oil combination the wood furnace is 25 years old and i am thinking that its time to change this The gasifier furnaces are the way to go but the project is going to be expensive

 

[WRVERMONT]

Hi John,
Your 700 gallon bulk tank can make a nice thermal storage tank. This tank would match nicely to a smaller boiler around 100,000 btu/H. If it turns out you would like to go with a larger boiler of around 150,000 btu/H, although not absolutely necessary, the system would benifit from a tank of around 1000 gallons.
Try to keep the storage located close to the largest loads. System may be expensive, but is well worth it. Good Luck in your planning. By the way, Hi Trevor. I look forward to seeing how your system is coming along later this summer.

 

[pybyr]

By the way, Hi Trevor. I look forward to seeing how your system is coming along later this summer.

Hi Peter- I need to get back to work on my storage; I sort of stalled a bit in thinking over the best way to do the inside porting in the tank so as to minimize mixing/ maintain stratification.

 

[WRVERMONT]

What about a scale model tank to study flow characteristics. I might be off the deep end on this- There are some hydrodynamics at hand here.

 

[tom in maine]

My sense is that you guys tend to over-analyze stratification in unpressurized tanks.
In a pressurized tank, it is easy enough to install enough ports to maintain the best stratification--hot out the top and return in the bottom.

In an unpressurized tank, the placement of the heat exchangers obviously plays a large roll.
Whether is it our tank or an STSS tank, you can note that the space heat exchanger goes from the top to the bottom of the tank. When you start to move heat in or out of the tank, heat moves in the right direction naturally--hot rises and cold drops to the bottom.
I believe space heat exchangers, working with a small delta T, help maintain stratification fairly well.

The DHW hx must be in the same area. The rather cold inlet temperature will drop a strata of cold water to the bottom and keep it there with minimal mixing. We had built large off peak electric hot water heaters around our Softanks. We and electric utilities spent a lot of time and money characterizing their performance. As they delivered hot water, the bottom part of the tank would reach cold water temperatures while the very top of the tank would maintain 180F for an extraordinary amount of time with almost no mixing until the tank was almost depleted of usable hot water. They worked about the same as pressure tanks did in the amount of heat delivered since we could offer rather inexpensive volume compared to pressurized tanks and not be fighting to get them in a basement.

If you are going to spend a lot of time thinking about building your own unpressurized tanks, spend it thinking about using as much foam insulation as you can (not styrofoam or EPS, but rather urethane or polyisocyanurate!) and making sure the structure is strong and will last.

There has been way too much conjecture about the heat exchanger design, that is more folkloric than thermodynamic. It is unfortunate to waste limited resources in the wrong place.
Check out the older posts on DIY tanks. Most of the hx's work very well. And remember that PEX should be 4-6x more lineal footage than copper. I would keep any PEX fittings out of the tank.

Happy Father's Day

Tom
Americansolartechnics.com

 

[pybyr]

My sense is that you guys tend to over-analyze stratification in unpressurized tanks.
In a pressurized tank, it is easy enough to install enough ports to maintain the best stratification--hot out the top and return in the bottom.

In an unpressurized tank, the placement of the heat exchangers obviously plays a large roll.
Whether is it our tank or an STSS tank, you can note that the space heat exchanger goes from the top to the bottom of the tank. When you start to move heat in or out of the tank, heat moves in the right direction naturally--hot rises and cold drops to the bottom.
I believe space heat exchangers, working with a small delta T, help maintain stratification fairly well.

The DHW hx must be in the same area. The rather cold inlet temperature will drop a strata of cold water to the bottom and keep it there with minimal mixing. We had built large off peak electric hot water heaters around our Softanks. We and electric utilities spent a lot of time and money characterizing their performance. As they delivered hot water, the bottom part of the tank would reach cold water temperatures while the very top of the tank would maintain 180F for an extraordinary amount of time with almost no mixing until the tank was almost depleted of usable hot water. They worked about the same as pressure tanks did in the amount of heat delivered since we could offer rather inexpensive volume compared to pressurized tanks and not be fighting to get them in a basement.

If you are going to spend a lot of time thinking about building your own unpressurized tanks, spend it thinking about using as much foam insulation as you can (not styrofoam or EPS, but rather urethane or polyisocyanurate!) and making sure the structure is strong and will last.

There has been way too much conjecture about the heat exchanger design, that is more folkloric than thermodynamic. It is unfortunate to waste limited resources in the wrong place.
Check out the older posts on DIY tanks. Most of the hx's work very well. And remember that PEX should be 4-6x more lineal footage than copper. I would keep any PEX fittings out of the tank.

Happy Father's Day

Tom
Americansolartechnics.com

Tom- your willingness to share your experience and knowledge here is terrific.

Your tanks, and many tanks, use the coil-in-a-tank approach for the space heat HX; in my instance, I intend to follow the lead of some others (such as WoodNotOil) and use a FlatPlate HX (5x12x70 plate) HX for the space heat HX. This will mean that there will be active flow to and from the tank to the plate HX, which is why I am trying to give particular thought to avoiding mixing/ maintaining stratification. I do intend to reverse flow on the tank side of the plate HX between loading the tank with heat and drawing heat from the tank-- so as to keep countercurrent flow heat transfer across the plate HX for maximum efficiency of heat exchange and so that hot water is being sent to the top of the tank when loading with heat, and drawn from the top of the tank when extracting heat.

If you (Tom- and/or for that matter, others with engineering horsepower and experience) think that I can worry less about stratification with my intended install, that's great-- I may be afflicted with the over-thinking that goes with doing something for the first time...

While I am asking about these design factors, my DHW coil in the tank is going to be 70 FT of corrugated 3/4 stainless (I know some say that the stainless and/ or corrugation is unnecessary, but I've already sourced it and have it on hand). It seems to me that I could arrange it in two ways- one a "spread out spiral" from the bottom of the wank up to the top (which will probably expose more of the tubing to the water surface- but the lower areas of the tank will be colder water) or I could coil it in a tighter configuration of multiple loops very close together, suspended near the top layers of water, which will tend to be and remain the hottest. I'm leaning towards the second (dense loops all near the top of the tank). Reactions/ suggestions?

Thanks & Happy Father's Day

 

[WRVERMONT]

The one advantage to using the "spread out spiral" from bottom to top for your DHW HX would be that this would promote a higher differential (stratification) between the bottom and top of the tank as cold well water first takes heat from the bottom of the tank. This would be especially good when you get to the refireing cycle because the water going back to the boiler will be cooler.

 

[john54]

really Hot thanks for the help .Would you go for the Eko 25 or the 40 with my size of storage tank.About the heat exchangers plate or coils in the tank?? If i use the plate type how big and were is the best place to mount it . If the coil how log and does it need to be.Does it go in the top and out the bottom of the storage tank. The holes in the bottom of the tank are one only

 

[North of 60]

Without pressurizing your storage you will have to have it located above the highest point ABOVE your heating system or only above the boiler if you are using a heat exchanger. Also think of freezing for storage location.
N of 60

I probably misunderstood. But my tank is about 5 to 6ft below my boiler. Distance between boiler and tank is about 100ft. Works fine.

Height of unpressurized storage in relation to boiler would only matter if you were running the same water in both boiler and storage tank, without a heat exchanger in between.

With most boilers, you wouldn't want to do that anyway, as, under such conditions, you're likely to have (and keep) a lot of oxygen in the boiler water, leading to premature corrosion of the boiler.

Once you have some type of heat exchanger separating the boiler water from the storage tank water, vertical difference in having the tank below the boiler becomes mostly irrelevant, unless you were talking huge vertical differences.

You guys meant what I knew. ;-) Got my wording mixed up. Yes a heat exchanger deletes that situation.
N of 60

 

[Floydian]

Hello John-

I think there are a few factors to figure out in deciding between an EKO 25 and 40. An accurate heat loss calculation of what you have and what your going to build should give you a good idea of what you need. If you need 60k btu/h at design temp, an EKO 25 is going to just meet that with no excess heat going to storage.(60k btu/h is about the average output of the 25 according to this site)

Another factor is what minimum temp water will your system need to deliver the necessary btu's. The slab in your future garage shouldn't need more than 110*F but what do you have in the house? Baseboards, panel radiators. (Over sizing panel rads will allow you to use much cooler-not sure about baseboards)
Knowing this will determine how much usable heat you can store in your tank.

There is certainly more to it all than this but I would want to know these things before spending many thousands of dollars.

Good luck,
Noah

 

[john54]

Noah I have a forced air wood oil they are hooked together with duct work the wood furnace is 25 years old and its time to get some thing new. I would like to keep the oil furnace for my back up using a hot water heat exchanger in the plenum for now and start to add panel rads in the house then eliminate the forced air set up. I would then get a oil fired boiler as back up. I see what you are saying about the size of the boiler . If the house uses 180 and the garage needs 120 is this something that is possible? GARAGE 1200 Sq FT HOUSE 1400 PLUS HOT WATER.