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Series versus parallel for pressurized storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by tuolumne, Jan 14, 2008.

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  1. tuolumne

    tuolumne New Member

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    I am using two 500 gallon propane tanks for thermal storage. I plan to have a hot charging line directly from the boiler (A), with a return port (B), and also to bring the house return water through the storage (C). When the house is not calling, water cycles from A to B. When the house is calling, hot water heads to the house first, and back through the tanks from C to B. When the boiler is cold, house demand circulates from C to A. If in parallel, A, B and C would be in the same locations on each tank. If in series, the locations could be the same (more piping) or maybe a mirror image. Where should I put the pipes to minimize stratification? As it stands, my scheme is as the sorry illustration shows; both tanks the same and in parallel.

    A
    C {OOOOOOO} B

    The only existing holes large enough are around A, so I'll need to add holes for most any scheme, unless I can work a bent pipe into one of the ports and attach it to a bushing.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Why would you want to reduce stratification?

    If you can take advantage of stratification, you can get much more usable heat out of the same tank(s). If I had a blank sheet of paper, I'd be inclined to plumb two tanks in series with top port connected to bottom, and with flow so that hot goes in the top of the first tank during charging, and cold goes in the bottom of the second tank during recovery:

    T1=top port, Tank 1 etc.

    Charging: Boiler -> T1(OOO)B1 -> T2(OOO)B2 -> Boiler

    Recovery: House <- T1(OOO)B1 <- T2(OOO)B2 <- House

    If my minimum usable temp is 120 and I have both tanks at 120, then I'm done. If I have stratification so that I have 160 at the top of tank 1, 120 at the bottom of tank 1 and the top of tank2, and 80 at the bottom of tank2, then I still have usable heat, even though the energy in the system is the same.
  3. tuolumne

    tuolumne New Member

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    OK, here is a parallel scheme. Boiler circulates through diverter until temp is 140, at which point the valve opens and allows tank charging of the tanks. These tanks are actually installed side by side, so the tank supply and return end up side be side on adjacent tanks. If the house calls, the house pump will take priority and the hot charging water bypasses the tanks. If the boiler is off line, the house will pull water from the "hot" side of the tank as you indicated. Does this diagram agree with your thinking?

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  4. Nofossil

    Nofossil Moderator Emeritus

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    That could work, though I'm skeptical about two pumps running together like that. I've attached a sketch that's more what I was thinking. With the EKO burning, flow goes clockwise through any combination of the mix valve, the house, and the storage. Flow through the storage is controlled by the storage zone valve. Both pumps are assumed to have check valves.

    During recovery, the storage circ runs and flow is counterclockwise through the house.

    This is pretty much what I'm running, and it works fine. The house zone valve(s) would typically be controlled by the house thermostat(s). The storage zone valve could be controlled by some combination of an aquastat on the EKO and relay logic based on the house no longer needing heat.

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  5. solarguy

    solarguy New Member

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    This all gets rather expensive but.....

    I would pipe the storage tanks in a reverse return piping fashion, this ensures equal flow
    thru both tanks. I would connect the supply from the wood boiler near the middle of the tank, the return from the boiler near the bottom of the tank. I would than have a seperate circuit that goes to the heating system to pull the hot water off the top of tank & use a modulating, temperature controlled 3 way valve on the supply off the tank to prevent premature cooling of the vessels. I would also utilize additional mixing valve controls, interlocked relays with sensors so when any zone in your home is calling for heat, the wood boiler diverts water to your heating system. When the zone is satisfied, the mixing valve diverts back to the storage tanks providing the boiler temperature is greater than the storage tank temperature by at least 10 degrees, if not the boiler just simmers. This works real real good but you also have to set up your system as outlined below to pull the whole thing together.

    I would control the whole heating system thru an outdoor boiler reset controller. If your
    existing heating system is zoned w/ circulators, get rid of them & go to zone valves
    connected to a primary/secondary piping/pumping arrangement. When using a primary/secondary piping/pumping arrangement you're not pulling 100% ambient return water thru your boiler or storage tank to cool it off because now your injecting your heat source.

    mechanical contractor/engineer<<<<<<
  6. tuolumne

    tuolumne New Member

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    I'll hijack my own thread. Thanks for the input thus far. This system is all new, so there is a lot of room for change. I have two types of heating in the house; two radiant floor zones, and three zones of wall panel radiators (Buderus). The radiators are sized at 140 degrees, and zones with multiple rooms have thermostatic mixing valves for some flexibility (boys vs. girls etc.) One radiant zone is a single large room (attic over garage) with two equal length loops. This is a wood floor, and lower temps are better. The other radiant zone is three rooms; the entry, laundry, and a loft above. The loft has a wall panel sized at 110 degrees, and the first floor rooms are both tile. The radiant is two loops, and the wall panel is a separate run. I'm not sure how well this will balance, so that zone probably needs a manifold with balancing valves. I could also feed the radiator first, then the two equal floor loops in parallel if this seems like a better idea. I would have manual balancing control at the radiator to adjust what percentage of the flow goes through the radiator.

    As you see, I am trying to run 3 different system temps (say 140, 110, and 95) and also incorporate outdoor reset with the cheapest method. I have a Taco RMB-1 from a different project that could be used to mix the system down using outdoor reset. My only question is how much piping this pump can handle. I think it's 1/25 HP. If I mixed the whole system down to 140 (or lower on a mild day) with this, I could temper the radiant zones down to their requirements. This is the cheapest method I can think of, but the wild card is the capacity of the RMB 1 pump. Also, I would have a system circulator running behind it in series. Does that help? Otherwise, if I use the RMB-1 for the radiant zones only, what is a cheap way to use outdoor reset and injection on the radiator zones?
  7. Nofossil

    Nofossil Moderator Emeritus

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    I kinda think you want to run hottest water through the radiator panels, then through the hotter radiant, then the cooler radiant. I don't think you can get by without a pump and mixing valve for each temperature, though.

    If you can pull this off, you're accomplishing a very good thing in that the return water to the storage tanks is less than 90 degrees.
  8. SteveJ

    SteveJ Member

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    Solarguy - could you provide a drawing of the system you are proposing?

    I see the injection and extraction points you suggest are consistent with a solar DHW tank but cannot see the stratification advantages in a 700gallon + storage tank using the middle extraction point and different coils - seems more costly than the single in/out setup like nofossil uses.

    What am I missing?

    Thanks,
    Steve
  9. Bob Rohr

    Bob Rohr Minister of Fire

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    Actually a tank full of 180F water will hold more BTU's then a stratified tank with 180F at the top and 120F at the bottom.

    Ideally you would have a distribution system that could operate with the coolest temperature possible. for radiant that may be a s low as 100, maybe 90F supply. 120- 140 for panel radiators.

    Then drag that 180F buffer down as low as humanly possible. This will drive up the efficiency and HX rate of the wood boiler and if solar is ever connected.

    the wider the delta t between return to a boiler, solar panel, etc al the more heat transfer and the longer the run cycles.

    all things considered the primary secondary piping I show above has the greatest potential to accomplish any and all of the desired sceniros, hands down.

    as for the perfect control logic?? it depends on what you want, how much you want to spend, and how complex you are comfortable with.

    Here is a formula to play with to determine buffer tank sizing.

    Tank volume = Vb X e X d X zone - Bl X t burn Divided by 8.33 x delta t

    In this example I used 3 cubic feet of white oak, burned at 70% efficiency. The current building load is 5000BTU/hr. The total design building load is 40,000 and a 3 hour burn time.

    Divided by 8.33 (factor for water) times tank charged to 200F minus lowest useable temperature of 120F = 390 gallons.

    8.33 X tank volume X delta T divided by total load is the formula for drawn down or coast time at full 40,000 BTU/ hr load = 6.49 hours

    Keep in mind heatloss from the tank is not included here. Insulation r- value and ambient air temperature is required to calc that..

    Thanks to John Sigenthaler PE for this formulas It's much easier with his HDS software program I showed in another post :)

    hr

    Correction 3 cubic feet of white oak, not lbs, sorry

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