Pressurized Storage Questions: Temperatures/Mixing?

49chevyman Posted By 49chevyman, Oct 29, 2017 at 5:55 PM

  1. 49chevyman

    49chevyman
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    Just started the system last week after a few upgrades. Things are going OK, but I am paying more attention to temperatures this season than I did last winter for my first season. The EKO 40 is burning great with strong gasification and reaching the 190F setpoint fairly quickly. I am trying to model my burns similar to what Maple1 and others are doing, with only one burn per day, or if weather is decent, maybe every other day, as long as the storage tank is hot. A typical burn this year has started with top of storage around 120 to 140F, bottom of storage around 80 to 100F. Start fire, boiler circulator kicks on at 155F, Danfoss valve seems to start letting some flow from bottom of storage around 160 to 170F or so at the top of the boiler. Top of storage tank will start heating up, but the boiler can reach the 190F setpoint after an hour or so and go into idle mode while the top of storage is only at 155F or so. Shouldn't the Danfoss valve open on up to let more storage and less boiler bypass flow through? Seems like this would let more cold water in from the bottom of storage and keep the boiler from reaching its setpoint and going into idle mode so quickly. There is a 'balancing' valve in the bypass line. It is a ball valve that appears a little more than half closed. I have not adjusted it. All I have to measure temps is a infrared point and shoot. Also, should the flow from the gasifier to storage go into the bottom or the top of the storage tank to best 'charge' the 1000 gallons of storage? Storage tank is old propane tank about 4 feet in diameter. It does seem to stratify well, with about 40F difference between top and bottom. I believe this is good for the two circulators that feed my water to air coils in my heat pump's air handlers, but maybe not so good for charging the storage tank. A picture of the system is below. Let me know if you have questions. I read a post on here about stratification versus mixing on storage, but I will be danged if I can find it now.
     

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  2. maple1

    maple1
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    Sounds like you might have to close that valve a little more.
     
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  3. 49chevyman

    49chevyman
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    Thanks Maple. I can try that. I have no idea why it is where it is. I will match mark the handle where it is now and try small moves more closed to see how it reacts. Where does your flow from the boiler enter your storage tank? Do you try to mix the tank during burns?
     
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  4. maple1

    maple1
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    Enters at top. Back to boiler from bottom. Those throttling valve/Danfoss setups have always been a bit of a mystery to me. I have Ts on my boiler supply & return tappings with tridicators in them, makes a quick visual check on supply & return temps easy. You will likely have to watch your return to boiler temp fairly close until you get it tuned in.
     
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  5. maple1

    maple1
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    And on the mixing, no not really. There is still some stratification at my end of burn, usually.
     
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  6. JohnDolz

    JohnDolz
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    Something to possibly think abut is the pipe size going to and from storage (and potentially the pump/circulator size). Not too long ago I switched out a 35kw boiler for a 60kw. When the 35kw was installed 1.25" piping was used, the manufacturer recommended 1.5" for the 60kw. To be safe I retrofitted as much 1.5" pipe as possible. The concern was that heat could not be removed from the boiler fast enough. Assuming your Danfoss is opening fully, moving the water/heat might be a place to look. Good luck.
     
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  7. Bob Rohr

    Bob Rohr
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    Piping look good, hopefully the air purger is in the main supply piping, not above the line like the drawing shows?

    1-1/4 if plenty for that boiler it puts out around 110,000 BTU/ hour if burning 80% efficient, optimistic efficiency :) So 11- 12gpm is easily moved in 1-1/4". Keep trying to throttle the dances bypass.
     
  8. jebatty

    jebatty
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    Having good data has been very helpful to me in solving boiler problems. I originally used the small milk thermometers for getting that data. They are very inexpensive. I used a couple of cable ties to secure the probe to the pipe and then wrapped with insulation. In your diagram, a probe on the boiler supply, on the return before the Danfoss, and on the return after the Danfoss, would give good data. Also would quickly show the effect of adjusting the balancing valve.

    Of concern to me is the 250' of piping between the boiler and the storage tank. That's a lot of pipe and pump head/pressure drop and low gpm might be the cause of restricted flow which is not allowing all the btus produced by the Eko to move to storage. What size pipe? What make/model is P1? What is your estimated flow rate and how did you compute it? Pipe of 1" ID will have a pump head of about 21' at 10 gpm on a straight run, no elbows, etc, and greater if elbows, etc. 10 gpm will move 100,000 btus at a delta-T of 20. The Eko 40 is rated at 137,000 btus, and at high burn you may need to move as much as 14 gpm at delta-T of 20. Pump head rises rapidly as flow rate exceeds 10 gpm.
     
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  9. maple1

    maple1
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    250'? Where did I miss that? That would indeed require some careful considerations to make sure you have flow for that distance.

    One more on mixing/stratification. If you are charging pretty well to full, stratification is much more important on the depletion side (when drawing from storage) than when charging - so mixing while charging isn't really a bad thing. Unless your loads need or would benefit greatly from higher supply temps while charging.
     
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  10. Fred61

    Fred61
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    One thing I found with using a ball valve for throttling flow is that flow is not linear with the position of the handle. Small adjustments in the position of the handle could have no effect on the flow or have too much of an effect.

    I changed out some ball valves for globe valves in some distribution lines and have had better luck controlling flow.
     
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  11. jebatty

    jebatty
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    Stratification on charging the storage tank may be very important if your flow rate is insufficient to move the high burn boiler output at delta-T of 20F and less. For example, if the delta-T is 30F, flow rate of about 9 pgm will move 140,000 btus. So, if you can supply the boiler with the colder bottom of storage water rather than higher temp mixed water, you may have more success operating your boiler without experiencing idling periods. And, if you can manage your wood loads so that the load is burning down (lower boiler btu output) as the return from storage temperature rises, you may be able to eliminate idling as well with a lower flow rate.
     
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  12. maple1

    maple1
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    My thinking has evolved to be more around laps through storage, since I started burning to storage 5 years ago. With boiler return temps being set at 140 for the first lap, two laps does me. Depending on how hard I'm burning, that puts me to somewhere between 180 & 190 at top, with dTs between 20 & 25. With the fire dying out before lap 2 is done. I still have flexibility left since my loading unit circ is a 3 speed - I only use speed 1 98% of the time. There is the odd time that my loads are using almost all my boiler supply water, when starting with cold storage and cold house that is recovering from setbacks or us being away overnight or longer. At those times, the boiler will see warmer return water from the zones, and in turn hotter supply temps. The overall boiler temps can creep up if that goes on long enough, to where supply gets to the 190-200 range. A rarity, but if it does happen that is the rare time I might switch my loading unit up a speed.
     
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  13. Bob Rohr

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    It's all about the basic hydronic formula heat transfer = 500 X flow rate X ∆T

    If you supply 160°, return 130° at 9 gpm flow looks like this 500 X 9 X (160-130)= 135,000 BTU/hr.

    500 is a doctor for water at 60° it changes slightly as temperature rises.

    So as the storage tank temperature rises, return temperature increases so does the rate of heat transfer 500 x 9 X (160-150)= 45,000 BTU/hr

    This is why it seems to take much longer to get the tank up to the final setpoint temperature, the energy transfer slows as the ∆T closes.

    The formula also helps show the misconception of running a fixed ∆T circulator, you limit heat transfer. True, you may run the pump longer, due to less heat transfer, but you limit the heat transfer on both ends.

    If a fixed speed circ can provide a 30∆ or higher with a cold load as shown above, why would you limit the systems ability to transfer energy at the boiler output ability?

    Plug in your own numbers and show me how a fixed 20∆ increases efficiency? Certainly not heat transfer efficiency, marketing spin :)
     
  14. jebatty

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    Bob, am I right that this comment is related to my prior posts? If so, and if I understand you correctly, I agree with you in theory, but I was focusing on the 250' pipe run and probable high pump head, which may be very difficult for the OP to easily deal with, as his piping option probably now is closed. Better design up front might have eliminated any problem related to inability to move high boiler output at low ::DTT because of undersized piping or undersized circulator.

    Also, if I assume 1" ID piping, 250' of pipe (+ add for fittings, etc.), 9 gpm = 18' of head which will move 135,000 btu at ::DTT=30; 14 gpm = 40' of head which will move 140,000 btu at ::DTT=20, 28 gpm = 151' of head which will move 140,000 btu at ::DTT=10, and if the sky has not been reached at this point, it soon will be. I have no direct experience with variable speed circulators, so maybe these are available to deal with the high pump head scenario in the OP's situation which exists as ::DTT closes.

    I think nearly every system likely has developing issues related to moving high btu output at low ::DTT.
     
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  15. TCaldwell

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    No to hijack the thread, what if the delta t circ is used with outdoor reset, Due to the constant running, at low water temps the delta is bound to close with a fixed speed circ, with the delta t circ it will slow down to maintain the desired delta and possibly the system could run smoother? I realize this isn’t the op’s objective but could a delta circ be of benefit for a odr system?
     
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  16. jebatty

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    I don't think this solves the OP's issue. The boiler load is the tank storage - supply to storage top and return from boiler bottom. Boiler output is independent based on the wood load and state of burn, about 140,000 btuh rated maximum. I doubt a delta circ could increase flow sufficiently to handle high output and low ::DTT as described above. What would be helpful is modulated output on the boiler, something which some boilers, like the Froling, can do to within a certain range; or a pellet boiler may be able to do to an even greater extent. But in the end, boiler output still may exceed the capacity of the load, as for example, when storage reaches 190F top to bottom.
     
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  17. TCaldwell

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    Jim, I guess I was responding to bob’s comment about market spin
     
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  18. Bob Rohr

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    Yes, I was just talking heat transfer 101 not specific to the issues. You are correct in questioning the high resistance with that 250' loop size.
     
  19. 49chevyman

    49chevyman
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    Guys - this has been extremely helpful. I believe Jim has nailed my problem. That 250 total feet of underground pex is 1" diameter on both the supply and return. The boiler circulator is a Bell and Gossett NRF-22. I have not found a curve for it yet, but shutoff is listed at 15 feet of head. BEP is listed at 10 gpm at 8 feet. Circulator is quiet when it runs, so I am guessing I am not deadheaded, but I bet it is not flowing much, and that is why I hit setpoint at the boiler instead of charging my storage tank. Not possible to change my underground piping at this time, but I could buy a new circulator. Any suggestions for a high head circulator? Does anyone know of a simple, in-line flow indicator that could be installed in the pex line to storage, so I could measure the flow directly? I would like to measure the differential pressure across the boiler circulator, but I do not have any taps.

    To answer some of the questions, the air purge is NOT located in the main line and IS as I have it drawn located off of a tee. No indications of cavitation noise in the system. When I have purged a little bit of water from the system, it is clear and bubble free. Pressure on the expansion tank was around 7.5 psig cold. I have not measured it hot yet. Makeup water is valved out. No leaks in the system and have not had to top it off last season or this yet.

    Would a loading unit like the Tekmar or Caleffi eliminate the issue with adjusting the balancing valve?
     
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  20. 49chevyman

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    According to pressure drop tables, I should hit the 15' shutoff head at around 6.5 gpm flow for 140F water and 250' of 1" pex with NO fittings. There are a couple of 1" elbows, and two ball valves in the lines.
     
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  21. JohnDolz

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    I use a Laddomat Loading Valve, have no balancing valve in my bypass loop - I assume a Caleffi or Tekmar would do the same job. Also I am going to take some credit for pointing out the flow concern, I just did it in my simpleminded way (I'm a Sales Guy, I bring no technical data to the discussion:)):).
     
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  22. jebatty

    jebatty
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    I'm not going to conclude the Danfoss is not an issue, but I do think that putting the focus on the Danfoss or balancing valve is like patching the ceiling because of a water leak when the cause of the leak is a hole in the roof. In other words, the real problem is the 250' of 1" pex. I think you solve the gpm problem first. You may find then that there is no Danfoss problem.

    1" pex actually has an ID of a little less than 7/8", which of course further increases your pump head/pressure drop. That means 45' of pump head for 250' of 1" pex at 10 gpm. And your pump head is even greater due to elbows and valves.

    You need 16 gpm to move 160,000 btuh at ::DTT=20::F. I don't think you can achieve this without a major change in your piping. You could make your system work satisfactorily at 10 gpm through careful wood load management to keep ::DTT>=30::F at maximum burn. Certainly not ideal, but workable. This would mean that as ::DTT starts to fall from 30::F, your boiler output has to also fall, that is, the burn would have to be falling from the burn rate that produces your rated output of 160,000 btuh.

    And you won't accomplish this with the NRF22. Two Taco 0013 in series, or equivalent, likely could move 10-11 gpm or maybe a little more, depending upon your actual pump head.
     
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  23. jebatty

    jebatty
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    The circulator curve for the Taco 0013.

    upload_2017-11-3_6-33-19.png
     
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  24. maple1

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    What do you mean pressure on the expansion tank? The tank should have a dry side & wet side - is that 7.5 the air or water pressure?. Did you set your precharge pressure (dry side) with an air guage to lowest system pressure, before you opened it to the system? My expansion tank came with too much air in it, I had to let some out. I think that is common. That would reduce your expansion capacity & cause your hot pressure to be higher than it would be otherwise.

    My loading unit (LK) has no balancing valve. So yes it would eliminate a balancing valve issue. That's why the Danfoss needing one has always seemed a mystery to me - have always thought if it was designed properly to work the way it sounded like it should work, it shouldn't need one.

    Not sure what kind of mind voodoo Jim has going on :) - I have reread the thread more than once & still don't see where you said you had 250' of run (could still have missed it). But good if it gets you on track.

    EDIT: OK, I guess I should have looked at the picture. Huh.
     
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  25. jebatty

    jebatty
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    I have lots of mind voodoo going on all the time. :)
     
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