Storage Charging / Stratification / Efficiency

[jebatty]

Since I haven't done this for awhile, a January 1, 2012, burn is a good time to log some data and show what happens during a gasser burn while charging 1050 gallons of horizontal, pressurized storage, with boiler and insulated storage in the heated building space.

The burn started at 11:45AM, but due to my error data logging did not start until 11:55AM. Logging was every 5 minutes. 137 lbs of wood and kindling were burned. Assumed energy available at 20% MC and 400F stack temp is 6050 btu/lb. Storage start temp about 115F; storage end temp about 190F. Energy stored = (190-115) x 8.33 x 1050 = 656,000 btu's. Energy available = 137 x 6050 = 829,000. Efficiency to storage = 656,000 / 829,000 = 79%.

No draw on storage during burn (circs shut off); 6.5 hours of burn; shop interior temp maintained at 62F; approximate building heat loss (from other measurements) is about 12,000 btuh; heat from boiler burn and loss from tank to heat building = 12,000 x 6.5 = 78,000 btu's. Total efficiency = (656,000 + 78,000) / 829,000 = 89%. This might represent what could be expected in added efficiency and heat gain from locating a boiler and storage in a typical heated space rather than an out-building.

The tank has 8 sensors installed on the surface of the tank, 4 on each end of the tank at the top (R1 and L1), down 12" (R2 and L2), down 24" (R3 and L3), and at the bottom (R4 and L4). Tank sensors are DS18B20. Supply to the tank is into a horizontal port on the L end of the tank and 6" down from the top. Return from the tank is from a horizontal port on the L end of the tank and 6" up from the bottom. The tank has a minimum of R30 insulation. All plumbing has insulation.

The chart also shows stack temperature (divided by 4 to display on the same chart as the tank temperatures), as measured with a K-type sensor inserted into the center of the stack 12" above the elbow exiting from the boiler. Average stack temperature is approximately 400F.

This burn represents a very typical burn in my system, about 6 hours long. The burn will provide heat from storage to heat the building for about 48-56 hours before another burn will be needed. Typical winter burns are every other day.

I hope you find this interesting and helpful. Please ask questions. Happy New Year!

 

[PassionForFire&Water]

Hi Jim,

2 questions:

- Looking at your stack temp curve, there is a significant decrease in stack temp around 13:25 and 14:55.
Any reasons for this?
Did you open the loading door?

- Why you measure every 5 minutes and not every 1 minute?

 

[Pat53]

Interesting, thanks. If I'm reading this right, your bottom sensors are showing about 160F when the burn is done? Also, what is your average Delta T during a burn?

Pat

 

[jebatty]

Looking at your stack temp curve, there is a significant decrease in stack temp around 13:25 and 14:55. Any reasons for this? Did you open the loading door?

Wood load burned down (aspen burned this time); and reloaded wood.

Why you measure every 5 minutes and not every 1 minute?

No particular reason. 1 minute probably would have been better, and for this short period of time that would have been a good idea. When I log data, I often do it over several days, and 1 minute data points adds up to lots of data to handle in my programs, which is the reason for 5 minute data points. I used 5 minutes out of habit.

your bottom sensors are showing about 160F when the burn is done?

Yes, bottom sensors about 160F. Sensors 12" up from bottom are above 190F and have been there for awhile. The hot water column at 190+F extends below the 12" line, exactly where is unknown, but based on prior experience it is likely that the 190F temp is quite close to the bottom of the tank. Also, the bottom 12" has about 290 gal of water, as does the top 12", the middle 12" about 420 gal. With the top 24 at about 192-196F, I estimated entire tank at 190F.

Also, what is your average Delta T during a burn?

On the boiler side with the Termovar, return water would normally be between 145-155F until system return got above this point, and boiler supply normally would be about +20-25 over Termovar return. On the tank side, boiler supply would be the same, but return from tank would be close to bottom sensor temp or somewhat higher, depending on actual water temp at 6" above the bottom of the tank.

 

[woodsmaster]

I wish my insurance would have let me put mine in the shop. I have a 12 x 14 well insulated shed and it stays over 85 in there all the time. A lot of wasted heat. I do like that the mess is out of the home and shop, and I wouldn't want the extra heat in the building in the summer when I'm heating DHW . Some day I hope to make some solar panels for DHW in the summer so that would be a non issue.

 

[jebatty]

I did have a sensor on the tank return before the Termovar, and going back to the data:
-- at 11:55am, the start of data logging, return was 111F
-- at 1:25pm, 1/4 into the burn, return was 111F
-- at 2:55pm, 1/2 into the burn, return was 116F
-- at 4:25pm, 3/4 into the burn, return was 159F
-- at 6:15pm, end of burn, return was 187F (also better verifies estimated 190F average tank temperature)

 

[jebatty]

There is a "mess," but this being my shop, it is of little consequence. Also, no DHW, but if there was, would need a way to vent the heat when it is not needed in the shop. There is no perfect world, at least yet.

 

[goosegunner]

Have you calculated what your typical storage time would be if you heated the building to home temps of say 70-72 degrees?

It would be great to have those long times between burns but the time would be much shorter with home vs shop. I wonder how much storage size would have to increase?

gg

 

[jebatty]

Have you calculated what your typical storage time would be if you heated the building to home temps of say 70-72 degrees?

I haven't. For my shop that would be much too warm. The radiant, warm floor makes the shop feel very warm, even if the air temp seems low.

It would be great to have those long times between burns but the time would be much shorter with home vs shop. I wonder how much storage size would have to increase?

Storage is all about usable water temperature and heat loss. My guess is that if I set the floor to 72F, I would need to set the mixing valve to 110F, I don't know how much my heat loss would increase, but I would lose about 10F of usable water temperature, which translates to 83,000 btu's for 1000 gal of storage, which at 12000 btuh heat loss is about 7 hour loss of heating from storage. My maximum heat loss last winter was about 17,000 btuh, typical was 12-14,000 btuh.

You would have to use your heat loss and minimum usable temperature to determine how much storage you would need to arrive at the time you would need between boiler firings.

 

[mole]

Jim, I have to tell you first that I'm a big fan of yours. I truly enjoy the quantity and quality of data you contribute. I recently bought the scale you posted so I can weigh my wood and calculate the efficiency on my seton. I haven't seen any real field data on seton efficiency, but I suspect it's a bit lower than the more popular two stage boilers. Anyway, I notice you use 6050 available btu/lb of wood at 20% moisture and 400F stack temp. I'm wondering where you derive the 400deg stack temp qualifier from and how the available btus/lb may differ at 450 or 475f which is more in line with my normal operating temps.
Thx,
JR

 

[jebatty]

Thank you. I rarely turn down flattery. Here is the link to my source. Energy in Wood I suspect there may be some wiggle room and other sources may have somewhat different numbers, but this article and method seemed to me to be very scientific and logical. I don't think the article gave an easy method to determine energy at other stack temps. Since this article I have adjusted my Tarm to more closely come to 400F, but this always seems to be a moving target. I use the 6050 and 400 consistently to keep my comments and these variable consistent.

 

[ewdudley]

I'm wondering ... how the available btus/lb may differ at 450 or 475f which is more in line with my normal operating temps.

It's about a 1% difference in the amount of heat going up the flue per 35 degF / 20 kelvin difference in flue temperature. If there's excess combustion air then it would be a greater loss because more air is being heated per unit of fuel burned.

So if the number is 6050 available btu per lb at 400 degF stack temperature, then you could expect about 97.857% of 6050 btu per pound at 475 degF stack temperature, or 5920 btu per lb.

 

[jebatty]

Eliot - thanks - I have posts in the past that mention about 3F per 100F of flue temp. Your post revived that memory. Happy New Year!

 

[ewdudley]

I have posts in the past that mention about 3F per 100F of flue temp. Your post revived that memory.

I hope repeating it makes it true, because I got the number from googling for our old posts!

"per kelvin" (flue | stack) efficiency site:hearth.com

I don't recall where I read the number originally, but I believe it makes sense if we look at the problem from the opposite side: what would the flue temperature be if none of the heat was 'lost' to the boiler?

IIRC they can calculate that temperature and it's something like 3600 degF for wood burned at atmospheric pressure with 21% oxygen content, starting from 70 degF. So for every 35 degF of flue temperature drop, 1% of the heat is lost to the boiler. At least that's my understanding, I can't back it up with solid references.

Cheers --ewd

 

[mole]

Thanks Jim,
From the article, it looks like the author is counting the heat of vaporization plus the amount of heat necessary to heat the water vapor to 400F. So I could add on the extra energy necessary to heat the vapor to 450F or 475. That seems sort of silly at this point, since I'll end up with lower available heat per pound of wood and the calculated efficiency would go up. But in reality as you and Eliot point out, the real world efficiency is dropping as stack temp rises. So for the sake of comparing my Seton efficiency with some of the other boilers out there, I'll stick with the 6050 btu/lb.

I'm going to get a few measurements of heat delivered to storage under different burn conditions and will post.
Thanks again.
JR