Gasser Burns - Reducing the Storage Recharge Guess

Jim, I know you have a number of temperature sensors across the height of your tank so you will know...
When you say 'starting' temp are you finding that your tank is usually 107 (or whatever it is when you find it convenient to fire a load) from top to bottom or are you mentally scanning across your sensors and averaging the range?

Jim how do you have your tanks insulated?

Calculating by weight is a pretty accurate approach. I've been doing it by 'educated guess' - I know about how much burn time I can expect from a firebox full.

I use a variant of my Delphi tool for calculating how much burn time I need. The Delphi tool uses your latitude and longitude to get NOAA forecast data, and uses plugged in values for heat loss per degree day and boiler output. From that it calculates burn time.

My private variant knows about my tank temperatures and gives me a more detailed set of recommendations as shown below. If I had a convenient way to weigh my firewood, I could combine Jim's calculations with my tool and hit the outer limits of wood-fired OCD.

Jim im just wondering about the energy used on heating the steel that your tank is made of . Or is that going a little too far.

Not quite on-topic but in the opening post there was a reference to gasser effieciency as 75-80%. What is a good real world estimate for efficiency for a modern high efficiency gasser? Lots advertise 90% these days. A swing of 15% can influence the caculations significantly.

Jim, I agree that I have a new view on burning wood after weighing my wood for a short period of time. (I doubt I will continue to weigh my wood forever but.....) I now view my wood consumption as "It is what it is" as long as my efficiency is on target. One Garn owner may use 3 cords of X to heat for the year, another 20 cords. If both have the same efficiency per pound, one has a lot larger load to satisfy no matter where that load disappears.

I like the fact that i can now fairly closely calculate the temperature gain/# which helps me top of the Garn in the evening so the coil in the plenum operates longer through the night.

I got the following response from Ken Oaks in a discussion we were having on efficiency.

"Testing wood is a very complex procedure and almost everyone leaves out variables that will skew the results significantly. The Garn we have been testing in the lab has been averaging around 80% overall efficient and a little better when loaded to the maximum capacity because it has a longer high fire rate and therefore reduces the impact of the less efficient start up period. That system has a computer with about 15 or 20 sensors and a program designed by Martin and the U of M engineering class. It is the same model for testing that ASTM is using. The most efficient wood burning possible is about 88% (I’m guessing at what Martin once told me) and anything above that will start condensing and that creates a whole new set of problems."

Hi Jim, you are wright with your calculations.

The only room for getting misleading numbers is the wood heating value someone would use in his calculations

For instance, the EPA hydronic heater program uses 8,550 BTU/Lbs of red oak (HV) and 7,478 BTU/Lbs of red oak (LV). This is on a DRY BASIS ! So, you are correct by bringing the moisture content into the mix.

6050 versus 8550 is a big difference, and it will therefor significantly change the calculated numbers.

What wood are you burning?

For wood pellets there are test methods used to test this heating value before each certification test, so they are much more accurate.

Also, ALL European testing information is based on Lower Heating Value, what gives a more favorable overall efficiency number of roughly 14% !
The US typically goes by Higher Heating Value. EPA calculations for OWH are done for both values, but the HV is used for certification.
This just shows you that it is very important how customers are comparing wood boilers (Apples to Apples).

An approximate method to calculate overall efficiency is to use the stack temp and firebox temp.
E = ((Stack Temp - Ambient Temp) / Firebox temp) - 1
E = ((400 - 65) / 2000 ) - 1 = 83% (I just took some numbers)
So the lower the stack temp the higher your efficiency, because all this energy went into the water or accumulated partially into the steel boiler construction.

I will try to check what some of the ASTM standards use and maybe the EN305

Hope all I'm writing here is correct, .... I just woke up and I could not resist getting into it.

Marc

Maybe we need a white paper on wood heat values and how they're used. My understanding is that wood itself has a heat value of approximately 8550 BTU per pound, with very little variation between species. If you have a 10lb piece of wood with 20% moisture content, you really only have 8lb of wood. At 8550 BTU/lb, that's 68,400 BTU available, or 6840 BTU/lb.

In this case, you could use 8550 BTU/lb and apply that to the dry weight of the wood, or 6840 BTU/lb and apply it to the actual weight. Either approach yields the same result.

In my view, any efficiency test should be based on the dry weight of the wood burned - 8 pounds in this case. If you burn 10 pounds of wood at 20% MC, then you've really only burned 8 pounds of wood. The total available BTUs should be 68,400.

Some tests deduct the latent heat of vaporization - the heat required to vaporize the water in the wood. Water needs 970 BTU/lb, or 1940 BTU for the 2 lbs in our example. If you deduct this, you would conclude that you have 66,400 BTUs available, or 6640 BTU/lb.

In my opinion, latent heat should NOT be deducted, because it's a true energy loss from the system, and it's theoretically possible to recover that heat.

It seems like 'high value' usually refers to the 8500 BTU/lb value. 'Low value' seems to mean a lot of different things.

The European tests that I've seen used 6820 BTU/lb of wood at 15% MC. That translates to 8023 BTU/lb of dry wood, or about 6% less than what I'd expect. Those tests showed 91% efficiency for a 280,000 BTU gasifier. Using 8550 BTU/lb, that would translate to a 'real' efficiency of 85%.

Jim im just wondering about the energy used on heating the steel that your tank is made of . Or is that going a little too far.

Click to expand...

1000 lbs. of steel only holds about as much heat (in the temp range we're dealing with here) as less than 15 gallons of water.

500 gallon propane tanks weigh about 1000 lbs. 1000 gallon tanks, I think, weigh about double that. So the steel part of the storage tank is only about 3% additional storage compared to the water volume.

Still, 3% more storage might mean sleeping in 20 minutes more on a cold morning. We 2nd shift guys think about stuff like that.