Gasifer Efficiency - hotter vs cooler supply temp

[jebatty]

I've noticed several posts stating, if I remember correctly, that firing to maintain a lower (165F+/-) boiler supply temp, if it satisfies demand, is more efficient that firing which results in a higher boiler supply temp. I don't recall whether or not these systems had storage.

After 2 days of experimenting with a new control system, which resulted in changing when I start the circulator after firing the Tarm, I noticed that:

1) Prior use started a low speed circ shortly after firing to recirculate boiler water and then start supply to the system when boiler supply reached 160F. Early burn recirculation meant that the entire boiler water content (about 54 gal with the Tarm) would also be close to 160F. With the boiler in high burn, return water from system at 120F or so, boiler would max out at about 170F supply until return started to rise, and then boiler supply also would rise. Probe flue temp 500-600F.

2) The experiment was to fire the Tarm to achieve boiler supply temp of 185F before starting the circ. With the boiler in high burn, return water from system at 120F or so, the boiler would hold a consistent supply of 183-190F (my circ setup increases flow at 185F and above), even as return rose from 120F to almost 170F. At that point I allowed the wood load to finally burn down, no boiler idling occurred, and 1000 gal storage ended up at 186F top to bottom. Probe flue temp 500-600F, same as before. This experiment was not intentional, but was accidental and was related to configuring a different control setup.

3) I did not weigh the amount of wood burned during either burn.

What is the conclusion? Since probe flue temp was the same during both burn periods, it would seem that burn efficiency was about the same in both cases. The experimental burn, however, resulted in much higher btu output. I can't calculate the increase exactly, as my flows change with boiler supply temp, but I was maintaining a 13-20F increase in supply temp over prior experience, and flow rates also were at least 4 gpm higher when supply was over 185F than when supply was 170F.

The increase in btu output must have resulted from a faster rate of burn, thus producing more btu's, and from extraction of those extra btu's to heated water, as probe flue temp was the same in both cases.

It would seem, therefore, that burn heat extraction to water efficiency increased during the experimental burn. This efficiency must be related to the firebox being surrounded by water at 183-190F vs 160-170F. A gassifer, at least the Tarm, does have water around most of the firebox.

From the experiment I would conclude that, if a system has a means to use the extra btu output, storage for example, maintaining a hotter supply temp will result in more btu output and faster storage charging without a decrease in burn efficiency, and therefore will result in shorter burn time, although more frequent loading of wood will be required during the burn cycle.

I have harbored a mild disappointment in that I was not able to achieve consistent btu output close to the 140,000 btu rating of the Tarm, except at very high delta-T's. This experiment indicates that the Tarm in fact probably can produce consistently high btu output approaching its btu rating by using the experimental burn procedure.

I would have to regard this report as somewhat anecdotal, as I did not keep accurate data during the burns, but I do pay quite close attention to operation of the Tarm, I have temperature measurements at a number of locations, I have a flowmeter in the system, and I believe that what I have related would be substantially supported by data. My confidence is such that I likely will continue this burn procedure.

 

[DaveBP]

Once again, Jim, very interesting experiment.

If you aren't modulating the draft fan or changing the draft speed any other way and the flue temps are about the same then I believe that the same amount of heat has gone up the chimney. That eliminates that variable from your comparison.

While the boiler was coming near 185F but the circulator hadn't yet started did you hear anything that might have suggested flash boiling inside the boiler? (The infamous banging that's been described on this forum). Having the circulator on at least low speed would help keep hot spots from forming and might help heat the boiler faster with the slow turbulence of the water but if it's not flashing it can't be harming anything, can it.

When the circulator finally kicks in, with supply temps at 180F or so, there can't be much return water from your tank coming in through the Termovar at first. Can you see the supply temp drop when the circ starts? That might be one of the effects of starting at the higher temp, that the tank return fraction will be less at first and not drop the boiler temp so much. The boiler doesn't have to catch back up to the start temp, it just has to maintain the same deltaT.

 

[jebatty]

Thanks, Dave, for the thinking and queries.

If you aren't modulating the draft fan or changing the draft speed any other way and the flue temps are about the same then I believe that the same amount of heat has gone up the chimney. That eliminates that variable from your comparison.

This is exactly true. In fact, I made no change in burn operation other than bringing boiler up to 188F (I rounded down before, actually 188F with a 3F differential; increased flow via second circ "on" at 188F and "off" at 185F) before starting a circ.

While the boiler was coming near 185F but the circulator hadn't yet started did you hear anything that might have suggested flash boiling inside the boiler?

None at all. A couple of days before I did vent a small amount of air from the top of the boiler. No banging or evidence of flash boiling.

When the circulator finally kicks in, with supply temps at 180F or so, there can't be much return water from your tank coming in through the Termovar at first. Can you see the supply temp drop when the circ starts? That might be one of the effects of starting at the higher temp, that the tank return fraction will be less at first and not drop the boiler temp so much. The boiler doesn't have to catch back up to the start temp, it just has to maintain the same deltaT.

I have the Termovar throttled down quite a bit, but enough to insure 140F minimum return temp. In this case with 120F return water, boiler return started at close to 150F. At the start temp of 188F, both of my circs, in series, are "on." Supply temp would climb to 190-191F, although the thermometer on the Tarm would read about 182F. Then supply temp would fall slowly to 185F when the 2nd circ is "off," flow dropping from about 9 gpm to about 4-1/2 gpm. Supply temp would continue to drop to about 182F and then start to climb again to 188F when the 2nd circ would kick back on again. This cycle repeated through the burn. Termovar return temp to the boiler climbed above 150F as storage return temp rose. Obvioulsy, 4-1/2 gpm was insufficient flow to absorb boiler output, and 9 gpm was slightly in excess of what was needed to do the same at the delta-T range of operation.

I think part of the fluctuation in temp comes from somewhat uneven heating of the water in the boiler and how the water flows through the boiler from return to supply.

I pretty much repeated the burn experiment earlier this morning. Top of storage tank was 165F, bottom of storage tank (boiler return) was 146F. One difference though is that I brushed the hx tubes first, as it had been about 3 weeks since the last cleaning. Supply temp rose to 188F, both circs "on." Temp continued to rise, this time to 194F (Tarm thermometer read about 187F), and then fell slowly to 185F when the 2nd circ is "off." Temp fell to 181F and then rose slowly to 188F, 2nd circ on, temp rose again to 193F, then started a very slow drop, etc. Flue probe temp was 550F and then rose slowly to 600F over the burn, a little cooler I think because of the hx tube brushing. But at 600F, supply temp was holding at about 190F, I was maintaining a delta-T=30-33F, and flow was a little over 9 gpm (135,000+ btuh output).

I have no doubt that I am achieving a materially higher level of btu output from the Tarm under these conditions than I was previously. Return water protection remains. Flue temps are no higher than before. My flue temps even with chain turbulators are higher than some operators realize, I think, because I burn mostly dry pine. I know I have good draft and good chimney clearance from obstructions.

 

[heaterman]

You probably won"t see any noticeable increase in efficiency unless you can fire into water that is less than 140-150*. Maybe a 1 or 2 points available at 150 vs 190 but that's about it. No real gains to be had unless you can collapse the flue gas and extract the btu's there. Gas and oil fired boilers are the same way.

 

[jebatty]

Heater, I see your point, but I think we're talking apples and oranges. My point is that if I let the Tarm come up to a higher temperature, in the case under discussion 188F vs 160F, before I start circulating, I am able to achieve sustained output at a much higher btu rate. I'm guessing that hotter water around the firebox at the start of circulation enables a hotter sustained burn with hotter water output.

What I experienced, with return water 120F, high burn, and same flue temp in both cases, is that if I fire the Tarm and start re-circulating until the Tarm is up to 160F and then start circulating to storage, the Tarm will gradually rise in output to about 170F, and pretty much hold there, until return from storage starts to rise above 120F (delta-T=50+/-). But if I let the Tarm go up to 188F before I start circulating, the Tarm will maintain an operating range of about 182-191F, and will stay in that range even as return from storage starts rises from 120F to almost 170F (delta-T=60 to 70 +/-). My controls add a 2nd circ in series which is "on" at 188F and "off" at 185F, and flow increases about 4 gpm's when the 2nd circuit is "on." Sustained btu output is substantially higher in the second case than in the first case.

 

[heaterman]

Heater, I see your point, but I think we're talking apples and oranges. My point is that if I let the Tarm come up to a higher temperature, in the case under discussion 188F vs 160F, before I start circulating, I am able to achieve sustained output at a much higher btu rate. I'm guessing that hotter water around the firebox at the start of circulation enables a hotter sustained burn with hotter water output.

What I experienced, with return water 120F, high burn, and same flue temp in both cases, is that if I fire the Tarm and start re-circulating until the Tarm is up to 160F and then start circulating to storage, the Tarm will gradually rise in output to about 170F, and pretty much hold there, until return from storage starts to rise above 120F (delta-T=50+/-). But if I let the Tarm go up to 188F before I start circulating, the Tarm will maintain an operating range of about 182-191F, and will stay in that range even as return from storage starts rises from 120F to almost 170F (delta-T=60 to 70 +/-). My controls add a 2nd circ in series which is "on" at 188F and "off" at 185F, and flow increases about 4 gpm's when the 2nd circuit is "on." Sustained btu output is substantially higher in the second case than in the first case.

Now that I've gone back and read through your first post before going off half cocked.............I see what you are describing and trying to attain........and I concur with your method. Sounds like in your case it's advantageous to let the boiler get up a good running start before the load is applied. This is not a bad thing as it seems to indicate your load and boiler output are matched pretty evenly.

 

[kabbott]

I have asked this before in another thread, and nofo seemed to think there was little difference.... I know in my own non gasser boiler that there seems to be
an increase in output/efficiency at higher water temps. I would assume this is simply because the cooler water temps just quench the fire so much. The thing
with a gasser is there are so many variables....does the pri/sec air need to be fine tuned for different water temps and so on.

From a heat transfer point of view the lower the temp and the higher the flow the better.... pretty simple there. But the question remains how much does this effect
the burn. I would also think that water temp would not affect the stack temp that much, Your talking 20, 30, or so degrees change in water temp and I would think
this would affect the 500 to 600 degree stack temp to much, def less than the 20 or 30 degrees. May be hard to measure.

The sec. chamber is insulated from the water jacket pretty well so it should not affect it to much. Could it be that the hotter water temps are allowing more
wood gas to be cooked off in the pri chamber and that is the reason for the higher output? unless it is a huge difference than the only way to really know is to
weigh the wood. Not sure about you but if I start weighing each piece of wood my wife will have me committed, she already thinks I'm nuts about the whole
"wood boiler & nfcs" deal.LOL

Kris

 

[jebatty]

kabbott, I think we see this pretty much the same way. I too think that hotter water surrounding the firebox results in faster production of wood gas, i.e the wood is being burned faster, resulting in a higher volume of gasification burn, and therefore more hot gases passing through the hx tubes and hotter water. As to the change in flue temp, would this not relate more to the ability of the hx tubes to extract to water the extra heat being produced than to the increase in temp around the firebox?

Last season I collected data every 15 minutes during several burns and also weighed the wood being burned. The data related to much different operating conditions so is not comparable to this thread. When I have nothing else to do, perhaps I will do that again this season fwiw.