Pellet Rocket temp. logger data and charts

[rectifier]

Well, I've been quiet lately, and the reason is... the stove has been running really well. I borrowed a 2-probe Fluke thermometer/datalogger and have been monitoring stove surface temperature as well as stack temperature over the last week, while changing a few variables.

A setting was determined where the gasifier was basically 'maxed out'. At this point, fresh pellets were visible right above the secondary air blast intakes, and the gasifier was balanced throughout the burn (no creep in the pyrolysis zone which is basically right at the secondary intakes) This was marked on the throttle as point 'A'.

Lower power tests were run at 'A-1/2' which is pushed in 1/2" from 'A'. The gasifier is not perfectly balanced, but the pyrolysis zone does not creep very fast - after 8hrs, it has only made it 1" above the initial point, which is more than acceptable.

The initial 2 tests (5,6) were made with a fan blowing on the stove body, to get some extra heat for the shop. Then it was realized that decreasing the stove temperature hurt the radiant output significantly and didn't result in much extra heat.

The next 2 (7,8) were made with no fan at all, and the final test on the graph (9) was done with the blower on the aluminum flex stack, after the stack temperature probe. This was the best configuration as the body radiates significant heat, and a lot of heat is removed from the exhaust before it hits the chimney.

Note that stack temps are INTERNAL in the center of the stack. A stack temp of 550F corresponds to a stovepipe temp of 330F (about an ordinary woodstove stack temperature). With the blower on the aluminum flex, I could hold my hand on the output chimney without burning it, while the stove was still pouring out heat at over 500F.

Image is large in pixels but small in bytesize - nobody likes squinting at blurry graphs so it is rendered at what should be a 'fullscreen' size on most any computer these days

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[rectifier]

Stove body temps, measured on the top of the cast iron 'barrel'.

Note the funny dip in Dec.6's run on both charts - caused by a forced backdraft when someone turned on the shop welding fan without kicking the door open for makeup air. That was a smoky afternoon...

This stove set at 'A' consumes 25lbs of pellets over 8hrs, an average power of 32kBTU/hr. This is enough to keep the shop at 60F with it being ~32F outside, with no interference from the natural gas heater.

The stove at 'A-1/2' consumes 16lbs of pellets over 8hrs, an average power of 22kBTU/hr. This is enough on a warmer day, but the furnace will still kick on occasionally. The stack temps are higher than stove temps at 'A-1/2', and I think a bit of excess air is getting by, the flame looks lean. I might come up with an improved carburetor setup which changes intake volume as it slides in and out - or I might not, it seems to work OK as is.

No smoke is produced by this stove except a little during warmup when the flame overflows the riser and scrapes the cold iron.

Efficiency is probably a little low, after all I am developing a burner here and not a stove. But we are going to try to find an old hot water tank and make a taller riser/bigger heat exchange surface to replace the motor housing. This should boost efficiency and power significantly, and let us keep that gas burner off, the pellets are much cheaper than running the old and busted gas furnace.

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[ytksgmt]

Impressive data.
Let me ask some questions.
You wrote
"The stove at 'A-1/2' consumes 16lbs of pellets over 8hrs, an average power of 22kBTU/hr."

You didn't mentioned about the differences of consuming speeds between Dec.7 and Dec.8,9.
You meant you didn't notice significant differences,right?
If so, the graph of Stack Temps indicates fan blowing is effective for absorbing heat from stove to room.
But, you also wrote
"Then it was realized that decreasing the stove temperature hurt the radiant output significantly and didn't result in much extra heat."
This means heat transfer by radiation is more effective than convection for heating human body,right?


Another question.
Both graphs show gradual declines of temperatures through 8 hours.
Your system doesn't have mass storage.
So, I can't say that rise of temperature of your mass storage caused the declines.
Effect of counter draft caused by warming up of motor barrel must fully appear within an hour or two after ignition, I guess.
What do you think caused the gradual declines?
Maybe, the layer of perlite around heat riser?
Heat from heat riser soaked / passed through the insulation layer over several hours?

[Deleted]

Posted by ytksgmt on Today at 7:26am

This means heat transfer by radiation is more effective than convection for heating human body,right?

Convection needs a lot surface to overpower radiation.
In general heat distribution by convection is more evenly.
With radiation one can have a warm front but a cold back.

[rectifier]

To answer the questions,

Consumption speeds for Dec.6,7,8 are near identical. They are all set at A-1/2 with other minor variables (7,8 were run with no other differences, and the graphs are near identical). This was done to prove whether throttle settings resulted in repeatable BTU output.

The fan on the stove body did extract more heat into the room. However, it has to be noted that the shop has a >20' ceiling. There is a lot of air to heat up in there! Also, it tends to end up at the top of the shop. We have fans to bring it back down, but there is a massive heat loss through the uninsulated roof. The radiant heat heats us and the shop equipment which then heats the area at floor level.

The fan was taken off the body more for issues of consistency and proper testing. The burner is designed to run without electricity - thus, there should be no fans used. The fan was later moved to the flex aluminum stack after the sensors, to emulate a piece of thermal mass/heat exchanger and recover the remaining heat without decreasing the radiative temperature of the stove body - this turned out to work much better than cooling the stove body.

Temperature decline - this is the 'bug' I am trying to fix in the stove. I don't blame the rocket system as it should be equilibrated in an hour or less. The perlite will have heat soaked in that time (only 1" thick) and is only serving to maintain a constant differential between 1200F riser and 700F stove body. I believe this dropoff is caused by ash accumulation, which closes off the gasifier throat as fuel is consumed. This is effectively the same as sliding the throttle in, which causes a decrease in power.
I have a plan to solve this that involves re-working the 'throttle' part of the stove to act like a ramjet. Roughly summed up it sounds like this: Duct the incoming air into a laminar stream, then send it through a venturi section before it hits the woodgas from the gasifier. When it hits the flame and burns, it expands and accelerates. Try to use this high-speed air to blow the ash away.