improving a donkey

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Hand Fed Coal Boiler: a hand fed donkey boiler
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Post by ryanc » Tue. Jul. 07, 2020 11:49 am

Hi guys Ive just found this site and have a lot to learn.
I have a fish farm in South Africa and need to heat the water for about 4 months a year.

Im currently using a home made donkey boiler, made out of a locally built rocket boiler. The rocket boiler is basically a 300 litre drum with a small firebox at the bottom and a 160 mm pipe through the center as a chimney.
It was hugely in efficient, all the heat went straight up, fine for heating water for a shower, but not much more.

I've turned the rocket on its side, and turned it into a donkey boiler running on coal nuts as they are cheaper and easier than wood here.
Im getting fires hot enough to bend the 25 x 5mm angle iron I used as a grate, but not a huge amount of heat in the water. I pump about 3000 liters an hour through the system and get a 5 + degree C temp difference

I need to rebuild the boiler, to replace the grate and make it more efficient. Im thinking of copying this firebox / grate design Downdraft Stove Concept

To get more heat into the water Im thinking of adding back and forth loops of 25mm x 4mm seamless pipe between the fire and the boiler drum, the water will go through this first, then into the drum.

Is it worth putting loops of water pipes under the fire grate, up the back wall and up to the top or will this cool the fire box too much?

The system is open ended so if the pump breaks and the water boils it can either flow into the fish farm, or vent through a stand pipe so Im not worried about steam, but am very cautious of it.

I'd really like to make a down draft boiler, but thats going to have to wait for next winter, my ideal would be something that can put out about 100kw worth of heat!

If anyone has links to a good bunch of plans or ideas please let me know.

franco b
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Post by franco b » Tue. Jul. 07, 2020 12:01 pm

Horizontal fire tube boiler will provide the most heat exchange surface.
Scotch marine type design.

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Post by KLook » Tue. Jul. 07, 2020 8:55 pm

Had to look that up Franco.....great information available.


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Posts: 2
Joined: Tue. Jul. 07, 2020 10:40 am
Hand Fed Coal Boiler: a hand fed donkey boiler
Coal Size/Type: bitumous small nuts
Other Heating: biogas

Post by ryanc » Wed. Jul. 08, 2020 1:35 am

HI Franco,
A fire tube is what Im building for stage 2, we use them a lot here for tobacco curing etc. Very expensive to buy though and need clean water so Im going to build a heavy duty version so I can go direct instead of using a heat exchanger.
Even getting small blowers like the Daytons arent imported here so it makes the design a touch more tricky!

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Post by McGiever » Wed. Aug. 19, 2020 11:23 am

If you might source heavy steel reinforcing rod used in concrete pours. they can out perform your angle irons..look for number 5 or larger...numbers equal 1/ a no. 5 = 5/8 in....metric, IDK :)
Number 8 would not be too large and very heavy duty.

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