[Stoves] CARITAS stove model TJ4B

[Crispin Pemberton-Pigott]

Dear Friends

 

Sorry for the poor quality photo but I wanted to show it to you with the
hopper open. This is a coal stove with a hopper based on the GTZ-7 stove
from Ulaanbaatar (2010). The model is a GTZ-7.6 but both the GTZ name has
changed and the model 7.6 was designed never built in Mongolia.

 

So, it gains life in Tajikistan. It is a cross draft stove with a hopper
feeding coal onto a grate inclined down (away and to the left as seen here)
at 30 degrees from the horizontal.

 



CARITAS TJ4B Cooking and chimney heating stove

 

The stove is little more than a metal shell with one central divider to
facilitate the location of the bricks. It contains 19 good quality high
temperature bricks recovered from old furnaces. In operation, the fire is
lit in a TLUD style inside the section directly under the cooking plates. It
is lit with split dry wood surrounded by a small amount of coal chips. These
chips are easily ignited and fall onto the bed of coal lying underneath.
This in turn lights the fuel at the bottom of the hopper. Air crosses
through the lower hopper creating a continuously pyrolysis of fuel near the
bottom of the grate. It functions a bit like a downdraft stove with the
gases forced to pass through a cup of red hot coke stationed to one side.
The flames then run upwards vertically to the cooking area. The gas path
then passes out the far end of the stove body into a chimney elbow.

 

If the firepower drops below the desired range, the ring on the movable
grate can be shaken in and out. This causes additional fuel to fall onto the
lower portion of the grate increasing the burn rate.  The door is normally
completely closed and holes up the grate. If it is opened fully, the grate
can be removed by pulling it straight back. Anything sitting on it is
dropped into the ash drawer below.

 

Here is the performance:

 



As this stove is supposed to heat via a metal chimney, it is important to
find two things: a high gas temperature and an efficiency that is not so
high as to leave little heat available. If the efficiency is high and the
gas temperature low, there is little more than can be extracted. This stove
should be operated in the >85% efficient range (by adding horizontal chimney
pipes).

 

The two fire-related numbers are the excess air level (about 100% which is
in the ideal range) and a low CO/CO2 ratio (?0.2% sustained).

 

With a space heating efficiency of about 53% it leaves another >30%
available for heating a second room using the chimney. The cooking
efficiency is typically 25% for this design, and that heat is considered
part of the space heating contribution.

 

Once the fire is established, the performance continues as shown above
pretty much indefinitely. As long as the ash is not lumpy, it will fall
through the grate which should be shaken occasionally, say, per 4-6 hours
depending on the coal.

 

The hopper can be refuelled at any time. It does not disturb the performance
unless the fire is nearly out.

 

Constructed from new materials, this stove costs about $50 to make. It will
last well because nearly everything that gets really hot is made from
bricks.  The heating power is about 3.5 kW from the stove with another 2 kW
available from the chimney. It really needs a cast iron top, held on by
screws. The bricks can be replaced by the owner if a lower quality brick was
used. The ones lining the fuel hopper do not all have to be such good ones.

 

Regards

Crispin

 

 

 

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