[Stoves] CARITAS stove model TJ4B

Fri Mar 18 23:34:51 CDT 2016

Dear Crispin,

It always seems scary to me to have a small coal burning stove inside a home due to possible CO poisoning. Seems all one needs is a cold stack that reduces air flow, wet coal or the top covers are not closed tightly - and problems. So is there a lot of deaths during the night with people burning coal? 

Regards

Frank

> On Mar 18, 2016, at 7:15 PM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
> 
> Dear Frank 
> 
> This type of stove is designed to operate continuously. It is very likely to go out overnight in most homes. 
> 
> There is no need to dump the ash out of the combustion chamber. The ash will drop through the grate and if it doesn't the gaps will be widened. The amount of ash is dependent on the coal composition. Nothing else. 
> 
> The CO in the stack is directly measured by the analyser so it is very low. When it is hot it is about 200 ppm. As the fire dies of course it rises bit there is a significant reduction if the bricks are heat conductive. Provided they bring stored heat ‎to the fire fast enough the CO can be kept burning far longer into the late fire. 
> 
> This aspect of combustion is not much discussed because it isn't sexy. The effect is this: the lengthened late burn gives the impression that the stove has stored a great deal of heat and people really like that. In fact it is still burning at low power. ‎Because of the extra burn quality there is very little unburned char at the end, maximising fuel efficiency. 
> 
> The CO/CO2 ratio of a dying fire stabilises at about ‎14%, sometimes as low as 12. It will in be high (30%) only in hot, air-choked combustion ‎or when dumping cold fuel on a hot fire. 
> 
> Regards 
> Crispin
> 
> Dear Crispin, 
> 
> Is it easy to stop and re-start the stove? Or do you need to dump and refill with new material? and how much ash is produced? I would think the CO would be extremely high in the stack?
> 
> Thanks
> 
> Frank
> 
> 
>> On Mar 18, 2016, at 3:12 PM, Crispin Pemberton-Pigott <crispinpigott at outlook.com <mailto:crispinpigott at outlook.com>> wrote:
>> 
>> 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.
>>  
>> <image003.jpg>
>> 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:
>>  
>> <image005.png>
>> 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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