[Stoves] MCS update

[Crispin Pemberton-Pigott]

Dear Friends of Burning Charcoal Efficiently

We are slowly ramping up the production of the Maputo Ceramic Stove and a
milestone was reached today when Ceramic Arte shipped 1000 stoves in a
single day. 

The actual production rate is only 56 per day 7 days a week. Within a couple
of weeks we should reach 150 per day.  There will be a minor launch at the
FACIM trade show at the end of the month.  Today the number of moulds in
which the stove body is formed was increased. The material used is ALMOD 60,
a type of plaster of Paris. That replication process seems to go seamlessly
in the hands of the junior staff.

With the traditional metal stoves the baseline consumption (not corrected
for moisture content) of charcoal (probably 10% WWB) is 195 gm per kg of
food prepared (all categories of food lumped together).  On average the
moisture in the charcoal measured in the baseline was 10% or very close to
it. We are presently collecting the last of the data from users who have had
a stove for more than a month to confirm the fuel savings.

Something that has speeded the analysis of data is a moisture balance into
which is dropped a sample of charcoal.  It measures the moisture content to
4 significant digits. Pretty cool.

The only production problems these days relate to the power being cut off
during a firing which messes up the firing cycle, and over-voltage on
occasion damaging wiring in the main panel. The main consequence is
over-firing. Under-fired stoves are simply re-fired.

Today we started a test to see if the charcoal can be sized to 0.5-1.0mm
looking rather like large castor sugar.  There is too much powder in the
PK11 mix which results in an over-consolidated fired product - not foamed
enough and this reduces the thermal shock resistance.  We are informed that
the best profile of the mix is 10% charcoal by weight, of the size
mentioned, corrected for moisture content.  The first test of this 'sized'
charcoal grains will be on the grates to see if it affects the cracking
rate.  So far a large number of the grates crack but do not break and
continue in service for a long time.  We are also trying a slot cut into the
grate from edge to centre to see if that performs well. The only alternative
to this seems to be to try to further reduce the thermal expansion
coefficient.  Fortunately the product is very strong and having a crack or
two only seems to give it resistance to breaking.  It is an odd finding
because it was guessed initially that a small crack would progressively lead
to failure of the part. No so.  One good crack leads to a long life
apparently.

The reported fracture rate is quite low in spite of offering people free
replacements if they bring in a cracked component.  More can be said about
that later as we have very accurate data coming in on cracks, breaks and
failures, plus what people did about it.

A small dimensional change is going to be made to the outer corner of the
grate that will allow it to be pulled off the ALMOD 60 mould more easily.
While the change is very minor to the product. The change to the tooling is
significant so we will have to remake the moulds.  Basically the problem is
that the grates are moulded with a radiused edge and the part of the mould
that shapes the curve tends to grip the lip of the grate as it dries.  We
can turn over the moulds more rapidly (needing fewer of them) if it is less
prone to sticking.

Drawings of the grate mould and the means for casting them are available
upon request.  I will post them as a set when they are finalised.

Regards
Crispin in Maputo