[Stoves] Somewhat urgent question¿ Interior ross sectional area of a plancha stove.

[Crispin Pemberton-Pigott]

Dear Charlie

You certainly touch on all the major points in a brief email.  You grasp of
the fundamentals is sound and I think you are going to have a good product.

Last first, the Baldosa tiles (if they really are) will do for the
combustion chamber. They are an interesting combination of ingredients that
has pretty good thermal shock resistance.

On the chimney front, please get
http://www.newdawnengineering.com/website/stove/tests/draft.xls and have a
look through. It is not as difficult as you might think - fill in the
information as best you can on the left section then look down.

The reason I suggest this is that if you have some understanding of the
draft, you will be able to make changes as the stove develops in the next
few months.

You inclination about the expansion of gases is quite correct.  The equal
area idea does not take this into consideration. As a result the area is
constant the velocity of the gases varies through the stove.  You can do it
the other way round, but it is not optimal either.

Think of it in this way:

Air in, is cool and dense.
It expands when combined with burning fuel which creates gaseous products
and is also hotter (more space needed)
The heat is given up to the plancha (and pots) reducing the temperature and
therefore volume
It goes up the chimney where it cools even more

The chimney is heat driven so the air can't be cold when it arrives (and it
fact is never is) but it can be chilled on the way up below the room
temperature and it will stall the gas flow, esp when lighting.

The volume of gas going up the chimney is much less than the volume in the
middle immediately after the fire.  There is a certain amount of energy
available from the chimney draft.  Expending this in the right place is a
large part getting a stove to work efficiently.  A chimney is an expense and
it should deliver a better cooking experience.

The air inlet may or may not by the limiting factor on air flow.  If you
make the constriction on the system the gap between the plancha and the
stove body, that will ensure the best heat transfer to the plate.  The other
parts of the gas flow path can be larger, roughly in accordance with the gas
volume at that point, but it is not all that important.  There are many
different things that work well.

Having a larger combustion chamber means you can have more fuel in it.
Having a larger air inlet means you _could_ get more air in it, but with a
chimney that is usually not a problem.  It is far more likely that you will
get in more air than you want, actually. A chimney is like a low power fan
and should be used to increase heat transfer and secondary air mixing (which
is not going to be a feature of your stove, probably).

The overall concept is that you should have adequate space to get in the
fuel, enough space to burn properly (bigger), send the gases to the heat
transfer area (which is the plate above the fire and the area between the
fire and the chimney, under the plate) then a chimney large enough to
accommodate the shrunken gases, but small enough to work well when it is hot
yet heated up.

If you were to make a very large chimney, it won't work at all when it is
cold and damp.  The smaller it is, the faster it will heat up and work
properly.  If you have it too small (because that is the only pipes you can
find) make it taller and the draft will increase, and the power available
will pull harder on the gases to compensate for the small size.  Keep the
gas speed in the chimney under 3 meters per second (see the draft calculator
for that).

So there isn't a single answer to the question.  I hope this will get you
started.

Something to watch for is that the fire burn rate does not exceed the
chimney's 'sucking ability'. If it does, smoke will feed back into the room
even as the chimney appears to be working.

If it works well, you might choke the chimney when everything is hot to see
home much you could restrict the flow without getting smoke feeding back.
Without instruments that is a good guide.  Run it hot and hard at max heat.
If there is no feedback, just, perfect.  Choke until there is.  Make notes.
Reduce the plancha-stove body gap a little and repeat.

As I said, usually there is too much air and that costs you fuel and time,
forever, for nothing gained. You can make a damper in the chimney by sliding
in a brick to partially close it.  Cheap, effective.

Best regards
Crispin in Johannesburg


-----Original Message-----
Subject: Somewhat urgent question¿ Interior ross sectional area of a plancha
stove.

I can get to town/electricity every few days and am on line on and off for a
few hours right now.  The dimensions of our planch stove design here in Peru
are somewhat limited by the dimensions of the types of hollow brick
available (I can only cut tile - or break bricks in a clumsy fashion - and
have only simple tools) so various dimensions of the stoves are limited.
Particularly the chimneys - only scrap iron pipe or stacked hollow bricks
are here - so I want to be a little flexible in the open area of the stove
at various places in the cross section.

One ¨rule¨of the rocket is constant cross sectional area throughout the
stove but it seems that the exit/chimney really only needs a larger area
than the rocket entrance - true?  I use the analogy of a river flowing into
a new channel - if the area diminishes then smoke backs up into the room.
And if the new channel is smaller then the flow out needs everything is
fine.  And if there is a temporary wide spot inside the stove then there is
no problem, as long as the exit is larger than the entrance?
   
  I am trying to increase the size of the combustion chamber as much as I
can, since the plancha will slow the time to boil (and other perceived
problems), to increase the firepower a little.  But I only have limited size
possibilities for either the entrance and exit, and I don´t yet know how to
exactly take into account that there is an expansion of gas as it heats, and
there is a net increase in gas volume during combustion.  How does the equal
area rule take such things into account¿  I am taking some risks, but as
long as I increse the chimney area I hope to be fine.  As always, I want the
best conditions to increase the burning of the smoke before it leaves.
   
  Any advice on whether commercial floor tiles can generally be the walls of
the combustion chamber¿  What exactly is baldosa tile¨¨ - specifications¿  I
tested mine (and also the hollow bricks) in a very hot fire with no
problems, but success in the longer run is too important to take too many
risks.
   
  thanks in advance!
   
  Charlie