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

[larry winiarski]

Hi Charlie

Nice to hear what you are doing!  Hope this helps

recently back from El Salvador and catching up on a months email!

Years ago I started advising people to design for approximately constant 
croos sectional are so as not to restrick the natural draft of the the 
stove.   Natural draft works on a miniscule pressure difference between the 
botoom and top of the chimney. we do not want to tadd undue pressure losses 
by constricting the flow or over expanding.  However as the hot gases give 
up their heat they become more dense so one can slighty reduce the 
cross-sectional area and still have the same mass flow at about the same 
velocity. When you  have good heat transfer a 25% reduction  under the 
plancha seems to work and the chimney diameter can be smaller.

A few years ago I found that the rustic , slop molded brick tile (baldosa) 
made a good combustion chamber  the best are slightly more than an inch 
thick and about 12" by !2".  The natual porosity of these rustic tiles seems 
to make them more resistant to thermal shock and they are more insulating 
than compressed or extruded flor tiles.  these rustic baldosas have served 
for several years in thousands of rocket stoves in  central america and the 
are  very cheap.  Hand made rustic clay  roofing tile also can be made into 
combustion chambers They are thinner and more fragile.  I think a good 
brickmaker useses his best mix for roofing tiles(tejas) and baldosas and 
drys them more carefully  in the shade.

It is difficult to mold the baldosas into the L shape as they want to crack 
while drying.  While it can be done I think it is better to cut then to 
shape after they are fired

God Bless

Larry
----- Original Message ----- 
From: "Charlie Sellers" <csellers42 at yahoo.com>
To: "Discussion of biomass cooking stoves" <stoves at listserv.repp.org>
Sent: Wednesday, August 08, 2007 10:35 AM
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



http://improvedstoves.blogspot.com/ - just R&D related to fuel efficient 
biomass stove issues
  http://travelswithcharlie.blogspot.com/ - most recent travel posts
http://new.photos.yahoo.com/csellers42/ - travel photos, of everywhere - 
click on the country albumns on the left
http://huiplesofguatemala.blogspot.com/ - my textile project in Guatemala - 
what colors!
http://travelswithcharlie2.blogspot.com/ - older travel posts, including 
Nepal travelogue
http://ewbappropriatetechnology.blogspot.com/ - just posts for the ATDT of 
the EWB-SFP; AT for developing countries


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