Fw: Verhaart response on DD questions

["Ron Larson" <ronallarson@qwest.net>]

Stovers:

 

    In my response today to Harmon, I went looking for the web site given below and realized for the first time that Piet's response below had not gone to the full list.  As I am pretty sure that Piet wanted it fully out and because it contains such good new information, I am forwarding it without comment (and not much would be given anyway - this response is very good).

 

Ron

 

----- Original Message -----

From: Peter Verhaart

To: Ron Larson

Sent: Saturday, December 29, 2001 12:19 AM

Subject: Re: Continuing on stove nomenclature and descriptions (Faemus??)

At 08:48 28/12/01 -0700, you wrote:

Peter (cc stoves):
 
    Thanks for the correction.  I visited the stoves site maintained by Alex (http://www.ikweb.com/enuff/public_html/DDBbq/DDB.htm)
   
    Yours is a nice looking unit - I only vaguely remember seeing it earlier (which is dated Feb. 7, 1999).  The design is somewhat similar to what I described in my following note of yesterday - but I like better your lower placement of the fuel-air sources.
 
    A few questions - 
 
        1.  Can you expand on the use of "cheat holes in the riser pipe".  I can't spy them.  How large?  How far up?  How does their existence help start the updraft mode?

There are 3 holes of 12 mm in the riser pipe, about 20 mm above the grate. A 30 mm wide strip of steel, 30 mm above the grate is welded on the riser pipe. When the fire is started it burns in updraft mode, very weakly so as there is no chimney draft.
Some of the time some of the flames will enter the 'cheat holes' and so create the beginning of chimney draft. As the average temperature of the gases downstream from the fire increases, so will the chimney draft and in a typical case it takes about 12 minutes to establish full downdraft mode.

        2.  Can you tell us more about the "slide to adjust the active grate area."   Does this change the amount of air flow?   I can't figure out the location.  Is it right below the visible grate - or further down?  How far down?  What range of areas are possible?

The slide covers the grate from fully open (about 120 * 120 mm) to fully closed. It is on top of the grate, when activated it pushes the burning fuel closer together. I use it about 1/3 closed for my one or two steaks. The available grate area can vary from 14400 mm^2 to zero.

        3.  What is the mechanism for removing ash?

The ash collects (mainly) in the dead end at the bottom of the riser pipe. A lid which doubles as a third pod can be detached to remove the ash.

        4.  In the Khan paper, wood blocks were added at rates like two small blocks every thirty seconds.   What is your typical fuel feed rate?  Have you calculated maximum and minimum power levels in kW?   Is there a "sweet spot"?   Have you ever measured CO or other emissions?

I probably add them at a similar rate. The procedure is to cover any hole in the burning fuelbed with a piece of wood. I did not experiment, as Hasan has done, with chimney height, I just took about 1.2 m (just guessing) because I knew from earlier experiments (with Hasan) that this would be sufficient.
I have not had the opportunity to measure CO or other emissions other than by nose. Things might look up in the near future though, a lecturer from Biology at the Central Queensland University has developed a  process to convert (?) shrub and tree cuttings into fuel and even briquettes, if I am informed correctly, not charcoal. The idea is, if I am not mistaken, to provide a fuel which contains no harmful bacteria or fungi.

        5.  Is there any insulation?   (all metal?)  Any estimate of efficiency?

Yes, a 1 cm thick layer of Kaowool on the bottom and sides of the body that carries the steel plate. The efficiency of a barbecue? kg of steak/kg of wood. Might be less than unity, certainly if you like your steak rare.

        6.  In the US, our barbecues are always (? - at least usually) open grates - not solid plates like yours (which is of course needed to maintain draft).  Is it typical in other locations where you have lived to have barbecues with solid surfaces?

In Australia most gas barbecues have both a solid surface as well as a grate over lumps of pumice or such. In my experience the solid surface is used predominantly.

        7.  I can see using your design as a "griddle", but also as a "plancha-type" - with ordinary cook pots and a maximum temperature need only of that for boiling water.  Do you have any experience or data on how the stove works that way? 

No, but I have been thinking of doing an experiment. I have even started making a stove with a hot plate, I might take it up again when the weather cools down a little, we are now regularly having temperatures of 40+ C.

        8.  How uniform is the temperature on the cooking surface?   Did you ever (or could you) try putting a large square basting pan on the cook surface and observe where boiling is occurring?  I'd like to know the max "Figure of Merit" (ratio of weight of water evaporated to the weight of fuel) you could obtain (and whether this changes much with the vigor of the boil).

The temperature is not uniform, it is highest right above the riser pipe. The plate is 10 mm steel. Aluminium of the same thickness would give a more uniform distribution and the steel plate would probably have a more uniform temperature if I welded fins on the downstream end of it. As it is, however, it has its advantages, the hot end is good for steaks, the downstream part better suited for sausages.

        9.  The plate thickness of 10 mm seems a bit large.  Any particular reason for that thickness?  How about side thicknesses?

See 8. It makes the stove quite heavy, reason more uniform temperature. Sides are 3 mm steel, being the only available material at the time.

        10.  I am wondering about your statement that it takes about 12 minutes to settle down.   What is happening during this period?  Are you building up a layer of charcoal below the grate?

The 12 minutes are used both to let the fire spread and to create hot gases in the chimney. During this period it has all the earmarks of a classical wood fire, flames, glow and smoke. There is only volatiles and air and flames below the grate, all the charcoal burns (sorry) up on the grate. It is a more or less stationary process with a (more or less) constant rate of burning charcoal, pyrolysis and combustion of volatiles. Our explanation of the clean combustion is a mix of high temperature, turbulence and short residence time resulting in clean combustion. We noticed at Eindhoven that the CO tends to increase the moment all wood is carbonised and no more volatiles are produced.

        11.  It looks like your design could be readily modified to achieve power control through air flow rather than fuel metering (which offers also the possibility of charcoal-making).  Have you ever seen such a design - and can you supply references?  Does your own work at Eindhoven exist on the web anywhere?  Published in a journal anywhere?

I don't see how that could be achieved. The air control is through the permeability of the fuelbed and, to some extent by the size and height of the chimney. For a charcoal making stove you would preferably have a process where you have a constant composition of the volatiles so you can design a burner for it.  To achieve that you need a more or less stationary process (like in the downdraft stove). It smells like mechanisation to me.
There is or should be one article: "Making do with the open fire", many aspects of which have been rediscovered by Dean Stil, for whom I have great respect for opening up this topic which 20 years ago was almost lethal. Then there is the collection of articles "Wood heat for cooking". Apart from that there is little. I didn't publish, so I perished.

Again my apologies for having not remembered your prior positive statements about your down-draft barbecue.  Besides the major advantage of getting the smoke out of one's eyes - are there any other benefits or disadvantages we should know about?

Yes, no smoke, just clean gas, mainly N2; CO2 and H2O and the merest traces of CO, which make it suitable for baking in direct contact with the clean gases, like described by Hasan Khan.

No apologies, Ron, thank you again for doing a great job over the (almost) past year and best wishes for the next