Paul Taylor on sat 28 dec 02
Dear Craig and All
About burners again - I know this discussion is not in the tradition
of the immediate and snappy rapport on the internet; but this is research
and design which takes more time and work than other posts - and since I am
setting fire to an explosive mixture a bit more consideration than
innovation may be best.
Just in case we have all forgotten the criteria I set for my test/small
production kiln 16 cuft packing space are
(needs) 1 Even temperature, 2 Atmosphere control, 3 soak and or ramp
control on cooling, and 3 The weather is so unpredictable and scrawly here
that you can not rely with any consistency, on the chimney to do any thing
but vent kiln gases.
(would be nice) 1 preheated air , 2 quiet burners.
I do not need : too many variables and uneven temperature . I am happy
enough to adjust settings manually so I do not need computerised controls.
So far I have come up with nothing but confusion. I think this is not
helped by the way natural gas is treated as the same as propane in most of
the books . I think kiln manufacturers consider them together because it
halves there inventory . A burner designed for natural gas at 11 wg will
fire with propane with no change in the system because LPG can be be
governed down to natural gases low WG pressure .
Since I am firing with propane (LPG) I can fire at a high pressure up t=
o
12 ppsi and higher. I am designing a burner that does not have to have the
compromise of low pressure that natural gas would force on me.
I am again thinking of preheating air. In the =B3Energy Efficient Potters=B2
book (whose authors i have not been able to communicate with) , the writer
uses nozzle mix burners because he claims that the integrity of the fuel is
destroyed by the preheated air but in Nils Lou=B9s book he uses a forced air
burner and reports as good a saving with out even hinting of any fuel
disintegration (called cracking). I am tempted to believe lois experience
especially as Mell seems to have a part in the book. So I am back to
thinking on forced air burners with preheating. also it has occurred to me
that using preheated air is more efficient because the volume of air is
increased through the burner giving greater flame speed thus better heat
distribution. I would be interested to know what extent the hot air being
thinner would help mix the gas and air or would the loss in friction betwee=
n
the two prevent mixing ???
From what I see the basic difference is between forced air and the
atmospheric burners is, positive pressure (forced air) and negative pressur=
e
(atmospheric) . =20
The advantage of the atmospheric burner is that it works with out
electricity and is very safe to run also I get the sort of gentle tun down
needed for cooling control.
but the engineering of venturie (atmospheric) burners has to be very exact
. What i suspect is, that these burners can not be run by primary air alone
---- but I would like some one to confirm this ???.
So the plan I have so far is to put two burner systems in the kiln ; a
forced air system; and enough of a high pressure venturies system to get m=
e
to biscuit temperature. I can put one burner above the other. The pipe work
may look less than elegant but I am not selling kilns. I would be increasin=
g
the size of the atmospheric burner that forms part of the safety system on
all forced air systems but I will not be considering it part of burner
safety. With the two systems I could include a bit of stainless steel wire
wool in the forced air burner for better fuel efficiency.
For all this bother I get a faster flame speed thus supposedly a more
even kiln firing. I get a measurable amount of fuel in proportion to air . =
I
get rid of a variable (secondary air) and in my area with the weather
changing by the hour it is very difficult to control secondary air bought i=
n
by negative pressure-- and I get an extra weeks wages in fuel saving per
year. If the electricity goes I can carry on firing past the temperature
where the pots would be spoilt with the other burners. I also get a very
good turndown and cooling control where I do not need such a violent flame.
all this is relatively cheap for me since I have enough second hand fir
bricks and flue to build a bigger chimney and can do all the plumbing mysel=
f
; plus I am not into fancy electronic controls which can always be added
later .
------=20
After Craig wrote about hybridising atmospheric burners to get greater
efficiency my idea for a time was to combine the two burner systems in one
burner ( this may also be Craig's idea).
If I build a primitive atmospheric system called an Alfred burner -
picture in olsons book - which would work all but inefficiently with the
addition of a lot of secondary air. And instead of using a T sectioned pip=
e
to let in primary air, I would use a cross section and put a pre heated air
pipe into the other side. I can now use either system atmospheric or forced
air in the burner. All I do is turn on the blower then close off the primar=
y
atmospheric air intake and close down the excess secondary air or visa vers=
a
depending on circumstances. This all can be done at any point during the
firing . Because the burners are essentially forced air I would not have to
put the fancy tapering on the burners - important in an efficient
atmospheric burner. Also the burner I have described is easily made out of
standard pipe fittings, a jet, and a small amount of jigging around with
threaders and a welder.
The only other thing playing on my mind is that I would not be able to do
any of this with two inch pipe burners , even with High pressure gas for a
16cu ft kiln . However if I could not finish a firing with the burners
running atmospherically it will not matter that much. All I need is to be
able to hold the firing while the electricity is reinstated or at worst get
the kiln past the temperature where the real damage to the ware occurs -
just before biscuit temperature.
I think what I do not like about the joint system is, that I feel my
knowledge of orifice (jet) sizes in conjunction with fuel pressure and
engineering structure is not good enough and so far I have had difficulty
getting the =8Cinfo=B9 I have. Also that I am probably doing both jobs
inadequately where as doing the two burner types separately I can use them
both at their optimum. But your comments would be welcome.
The odd thing is in that considering the hybrid system I suddenly realise
that by using a cross section pipe as opposed to a T section pipe when I
build my burner I can change the jets with out taking the whole system to
bits. so I could do a little research with the joint system if the mood
takes me.
To conclude it is very difficult to remember that even temperature,
precise and measurable evenly distributed atmosphere is my main criteria.
When all the other considerations like preheating air and safety systems ar=
e
to be thought about its difficult to keep a sense of priority. I want
something better than I have with my atmospheric burners because my pots an=
d
glaze need to be fired precisely and I am willing to pay for that .
The essential questions I have is . Does the secondary air brought into
the kiln by an external burner head depend on kiln pressure or is it
dependant on the venturie action of the burner?? OR To what extent does i=
t
make a variable that matters (remembering my criteria for precise atmospher=
e
control )?? If I can use 100% primary air in a forced air burner by using a=
n
internal ceramic burner head (as I use at the moment )I probably will. but
if the secondary air cooling down an external burner tip is not subject to
slight fluctuations in kiln presser I will change my mind.
Do forced air burners give a more even temperature through out the kiln o=
r
is their advantage just fuel saving ???
If any one has the perfect fit between jet size and gas pressure to give
me the optimum flame (forced air) or tables (I will try the gas supplier).
Again I am not quiet sure whether higher pressure is better. As said I do
not think natural gas pressure is a necessary presser to use; that could be
more of a commercial consideration for kiln manufacturers ???
-- =20
Regards from Paul Taylor
'Craftmanship is art'
http://www.anu.ie/westportpottery
phone 098 21239
Paul Taylor
Westport pottery=20
Liscarney=20
Westport=20
County Mayo
Ireland
Craig Martell on mon 30 dec 02
Paul was talking about burners and said:
>I think this is not helped by the way natural gas is treated as the same
>as propane in most of
>the books . I think kiln manufacturers consider them together because it
>halves there inventory . A burner designed for natural gas at 11 wg will
>fire with propane with no change in the system because LPG can be be
>governed down to natural gases low WG pressure
Hello Paul:
Propane is a very hi btu gas and it takes a lot more air to combust
properly so it is different than natural gas and requires a burner that
will move a lot of air. I'm sure you know that air is only about 20%
oxygen and the rest is mostly nitrogen. Nitrogen does nothing to aid
combustion and that's one of the reasons burners are designed to move air
primarily. The small orifice/high pressure propane burners will entrain
more air than a lower pressure natural gas burner with a bigger
orifice. That's the big difference. You need more air to combust propane
efficiently.
> From what I see the basic difference is between forced air and the
>atmospheric burners is, positive pressure (forced air) and negative pressure
>(atmospheric) .
That's what I've learned too. Forced air burners are a lot easier to
design and make but you have to contend with blowers, solenoid valves,
rheostats, etc. To move as much air naturally, with a venturi burner,
there's a lot more engineering and knowledge of physics required. I guess
that point makes us appreciate a well designed, efficient venturi.
>but the engineering of venturie (atmospheric) burners has to be very exact
>. What i suspect is, that these burners can not be run by primary air alone
>---- but I would like some one to confirm this ???.
I don't know who would give you confirmation in a quantitative
sense. Venturis in general don't have the hi efficiency that forced air
burners will have. You therefore will need good secondary air and a fair
amount of control to regulate this. That's not really a bad thing or a
major drawback to using venturis. Just the nature of the beast. Venturis
need to be designed to work as well as they possibly can and kilns need to
be designed well to use the secondary air.
>The essential questions I have is . Does the secondary air brought into
>the kiln by an external burner head depend on kiln pressure or is it
>dependant on the venturie action of the burner??
Secondary air is controlled by the size and relationships of the inlet
ports and the exit flue, plus the area of the stack and the height of the
stack. The amount of secondary air you need is related to burner
efficiency. The more efficient your burners, the less secondary air you need.
>Do forced air burners give a more even temperature through out the kiln or
>is their advantage just fuel saving ???
Their advantage, as I see things, is availability and contol of primary air
which makes it easier to achive stoichiometric combustion, and fuel
efficiency. Even temperature is achived with a good heat source, but
mainly with good kiln design.
regards, Craig Martell Hopewell, Oregon
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