Hank Murrow on fri 3 jan 03
From: Hank Murrow
Date: Fri Jan 3, 2003 10:06:35 PM US/Pacific
To: paul taylor
Subject: your burner question. Finally
On Saturday, December 28, 2002, at 09:33 AM, Paul Taylor wrote:
> Dear Craig and All
>
> About burners again - (needs) 1 Even temperature, 2=20
> Atmosphere control, 3 soak and or ramp
control on cooling, and 3 The weather is so unpredictable and scrawly=20
here that you can not rely with any consistency, on the chimney to do=20
any thing but vent kiln gases.
***** Even temperatures and atmosphere control are more a result of=20
design/build decisions than burner configuration. Soak control is easy=20=
either way. In Aspen, where weather was often violent, we had no=20
difficulty with our chimneys. Where are you located?
(would be nice) 1 preheated air , 2 quiet burners.
****** Low pressure will be quieter.
I do not need : too many variables and uneven temperature . I am happy
enough to adjust settings manually so I do not need computerised=20
controls.
***** You are making a pretty good case here for a natural draft=20
configuration........no electricity to fail, no blowers to fail.
Since I am firing with propane (LPG) I can fire at a high pressure up=20=
to 12 ppsi and higher. I am designing a burner that does not have to=20
have the compromise of low pressure that natural gas would force on me.
***** In my experience of around 120 kilns built, there is no such=20
compromise. I have found that propane is most effective and quiet at=20
10" water column pressure.
=46rom what I see the basic difference is between forced air and the=20
atmospheric burners is, positive pressure (forced air) and negative=20
pressure (atmospheric). The advantage of the atmospheric burner is that=20=
it works with out electricity and is very safe to run also I get the=20
sort of gentle tun down needed for cooling control.
**** Amen, brother.
But the engineering of venturi (atmospheric) burners has to be very=20
exact. What I suspect is, that these burners can not be run by primary=20=
air alone---- but I would like some one to confirm this ???.
***** Though they can be run on 100% primary, I often run them with 50%=20=
primary and 50% secondary, the key is to be very careful in casting=20
your burner ports according to the specs of the Manufacturer. If you do=20=
this correctly, secondary air will exactly match primary air at all=20
times and settings.
If I build a primitive atmospheric system called an Alfred burner=20
-picture in olsons book - which would work all but inefficiently with=20
the addition of a lot of secondary air. Also the burner I have=20
described is easily made out of
standard pipe fittings, a jet, and a small amount of jigging around=20
with threaders and a welder.
***** You are correct to call the Alfred type primitive. I have found=20
them to have very poor turndown. And the flame often burns inside the=20
burner, rather than at the tip. Bad.
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=20
difficulty
getting the =91info=92 I have. Also that I am probably doing both jobs
inadequately where as doing the two burner types separately I can use=20
them
both at their optimum. But your comments would be welcome.
***** OK, here they are: Design an atmospheric burner system using=20
Eclipse mixers sized to accommodate 50/50 mix of primary/secondary air.=20=
Cast your burner ports with a super duty castable like Mizzou. They=20
will all be exactly the same if you make a good mold. Manifold two or=20
more tips for each mixer, one mixer for each side of the kiln. Use=20
Sticktite flame retention tips. Run the system on 10" wc. Build a=20
generous chimney with the damper as high up as practical. Add some sort=20=
of shutter to close off the burner ports to allow slower cooling as=20
demanded. Use a U-tube manometer to measure gas pressure (you can make=20=
yourself). Eclipse mixers have easily removable orifii if plumbed with=20=
this in mind.
To conclude it is very difficult to remember that even temperature,
precise and measurable evenly distributed atmosphere is my main=20
criteria.
When all the other considerations like preheating air and safety=20
systems are
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=20
pots and
glaze need to be fired precisely and I am willing to pay for that .
***** I think your experience has taught you to be suspicious of the=20
system you had. I would design a better atmospheric system.
The essential questions I have is . Does the secondary air brought=20
into
the kiln by an external burner head depend on kiln pressure or is it
dependant on the venturie action of the burner??
***** The Eclipse atmospheric system draws in primary air in proportion=20=
to the gas flow. Secondary air flow is dependent on the design of the=20
cast ports.
OR To what extent does it make a variable that matters (remembering my=20=
criteria for precise atmosphere
control )??
***** The damper will effectively control the secondary air.
Do forced air burners give a more even temperature through out the=20
kiln or
is their advantage just fuel saving ???
**** I would dispute this finding. My cubic meter kiln uses 18 therms=20
of Natural gas.
In sum, you will learn a lot by taking on this project, as I did over=20
the suite of kilns I've built. So go ahead with your best plan. I offer=20=
my thoughts as one who has traveled the road a bit further.
Cheers, Hank in Eugene=
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