Dave Finkelnburg on tue 8 may 01
Matt,
You asked about using the passive damper. In general, I believe the
active damper works best for a flue that is small relative to the size of
the kiln, and a passive damper is the one to use if the flue is oversized.
Think of any damper as a control valve. It controls the amount of
secondary air being pulled into the kiln around the burners, or in your
case, through the fire box.
Whether the damper is active (a "gate" you slide to close off part of
the flue) or passive (an opening you make larger or smaller so air is drawn
in there rather than from farther back in the kiln like at the burner port
or fire box) the idea is the same. Which one to use depends on which gives
you the best control.
Usually the gate-type damper doesn't really begin to have any effect on
controlling the amount of air coming into the kiln until the damper is at
least half closed. That's because the gate works by causing a pressure drop
across the gate. The velocity of the flue gas needs to increase
significantly (like double) before the amount of gas going around the end of
the damper begins to drop much. If the flue is very small, so the velocity
is high through the flue, then the damper gives you great control. If the
flue is oversized, flue gas velocity is low at the damper until the damper
is almost closed. Then all the control is in the last, say 10%, of the open
area between the damper end and the flue wall. That means at last you get
control, and a tiny change in the damper position makes a big change in the
amount of secondary air--big change in amount of reduction. Such oversized
flues are a real pain when trying to set the active damper because a tiny
bump and you get a big change. This is where the passive damper can be a
real help.
I hope this makes some sense. In the end, you have to judge whether you
are getting the right mix of combustion air and fuel. Figuring how much air
to let into the kiln reminds me of my wife's sage advice to me on how much
shortening to add to the pie dough. "Oh," she said casually, "use enough.
But not too much!" In other words, you learn from experience. :-)
Good firing!
Dave Finkelnburg in Idaho
Dave Finkelnburg on wed 9 may 01
Matt,
Thank you for your gracious comments about my reply to your damper
question.
Your next two questions reveal how little I really know! :-)
You asked,
>1> Do you find that closing the damper (or reducing draft in other ways)
>DOES cause the lower part of a downdraft kiln to heat up more rapidly?
Yes.
>2> HOW do you think that reducing draft would make the bottom heat
>differently?
IF your kiln is in oxidation (more air is coming in through the ports or
firebox than is necessary to burn the fuel) then any excess air is cooling
the kiln. All the incoming air is cooler and denser than the gases already
inside the kiln, so it's logical to imagine that at least some of the
incoming excess air will take the shortest path, across the kiln floor, to
the exit flue. Thus, you have a cold kiln floor, while ware closer to the
dome of the kiln remains hotter.
Closing down the damper reduces the amount of excess air coming into the
kiln, and very importantly, reduces the total volume of gases passing
through the kiln. This allows the other gases to remain in the kiln longer,
where the fuel has more chance to meet up with oxygen molecules and burn and
release heat.
This double benefit of fuel in the kiln longer and less cold air coming
into the kiln reaches it's maximum benefit at a "neutral" flame where fuel
and oxygen from the air just balance.
Any natural draft kiln with the burner ports or firebox near the floor,
and exit flue also near the floor, will tend to have the floor be the cool
spot of the kiln. The faster the firing the more apparent this will be.
Slowing the firing down near the end will allow radiant heat to bounce
around and even out the temperature throughout the kiln, until the coldness
near the floor can become insignificant.
I have to end this, Matt, by admitting that I had terrible problems
getting my gas downdraft to fire evenly top to bottom and front to back, and
everything I did to fix the problem I learned from ClayArters. I'm sure oth
ers will have better answers to your questions than I do.
Good firing!
Dave Finkelnburg
Matt MacIntire on wed 9 may 01
Dave,
I have read many books about kilns, but I have never read such an clear
explanation of how a damper works. I thank you for taking the time to
clarify this for me.
Your comments make perfect sense and are quite helpful. From your
explanation, I understand much better how these two adjustments will work
together. As I learn to use these better, I will have some framework for
making reasonably intelligent decisions about how to make adjustments.
Our chimney has a huge cross section. It is the full width of the kiln,
gradually tapering to a normal size chimney at about 6 feet. The two
horizontal dampers are in a middle section and I'd guestimate the cross
section at the dampers is around 4-5 square feet. I had noticed that the
damper adjustments have little effect until they begin to get at least half
closed. Now I understand a much better how this works.
I wonder if you would comment on another aspect of firing control that I
have always wondered about...
When firing a downdraft kiln, and the bottom of the kiln is not getting as
hot as the top, I have always heard folks say that closing the damper would
"force the heat down." Though closing the damper does seem to have some
(minimal) effect in my observation, I was never satisfied with this
simplistic explanation of why this might help. Two questions ensue...
1> Do you find that closing the damper (or reducing draft in other ways)
DOES cause the lower part of a downdraft kiln to heat up more rapidly?
2> HOW do you think that reducing draft would make the bottom heat
differently?
While I don't buy the "forcing" it down line, it is clear that altering the
draft will surely alter the main path of the flame itself. This would
certainly effect how different parts of the kiln heat up. I'd be very
curious to learn your thoughts on this.
Thank you for your help.
Matt
Hank Murrow on wed 9 may 01
Dear Dave;
REAL NICE POST, GUY!
Hank in Eugene
>Matt,
> You asked about using the passive damper. In general, I believe the
>active damper works best for a flue that is small relative to the size of
>the kiln, and a passive damper is the one to use if the flue is oversized.
> Think of any damper as a control valve. It controls the amount of
>secondary air being pulled into the kiln around the burners, or in your
>case, through the fire box.
> Whether the damper is active (a "gate" you slide to close off part of
>the flue) or passive (an opening you make larger or smaller so air is drawn
>in there rather than from farther back in the kiln like at the burner port
>or fire box) the idea is the same. Which one to use depends on which gives
>you the best control.
> Usually the gate-type damper doesn't really begin to have any effect on
>controlling the amount of air coming into the kiln until the damper is at
>least half closed. That's because the gate works by causing a pressure drop
>across the gate. The velocity of the flue gas needs to increase
>significantly (like double) before the amount of gas going around the end of
>the damper begins to drop much. If the flue is very small, so the velocity
>is high through the flue, then the damper gives you great control. If the
>flue is oversized, flue gas velocity is low at the damper until the damper
>is almost closed. Then all the control is in the last, say 10%, of the open
>area between the damper end and the flue wall. That means at last you get
>control, and a tiny change in the damper position makes a big change in the
>amount of secondary air--big change in amount of reduction. Such oversized
>flues are a real pain when trying to set the active damper because a tiny
>bump and you get a big change. This is where the passive damper can be a
>real help.
> I hope this makes some sense. In the end, you have to judge whether you
>are getting the right mix of combustion air and fuel. Figuring how much air
>to let into the kiln reminds me of my wife's sage advice to me on how much
>shortening to add to the pie dough. "Oh," she said casually, "use enough.
>But not too much!" In other words, you learn from experience. :-)
> Good firing!
> Dave Finkelnburg in Idaho
vince pitelka on wed 9 may 01
> When firing a downdraft kiln, and the bottom of the kiln is not getting as
> hot as the top, I have always heard folks say that closing the damper
would
> "force the heat down." Though closing the damper does seem to have some
> (minimal) effect in my observation, I was never satisfied with this
> simplistic explanation of why this might help.
Matt -
Whoever told you that had their information backwards. Think of the draw at
the flue opening. If you close the damper, the flames and/or heat from the
burners will tend to circulate upwards into the chamber bringing more heat
upwards. If you open the damper, the draw will pull the flames directly to
the flue, concentrating more heat at the bottom. With an updraft kiln it is
exactly the opposite - opening the damper pulls the heat to the top, closing
concentrates it at the bottom. Of course opening the damper excessively in
either case sucks in too much secondary air, decreasing efficiency, and
closing it too much causes reduction, decreasing efficiency. You can
usually make necessary adjustments with a neutral or slightly reducing
flame, so that you maintain combustion efficiency.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
craig clark on wed 9 may 01
Matt, one of the best explantions that I ever recieved about the even
firing of any kiln involved picturing the shape of a flame, the hottest part
of that flame, and the length of the flame. This was given in reference to
gas fired kilns but I believe the general description of what happens with a
flame may be applicable to a wood fired kiln as well.
When you adjust the dampers you are adjusting the secondary air of the
kiln. By doing this you are affecting the atmosphere in the kiln by
effectively lowering or increasing the amount of oxygen that is available
for combustion in the kilns cavity. This inturn affects the amount of free
carbon in the atmosphere and the length of the flame. Opening the dampers
shortens the flame. Closing the dampers lengthens the flame. Keep in mind
the length of the flame and where you want the hottest part of the flame to
be in your kiln if you are after an even firing.
It is the balance between the adjustment of your primary air, at the
burners, the flow of the gas at the burners, and the adjustment of the
secondary air that brings about an even heat. This is a balance that varies
from kiln to kiln. If the bottom of your downdraft is firing cool try
lengthening the flame, that always worked on our salt kiln. As you progress
in the firing try and keep the bottom a little ahead of the top and just
open things up a bit at the end. The top will catch up to the bottom quite
quickly and then you can re-reduce if need be.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
-----Original Message-----
From: Matt MacIntire
To: CLAYART@LSV.CERAMICS.ORG
Date: Wednesday, May 09, 2001 12:27 PM
Subject: Re: wood kiln--passive damper (long)
>Dave,
>
>I have read many books about kilns, but I have never read such an clear
>explanation of how a damper works. I thank you for taking the time to
>clarify this for me.
>
>Your comments make perfect sense and are quite helpful. From your
>explanation, I understand much better how these two adjustments will work
>together. As I learn to use these better, I will have some framework for
>making reasonably intelligent decisions about how to make adjustments.
>
>Our chimney has a huge cross section. It is the full width of the kiln,
>gradually tapering to a normal size chimney at about 6 feet. The two
>horizontal dampers are in a middle section and I'd guestimate the cross
>section at the dampers is around 4-5 square feet. I had noticed that the
>damper adjustments have little effect until they begin to get at least half
>closed. Now I understand a much better how this works.
>
>I wonder if you would comment on another aspect of firing control that I
>have always wondered about...
>
>When firing a downdraft kiln, and the bottom of the kiln is not getting as
>hot as the top, I have always heard folks say that closing the damper would
>"force the heat down." Though closing the damper does seem to have some
>(minimal) effect in my observation, I was never satisfied with this
>simplistic explanation of why this might help. Two questions ensue...
>
>1> Do you find that closing the damper (or reducing draft in other ways)
>DOES cause the lower part of a downdraft kiln to heat up more rapidly?
>
>2> HOW do you think that reducing draft would make the bottom heat
>differently?
>
>While I don't buy the "forcing" it down line, it is clear that altering the
>draft will surely alter the main path of the flame itself. This would
>certainly effect how different parts of the kiln heat up. I'd be very
>curious to learn your thoughts on this.
>
>Thank you for your help.
>
>
>Matt
>
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>
Matt MacIntire on thu 10 may 01
Vince,
Thanks for your informative post.
I had heard this mis-information from several different people who have done
more firing than I have. I imagine they were confused by the difference
between updraft and downdraft kilns, as you suggested. Somehow this idea
never sounded right to me, hence my recent question. I'm glad to finally
get some clarification.
In my mind I did visualize the path of the flame such as you described, with
increased draft sucking the flame across the bottom of the kiln faster. But
since several people gave me the same erroneous information, I doubted my
own thinking. Thanks for clearing this up.
What a great resource the list has been to me lately. Everyone is so
willing to share their time and experience. I have learned a great deal
from these recent discussions. I really appreciate everyone's help.
Matt
Matt MacIntire on thu 10 may 01
Craig,
On first reading, the way you visualize the flame length seemed a bit
different from how I was focusing on the draft in the kiln. Yet the more I
thought about it, the more I could see that thinking about the flame length
is entirely consistent with thinking mainly about the draft, such as Vince
described in his message.
Increasing the draft increases the secondary air and shortens the flame.
Same result, just slightly different terminology and descriptive language.
I feel that I understand how to regulate the fire much better now. My
thanks to you, and others who have helped me learn about controlling a wood
fire.
Matt
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